DHC-6 TWIN OTTER

MAINTENANCE TRAINING
MANUAL
VOLUME 2
ATA 24, 36, 21, 35, 29, 32, 27, 30, 33, 31, 23, 34, 22
REVISION 0.4

FlightSafety International, Inc.

Marine Air Terminal, LaGuardia Airport
Flushing, New York 11371
(718) 565-4100
www.FlightSafety.com
 FOR TRAINING PURPOSES ONLY

NOTICE

The material contained in this training manual is based on information obtained from
the aircraft manufacturer’s Maintenance Manuals and Pilot Manuals. It is to be used for
familiarization and training purposes only.

At the time of printing it contained then-current information. In the event of conflict between
data provided herein and that in publications issued by the manufacturer or the FAA, that
of the manufacturer or the FAA shall take precedence.

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you might have for improving this manual or any other aspect of our training program.

FOR TRAINING PURPOSES ONLY

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Courses for the Dash 6 Series and other deHavilland aircraft are taught at the following
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 INSERT LATEST REVISED PAGES, DESTROY SUPERSEDED PAGES
LIST OF EFFECTIVE PAGES

Dates of issue for original and changed pages are:

Second Edition..... 0.0..... September 2014 Revision............... 0.3......... October 2015

Revision............... 0.1......... October 2014 Revision............... 0.4...... November 2015
Revision............... 0.2............... April 2015

THIS PUBLICATION CONSISTS OF THE FOLLOWING:

Page *Revision Page *Revision

No. No. No. No.

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 CONTENTS
VOLUME 2
Chapter Title ATA Number
ELECTRICAL POWER 24
PNEUMATIC 36
AIR CONDITIONING 21
OXYGEN 35
HYDRAULICS 29
LANDING GEAR 32
FLIGHT CONTROL 27
ICE PROTECTION 30
LIGHTING 33
INDICATING AND RECORDING 31
COMMUNICATION 23
NAVIGATION 34
AUTOFLIGHT 22
APPENDICES
 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CHAPTER 24
ELECTRICAL
CONTENTS
Page

24-00-00 ELECTRICAL POWER SYSTEMS............................................................. 24-1

Introduction......................................................................................................... 24-1
General................................................................................................................ 24-5
Safety Precautions................................................................................................ 24-7
Controls and Description...................................................................................... 24-9
Battery/Off/External Switch.......................................................................... 24-9
Master Switch............................................................................................... 24-9
Cover Guarded Normal Manual Switch......................................................... 24-9
Starter Switch............................................................................................... 24-9
Generator Switches....................................................................................... 24-9
BUS TIE Switch............................................................................................ 24-9
Main Battery............................................................................................... 24-11
Battery Venting........................................................................................... 24-13
Auxiliary Battery........................................................................................ 24-15
NiCad Battery Temperature Monitoring System.......................................... 24-17
External Power................................................................................................... 24-18
24-00-00 MAINTENANCE PRACTICES.................................................................. 24-19
Servicing............................................................................................................ 24-19
Application of External Power..................................................................... 24-19
Removal of External Power......................................................................... 24-19
Hot Battery Bus................................................................................................. 24-19

FOR TRAINING PURPOSES ONLY 24-i

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TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page

Bus Feed Current Limiters.......................................................................... 24-21

Starter/Generators.............................................................................................. 24-23
Protection.................................................................................................... 24-23
Starting....................................................................................................... 24-23
Generation.................................................................................................. 24-23
Indication.................................................................................................... 24-25
Generator and Starter Relays....................................................................... 24-26
Voltage Regulators...................................................................................... 24-26
Overvoltage Relays..................................................................................... 24-27
24-30-00 DC METERS............................................................................................. 24-29
Voltmeter........................................................................................................... 24-29
DC Load Meter.................................................................................................. 24-31
Reverse-Current Relays...................................................................................... 24-33
Distribution........................................................................................................ 24-35
Circuit Breakers.......................................................................................... 24-36
Main Distribution Box................................................................................ 24-37
Power Distribution and Generator Control Box (Post Mod 6/1274)............. 24-38
Power Distribution and Generator Control Box (Post Mod 6/1591)............. 24-39
Power Distribution and Generator Control Box........................................... 24-40
Generator Field Control Relay (LGFC or RGFC)........................................ 24-42
Reverse Current Relay................................................................................. 24-42
24-20-00 AC POWER SYSTEM................................................................................ 24-45
General.............................................................................................................. 24-45
Switch Control............................................................................................ 24-45
Indication.................................................................................................... 24-45

24-ii FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page

Inverters............................................................................................................. 24-47
Standard Installation................................................................................... 24-47
24-21-00 MAINTENANCE PRACTICES.................................................................. 24-48
Adjustment/Test................................................................................................. 24-48
Operational Test Inverter System................................................................. 24-48
24-23-00 STATIC INVERTER 400VA SYSTEM....................................................... 24-51
General.............................................................................................................. 24-51
Operation........................................................................................................... 24-51
24-24-00 STATIC INVERTERS S.O.O. 6142............................................................ 24-53
AC Equipment............................................................................................. 24-53
24-30-00 MAINTENANCE PRACTICES.................................................................. 24-55
Wire Coding....................................................................................................... 24-55
Limitations......................................................................................................... 24-57
Troubleshooting................................................................................................. 24-57
AC Power System............................................................................................... 24-60
Inverters............................................................................................................. 24-60
Standard Installation................................................................................... 24-60
Static Inverter - 65VA System............................................................................ 24-63
Description.................................................................................................. 24-63
Operation.................................................................................................... 24-63

FOR TRAINING PURPOSES ONLY 24-iii

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

ILLUSTRATIONS
Figure Title Page

24-1 Electrical System Controls.........................................................................24-2

24-2 Electrical System Schematic with RCCB...................................................24-4
24-3 Electrical System Post Mod 6/1651 with Current Limiters.........................24-6
24-4 Pilot Overhead DC Control Panel..............................................................24-8
24-5 Generator and Bus Tie Switches................................................................24-8
24-6 Main Battery............................................................................................24-10
24-7 Battery Venting System............................................................................24-12
24-8 Battery Sump Drain.................................................................................24-12
24-9 Auxiliary Battery Installation..................................................................24-14
24-10 Battery Temperature Monitor Panel.........................................................24-16
24-11 Battery Temperature Monitor Sensor Locations.......................................24-16
24-12 External Power Receptacle.......................................................................24-18
24-13 External Power Cable...............................................................................24-18
24-14 Hot Battery Bus.......................................................................................24-19
24-15 Bus Feed Current Limiters (1 of 2)..........................................................24-20
24-16 Bus Feed Current Limiters (2 of 2)..........................................................24-20
24-17 DC Power and Engine Starter Panel.........................................................24-21
24-18 Starter Generator Installation...................................................................24-22
24-19 Generator Brush and Spring Position.......................................................24-22
24-20 DC Power and Engine Start Panel............................................................24-24
24-21 Generator and Bus Tie Controls...............................................................24-24
24-22 
Generator and Starter Relay Location......................................................24-26
24-23 New Voltage Regulator............................................................................24-26

FOR TRAINING PURPOSES ONLY 24-v

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure Title Page

24-24 Old Style Voltage Regulators...................................................................24-26

24-25 Overvoltage Relay Panel..........................................................................24-27
24-26 DC Volts Meter Panel..............................................................................24-28
24-27 DC Load Meter Panel..............................................................................24-30
24-28 DC Generator Shunt................................................................................24-31
24-29 Reverse-Current Relay Locations.............................................................24-32
24-30 DC Contactor Box...................................................................................24-33
24-31 Distribution Diagram...............................................................................24-34
24-32 Circuit Breaker Panels.............................................................................24-36
24-33 Main Distribution Box.............................................................................24-37
24-34 Power Distribution and Control Box (Post Mod 6/1274)..........................24-38
24-35 Power Distribution and Control Box (Post Mod 6/1591)..........................24-39
24-36 Power Distributor and Generator Control Box (Post Mod 6/1651)............24-40
24-37 DC Bus Feed Circuits..............................................................................24-41
24-38 MMEL - Electrical Power........................................................................24-43
24-39 Inverter Single Switch.............................................................................24-44
24-40 Inverter Dual Switch (Optional Mod)......................................................24-44
24-41 Inverter Installation.................................................................................24-46
24-42 Rear Fuselage Interior.............................................................................24-47
24-43 400VA Inverter System Schematic...........................................................24-50
24-44 Dual Inverter Schematic..........................................................................24-52
24-45 Wire Coding Example..............................................................................24-54
24-46 65VA Inverter System - Schematic (Pre Mod 6/1274)..............................24-62
24-47 65VA Inverter System - Schematic (Mod 6/1274)....................................24-64

24-vi FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

TABLES
Table Title Page

24-1 Temperature/Voltage Settings............................................................24-26

24-2 DC Generation Troubleshooting (Sheet 1 of 3)....................................24-56
24-3 Generator Load Limitations..............................................................24-57
24-4 DC Generation Troubleshooting (Sheet 2 of 3)....................................24-58
24-5 DC Generation Troubleshooting (Sheet 3 of 3)....................................24-59

FOR TRAINING PURPOSES ONLY 24-vii

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CHAPTER 24
ELECTRICAL

24-00-00 ELECTRICAL POWER SYSTEMS

INTRODUCTION
The DHC-6 Twin Otter Series 300 electrical system functions primarily on DC power.
28VDC power to inverters produce AC power used for avionics and some engine
instruments.
Aircraft 28VDC electrical power is provided by two starter-generator systems in conjunction
with two 24 Volt batteries (main and auxiliary). The DC system is a single wire type
installation using the airframe structure as ground return. The main battery supplies all DC
power necessary when the generators are selected off and is also used to start the engines.
The auxiliary battery provides an independent source of power for the engine start and
ignition control circuits.
The user should consult the Aircraft Maintenance Manual (AMM), applicable AFM
supplements and vendor manuals for additional information on specific manufacturers
installations not included in this chapter.

FOR TRAINING PURPOSES ONLY 24-1

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 24-1. Electrical System Controls

24-2 FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Aircraft alternating current (AC) is NOTES

provided as follows:
•• 250VA static inverter system.
•• 400VA static inverter system.
Each inverter gives 115/26 Volt, 400 Hz
single phase outputs from a 28VDC input.
With transformers fitted, the 250VA and
400VA systems can provide an additional
26 Volt output for extra AC instruments.
An external 28VDC power source can be
connected to the aircraft for engine starting
and test purposes.
DC circuits are protected by thermal push-pull
circuit breakers and the AC circuits by fuses.

FOR TRAINING PURPOSES ONLY 24-3

24-4 24 ELECTRICAL POWER

L GEN R GEN
DC MASTER
OFF OFF
ON ON RESET NO 1
OFF INV
RESET STARTER
LEFT RESET
GENERATOR
STARTER
GENERATOR
EXT
L GEN EXT R GEN NO 2
O/H PWR OFF INV
O/H
BATT
BATT

TWIN OTTER SERIES

SHUNT

REVERSE - MAIN
REVERSE
CURRENT MAIN CURRENT
BATTERY
RELAY BATT RELAY
RELAY
+ RADIO AC
RADIO
AC SUPPLY
BATT/EXT MAIN PROVISION
PWR BUS BATT BUS
CONT REL
REVERSE 115-VAC
FOR TRAINING PURPOSES ONLY

CURRENT 400-Hz
CB BUS

EXT
PWR
RELAY EXT PWR 26-VAC XFMR
RECEPTACLE 400-Hz
BUS

BATT AC
FAIL INV INV
RELAY NO 1 NO 2
L GEN R GEN

MAINTENANCE TRAINING MANUAL

INDICATOR SELECT

CAUTION
DC VOLTS DC LOAD
400 Hz LT

AUX BATT AUX

RELAY + BATT
INV
- RELAY
L ENG R ENG
START START
L ENG R ENG
IGN IGN

BUS-TIE
RELAY

NO 2 INV CONT
VOLTMETER

400-Hz FAIL

NO 2 INV
NO 1 INV

NORM BUS
TIE

OPEN

LEGEND
GENERATOR POWER AUXILIARY POWER NO 1 INVERTER 26-VAC POWER
BATTERY POWER EXTERNAL POWER NO 2 INVERTER 115 VAC POWER

Figure 24-2. Electrical System Schematic with RCCB

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

GENERAL NOTES
Refer to:

•• F i g u r e 2 4 - 2 . E l e c t r i c a l S y s t e m
Schematic with RCCB.
•• Figure 24-3. Electrical System Post
Mod 6/1651 with Current Limiters.

The DC generation system provides the aircraft

with the primary source of 28VDC power, and
also provides the starting facility for starting
the aircraft engines (refer to “Starting with
Battery Power with Fuel Selected On” page
24-12 in MSM). The system consists of left
and right starter-generator systems designed
to either operate in parallel during normal
operation or, if required, both systems can
be operated as completely separate power
systems as determined by a BUS TIE switch.
A secondary source of DC power is provided
by a 24 Volt main battery and, in addition, a
24 Volt auxiliary battery gives an additional
independent source of DC power for the start
and ignition control circuits.

The auxiliary bus is supplied through a relay

from the auxiliary battery and through a diode
from the left DC bus. Diodes are incorporated
for AC rectification or to block a DC circuit
feedback.

Generator line voltage is indicated by a

voltmeter, and generator load and battery
charge/discharge condition is indicated
on a load meter. Each generator system is
provided with GENERATOR fail lights
and GENERATOR OVERHEAT lights and
associated sensors can be fitted.

An external power receptacle is provided for

connection to a 28VDC ground supply for
engine starting and test purposes. Figure 24-3
shows the electrical system controls with a
current limiter configuration and Figure 24-2
illustrates the electrical system in schematic
form showing the RCCB configuration.

FOR TRAINING PURPOSES ONLY 24-5

24-6 24 ELECTRICAL POWER

GEN GEN
RESET L GEN R GEN RESET
OFF DC MASTER OFF

ON ON
RESET OFF
RESET RIGHT
STARTER-
GENERATOR

EXT R GEN
L GEN SHUNT
EXT
LEFT SHUNT OFF
PWR
STARTER R GENERATOR
L GENERATOR BATT
GENERATOR

BATT REVERSE
START REVERSE

TWIN OTTER SERIES

SHUNT CURRENT
SWITCH R CURRENT
CUTOUT
CUTOUT MAIN
BATT
L RELAY
IGNITER NO MAIN
SWITCH BATT/EXT MAIN BATTERY
1 PWR BUS BATT BUS
BOTH
FOR TRAINING PURPOSES ONLY

2
MAN IGN
MODE

MOD 6/1651
EXT
PWR
RELAY

EXT PWR
1 2 RECEPTACLE

MAINTENANCE TRAINING MANUAL

BATT

L GEN R GEN

INDICATOR SELECT

DC VOLTS

AUX BATT
AUXILIARY
RELAY
BATTERY
DC LOAD
L ENG
START
R ENG
IGN
L ENG
R ENG IGN
START
BUS-TIE
VOLTMETER RELAY

AUX BATT BUS

400 Hz FAIL

NO.1 INV
NORM BUS
TIE

OPEN

L 28-VDC BUS

Figure 24-3. Electrical System Post Mod 6/1651 with Current Limiters
 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

SAFETY PRECAUTIONS NOTES

The following safety precautions should be
observed prior to the removal or installation of
any electrical equipment in the airplane:

1. Refer to the appropriate wiring diagram in

the Wiring Diagram Manual, PSM 1-63-
2W, prior to removal and installation of any
electrical equipment or components and when
disconnecting or connecting electrical wiring.
2. Ensure that the DC MASTER and BATTERY-
EXTERNAL switches are set to OFF.
3. Disconnect the external power source,
pull the COMP LTS circuit breaker, and
disconnect the main battery. Pull out the
applicable circuit breaker, and insert a clip
to prevent resetting.
4. If units removed are not to be immediately
replaced, ensure that the connector ends are
capped and stowed and that all loose wires
are insulated and stowed.
5. After installation, remove the clip and push
in the circuit breaker; perform a functional
test on the unit.

Removal and installation procedures for the

electrical equipment are self-evident except
for the main and auxiliary batteries. Equipment
grounding and bonding surfaces must be
cleaned, and after installation any bared
surfaces must be finished. When test or repair
requires removal of cables from terminals, the
terminal nuts must be tightened to the proper
torque values when the cables are reconnected.

FOR TRAINING PURPOSES ONLY 24-7

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 24-4. Pilot Overhead DC Control Panel

Figure 24-5. Generator and Bus Tie Switches

24-8 FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CONTROLS AND DESCRIPTION Generator Switches

Refer to Figure 24-5. Generator and Bus Tie
Refer to Figure 24-4. Pilot Overhead DC
Switches.
Control Panel.
Two GENERATOR switches, labeled LEFT
Battery/Off/External Switch and RIGHT, are on the overhead console
(Figure 24-5). Each is a three-position switch
The purpose of this switch is to allow the pilot
with OFF, ON, and RESET positions.
to select the source of power to be applied to the
aircraft buses.
The ON position connects the applicable
generator output to the electrical bus through
Master Switch a reverse current relay and a voltage regulator.
OFF disconnects the generator. RESET is
The purpose of this switch is to complete selected
a spring-loaded position used to bring the
circuits and to connect all DC power sources to
generator on-line.
ground.
Control voltage used to close the reverse current
Cover Guarded Normal Manual relays when the generators are brought on line
is supplied by two 10-amp circuit breakers
Switch labeled GEN CONTROL L and R, which
This switch on the DC Control panel over are located on the overhead circuit breaker
the pilot head, is used to allow the normal panel. Voltage used to excite the generator
electrical sequencing during and engine start field circuits is protected by two 5-amp circuit
by controlling either K7 or K8. breakers labeled GEN FIELD, which are in the
engine nacelles, just aft of the oil dipstick door.
When the switch is selected to the MANUAL
position, all start circuits are inhibited except K7
and K8. As both relays are energized the ignitor
BUS TIE Switch
circuits are energized but the inhibiting of A single pole two-position switch on the
Paralleling circuits and load meter are bypassed. overhead console (Figure 24-5). Its function
is to control K3, which opens or closes the
connection between the left and right main
Starter Switch buses. The relay coil is in the generator control
A three-position, toggle lock switch labeled box, between K1 and K2. Either main bus can
LEFT - START - RIGHT controls power power K3.
supplied to the starter/generators. The switch
is spring loaded to the unmarked centre OFF
position. When the switch is engaged, a control
voltage is supplied from the auxiliary bus to
the selected reverse current relay, causing it
to engage and electrically connect the selected
starter generator circuits only to the batteries
or external power.

FOR TRAINING PURPOSES ONLY 24-9

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 24-6. Main Battery

24-10 FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Main Battery NOTES

Refer to Figure 24-6. Main Battery.

The main battery is a 24 Volt, 40-amp hour,

nickel cadmium type (20 cell), below the floor
in the rear baggage compartment. The battery
is provided with a cover and sealing gasket
to prevent electrolyte spillage into the battery
compartment.

NOTE
If Mod 6/1611 is installed this
battery is basic to the aircraft.

•• S.O.O. Mod 6091 – 24 Volt, 36-amp-

hour, lead-acid type.

FOR TRAINING PURPOSES ONLY 24-11

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

ADAPTER*

HOLDDOWN STRAP
CLAMP TIEDOWN BOLTS

VENT HOSE

BATTERY
CONNECTOR

ADAPTER*

TEMPERATURE SENSOR
SUMP JAR CONNECTOR (MOD 6/1479)
(TYPICAL)

ADAPTER*
BATTERY

VENT HOSE

Figure 24-7. Battery Venting System

Figure 24-8. Battery Sump Drain

24-12 FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Battery Venting NOTES

Refer to:

•• Figure 24-7. Battery Venting System.

•• Figure 24-8. Battery Sump Drain.

The venting system is included to vent gas

pressure build up within the battery box,
especially during the recharging phase after
engine start. Gas is vented from the battery
box into a glass sump jar, then exits through
a vent hose opening in the fuselage under
the rear baggage compartment. The sump
jar, is a sealed glass container with a one-
inch thick felt pad on the bottom, traps any
electrolyte which maybe released by the battery
through the venting system. The pad must
be saturated with 3% boric acid and distilled
water solution for NiCad batteries. For lead-
acid batteries a 1-part sodium bicarbonate
8-parts distilled water solution to neutralize any
electrolyte spillage that may occur. The vent
tube from the sump jar outlet to the fuselage
skin, correctly installed, prevents electrolyte
contamination adversely effecting adjacent
structure. To maintain certification standards
a venting system must be installed even if the
recombinant style sealed lead acid battery is
installed.

NOTE
Always ensure that the proper
neutralizing solution is used in
the sump jar, (e.g.; For aircraft
fitted with a lead-acid battery
use sodium bicarbonate [baking
soda]) and for aircraft fitted
with a NiCad battery use boric
acid. Do not use tap water as
the purification chemicals may
inhibit the neutralizing agents.

FOR TRAINING PURPOSES ONLY 24-13

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 24-9. Auxiliary Battery Installation

24-14 FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Auxiliary Battery NOTES

Refer to Figure 24-9. Auxiliary Battery
Installation.

The auxiliary battery is a 24V, 3.6-amp-hour

NiCad battery to provide a separate power
source for engine start control relays and
Igniter and glow plug) operation. With the
auxiliary battery installed, faster and cooler
engine starts are possible, because auxiliary
battery voltage supplied to the glow plugs and
start control relays does not suffer from the
same voltage drop that occurs when the main
battery is engaged to turn the starter/generator.

The auxiliary battery is installed on the right

side of the forward wall of the rear baggage
compartment (Figure 24-9). The auxiliary
battery should not be removed from the aircraft
without regulatory approval, because the
battery was part of the Series 300 approved
equipment when the aircraft was certified.
deHavilland does not recommend removal of
the auxiliary battery, as removing the battery
eliminates the benefits provided, especially
when ground power equipment is not always
available.

FOR TRAINING PURPOSES ONLY 24-15

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 24-10. Battery Temperature Monitor Panel

HEAVY-DUTY NICAD LIGHT-DUTY NICAD

MARATHON 29094 (MOD 6/1479)

Figure 24-11. Battery Temperature Monitor Sensor Locations

24-16 FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

NiCad Battery Temperature The warning light is also connected to the

caution lights dim/test circuit, and the indicator
Monitoring System integral light is connected to the caution light
A battery temperature monitor system (Figure dimming control circuits. The monitoring
24-10 and Figure 24-11) is provided as standard system is powered from the left 28VDC bus,
equipment for aircraft using NiCad batteries. through a BATT O/HEAT circuit breaker on
A FAA regulation required all main NiCad the overhead circuit-breaker panel. The monitor
batteries, to provide the pilot with a continuous has internal lights for night operation, and
battery temperature indication and early brightness of the red warning lamp is controlled
warning of a battery high temperature condition by the position of the CAUTION LTS switch.
occurring. This enabled action to be taken by
the crew to reduce the battery temperature,
avoiding possible thermal runaway, overheat
conditions and subsequent battery failure. The
regulation was a result of a battery overheat
incident on another aircraft type which resulted
in structural damage. However, the likelihood of
a battery overtemperature occurring is greatest
following engine start, when the battery is being
recharged.

The Marathon brand monitor installed in Twin

Otters consists of two sensors each mounted
separately on the battery inter-cell connecting
links nearest to the centrally located cells.
Maximum temperature usually occurs in the
centre of the battery. The sensor signals are
fed to two separate systems, a red warning light
and a dial type battery temperature indicator.

The monitor warning light and indicator are

both mounted on a panel together with LAMP
and IND circuit breakers and a push button
TEST switch.

The monitor is installed below the co-pilot

flight panel, adjacent to the centre pedestal. The
monitor has a push button test facility which,
through an integral sensor heater, provides a
simulated overheat condition to verify the correct
function of the warning light and indicator.

When the TEST button is actuated, a simulated

overheat condition is provided by an integral
sensor heater, which verifies the correct
function of the warning light and indicator.

FOR TRAINING PURPOSES ONLY 24-17

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

EXTERNAL POWER
Refer to:

•• Figure 24-12. External Power

Receptacle.
•• Figure 24-13. External Power Cable.

The external power receptacle is located on the

left aft side of the fuselage.

A spring-loaded access panel covers the

receptacles. Use of external power allows all
ground testing and engine starting procedures
to be performed without using the airplane
main and auxiliary batteries. Figure 24-12. External Power Receptacle

Figure 24-13. External Power Cable

24-18 FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of

the maintenance practices and is intended for
training purposes only.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2.

24-00-00 MAINTENANCE
PRACTICES
SERVICING
Application of External Power
To apply power to the aircraft buses from a
ground power source, proceed as follows:

1. Connect 28VDC power source to external

power receptacle.
2. Set EXTERNAL–OFF–BATTERY switch Figure 24-14. Hot Battery Bus
to EXTERNAL.
3. Set DC MASTER switch to on.
4. Set BUS TIE switch to NORMAL.
5. Check voltmeter for 28VDC reading.

Removal of External Power

1. Set DC MASTER switch to OFF.
2. Set EXTERNAL–OFF–BATTERY switch
to OFF.
3. Disconnect ground power source from
external power receptacle.

HOT BATTERY BUS

The battery is directly connected to the cabin
rear bulkhead circuit breaker panel. Normally
there are only two circuit breakers on this bus,
the passenger door entrance lights and baggage
compartment lighting (Figure 24-14). If the
aircraft left the factory with an optional coffee
maker installed, the coffee maker (sometimes
referred to as a hot cup) is also powered from
the hot battery bus. Battery power is always
available to the hot battery bus.

FOR TRAINING PURPOSES ONLY 24-19

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TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 24-15. Bus Feed Current Limiters (1 of 2)

Figure 24-16. Bus Feed Current Limiters (2 of 2)

24-20 FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Bus Feed Current Limiters NOTES

The current limiters are in the power distribution
and generator control box. Three additional
current limiters were also installed at the opposite
end of the power supply cables, in the battery bay
compartment. The current limiters, sometimes
referred to as battery feeders, are time delay slow
blow fuses (Figure 24-15 and Figure 24-16). The
time delay design permits high current flows, in
the range of 800 and 1000-amps, to the starter
generators during the short time needed to start
the engine. Should a high reverse current charge
in excess of 450-amps occur for a prolonged time
during battery recharging, the current limiters
will melt, electrically isolating the main battery
bus and battery from the generator.

NOTE
If one current limiter is open
all services including start are
available. If two current limiters
are open all services are available,
but the last limiter will blow when
a start attempt is initialized. There
will be no advance warning.

Figure 24-17. DC Power and Engine Starter Panel

FOR TRAINING PURPOSES ONLY 24-21

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TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 24-18. Starter Generator Installation

Figure 24-19. Generator Brush and Spring Position

24-22 FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

STARTER/GENERATORS draw, high torque and restricted RPM. When the

motor RPM is stabilized at a sufficient speed,
A 200-ampere starter/generator (Figure 24-18 and it is converted to a series wound motor. The
Figure 24-19) is mounted on the upper rear face conversion to series wound mode is implemented
of the engine accessory gear case with a quick- by a microswitch located next to the Fuel lever.
disconnect kit. The kit consists of a drive-end pad The conversion commences with the addition of
and a clamp. The pad is secured to the accessory fuel. The characteristics of a series wound motor
gear case with four studs, washers, and nuts. are low current draw, low torque and unrestricted
The clamp is used to attach the starter-generator RPM. Releasing the start switch at normal engine
to the drive-end pad. The starter-generator is idle speed terminates the motor phase.
cooled by air drawn from the engine intake area
forward of the engine rear fire seal. The cooling
airhose is routed and clamped to a duct assembly
NOTE
on the rear of the starter-generator. The airflow When the engine is at idle rpm
travels forward through the starter-generator (NG) +15%, the generator can be
augmented by an integral four-bladed fan and selected on line.
is exhausted through a screen at the drive end.
The starter generators each have a nominal
regulated output of 28.5 + 0.2V, at 200-amps,
Generation
to supply the aircraft electrical system. Under Refer to MSM.
an STC a 250-amp starter generator has been
installed in several geophysical survey aircraft Following engine start, a generator begins output
with special equipment. This configuration also when the GENERATOR switch is positioned
required heavier wiring and a 250-amp shunt for to RESET and then released to ON. When the
the increased generator output. generator control switch is selected to RESET
the generator shunt field are excited to increase
generator voltage and current output for system
CAUTION demand. The field circuits of the starter-generators
are protected by 10-amp circuit breakers labeled
To avoid damage to the cooling
GEN CONTROL L and GEN CONTROL R,
fan, do not stand the generator on
located on the overhead console circuit breaker
end during removal/installation.
panel. If the output voltage is greater than the
over voltage protection.
Protection
Typically, one generator can easily supply
The generator is provided with several types
all aircraft electrical needs, as long as air
of protection. A voltage regulator regulates
conditioning and de-ice equipment is not
generator output. Reverse current and undervoltage
being used and the battery does not require
protection are provided by the reverse current
recharging. Loss of one generator in flight
relay; overvoltage protection is by an overvoltage
normally does not require the pilot to shed
relay. The reverse current circuit breaker ensures
loads, with the exception of air conditioning,
the generator does not give too much power to the
which must be turned off, and conditions
battery. The generator-reset circuit is protected
when de-icing equipment is being used. The
by a 5-amp circuit breaker labeled GEN RESET
remaining generator load should be carefully
located inside the engine nacelle upper cowling.
monitored for the duration of the flight.
Starting
On initial start the starter-generator is a compound
series wound motor driven by the aircraft battery
or external power. The characteristics of a
compound series wound motor are high current

FOR TRAINING PURPOSES ONLY 24-23

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TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 24-20. DC Power and Engine Start Panel

Figure 24-21. Generator and Bus Tie Controls

24-24 FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Indication
System line voltage is indicated by a-voltmeter,
and generator load and battery charge/
discharge condition is indicated on a load
meter. Each generator system is provided with
GENERATOR fail lights and, as an option,
GENERATOR OVERHEAT lights with
associated sensors.

The generator caution light(s) will come on

any time the Generator is not connected to the
system.

The generator overheat caution light(s),

if installed, will come on if the internal
temperature of the generator exceeds 300°F.
This usually caused by internal failure or the
generator is not receiving adequate cooling air.

FOR TRAINING PURPOSES ONLY 24-25

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Generator and Starter Relays

Left and right generator and starter relays
(Figure 24-22) are located on the forward
face of each engine firewall. The generator
relay (MS24171Dl) is a 200-ampere, SPST,
continuous-duty relay; the starter relay
(MS24172Dl) is a 200-ampere, SPST,
intermittent-relay. The relays are identical in
appearance but are not interchangeable.

NOTE
When Mod 6/1590 or 6/1636
is incorporated the appropriate
generator switch must be selected Figure 24-22. G
 enerator and Starter Relay
to OFF, to de-energize the Location
latched generator field control
relay, before selecting Reset to
energize the generator field.

Voltage Regulators
Voltage regulators in each generator circuit control
the generator output at 28.5V nominal over the
full range of generator speed, load, and operating
temperature (Figure 24-23 and Figure 24-24).
They must be adjusted to accommodate extreme
ambient temperatures. Refer to Table 24-1.

An equalizer circuit in each voltage regulator

ensures equal loading (within 10 to 20-amperes)
when the generators are operating in parallel
(relays K5 and K6 energized and the BUS TIE Figure 24-23. New Voltage Regulator
switch in NORMAL). The equalizer circuit is
disconnected during engine starting (if only
relay K7 or K8 are energized individually).
When both relays K7 and K8 are energized
(IGNITION switch set to MANUAL), the
equalizer circuit is connected.

Temperature Voltage Setting

32oF 29.0 VOLTS

33 to 80oF 28.5 VOLTS

81oF 28.0 VOLTS

Table 24-1. Temperature/Voltage Settings Figure 24-24. Old Style Voltage Regulators

24-26 FOR TRAINING PURPOSES ONLY

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TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

NOTE overvoltage relay operates to disconnect the

generator from the bus by de-energizing the
In the event of a generator
associated generator field control relay. This,
going off line, due to a generator
in turn, de-energizes the reverse-current relay,
overvoltage or line failure,
causing the generator fail relay PD-K5 or
selecting the appropriate generator
PD-K6 contacts to close, resulting in the L or R
control switch momentarily
GENERATOR caution light to come on.
to RESET (Pre Mod 6/1590 or
6/1636) applies direct generator
Early overvoltage relays were susceptible to
output to the generator field
extreme cold weather, especially when parked
allowing the generator to build
overnight without cabin heat. Although an
up. Releasing the switch to on
alternative relay was introduced by mod 6/1539
energizes the reverse current relay
(TAB 658/4) at aircraft serial number 521 the
to reconnect the generator on line.
overvoltage relay was eliminated after aircraft
serial number 531 by mod 6/1590.
Beginning with aircraft serial number 531 a
new voltage regulator by mod 6/1590 (S/B
6/347 Rev B) with a 31.5 volts over voltage
sensing capability was provided.

The original two overvoltage relays in the

cabin roof were no longer required. At the same
time, the voltage regulator location was moved
from the battery bay to the aft right side face
of the cabin rear bulkhead station 332, near the
auxiliary battery, to eliminate grounding and
contaminated contact problems due to water
infiltration. However, for operators who wished
to refit the new voltage regulators, but could
not locate them on the baggage compartment
bulkhead due to existing avionics or other
equipment installed in the new location, a
modification was available by mod 6/1636 (S/B
6/366) to allow the new voltage regulators to
be installed in the original battery bay location.

Overvoltage Relays
With the introduction of the two new voltage
regulators, the overvoltage relays were replaced
with two standard relays. Then, in the event of
an overvoltage condition developing, these Figure 24-25. Overvoltage Relay Panel
relays are energized by the voltage regulator
overvoltage sensing circuit to de-energize the
associated generator field relay.

Overvoltage relays (Figure 24-25), prior to mod

6/1590, were located adjacent to the power
distribution and generator control box connected
in each generator circuit. In the event that the
generator voltage is excessive, the associated

FOR TRAINING PURPOSES ONLY 24-27

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TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 24-26. DC Volts Meter Panel

24-28 FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

24-30-00 DC METERS NOTES

VOLTMETER
A DC volt meter, DC load meter, and meter
select switch are installed on the DC meter
panel (Figure 24-26) located below the
caution lights panel. The meter panel select
toggle switch labeled IND SELECT is spring-
loaded to the centre BAT position and also
has L GEN and R GEN positions. Meter
shunts (200-amperes for each generator and
100-amperes for the battery) are included in
the meter circuit.

The DC volt meter dial is marked DC-volts

and has a scale graduated in 1-volt increments
from 0 to 30, with numerals at 10V intervals.
The-voltmeter is connected to the left DC
bus, protected by the 5-ampere circuit breaker
labeled VM on the main circuit-breaker panel,
and indicates bus voltage from the left DC bus
power source. To check the voltage output of
one of the three sources of power normally
available in flight (battery, left generator, or
right generator), it is necessary to shut off or
isolate the other two sources from the left bus.

FOR TRAINING PURPOSES ONLY 24-29

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 24-27. DC Load Meter Panel

24-30 FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

DC LOAD METER NOTES

The DC load meter dial is marked DC LOAD
and has a scale graduated in positive and
negative units of 0.1 over a range of -1.0 to
+1.0, with numerals at .4, .8, and 1.0 on each
side of the zero reference (Figure 24-27). The
meter shows battery charge or discharge when
the meter switch is set to the spring loaded
BAT (Centred) position, or individual generator
loading when set to the L or R GEN position.
The load meter is disconnected during engine
starting but remains connected in the manual
ignition mode. To read the load meter in the
BAT position, multiply the reading by 100,
and in the L or R GEN position multiply the
reading by 200 as the value of the battery charge
or discharge or generator load is expressed in
units of 0.1. Therefore, a unit of 0.1 represents
10-amps of current for battery reading (0.1 x
100 = 10 amps) 20-amps of current for generator
reading (0.1 x 200 = 20 amps.

Figure 24-28. DC Generator Shunt

FOR TRAINING PURPOSES ONLY 24-31

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 24-29. Reverse-Current Relay Locations

24-32 FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

REVERSE-CURRENT RELAYS
Reverse current relays, K1 and K2, connect
starting power to the left and right starter/
generators, and also connect generator output
to the DC buses and batteries. The relays are
mounted in the power distribution and generator
control box in the cabin roof. Each relay unit
comprises three relays. Relay 1 is energized
(22 volts minimum pull-in and 18 volts drop-
out) through terminal marked SW. Relay 1
closes relay 2, which closes relay 3. Relay 3
senses differential voltage (0.35 to 0.65 volt for
pull-in), and reverse current (9 to 25 amperes
for drop-out) between terminals marked GEN
and BATT. When a starting circuit is applied
to terminal marked APP, relays 1 and 2 are Figure 24-30. DC Contactor Box
bypassed, and relay 3 is directly energized,
connecting DC power from the BATT terminal
to the GEN terminal for starting power input to
the starter-generator. When the starting circuit
is disconnected, the generator circuit is applied
to the SW terminal, and normal relay operation
becomes effective for generator power output
through the GEN terminal, and relay 3, to the
BATT terminal. With relay 3 energized, an
output circuit on the terminal marked IND is
used for the generator fail relay circuit (PD-K5
or PD-K6).

FOR TRAINING PURPOSES ONLY 24-33

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 24-31. Distribution Diagram
24-34 FOR TRAINING PURPOSES ONLY
 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

DISTRIBUTION NOTES
DC power distribution is through a multiple
bus system consisting of left and right 28VDC
buses and main battery, battery/external power,
and auxiliary battery buses (Figure 24-31). The
left generator is connected to the left DC bus
and the right generator to the right DC bus by
LEFT and RIGHT GENERATOR switches
with OFF, ON, and RESET positions.

Power is distributed to the left and right DC

buses through bus-feed circuit breakers in
the power distribution and generator control
box and through circuit breakers on the main
overhead and radio distribution panels. The
power supplies electrical circuits through
individual system circuit breakers mounted
on panels in the flight compartment, the main
panels on the bulkhead to the left of the pilot,
smaller panels on the overhead console and the
radio panels on the centre pylon. AC power is
distributed through fuses on the fuse panel on
the overhead console.

FOR TRAINING PURPOSES ONLY 24-35

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Circuit Breakers right K2 reverse current relay will provide

power through the three 50-amp CB4, CB6,
Aircraft now have an additional number of
CB8 and three 30-amp CB12, CB14 and CB16
50-amp and 30-amp circuit breakers in the
circuit breakers on the power distribution and
power supply system. Six 30-amp circuit
generator control box circuit breaker panel.
breakers CB13, CB15, CB17, CB12, CB14
and CB16 are on a panel with the existing
The additional in-line circuit breakers provide
six 50-amp circuit breakers CB5, CB7, CB9,
protection by isolating individual system
CB4, CB6 and CB8 attached to the power
circuit faults from either side to ensure that
distribution and generator control box.
any one individual problem will not cause a
total electrical power failure.
Six additional 50-amp circuit breakers
identified (CB5A, CB7A, CB9A, CB4A, CB6A
and CB8A) are on a separate panel adjacent to
the main circuit breaker panel. They are in line
between the existing 50-amp circuit breakers
(CB5, CB7, CB9, CB4, CB6 and CB8) in the
power distribution and generator control box
and the main circuit breaker panel left and right
NOTES DC buses. A further six 30-amp rated
circuit breakers identified (CB13B, CB15B,
CB17B, CB12B, CB14B and CB16B) are on a
separate panel adjacent to the overhead console
circuit breaker panel.

They are in line and between the new 30-amp

circuit breakers (CB13, CB15, CB17, CB12,
CB14 and CB16) on the power distribution
and generator control box and the forward
radio circuit breaker panel. The left K1 reverse
current relay provides power through the three
50-amp CB5, CB7, CB9 and three 30-amp
CB13, CB15 and CB17 circuit breakers. The

POWER DISTRIBUTION
POWER DISTRIBUTION

CB CB
50 50 50 30 13B 30 12B

CB5A CB7A CB9A

CB CB
30 15B 30 14B

50 50 50 CB CB
30 17B
30 16B
CB4A CB6A CB8A

Figure 24-32. Circuit Breaker Panels

24-36 FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

COMPONENTS or K11A Mod 6/1605), boost pump fwd 1 and

2 (K15 and K14) and start control left and right
Main Distribution Box No.2 (K16 and K17). In aircraft with engine air
intake de-icing, two additional relays K12 and
The main distribution box (Figure 24-33) is
K13 will be installed. Rubber grommets line
an aluminum alloy box, closed by a cover,
the access holes in the box for cable protection.
secured with quick-release fasteners. Mounted
within the box are 15 relays (K1 to K11 and
K14 to K17), a bank of resistors, and several
diodes. Basic relays located in the box consist
of landing light 1 and 2 (K1 and K2), boost
pump aft 1 and 2 (K3 and K4), generator field
left and right (K5 and K6), ignition and start
left and right (K7 and K8), Start control No 1
left and right (K9 and K10), Hyd pump (K11

Figure 24-33. Main Distribution Box

FOR TRAINING PURPOSES ONLY 24-37

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Power Distribution and Generator

Control Box (Post Mod 6/1274)
Aircraft with Power Distribution and Generator
Control Boxes modified to 6/1274 are shown
(Figure 24-34) with a second panel attached
to the box to locate six 30-amp rated circuit
breakers as required by mod 6/1274. The K1
and K2 Reverse Current Relays are shown
incorporating the fire resistant isolation shield
guards by mod 6/1598. The figure identifies
the 150-amp current limiter location should the
aircraft include the Air Conditioning optional
mod S.O.O. 6109. Other components within the
box remain unchanged.

Figure 24-34. Power Distribution and Control Box (Post Mod 6/1274)

24-38 FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Power Distribution and Generator The 30 and 50-amp circuit breakers are now
grouped together at each side of the circuit
Control Box (Post Mod 6/1591) breaker panel relating to left and right DC bus
The post mod 6/1591 Power Distribution and power supply. The left and right PD-K5 and
Generator Control Box (Figure 24-35) positions PD-K6 generator fail relays and CR1 and CR2
the K1 and K2 Reverse Current Relays at blocking diodes are now located on either side
each end of the box completely separating of the box. The aircraft electrical power system
both relays. The separation ensures that an wiring is routed with three connectors (J1001,
overheat condition with one relay will not J1002 and J1003) and other leads through
affect the operation of the other relay. A single access holes lined with grommets into the box.
circuit breaker panel is now attached to the The Reverse Current Circuit Breaker and the
box replacing two individual panels initially Bus Tie K3 relay have been repositioned to
required by mod 6/1274 to accommodate the accommodate the relocation of the RCR relays
six 50-amp and six 30-amp circuit breaker and wiring changes.
positions and other optional system locations.

Figure 24-35. Power Distribution and Control Box (Post Mod 6/1591)

FOR TRAINING PURPOSES ONLY 24-39

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Power Distribution and Generator condition of one relay will not effect the other
relay. The left and right 30 and 50-amp circuit
Control Box breaker location groups are retained on the
The latest mod for the power distribution and circuit breaker panel relating to the DC bus
generator control box (Figure 24-36) replaced power supply.
the reverse current circuit breaker (RCCB)
with three 150-amp current limiters to alleviate MSM Figures “DC generation” illustrate the
delivery concerns encounter with the RCCB. DC generation system with and without Mods
Three additional current limiters are at the 6/1590 and 6/1636. MSM Figure “Power Only
opposite end of the power supply cables in Battery and Auxiliary (Sheet 1 of 6)” illustrates
the battery bay area. The K1 and K2 reverse the DC power starting and generating system.
current relays, installed at each end of the box,
maintain relay isolation to ensure an overheat

Figure 24-36. Power Distributor and Generator Control Box (Post Mod 6/1651)

24-40 FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 24-37. DC Bus Feed Circuits

FOR TRAINING PURPOSES ONLY 24-41

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Generator Field Control Relay NOTES

(LGFC or RGFC)
These will only be found if Mod 6/1590 or
6/1636 has been incorporated. Their function
is to latch the generator field relay (K5 or K6)
to the on position during generator operations.

Depending on mod status of the voltage

regulator will either monitor for overvoltage
situations and operate the external overvoltage
relays or monitor and control the overvoltage
through the (LGFC or RGFC).

NOTE
If after the first start and the
battery’s condition is in doubt, the
generator must be brought on line
using the approved procedures
and the battery must be recharged
until the load meter reads +.4 or
less (battery position) to ensure
there is an adequate charge for
the next start.

Reference PSM 1-63-1A Section

2 Para 2-2-2 Caution.

Reverse Current Relay

This action starts the reverse current relay
checks which include under voltage and
polarity. If the generator output voltage is
sufficient, then the RCCR connects the
generator to the bus for further sensing. The
next check is for generator voltage to be at least
1/2V higher than the bus and no reverse current
flow. If these parameters are met, then the main
contactor is energized closed and the generator
caution light goes out. The generator can now
be checked for load.

CAUTION
Do not reduce the power plant
to idle until the generator load
is <.5 indicated on the DC Load
meter with the switch selected to
the appropriate generator.

24-42 FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Aircraft: Revision No. 07 Page:

DE HAVILLAND DHC-6, SERIES 100, 200 & 300 Date: Sep. 18, 1998 24-1 of 1
System & 1. 2. Number Installed
Sequence 3. Number Required for Dispatch
Numbers 4. Remarks or Exceptions

24 ELECTRICAL POWER

1 L GENERATOR and R B 2 1 (O) Provided the corresponding loadmeter

GENERATOR Caution operates normally.
Lights

2 A.C. Inverters B 2 1 One may be inoperative for day VMC.

B 2 1 (M) One may be inoperative for day or night

provided flight instruments do not require
A.C. power.

3 A.C. Inverter 400 Cycle B 1 0 May be inoperative for day VMC.

Caution Light

Series 300 S only. B 2 1

B 2 0 Both may be inoperative for day VMC.

4 Battery Temperature Monitor B 1 1 (O) Either the warning light or the temperature
and Warning System (Ni- indicator must operate normally.
Cad battery)

5 Auxiliary Battery B - 0 (M) Provided it is disconnected from the

electrical system.
***

6 Auxiliary Power Source for B - 0

Gyroscopic Pitch and Bank
Indicator
***

Figure 24-38. MMEL - Electrical Power

FOR TRAINING PURPOSES ONLY 24-43

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

½A ½A

½A

RESET PROPS

Figure 24-39. Inverter Single Switch

GYRO ½ A ½A ½A
R BUS COMP
NORM

OFF L BUS
NORM
½A ½A ½A
EMER
OFF

EMER
½A

RESET PROPS

Figure 24-40. Inverter Dual Switch (Optional Mod)

24-44 FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

24-20-00 AC POWER CONTROLS

SYSTEM Switch Control
A two-position INVERTER switch (Figure
24-39 and Figure 24-40) with No.1 and No.2
GENERAL positions is on the overhead (fuse panel). The
switch provides for the selection of either of
Alternating current (AC) is provided by a static
the two inverters. Both inverters are powered
inverter system. The system consists of two
by their respective DC buses through the
static inverters supplied from a 28VDC input. A
inverter control relay. When the No.2 inverter
variation of systems can be fitted to the aircraft
is selected, it is energized through the INVR
as follows:
2 CONT circuit breaker located on the main
circuit-breaker panel.
•• 250VA inverter - Provides 175VA at
115 Volts and 75VA at 26 Volts.
When S.O.O. 6142 is incorporated, for British
•• 2 5 0 V A i n v e r t e r ( w i t h 1 0 0 V A CAA certification, two switches under the label
transformer) - Provides 75VA at 115 “INVERTERS” are on the overhead console
Volts with 175VA at 26 Volts. fuse panel and control the 400-hertz AC buses.
The L BUS and R BUS switches have NORM,
•• 400VA inverter - Provides 115 Volt
OFF, and EMER positions. Each switch should
output.
normally be positioned to NORM. When S.O.O.
•• 4 0 0 V A i n v e r t e r ( w i t h 2 0 0 V A 6127 is incorporated, for Australian CAA
transformer) - Provides 200VA at 115 certification, a single inverter switch is located
Volts with 200VA at 26 Volts. on the overhead console fuse panel with switch
selections labeled No.1 and No.2.
Series 300 aircraft adopted a 250VA inverter
as standard equipment. In addition, for aircraft
with special avionics equipment, a 400VA
Indication
inverter could be installed by EO68189. The A 400 CYCLE caution light on the caution
inverters with transformers were able to provide panel comes on if an inverter failure occurs.
an extra 26V output to accommodate additional The caution light circuit is protected by a 400
AC instruments. As only one inverter operates cycle FAIL circuit breaker on the main circuit-
at any one time, it is recommended that both breaker panel, and the 26VAC, 400-hertz
inverters be equally operated with alternate sensing relay is in the caution light dimming
selections from day to day. The non-operating box and protected by a l-ampere 400 cycle
inverter remaining as a standby in the event that FAIL fuse on the fuse panel.
the selected inverter fails during flight.

FOR TRAINING PURPOSES ONLY 24-45

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 2: Inverter Installation

Figure 24-41. Inverter Installation

24-46 FOR TRAINING PURPOSES ONLY

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TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

INVERTERS NUMBER 2, and the right generator is off line

with the BUS TIE switch open, power supply
Standard Installation to the number 2 inverter will be lost, resulting
in the loss of all AC operated avionics and
Refer to:
indications.
•• Figure 24-41. Inverter Installation.
If inverter number 1 is selected, and the left
•• Figure 24-42. Rear Fuselage Interior. generator is off line with the bus tie open,
battery power will still be available on the left
The 400 Hz single-phase static inverters are bus (assuming the BATTERY/EXTERNAL
installed in the rear fuselage area to provide switch is at BATTERY), and AC power will
115V and 26VAC outputs. not be lost.
The number 1 inverter is supplied power
through a 7.5-amp circuit breaker from the
left DC bus. This provides a higher degree
of protection from the possibility of a hazard
condition arising due to overloading under
certain fault conditions.

The number 2 inverter receives 28VDC power

from the right DC bus through a 7.5-amp
circuit breaker labeled INVERTER 2, also on
the overhead circuit breaker panel.

The inverter control relay is above the

inverters. The inverter selector switch, labeled
INVERTER 1 and INVERTER 2, originally
on the overhead console switch panel, was
relocated to the AC fuse panel above the left
crew seat. The switch relocation was part of Figure 24-42. Rear Fuselage Interior
the changes made to reduce the probability of
a loss of electrical power to the distribution
buses under fault conditions.

A larger capacity 250VA Static Inverter with

26V transformers became standard equipment.
This change providing an increased AC power
output for additional avionics equipment. The
DC input power supplied to each inverter is
protected by 20-amp circuit breakers labeled
INVERTER 1 and INVERTER 2 on the
overhead circuit breaker panel. The inverter
selector switch, which controls the inverter
relay, receives power from the right DC bus
through a 5-amp circuit breaker labeled INV 2
CONT on the main circuit breaker panel.

It is important to be aware of which inverter

is selected, prior to opening the BUS TIE
switch. If the INVERTER switch is selected to

FOR TRAINING PURPOSES ONLY 24-47

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of NOTES

the maintenance practices and is intended for
training purposes only.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2.

24-21-00 MAINTENANCE
PRACTICES
ADJUSTMENT/TEST
Operational Test Inverter System
1. Connect external power to the aircraft.
2. Select INVERTER switch to No.1.
3. Check fuel quantity indicators are active.
4. Pull INV 1 breaker and check 400~FAIL
caution light comes on.
5. Reset breaker and check caution light out.
6. Repeat for other inverter.
7. Pull INV 2 circuit breaker and check 400~FAIL
caution light comes on. Reset circuit breaker
and check caution light goes out.
8. Select INVERTER control switch to No.1.
Check 400~FAIL light remains off.
9. Disconnect external power source.

24-48 FOR TRAINING PURPOSES ONLY

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TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

PAGE INTENTIONALLY LEFT BLANK

FOR TRAINING PURPOSES ONLY 24-49

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TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 24-43. 400VA Inverter System Schematic
24-50 FOR TRAINING PURPOSES ONLY
 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

24-23-00 STATIC The 26V output from the autotransformer is fed

to the AC failure relay K8 (which breaks the
INVERTER 400VA circuit of the 400 CYCLE fail caution light)
SYSTEM and through the control switch and relevant
fuses. The radio AC relay connects AC power
to the radio system.
GENERAL
With the INVERTER control switch set to
The 400VA static inverter system (Figure No.2, the inverter control relay is energized,
24-43) consists of two static inverters (No.1 and connecting power from the right DC bus to the
No.2), a control relay on the left side of the rear No.2 inverter via the control relay contacts.
fuselage, and an INVERTER control switch on As with the No.1 inverter, the 115- and 26V
the overhead fuse panel. Each inverter provides outputs are fed to the AC services via the
115V, 400VA, 400 Hz, single-phase AC output control switch and relevant fuses, and the
from a 28VDC input. The No.1 inverter is autotransformer output is connected to the AC
supplied from the left DC bus, and the No.2 failure relay. The radio AC relay is energized
inverter is supplied from the right DC bus. to transfer radio circuits to the No.2 inverter.

For AC instruments and radio circuits, two Failure of an operating inverter de-energizes
autotransformers are installed to provide 26V, the AC failure relay, completing the circuits
400 Hz output. The autotransformers are and bringing on the 400 CYCLE fail caution
connected to the 115V output of the inverters. A light. A capacitor parallels the 26V (rectifier)
radio AC relay connects 115VAC and 26VAC to input to relay K8, which provides a hold-on
the radio system. circuit during inverter switch over.

The power supplied to the inverters is protected

by 20-amp circuit breakers located on the
overhead circuit-breaker panel. Power supplied
to the control relay is through a 5-amp circuit
breaker on the main circuit-breaker panel. Failure
of an operating inverter is indicated by a 400
CYCLE fail light on the caution lights panel. The
light is powered through a 5-amp circuit breaker
on the main circuit-breaker panel in conjunction
with an AC failure relay located in the caution
lights dimming control box. The AC failure relay
is energized by the 26V output of the operating
autotransformer.

OPERATION
Refer to MSM.

With the INVERTER control switch set to

No.1, power from the battery/external or left
DC bus is fed through the normally closed
contacts of the inverter control relay to the
No.1 inverter. The 115V output is then fed to
the AC services through the control switch and
relevant fuses.

FOR TRAINING PURPOSES ONLY 24-51

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 24-44. Dual Inverter Schematic
24-52 FOR TRAINING PURPOSES ONLY
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TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

24-24-00 STATIC AC Equipment

INVERTERS S.O.O. 6142 The FUEL FLOW, OIL PRESS, FUEL
QUANTITY, and TORQUE PRESS gages are
AC powered. Typically, both attitude indicators
The British CAA Series 310 Certification by
and both directional gyros will also be AC
optional mod S.O.O. 6142 required creation of
powered. Some avionics, such as some models
separate left and right 400 cycle buses. Unlike
of HSIs, may use a combination of AC power
the standard Twin Otter AC system, both
for the compass card and DC power for the
inverters function at all times, each supplying
CDI. Fuses for all of these services are found
their own AC buses (Figure 24-44).
on the AC fuse panel, above the captain’s seat.
The left inverter supplies power to the left AC
Avionics installation in Twin Otter aircraft
bus, powering the pilot directional gyro and
varies greatly, and the majority of aircraft have
attitude indicator, and both engine fuel quantity,
had additional or modified avionics installed
fuel flow, oil pressure, and torque pressure
since the aircraft left the factory. It is a good
indicators. The right inverter supplies the right
practice to make a careful inventory of which
AC bus to power the co-pilot directional and
equipment is AC powered in each individual
attitude indicators. Other avionics may be
aircraft, to avoid uncertainty should an AC
added to either bus.
problem occur in flight. This may easily be
done together with your maintenance technician
There are two inverter control switches, one for
while the aircraft is on the ground, by pulling
each inverter. These are labeled INVERTER R
both INVERTER circuit breakers and observing
BUS and L BUS, with three switch positions for
which services are lost.
each, labeled NORM, OFF, and EMER. Both
switches are selected to the NORM position for
dual operation.

Two caution lights are provided on the

annunciator panel, one for each inverter,
labeled 400 CYCLE and 400 CYCLE. Should
an inverter failure caution light come on, the
switch for the affected inverter should be
selected to the OFF position. If the affected
400 CYCLE caution light goes out, then the
instruments and services powered from that bus
are lost. This condition would most commonly
be caused by a ground fault.

Should the 400-cycle caution remain on after

the affected inverter has been switched to OFF,
the affected inverter switch may be selected
to the EMER position. AC power from the
remaining opposite side inverter will be routed
to the bus with the failed inverter. The one
remaining inverter will supply all AC services
on the aircraft. The 400 CYCLE caution light
for the failed inverter will remain on when AC
supply is re-established from the remaining
inverter.

FOR TRAINING PURPOSES ONLY 24-53

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TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

1 E 251 C 20 N

PREFIX SUFFIX LETTER

CIRCUIT FUNCTION LETTER WIRE SIZE

WIRE NUMBER WIRE SEGMENT LETTER

1E250 1E251

1E252N

1E250A 1E250B 1E250C 1E251A 1E251B 1E251C

1E252AN

TB- J- P- J- P- J- P-

Figure 24-45. Wire Coding Example

24-54 FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of •• RN-VOR/GS

the maintenance practices and is intended for
•• RM-MB
training purposes only.
•• VAN-DME
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2. •• F-Compass
•• C-Autopilot
24-30-00 MAINTENANCE •• RZ-Audio
PRACTICES •• RC-VHF and HF
•• RD-ADF.
WIRE CODING
The wire number is used to differentiate
Circuit wiring is identified by a code stamped between wires in a circuit. Wires of the same
at intervals throughout the length of each wire. circuit with a common terminal connection
The wire codes are composed of several parts have the same wire number.
as explained in (Figure 24-45).
The wire segment letters are used to differentiate
The prefix number is not always present. It between conductor segments within a wire. In
precedes the circuit function letter for dual general, the wire number starts with the letter
systems. Unit No.1 indicates a left system and A to identify the first segment at the power
unit No.2 a right system. For parallel system source and continues in alphabetical sequence.
circuits associated with the two engines, the wire Letters O and I are not used. When more than
code unit numbers are 1 and 2, corresponding to 24 segments are required, double letters AA,
the left and right engines respectively. AB, etc. are used.

The circuit function letter identifies the circuit The wire size number is used to identify
to which the wire belongs. The circuit function the size of the wire or cable using AWG
letters are listed below: (American wire gage). For coaxial cables and
thermocouple wires, no wire size number is
•• C-Flight control included in the code. The suffix letter N is
used with the wire identification code to denote
•• D-Voltmeter and ammeter
wires which complete a ground circuit.
•• E-Engine and fuel
•• F-Flight instrument
•• H-Heating system
•• J-Not used
•• K-Propeller and deflector
•• L-Lighting
•• M-Windshield wiper and washer
•• P-DC power supply, starting, and
ignition
•• Q-Pressure (fuel and oil)
•• V-AC power supply
•• W-Emergency

FOR TRAINING PURPOSES ONLY 24-55

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TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

SYMPTOM POSSIBLE CAUSE CHECK REMEDIAL ACTION

1. Low main battery output. Poor connections of main Connections of ground Torque nuts to 70-75 pound-
battery ground line. line to shunt and shunt to inches. Ensure shunt contact
airframe are secure. surfaces are clean.

Low main and auxiliary Feedback if either battery Check if Mod 6/1283 is Maintain both batteries in
battery. is left discharged (Pre Mod embodied. fully charged condition.
6/1283). Mod 6/1283 ensures against
occurrence.

No main battery relay Open circuit diode in main Power is available at Replace defective diode.
output. battery relay control circuit terminal A1 of main
(Mod 6/1283). battery relay.

No auxiliary battery relay Open circuit diode in Power is available at Replace defective diode.
output. auxiliary battery relay terminal A2 of auxiliary
control circuit (Mod battery relay (with
6/1283). reverse-current circuit
breaker open).

2. Starter will not operate Ignition switch is in Set correctly.

when start switch is NORM position.
selected.

Defective relay K16 (left No power is available at Replace defective relay.

engine) or K17 (right engine). terminal B1 of relay.

Open circuit diode No power is available at Replace defective diode.

CR5 - (left engine) APP terminal of reverse- Check diode mounting
CR6 - (right engine). current relay, but power holes are properly deburred
is available at terminal B1 and mica washers are
of either K16 or K17 relay. undamaged and not
providing short to ground.

Reverse-current relay fails Power is available at Replace reverse-current

to close. APP terminal but not at relay.
GEN terminal of reverse-
current relay.

Defective starter relay. Power is available at Check functioning of starter

GEN terminal of reverse- relay. Replace if confirmed
current relay. defective.

Defective relay K9 (left With relay energized Replace defective relay.

engine) or K10 (right terminals A2 and A3 are
engine). open from ground.

No start control or Flat auxiliary battery plus Power is available at Service battery as per
Ignition. defective blocking diode terminal A2 of auxiliary manufacturer’s instructions.
between main and auxiliary battery relay. Check blocking diode for
bus. continuity. Replace if open
circuit.

Table 24-2. DC Generation Troubleshooting (Sheet 1 of 3)

24-56 FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

LIMITATIONS It must be appreciated that the table does not

represent a complete troubleshooting guide
The load limitations on each generator are but reflects the malfunctions which experience
shown in Figure 24-5. has shown to be the most likely to cause the
symptoms discussed.
Loadmeter Minimum NG Conditions
The following table is to be used in conjunction
Reading
with the circuit schematics and diagrams and
0 - 0.5 Idle NG component location information contained
within the airplane maintenance manuals.
0.5 - 1.0 Idle NG + 15%

0.8 Ground - From 45°F

- 125°F

1.0 Ground - Up to 45°F

1.0 Flight - Up to 125°F

Table 24-3. Generator Load Limitations

In single-generator operation, if generator load

exceeds the limitations listed in Table 24-3,
nonessential services (e.g., cabin lights, reading
lights, fans, Nav/communication systems, etc.)
should be manually switched off within two
minutes to meet the load limitations specified.

CAUTION
Repeated attempts to reset a
failed generator could result
in an overheat condition at the
generator shunt field. Therefore,
no more than two attempts should
be made to reset a generator.

TROUBLESHOOTING
Table 24-2 through Table 24-5 lists information
to assist operators in the recognition of the
causes, with subsequent rectification action
required, of the more commonly reported
malfunctions in the Twin Otter’s main electrical
system. Use of the information in this table
should help prevent the unnecessary removal
for investigation and repair of items which
are actually serviceable or which require only
minor adjustment.

FOR TRAINING PURPOSES ONLY 24-57

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

SYMPTOM POSSIBLE CAUSE CHECK REMEDIAL ACTION

3. Generator fail light is not Reverse-current relay Continuity exists between Replace reverse-current
on before start cycle or contacts welded. This terminals BAT and GEN relay. Check battery
does not come on again condition results from of reverse-current relay charge state. Check starter
on completion of start undertaking starts (internal) with relay de-energized. generator for serviceability.
cycle with the generator with auxiliary and main
switch off. batteries in a low-charge
condition and the resultant
Generator motors on contact chattering and also
shutdown. from the practice of cross-
generator starting.
If these symptoms occur,
immediately select bus-
tie switch open.
On engine shutdown Reverse-current relay To prevent battery and
generator fail caution contact welded - see above operating generator
light is out and fails to for cause. monitoring starter-generator:
come on when generator 1. Select bus-tie switch
switch is selected off. open.
Generator motors on 2. If L GEN fail light does
engine shutdown. not come on, select
BATTERY-EXTERNAL
switch to OFF to isolate
the left starter-generator
from the battery and
operating generator.

Replace defective reverse-

current relay.

4. Generator will not come Defective switch. Continuity of switch is Replace switch if defective.
on line when GEN switch satisfactory.
is set to RESET and then
to ON.
Dirty contacts on voltage Voltage regulator pins Clean as required.
regulator or base. and base contacts are
free of contamination.

Generator will not come Overvoltage relay contacts There is continuity Replace if defective.
on line when GEN switch are open, denying power to between terminals P and Experience indicates that
is set to RESET and then generator field relay, with T on overvoltage relay. some relays open when
to ON. no overvoltage condition exposed to cold-weather
present. conditions but return to
normal condition by lightly
tapping units as soon as
possible.

Table 24-4. DC Generation Troubleshooting (Sheet 2 of 3)

24-58 FOR TRAINING PURPOSES ONLY

 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

SYMPTOM POSSIBLE CAUSE CHECK REMEDIAL ACTION

Open circuit diode Power is at terminal T Replace diode if defective.
CR3 (left engine) on overvoltage relay
CR4 (right engine) and continuity is across
diode.

Defective relay K9 (left With relay de-energized Replace defective relay.

engine) or K10 (right contacts A2 and A3 read
engine). to ground.

Inoperative field exciter Check voltmeter circuit Replace defective resistor

circuit. breaker is made. Check (100 ohms) if necessary.
continuity is across
resistors R1 (left engine)
or R3 (right engine).

5. Generator will not stay Incorrect setting of voltage Check voltage regulator Carry out voltage regulator
on line. regulator, permitting base contacts. Check adjustment procedure.
overvoltage relay to overvoltage relay Replace overvoltage relay if
operate under transient contacts. required.
overvoltage conditions.
Reverse-current circuit Reset - Refer to item 7.
breaker open.

Excessive load imbalance. Refer to item 6.

6. Generator fails to Improperly adjusted Adjust voltage regulator.

load-share. voltage regulator. Difference in regulator
settings should be less than
0.2 volt.

Poor connection of Check security of ground Torque nuts to 140-145

generator ground line in line to shunt and of shunt pound-inches. Ensure shunt
rear of nacelle. to airframe. contact surfaces are clean.

Reverse-current circuit Both generators switched Reset (always set generators

breaker repeatedly tripping on simultaneously. to ON separately). Check
when generators are battery charge condition.
brought on line. Low-battery-charge
condition.

8. Damage to external Reverse polarity of ground Replace damaged

power relay, battery power unit (Pre Mod components. As an insurance
relay, battery shunt, and 6/1293). against occurrence, install
associated bus bars. Mod 6/1293.

9. No external power input. Open circuit diode - Power is available at Replace diode if defective.
external power controls terminal X1 of external
input circuit (Mod 6/1293). power relay.

Table 24-5. DC Generation Troubleshooting (Sheet 3 of 3)

FOR TRAINING PURPOSES ONLY 24-59

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

AC POWER SYSTEM NOTES

Aircraft were initially supplied with two 65VA
static inverters installed in the rear fuselage,
behind the baggage compartment shelf. The
two inverters each providing a separate source
of 400 Hz single phase output power from a
28VDC input for the 26VAC and 115VAC
operated instruments.

INVERTERS
Standard Installation
The number 1 inverter was initially supplied
with 28VDC power from the battery/external
power bus, through a 7.5-amp circuit breaker
labeled INVERTER 1, on the overhead circuit
breaker panel.

24-60 FOR TRAINING PURPOSES ONLY

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PAGE INTENTIONALLY LEFT BLANK

FOR TRAINING PURPOSES ONLY 24-61

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TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 24-46. 65VA Inverter System - Schematic (Pre Mod 6/1274)
24-62 FOR TRAINING PURPOSES ONLY
 24 ELECTRICAL POWER
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

STATIC INVERTER - 65VA Operation

SYSTEM With the INVERTER control switch set to
No.1, power from the external/battery power
Description bus (Pre Mod 6/1292 and Pre Mod 6/1372)
or the left DC bus (Post Mod 6/1292 or Mod
Refer to:
6/1372) is fed via the normally closed contacts
of the inverter control relay to No.1 inverter.
•• Figure 24-46. 65VA Inverter System -
The 115-volt and 26-volt outputs are then fed
Schematic (Pre Mod 6/1274).
to the AC services via the control switch (Pre
•• Figure 24-41. Inverter Installation. Mod 6/1274) and relevant fuses, and also to the
AC failure relay K8 (which breaks the circuit
•• Figure 24-47. 65VA Inverter System -
of the 400~FAIL caution light). With Mod
Schematic (Mod 6/1274).
6/1274, the AC outputs for radio circuits are
carried by the radio AC control relay.
The 65VA static inverter system consists of
two static inverters (No.1 and No.2) and a
With the INVERTER control switch set to
control relay located on the left side of the rear
No.2, the inverter control relay (and radio
fuselage (refer to Figure 24-41), and a control
AC control relay Mod 6/1274) are energized
switch marked INVERTER No.1 and No.2 on
connecting power from the right DC bus to
the overhead switch panel (Pre Mod 6/1274), or
No.2 inverter via the control relay contacts.
on the fuse panel (Mod 6/1274). Each inverter
As with No.1 inverter the 115-volt and 26-volt
provides 115 Volt and 26 Volt, 400 Hz single-
outputs are fed to the AC services via the
phase AC outputs from a 28VDC input. No.1
control switch (and radio AC control relay,
inverter is supplied from the battery/external
Mod 6/1274) and relevant fuses, and also to the
power bus (Pre Mod 6/1292 and Pre Mod
AC failure relay.
6/1372) or the left DC bus (Post Mod 6/1292
or Mod 6/1372) and No.2 inverter from the
Failure of an operating inverter de-energizes
right DC bus.
the AC failure relay completing the circuit to
the 400~FAIL caution light which comes on. A
The power supplies to the inverters are protected
capacitor parallels the 26-volt (rectified) input
by 7.5–ampere circuit breakers on the overhead
to relay K8 which provides a hold-on circuit
circuit breaker panel, and the power supply
during inverter switchover.
to the control relay and a radio AC control
relay (Mod 6/1274) is protected by a 5–ampere
circuit breaker on the main circuit breaker panel.
Failure of an operating inverter is indicated by
a 400~FAIL light on the caution lights panel,
which is fed through a 5–ampere circuit breaker
on the main circuit breaker panel, in conjunction
with an AC failure relay located in the caution
lights dimming control box. The AC failure
relay is energized by the 26-volt output of the
operation inverter.

FOR TRAINING PURPOSES ONLY 24-63

24 ELECTRICAL POWER

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 24-47. 65VA Inverter System - Schematic (Mod 6/1274)
24-64 FOR TRAINING PURPOSES ONLY
 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CHAPTER 36
PNEUMATIC
CONTENTS
Page

36-00-00 PNEUMATIC............................................................................................... 36-1

36 PNEUMATIC
Introduction......................................................................................................... 36-1
36-10-00 DISTRIBUTION (MODS S.O.O. 6004/6085)............................................... 36-3
General................................................................................................................ 36-3
Heat Exchanger............................................................................................. 36-3
Strainer......................................................................................................... 36-3
Dual Pressure Switch.................................................................................... 36-3
Low Pressure Switch..................................................................................... 36-3
Pneumatic Package........................................................................................ 36-5
Pressure Regulator........................................................................................ 36-5
Operation............................................................................................................. 36-7
36-10-00 MAINTENANCE PRACTICES.................................................................... 36-7
Servicing.............................................................................................................. 36-7
Strainer......................................................................................................... 36-7
36-20-00 INDICATING.............................................................................................. 36-9
General................................................................................................................ 36-9

FOR TRAINING PURPOSES ONLY 36-i

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

ILLUSTRATIONS
Figure Title Page

36-1 Pneumatic Package....................................................................................36-2

36-2 Pneumatic Package Location.....................................................................36-4
36-3 Pressure Regulator.....................................................................................36-4

36 PNEUMATIC
36-4 18-PSI Pneumatic Package Schematic.......................................................36-6
36-5 Dual Pressure Switch - Wiring Schematic..................................................36-8
36-6 Caution Lights...........................................................................................36-9

FOR TRAINING PURPOSES ONLY 36-iii

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CHAPTER 36
PNEUMATIC

36 PNEUMATIC
36-00-00 PNEUMATIC
INTRODUCTION
The pneumatic system consists of a low pressure pneumatic package which is only required
when a customer option airframe de-icing is installed.

FOR TRAINING PURPOSES ONLY 36-1

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
36 PNEUMATIC

Figure 1: Pneumatic Package

Figure 36-1. Pneumatic Package

36-2 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

36-10-00 DISTRIBUTION Low Pressure Switch

(MODS S.O.O. 6004/6085) The low pressure switch is set to bring on
a caution light should the system pressure
drop to between 13 and 15 psi. When pressure
GENERAL increases to between 16 and 18 psi, the switch
contacts open to extinguish the caution light.
Refer to Figure 36-1. Pneumatic Package.

The pneumatic package is located on the left side

36 PNEUMATIC
of the cabin roof at station 177.00 approximately.
The pressure supply or the pneumatic package is
tapped from the bleed air system line on the left
side of the cabin roof.

Heat Exchanger
The heat exchanger is a rearward facing
airscoop mounted on top of the fuselage at
station 176.50. The double walls of the airscoop
and the internal baffle system form the heat
exchanger. Bleed air enters the heat exchanger
through the outboard pipe, and is directed
through the baffle system which exposes the air
to the entire surface area of the inner and outer
walls of the scoop, where cooling air passing
over these surfaces dissipates the heat from
the bleed air. The cooled air leaves the heat
exchanger through the inboard pipe.

Strainer
The strainer incorporates a 60 mesh wire
element and is fitted to the intake port of the
pressure regulator in the cooled air line from
the heat exchanger to prevent impurities in the
bleed air from entering the pneumatic system.

Dual Pressure Switch

The dual pressure switch assembly consists of
two switches, each connected to the automatic
temperature controller. When the pressure to the
pneumatic system fails below about 25 psi, one
switch opens to prevent the heater from increasing
its share of the flow. If the pressure falls below 20
psi, the second switch actuates to close the hot air
valve and increase the pneumatic system pressure.
The switches do not affect the heater when the
automatic temperature controller is being used in
its MANUAL mode.

FOR TRAINING PURPOSES ONLY 36-3

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
36 PNEUMATIC

Dual Pressure Switch Low Pressure Switch

Figure 36-2. Pneumatic Package Location

Figure 36-3. Pressure Regulator

36-4 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Pneumatic Package NOTES

Refer to Figure 36-2. Pneumatic Package
Location.

The pneumatic package consists of a heat

exchanger, a pressure regulator, a strainer, a
dual pressure switch assembly, a low pressure
switch, and connecting pipe lines.

36 PNEUMATIC
Pressure Regulator
Refer to Figure 36-3. Pressure Regulator.

The pressure regulator is mounted on a plate

which is secured to the cabin roof. It is set
to regulate the pneumatic system at 18 psi. A
safety relief valve on the pressure regulator
is set to relieve pressure at 25 psi should the
regulator section fail.

FOR TRAINING PURPOSES ONLY 36-5

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

AUTOMATIC
TEMPERATURE
CONTROLLER

HOT-AIR
VALVE

DEICING
36 PNEUMATIC

AUTOPILOT

DUAL PRESSURE
PRESSURE REGULATOR
PRESSURE
SWITCH AND PRESSURE
SWITCH
RELIEF VALVE
13-15 PSI

HEAT STRAINER
EXCHANGER

LOW PRESS
CAUTION LIGHT

TO LEFT ENGINE TO RIGHT ENGINE

INTAKE DEFLECTOR INTAKE DEFLECTOR

LEFT RIGHT
ENGINE ENGINE

BLEED-AIR BLEED-AIR
SHUTOFF SHUTOFF
VALVE VALVE

Figure 36-4. 18-PSI Pneumatic Package Schematic

36-6 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

OPERATION *The following is an abbreviated description of

the maintenance practices and is intended for
Refer to Figure 36-4. 18-PSI Pneumatic training purposes only.
Package Schematic.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2.
Air is bled from a bleed port on the gas
generator case of the left and right engines. The
bleed air is directed into a common manifold 36-10-00 MAINTENANCE
for distribution to the installed pneumatic
subsystems.
PRACTICES

36 PNEUMATIC
Bleed air is tapped off the common manifold SERVICING
and cooled by being routed through the heat
exchanger. Air exiting the heat exchanger is Strainer
directed to the strainer and dual pressure switch.
1. Unscrew strainer. Remove wire element.
An in-line regulator then reduces bleed-air
pressure to 18 psi. The regulated air pressure 2. Clean wire element and strainer.
is then directed to various services (airframe
3. Position wire element in strainer, install
de-ice, etc.) and to a low-pressure switch. The
and secure.
dual pressure-sensing switch ensures that if
bleed-air pressure drops to 25 psi, the hot-
air valve can demand no additional air. If
pressure drops to 20 psi, the dual pressure-
switch removes all hot-air valve control from
the automatic temperature controller and
completely closes the hot air valve. Completely
closing this valve directs all air to the pneumatic
package. A pressure drop after the regulator
below 15 psi causes the low pressure light on
the caution panel to come on. The light goes out
if pressure rises above 16 to 18 psi.

Figure 36-4 shows the 18-psi pneumatic

package schematic.

FOR TRAINING PURPOSES ONLY 36-7

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
36 PNEUMATIC

Figure 1: Dual Pressure Switch – Wiring Schematic

Figure 36-5. Dual Pressure Switch - Wiring Schematic

36-8 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

36-20-00 INDICATING NOTES

GENERAL
Refer to:

•• Figure 36-5. Dual Pressure Switch -

Wiring Schematic.

36 PNEUMATIC
•• Figure 36-6. Caution Lights.

The pneumatic indicating system is basically

a low pressure warning system. The system
consists of a low pressure caution light which
comes on when a low pressure switch actuates
if regulated pressure drops to between 13 and
15 psi. On increasing pressure, the pressure
switch actuates between 16 and 18 psi, the
caution light will go out. The pressure switch
is connected into the pressure line from the
pressure regulator.

The caution light is on the caution lights panel

in the flight compartment, and bright and dim
is selected through the caution lights dimming
control box.

Figure 36-6. Caution Lights

FOR TRAINING PURPOSES ONLY 36-9

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CHAPTER 21
AIR CONDITIONING
CONTENTS
Page

21-00-00 AIR CONDITIONING................................................................................. 21-1

Introduction......................................................................................................... 21-1
General................................................................................................................ 21-3
Heating System............................................................................................. 21-3
Cooling System............................................................................................. 21-3
Air Conditioner System................................................................................. 21-3

21 AIR CONDITIONING
Air Exhausting.............................................................................................. 21-3
Temperature Control System......................................................................... 21-3
Engine Bleed Air........................................................................................... 21-3
21-00-00 BLEED-AIR SYSTEM................................................................................ 21-5
General................................................................................................................ 21-5
Component Description........................................................................................ 21-7
Engine Bleed Air Ducts................................................................................. 21-7
Bleed-Air Shut-Off Valve Operation.............................................................. 21-7
Wing Bleed Air Duct Systems....................................................................... 21-9
Initial Bleed Air Duct.................................................................................... 21-9
“De Vore” Bleed Air Duct Post Mod 6/1482.................................................. 21-9
Inline Check Valves..................................................................................... 21-11
Fuselage Bleed-Air Duct Assembly............................................................. 21-11
21-40-00 HEATING SYSTEM.................................................................................. 21-13
General.............................................................................................................. 21-13

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Page
Description......................................................................................................... 21-13
Bleed Air Pipe Assembly............................................................................. 21-13
Hot Air Valve.............................................................................................. 21-13
Flow Limiting Orifice................................................................................. 21-13
Expansion Chamber.................................................................................... 21-14
Ejector........................................................................................................ 21-14
Silencer....................................................................................................... 21-15
Cabin Air Control Valve.............................................................................. 21-16
Ducts........................................................................................................... 21-17
Outlets........................................................................................................ 21-17
21 AIR CONDITIONING

Maximum Heating System.......................................................................... 21-19

21-50-00 PRIMARY COOLING SYSTEM................................................................ 21-21
General.............................................................................................................. 21-21
Ram-Air Scoop and Main Duct................................................................... 21-21
21-50-00 MAINTENANCE PRACTICES.................................................................. 21-21
Inspection/Check................................................................................................ 21-21
Fan Duct...................................................................................................... 21-23
Cooling System Ducts................................................................................. 21-23
Cool-Air Outlets......................................................................................... 21-23
Ram-Air Valve............................................................................................ 21-23
Maximum Cooling...................................................................................... 21-25
Ventilation................................................................................................... 21-27
21-60-00 TEMPERATURE CONTROL..................................................................... 21-29
General.............................................................................................................. 21-29
Component Description...................................................................................... 21-29

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Page

Control Panel.............................................................................................. 21-29

Automatic Temperature Controller.............................................................. 21-29
Cabin Temperature Sensor........................................................................... 21-30
Duct Temperature Sensor............................................................................. 21-30
OAT Sensor................................................................................................. 21-30
Hot-Air Valve Potentiometer....................................................................... 21-30
Duct Overheat Sensor.................................................................................. 21-31
21-60-00 MAINTENANCE PRACTICES.................................................................. 21-32
Adjustment/Test................................................................................................. 21-32
Test Temperature Control System Controls.................................................. 21-32

21 AIR CONDITIONING
Operational Test - Cabin Temperature Sensor.............................................. 21-32
Functionally Test of the Temperature Control System.................................. 21-32
Inspection/Check................................................................................................ 21-33
Inspection of Air Conditioner Inlet and Exhaust Outlets.............................. 21-33
Automatic Temperature Control Mode......................................................... 21-35
Manual Temperature Control Mode............................................................. 21-37
21-55-00 COOLING/DEMISTING FANS................................................................. 21-37
General.............................................................................................................. 21-37
Flight Compartment Fans............................................................................ 21-37
21-00-00 REFRIGERATION SYSTEM..................................................................... 21-39
General.............................................................................................................. 21-39
Description and Operation.................................................................................. 21-41
Operating Principles........................................................................................... 21-43
Component Description...................................................................................... 21-43
Compressor Motor...................................................................................... 21-43

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Page
Compressor................................................................................................. 21-43
Condenser................................................................................................... 21-43
Condenser Fan............................................................................................. 21-43
Evaporator................................................................................................... 21-44
Receiver-Dryer............................................................................................ 21-44
Pressure Switch........................................................................................... 21-44
Thermostat Switch...................................................................................... 21-44
Relays......................................................................................................... 21-44
Circuit Breakers and Current Limiter.......................................................... 21-44
Main Duct................................................................................................... 21-44
21 AIR CONDITIONING

Distribution Ducts....................................................................................... 21-44

Air Conditioner Inlet and Exhaust Outlet Inspection................................... 21-44
Alternative Air Conditioning Unit............................................................... 21-44
21-00-00 MAINTENANCE PRACTICES.................................................................. 21-45
Servicing............................................................................................................ 21-45
21-00-00 DIFFERENCES SERIES 100/200 AIRCRAFT.......................................... 21-49
Wing Bleed Air Temperature Control Mod 6/1266.............................................. 21-49

21-iv FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

ILLUSTRATIONS
Figure Title Page

21-1 Aircraft Bleed Air Heating and Cooling System........................................21-2

21-2 Temperature Controls.................................................................................21-2
21-3 Aircraft Bleed Air Heating and Cooling System........................................21-4
21-4 Bleed Air Source.......................................................................................21-4
21-5 Bleed Air Duct...........................................................................................21-4
21-6 Bleed-Air Shut-Off Valve...........................................................................21-6
21-7 Wing Bleed-Air Systems............................................................................21-8
21-8 Mod 6/1462 De Vore Shroud Bleed Pipes..................................................21-8

21 AIR CONDITIONING
21-9 Fuselage Bleed-Air Duct System.............................................................21-10
21-10 In-line Check Valve.................................................................................21-10
21-11 Low Pressure Regulator ........................................................................21-10
21-12 Heating System Installation.....................................................................21-12
21-13 Hot Air Valve...........................................................................................21-13
21-14 Expansion Chamber.................................................................................21-14
21-15 Ejector.....................................................................................................21-14
21-16 
Ejector and Recirculated Air Duct Installation.........................................21-14
21-17 Recirc Air Inlet........................................................................................21-14
21-18 Silencer...................................................................................................21-15
21-19 Silencer Location.....................................................................................21-15
21-20 
Cabin Air Valve Control Knob Location..................................................21-16
21-21 Cabin Air Control Valve...........................................................................21-16
21-22 Heating System - Component Location....................................................21-17
21-23 Air-Conditioning Operation (Maximum Heating)....................................21-18

Revision 0.3
FOR TRAINING PURPOSES ONLY 21-v
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Figure Title Page

21-24 Cockpit Foot Warmer...............................................................................21-19

21-25 Crew Heating Outlet................................................................................21-19
21-26 Cabin Ducting..........................................................................................21-19
21-27 Ram-Air Scoop........................................................................................21-20
21-28 Ram Air Control......................................................................................21-20
21-29 Main Duct Fan Switch.............................................................................21-20
21-30 Fan Duct and Ram-Air Valve...................................................................21-22
21-31 Cooling System Duct...............................................................................21-22
21-32 Ram-Air Valve Control............................................................................21-22
21-33 Gasper Outlets Cabin Cooling.................................................................21-23
21 AIR CONDITIONING

21-34 Air-Conditioning Operation (Maximum Cooling)....................................21-24

21-35 Fresh Air Exhaust....................................................................................21-26
21-36 Exhaust Vent............................................................................................21-27
21-37 Station 177 Internal Exhaust....................................................................21-27
21-38 Temperature Control Component Locations.............................................21-28
21-39 Temperature Control Panel.......................................................................21-29
21-40 
Automatic Temperature Controller...........................................................21-29
21-41 Cabin Temperature Sensor.......................................................................21-30
21-42 Duct Temperature Sensor.........................................................................21-30
21-43 OAT Sensor.............................................................................................21-30
21-44 
Hot-Air Valve and Potentiometer.............................................................21-30
21-45 Duct Overheat Switch..............................................................................21-31
21-46 Caution Lights.........................................................................................21-31
21-47 Air Conditioning - Temperature Control Auto Mode................................21-34
21-48 Air Conditioning - Temperature Control Manual Mode............................21-36

21-vi FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure Title Page

21-49 Flight Compartment Fans.........................................................................21-37

21-50 Refrigeration System Installation.............................................................21-38
21-51 Refrigeration System Schematic..............................................................21-40
21-52 Refrigeration System Operating Principles..............................................21-42
21-53 MMEL - Air Conditioning.......................................................................21-46
21-54 
Pre-Mod 6/1070 Ram-Air Scoop (Naca Scoop)........................................21-48
21-55 
Pre-Mod 6/1070 Cold Air Scoop..............................................................21-48
21-56 Wing Bleed Air Systems (Series 100/200 Differences)............................21-48

21 AIR CONDITIONING

FOR TRAINING PURPOSES ONLY 21-vii

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CHAPTER 21
AIR CONDITIONING

21 AIR CONDITIONING
21-00-00 AIR CONDITIONING
INTRODUCTION
This chapter describes the air-conditioning and pneumatic systems installed in the DHC-6
Twin Otter Series 300 airplanes. For descriptive purposes, air conditioning is divided into
bleed air, primary heating, primary cooling, ventilation, and temperature control systems.
The user should consult the Maintenance Manual, applicable AFM supplements and vendor
manuals for additional information on specific manufacturers installations not included in
this chapter.

FOR TRAINING PURPOSES ONLY 21-1

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21 AIR CONDITIONING

BLEED-AIR BLEED-AIR
DUCT DUCT
BLEED-AIR
SHUTOFF
VALVE

Figure 21-1. Aircraft Bleed Air Heating and Cooling System

Figure 21-2. Temperature Controls

21-2 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

GENERAL Air Exhausting

The exhaust air vents through a vent assembly
Figure 21-1 shows the engine bleed air installation.
in the cabin roof. Stale air also passes through
vents in the luggage compartment, and leaves
The air conditioning system, maintains a
the aircraft at the tail cone.
temperature controlled supply of warm air
and a supply of cool ambient ventilating air to
the flight compartment and cabin. The system Temperature Control System
comprises the heating, cooling, air exhausting
The temperature control system controls the
and temperature control systems.
opening and closing of the hot air valve which
controls bleed and maintains a selected cabin
Heating System temperature.
Compressed and heated bleed air from the
engine compressors is controlled and mixed Engine Bleed Air
with secondary air (ambient, fan pressure or
Air is obtained from the engine P3 bleed port
recirculated air).
on the gas generator case of the left and right
engines. Bleed air is directed through both
Cooling System wings into a common manifold in the fuselage
(Figure 21-1) for distribution to the installed
Ram air, ducted from an externally mounted

21 AIR CONDITIONING
pneumatic subsystems.
scoop supplies the heating system, and to
louver outlets on both sides of the cabin above
the windows. To provide a supply of cool air
when the aircraft is stationary, an electric fan
is installed in the main ram air duct. Additional
flight compartment ventilation can be obtained
by opening the flight compartment windows.
Two electric fans, provide for windshield
de-misting and additional cooling.

Air Conditioner System

At the customer’s option, a J.B. System Inc.
1000 Series Air Conditioner System (Mod
S.O.O. 6109) may be installed. The system is
essentially a heat transfer unit that provides the
means of circulating and cooling the air within
the aircraft interior.

FOR TRAINING PURPOSES ONLY 21-3

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
21 AIR CONDITIONING

Figure 21-3. Aircraft Bleed Air Heating and Cooling System

Figure 21-4. Bleed Air Source Figure 21-5. Bleed Air Duct

21-4 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

21-00-00 BLEED-AIR NOTES

SYSTEM
GENERAL
The P3 bleed air used for cabin heating and
airframe deicing is obtained from the engine
compressors through ducts attached to the gas
generator casing at the 11 o’clock positions.
The air supply is controlled by bleed air shut-off
valves (Figure 21-3 and Figure 21-6) installed
in the ducts forward of the nacelle firewalls
(Figure 21-3). The valves are controlled by
toggle switches labeled BLEED AIR L and
BLEED AIR R on the overhead console (Figure
21-2). The system is protected by 5A circuit
breakers labeled BLEED AIR L and BLEED
AIR R in the main circuit breaker panel.

21 AIR CONDITIONING

FOR TRAINING PURPOSES ONLY 21-5

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21 AIR CONDITIONING

Downstream
Duct

Figure 21-6. Bleed-Air Shut-Off Valve

21-6 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

COMPONENT DESCRIPTION NOTES

Engine Bleed Air Ducts
The duct routes bleed air from the bleed air port
on the engine to the bleed air shut off valve
(Figure 21-3).

Bleed-Air Shut-Off Valve Operation

The solenoid operated bleed-air shut-off valve
controls the air flow from the bleed port (Figure
21-6). When the solenoid valve is energized,
port A (at ambient pressure) is connected to
port B.

When the solenoid valve coil is de-energized,

port “A” closes and through internal passages
pressure on the cylinder side of the piston
increases to equal upstream pressure. In
conjunction with the compression spring, the

21 AIR CONDITIONING
bleed air valve closes.

NOTE
Engine bleed-air temperature
and pressure at the bleed-air
port vary according to engine
power setting. The pilot may
see “subject to atmospheric
conditions and altitude” an
increase in T5 as air normally
used for combustion and cooling
is diverted to other services.

FOR TRAINING PURPOSES ONLY 21-7

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

TO LH TO RH
ENGINE ENGINE
INTAKE INTAKE
DEFLECTOR DEFLECTOR
LH RH
ENGINE DE VORE ENGINE
SHROUD

BLEED-AIR BLEED-AIR
SHUTOFF SHUTOFF
VALVE OVERBOARD CHECK VALVES OVERBOARD VALVE
DRAIN VENT DRAIN VENT
DE VORE SHROUD

LEGEND
21 AIR CONDITIONING

HOT AIR AMBIENT

Figure 21-7. Wing Bleed-Air Systems

Figure 21-8. Mod 6/1462 De Vore Shroud Bleed Pipes

21-8 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Wing Bleed Air Duct Systems NOTES

Air from the bleed-air shut-off valve is
directed to a common manifold in the cabin
compartment by one of two wing bleed-air
systems (Figure 21-7).

Initial Bleed Air Duct

The original system (top schematic) transfers
engine hot bleed air at engine compressor
temperature directly through the wing ducts into
the fuselage manifold and bleed air duct area.

“De Vore” Bleed Air Duct Post

Mod 6/1482
The lower schematic in Figure 21-7 shows
the present wing bleed-air configuration. The
original wing bleed-air duct from the engine
nacelle to the fuselage is replaced by a De Vore

21 AIR CONDITIONING
shroud and inner duct (Figure 21-8) by SFAR 23
mod 6/1482 at aircraft 411. This design provides
an air space between the inner air duct supplying
the engine hot bleed air and the outer shroud.
Should a rupture occur to the inner air duct the
hot bleed air is immediately vented outboard
through a vent pipe between the outer shroud
and the wing lower skin. A later improvement by
mod 6/1614 (S/B 6/355) at aircraft 514 replaced
the original “De Vore” duct with a new “De
Vore” duct complete with bellows to eliminate
cracking. Material expansion caused by heat
passing through the duct is now compensated
with bellow movement.

FOR TRAINING PURPOSES ONLY 21-9

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

RH SHUT OFF
VALVE CHECK
COMMON VALVES
BLEED AIR
PIPE ASSEMBLY

WINDSHIELD
HEATING
OUTLET

CABIN HEATING DUCTS

21 AIR CONDITIONING

LH SHUT OFF
VALVE

Figure 21-9. Fuselage Bleed-Air Duct System

Figure 21-10. In-line Check Valve Figure 21-11. Low Pressure Regulator

21-10 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Inline Check Valves NOTES

Refer to:

•• Figure 21-9. Fuselage Bleed-Air Duct

System.
•• Figure 21-10. In-line Check Valve.

Check valves are installed in each end of the tee

duct to prevent a differential pressure situation
occurring (reverse flow) should a single engine
flame out.

Fuselage Bleed-Air Duct Assembly

From the tee duct a common supply duct is
routed forward, above the cabin ceiling and
down the left side of the cabin to the flight
compartment bulkhead, to connect with the hot-
air valve under the flight compartment floor. The
ducts in the fuselage are sealed and insulated

21 AIR CONDITIONING
with fiberglass to exclude fluids and reduce
the effect of heat with adjacent structure. A
tapping into the bleed-air duct, in the cabin roof
at station 177.00, provides a pressure point for
an 18-psi pressure reduction unit.

FOR TRAINING PURPOSES ONLY 21-11

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

RH SHUT OFF
VALVE CHECK
COMMON VALVES
BLEED AIR
PIPE ASSEMBLY

WINDSHIELD
HEATING
OUTLET

CABIN HEATING DUCTS

21 AIR CONDITIONING

LH SHUT OFF
VALVE

CABIN AIR
VALVE KNOB CABIN AIR
CREW HEATING
OUTLETS CONTROL VALVE

RAM AIR VALVE

TO CABIN
HEATING DUCTS

CREW FOOT WARMERS

EXPANSION RECIRCULATED HOT AIR VALVE

RAM AIR CHAMBER AIR INTAKE
(AMBIENT) EJECTOR SILENCER

Figure 21-12. Heating System Installation

21-12 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

21-40-00 HEATING
SYSTEM
GENERAL
The heating system utilizes bleed air from the
two engine compressors to mix with secondary
air (ram, fan pressure, or recirculated air) to a
selected temperature and distributes it to outlets
in the cabin and flight compartment.

DESCRIPTION Figure 21-13. Hot Air Valve

Refer to Figure 21-12. Heating System
Installation.

The heating system consists of hot air valve,

Flow Limiting Orifice
expansion chamber, ejector, silencer, cabin air A flow-limiting orifice in the bleed-air duct
control valve, ducts and outlets. before the hot-air valve, limits the bleed

21 AIR CONDITIONING
airflow to 4% of the bleed-air flow available
from the two engines. The orifice maintains a
Bleed Air Pipe Assembly back pressure in the bleed-air duct to ensure air
A tapping in the bleed air piping, in the cabin pressure is available for the pneumatic operated
roof at station 177.00, provides for a customer airframe de-ice system.
option low pressure (18 psi) pneumatic system.
Pipes are insulated with fiberglass and are
sealed to exclude fluids.

Hot Air Valve

Refer to Figure 21-13. Hot Air Valve.

The hot air valve, connected between the

bleed air pipe assembly and a pipe connected
to the expansion chamber, is below the pilot
seat under the flight compartment floor. The
valve is a motorized butterfly type which takes
approximately 30 seconds to move through its
full 90 degree range of travel.

FOR TRAINING PURPOSES ONLY 21-13

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Expansion Chamber EXPANSION

CHAMBER OUTLET TO
Figure 21-14. Expansion Chamber. CONNECTION SILENCER

The expansion chamber, which reduces bleed

air noise, is a duct into which protrudes a pipe
connected to the inlet end of the duct; the pipe
from the hot air valve connects to this end. The
outlet end of the expansion chamber incorporates
a group of nozzles which protrude into the ejector
and a flange which bolts to the ejector.
RECIRCULATED
AIR
FROM HOT
AIR VALVE
Figure 21-15. Ejector
21 AIR CONDITIONING

PROTRUDES
INTO EJECTOR

RECIRCULATED
AIR
Figure 21-14. Expansion Chamber

EJECTOR

Ejector
Refer to Figure 21-15. Ejector. Figure 21-16. E
 jector and Recirculated
Air Duct Installation
The ejector is located laterally across the
fuselage below the flight compartment floor
and forms the mixing chamber for recirculated
air. The nozzles, protruding into the ejector
and the ejector diffuser, induce a suction
which draws in recirculated air from the flight
compartment when ram or fan pressure air is
not being used as the secondary air source. The
recirculated air duct, connected between an
intake in the flight compartment footwell and
the rear of the ejector, incorporates an integral
silencer and check valve.

Figure 21-17. Recirc Air Inlet

21-14 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Silencer RAM AIR AND

EJECTOR
Refer to: INPUT AIR

•• Figure 21-18. Silencer.

•• Figure 21-19. Silencer Location.

The silencer is located longitudinally below the

flight compartment, its inlet connects to the ram
air fan duct and ejector outlet. The silencer outlet,
which incorporates the cabin air control valve, CREW
connects to the distribution ducting. Two smaller HEATING
diameter outlets on the silencer provide for the OUTLET
connection of flight compartment, windshield and AND CREW
FOOT
foot warmer ducts. Provision is also made on the WARMERS
silencer for the installation of temperature control
system components. SILENCER OUTLET
TO CABIN AIR
CONTROL VALVE

21 AIR CONDITIONING
Figure 21-18. Silencer

CABIN AIR
VALVE KNOB CABIN AIR
CREW HEATING
OUTLETS CONTROL VALVE

RAM AIR VALVE

TO CABIN
HEATING DUCTS

CREW FOOT WARMERS

EXPANSION RECIRCULATED HOT AIR VALVE

CHAMBER AIR INTAKE
EJECTOR SILENCER

Figure 21-19. Silencer Location

Revision 0.3
FOR TRAINING PURPOSES ONLY 21-15
 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Cabin Air Control Valve COCKPIT

FLOOR
Refer to:

•• Figure 21-20. Cabin Air Valve Control

Knob Location.
•• Figure 21-21. Cabin Air Control Valve.
COCKPIT
The cabin air control valve is in the silencer BULKHEAD
outlet duct and controls the supply of warm
or (with hot air valve and engine bleed valves
closed) the cooling air supply to the cabin. The
valve is manually operated by a knob behind
the co-pilot seat. Closing the air control (by
pulling the knob fully up), allows all available
flow to be directed into the flight compartment.
Figure 21-20. C
 abin Air Valve Control Knob
Location
21 AIR CONDITIONING

Figure 21-21. Cabin Air Control Valve

21-16 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Ducts
The heating system ducts comprise an
installation of aluminum alloy and polycarbonate
tubes which lead from the silencer to the pilot
and co-pilot foot warmers, the windshield heater
and cabin heater outlets.

Outlets
The heating system outlets connect to the
terminating ends of the windshield, crew and
cabin heating system ducts. The outlets are of
polycarbonate material and formed to provide
a suitably diffused air flow. The windshield
outlet is integrated with the glare shield above
the instrument panel, and the crew foot warmer
outlets are at floor level and direct their flow
towards the rudder pedals. The cabin outlet
ducts are box section ducts which extend
from the flight compartment/cabin bulkhead

21 AIR CONDITIONING
rearwards at the base of each wall.

CABIN AIR
VALVE KNOB CABIN AIR
CREW HEATING
OUTLETS CONTROL VALVE

RAM AIR VALVE

TO CABIN
HEATING DUCTS

CREW FOOT WARMERS

EXPANSION RECIRCULATED HOT AIR VALVE

CHAMBER AIR INTAKE
EJECTOR SILENCER

Figure 21-22. Heating System - Component Location

FOR TRAINING PURPOSES ONLY 21-17

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

OVERHEAD CONSOLE
FRONT OF CABIN
BEHIND HEADLINER

AUTOMATIC
TEMPERATURE CAUTION LIGHTS
CONTROLLER PANEL

LEGEND
DUAL
HOT BLEED AIR
PRESSURE
SWITCH RECIRCULATED AIR
CONDITIONED AIR
AMBIENT AIR
WINDSHIELD HEATER OUTLETS
RAM ELECTRICAL
AIR MODULATING VALVE
O.A.T
SENSOR FOOT WARMERS
21 AIR CONDITIONING

RAM AIR
MAIN DUCT FAN VALVE
(MANUAL)
DUCT
EJECTOR OVERHEAT
SWITCH
HOT AIR DUCT
VALVE CHECK TEMP
(MOTORIZED) VALVE SENSOR

PILOT’S CO-PILOT’S
HEATER RECIRCULATED HEATER
OUTLET AIR INTAKE OUTLET

SILENCER
BLEED CABIN AIR CONTROL
SUPPLY VALVE (MANUAL)

CABIN
CABIN TEMP

CABIN CABIN CEILING SENSOR

BASEBOARD EXHAUST FAN
LOUBERS
HEATER VENT
OUTLETS

Figure 21-23. Air-Conditioning Operation (Maximum Heating)

21-18 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Maximum Heating System

Figure 21-23 illustrates automatic operation in
a maximum heating mode.

The hot-air modulating valve is shown fully

open to allow bleed air enter the jet pump
(ejector) plenum at a high velocity. The
manually operated ram-air valve is shown fully
closed preventing external cold ram air entering
the silencer area to mix with the warm air. The
jet pump velocity from the expansion chamber
into the ejector also induces cabin air into the
ejector area to mix with the hot bleed air prior
to determine the final air temperature.
Figure 21-24. Cockpit Foot Warmer
Conditioned air noise level is reduced to a
minimum as it passes through the silencer.
The manually operated cabin air control valve
outboard from the silencer, below the co-pilot
seat floor, is shown in the open condition to

21 AIR CONDITIONING
allow a warm air flow to enter the main outlet
duct before branching left and right to both sides
of the cabin baseboard heater outlets at floor
level. Separate outlets from the silencer supply
air to the flight compartment. The areas serviced
are the pilot and co-pilot conditioned-air outlets,
foot warmers and main windshield defogging
system. Since the cabin distribution ducting is
larger than that of the flight compartment, the
major portion of the warm airflow will enter the
cabin distribution system.
Figure 21-25. Crew Heating Outlet
The flight compartment temperature can be
increased by altering the position of the cabin
air control valve with the valve operating
lever behind the co-pilot seat (Figure 21-20).
Pulling the lever upwards moves the control
valve toward the closed position decreasing the
volume of warm air to the cabin and increasing
the air volume to the crew conditioned-
air outlets, foot warmers, and windshield
defogging areas.

Figure 21-26. Cabin Ducting

FOR TRAINING PURPOSES ONLY 21-19

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
21 AIR CONDITIONING

Figure 21-27. Ram-Air Scoop

OPEN OPEN

R R
A A
M M

A A
I I
R R
CLOSE CLOSE

Figure 21-28. Ram Air Control Figure 21-29. Main Duct Fan Switch

21-20 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

21-50-00 PRIMARY *The following is an abbreviated description of

the maintenance practices and is intended for
COOLING SYSTEM training purposes only.
For a more detailed description of the practice,
GENERAL refer to the task in the Viking AMM PSM 1-63-2.

Cooling the cabin area during flight is achieved

with cool ambient ram-air entering the air
21-50-00 MAINTENANCE
duct through a protruding air scoop on the PRACTICES
left side of the nose section (Figure 21-27).
Cool air bypasses the silencer chamber to
supply air directly to ducts above the left and
INSPECTION/CHECK
right sides of the fuselage (Figure 21-33). The
cabin cool airflow is individually controlled
Ram-Air Scoop Inspection
for passenger’s comfort with gasper outlet
Inspect the Ram air inlet (Pre Mod 6/1070)
adjustment. The volume of cool ram air
screen, including (Mod 6/1070) duct inlet, for
entering the cabin area can be adjusted with
condition and free of obstruction.
the position of the manually operated ram-
air valve. The valve position is controlled by
a lever on the flight compartment pedestal Cabin Vent Fan

21 AIR CONDITIONING
(Figure 21-28) adjacent to the pilot. During
ground operation a fan replaces the ram air An electrically operated fan intended for ground
scoop effect during flight to provide an external operation was introduced by mod 6/1181, at
air supply to the cabin at ambient temperature aircraft 136, is installed between the main
Early pre mod 6/1181 aircraft prior to 136 do duct and external air scoop below the flight
not incorporate the ground operable fan. Cabin compartment floor. The fan is protected by a
cooling is only obtained during flight with 20-amp circuit breaker labeled CABIN VENT
protruding air scoops on each side of the upper FAN on the overhead console circuit-breaker
fuselage area aft of the flight compartment. panel. The fan is controlled by a vent fan switch
Although larger scoops with small fans were (Figure 21-28) labeled CABIN VENT FAN or
installed on a number of aircraft to reduce the MAIN DUCT FAN on the flight compartment
cabin temperature during ground operation, the pedestal. The switch is protected by a 5-amp
change did not provide the necessary degree circuit breaker labeled CABIN VENT FAN
of passenger comfort in high temperature on the overhead console circuit breaker panel.
environments. Access to the fan is through a side panel below
the flight compartment left door.
COMPONENT DESCRIPTION
NOTE
Ram-Air Scoop and Main Duct When changing the brushes on
the vent fan there is a mandatory
The ram-air scoop (Figure 21-27) by mod
run in time to be observed.
6/1070 at aircraft 136 is a protruding air intake
on the left side of the fuselage nose supplying
external cold ambient temperature ram air. A
polycarbonate main duct connects the ram-air
scoop to the main duct fan.

FOR TRAINING PURPOSES ONLY 21-21

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
21 AIR CONDITIONING

Figure 21-30. Fan Duct and Ram-Air Valve Figure 21-31. Cooling System Duct

Figure 21-32. Ram-Air Valve Control

21-22 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Fan Duct Ram-Air Valve

The Y-shaped fan duct (Figure 21-30) connects A butterfly ram-air valve, in the fan duct,
the main duct fan, heating system silencer is manually operated by a lever in the flight
inlet, and the cabin cool air duct supply. A compartment pedestal adjacent to the pilot
manual operated ram-air valve is installed in (Figure 21-28 and Figure 21-32) to regulate
the heating system prior to the ejector outlet ram-air flow to suit heating system requirements.
and silencer chamber.

Cooling System Ducts

The cooling system ducting connects to the fan
duct and is routed under the flight compartment
floor (Figure 21-31) and up both sides of the
cabin behind the flight compartment/cabin
bulkhead to join box section ducts running
along both sides of the cabin above the cabin
windows. The box section ducts house the gasper
air outlets.

Cool-Air Outlets

21 AIR CONDITIONING
Individual cool-air (gasper) outlets (Figure
21-33) are above the left and right sides of the
fuselage above the window level. The gasper
air outlet is passenger adjusted for personal
comfort.

Figure 21-33. Gasper Outlets Cabin Cooling

FOR TRAINING PURPOSES ONLY 21-23

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

OVERHEAD CONSOLE
FRONT OF CABIN
BEHIND HEADLINER

AUTOMATIC
TEMPERATURE CAUTION LIGHTS
CONTROLLER PANEL

LEGEND
DUAL
HOT BLEED AIR
PRESSURE
SWITCH RECIRCULATED AIR
CONDITIONED AIR
AMBIENT AIR
WINDSHIELD HEATER OUTLETS
RAM ELECTRICAL
AIR MODULATING VALVE
O.A.T
SENSOR FOOT WARMERS
21 AIR CONDITIONING

RAM AIR
MAIN DUCT FAN VALVE
(MANUAL)
DUCT
EJECTOR OVERHEAT
SWITCH
HOT AIR DUCT
VALVE CHECK TEMP
(MOTORIZED) VALVE SENSOR

PILOT’S CO-PILOT’S
HEATER RECIRCULATED HEATER
OUTLET AIR INTAKE OUTLET

SILENCER

CABIN AIR CONTROL

VALVE (MANUAL)

CABIN
CABIN TEMP

CABIN
CABIN CEILING SENSOR
BASEBOARD EXHAUST FAN
LOUBERS
HEATER VENT
OUTLETS

Figure 21-34. Air-Conditioning Operation (Maximum Cooling)

21-24 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Maximum Cooling NOTES

Figure 21-34 illustrates the operation in the
maximum cooling mode during flight. The
motorized hot-air valve is in the fully closed
position eliminating further hot bleed air to the
ejector chamber. The ram air and cabin air control
valves are both in the open position permitting
cool ram air to flow through the silencer chamber
to the cabin baseboard heater outlets at the same
time as flowing from the cabin open gasper outlets.

The maximum cooling mode, also defined

as fresh air ventilation or cabin air purging,
should it be necessary to remove annoying
bleed air or fuel odors from the cabin.

21 AIR CONDITIONING

FOR TRAINING PURPOSES ONLY 21-25

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

FRESH AIR
EXHAUST
21 AIR CONDITIONING

REAR BAGGAGE AREA

Figure 21-35. Fresh Air Exhaust

21-26 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Ventilation
Stale air is exhausted through a grille in the
headliner to above the ceiling area to exit through
the rearward-facing vent (Figure 21-35, Figure
21-36 and Figure 21-37) installed on the fuselage
roof at station 177.0. The airflow over the vent
during flight produces a venturi force to induce
a flow of stale or contaminated air through the
vent to atmosphere. Vents in the rear baggage
compartment area also provide a path to exhaust
cabin air.

Figure 21-36. Exhaust Vent

21 AIR CONDITIONING
Figure 21-37. Station 177 Internal Exhaust

FOR TRAINING PURPOSES ONLY 21-27

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
21 AIR CONDITIONING

Figure 21-38. Temperature Control Component Locations

21-28 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

21-60-00 TEMPERATURE
CONTROL
GENERAL
An electronic temperature control system
(Figure 21-38) is installed to operate in
automatic or manual modes. The function of
the automatic temperature control system is
to provide and maintain cabin temperature by
regulating engine bleed airflow through control
of the motorized hot air valve.

The temperature control system consists of a

controller, temperature control panel, automatic
temperature controller, cabin temperature
sensor, duct temperature sensor, outside air
temperature (OAT) sensor, hot-air valve
potentiometer, and overheat switch.

21 AIR CONDITIONING
COMPONENT DESCRIPTION Figure 21-39. Temperature Control Panel
Control Panel
The control panel on the overhead console and
provides manual or automatic mode control
(Figure 21-39). The panel includes a mode
selector switch labeled MANUAL-OFF-AUTO
including a temperature selector rheostat
labeled COOL and WARM and a manual
temperature control switch labeled MANUAL
COOL - HOLD - MANUAL WARM.

Automatic Temperature
Controller
The automatic temperature controller is above
the cabin ceiling (Figure 21-40). The controller
senses temperature in the automatic mode and
adjusts cabin temperature by controlling the
amount of hot air flow to the cabin system Figure 21-40. Automatic Temperature
through the motorized hot air valve. Controller

FOR TRAINING PURPOSES ONLY 21-29

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Cabin Temperature Sensor

The cabin temperature sensor on the aft right
side face of cabin forward bulkhead station
110.0 (Figure 21-41), relays cabin temperature
information to the automatic temperature
controller. A small fan inside the unit
continually circulates cabin air over the sensor
for instant temperature recording when cabin
temperature is above 21°C.

Figure 21-42. Duct Temperature Sensor

21 AIR CONDITIONING

Figure 21-41. Cabin Temperature Sensor

Duct Temperature Sensor Figure 21-43. OAT Sensor

A temperature sensor in the silencer duct
(Figure 21-42) senses early temperature
variations to enhance cabin temperature
automatic control capability.

OAT Sensor
The OAT sensor in the ram-air scoop (Figure
21-43) detects ambient ram-air temperature.

Hot-Air Valve Potentiometer

The hot air valve potentiometer inside the
hot-air valve (Figure 21-44) senses gate
valve position for the automatic temperature
controller.
Figure 21-44. H
 ot-Air Valve and
Potentiometer

21-30 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Duct Overheat Sensor NOTES

Refer to:

•• Figure 21-45. Duct Overheat Switch.

•• Figure 21-46. Caution Lights.
The duct overheat sensing switch in the silencer
(Figure 21-46) brings on a caution light labeled
DUCT OVERHEAT on the annunciator caution
light panel when the silencer duct reaches a
temperature of 300°F). The light will go out
when the silencer duct temperature drops below
the 300°F level. Power for the caution light is
obtained from the left DC bus through a 5-amp
circuit breaker labeled BLEED AIR on the
main circuit breaker panel.

21 AIR CONDITIONING
Figure 21-45. Duct Overheat Switch

Figure 21-46. Caution Lights

FOR TRAINING PURPOSES ONLY 21-31

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of AUTO, gradually release pressure and hold
the maintenance practices and is intended for at 17.5 psi. Check that hot air valve does
training purposes only. not run when TEMP CONTROL knob is
selected to any temperature.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2. 7. W h e r e a p p l i c a b l e , r e l e a s e p r e s s u r e ,
disconnect pressure source, and reconnect
21-60-00 MAINTENANCE heat exchanger inlet.

PRACTICES Operational Test - Cabin

Temperature Sensor
ADJUSTMENT/TEST
Sensor Blower
Test Temperature Control System 1. Ensure CABIN HT/VENT circuit breaker
Controls is in.
1. On aircraft with customer option equipment 2. Select temperature control switch on
requiring the low pressure (18 psi) overhead console to AUTO.
pneumatic system, the heat exchanger
3. Observe operation of blower in cabin
inlet must be disconnected and a constant
temperature sensor housing.
pressure of 30 psi connected to the heat
21 AIR CONDITIONING

exchanger inlet port. 4. Select temperature control switch to OFF.

2. On the overhead console, select the
MANUAL-OFF-AUTO switch to
Functionally Test of the
MANUAL and hold the MANUAL COOL- Temperature Control System
HOLD -MANUAL WARM switch down to
MANUAL WARM. Check audibly that the NOTE
hot air valve actuator under the floor below
This test can be carried out on
the pilot seat, runs for 30 ± 8 seconds.
the ground with engines running
3. S e l e c t t h e M A N U A L C O O L - H O L D - provided the cabin temperature
MANUAL WARM switch to MANUAL is not over 70°F. When the
COOL, and check that hot air valve actuator cabin temperature is over 70°F
again runs for 30 ± 8 seconds. the tests should be carried out
in flight at an altitude where the
4. Select the MANUAL-OFF-AUTO switch to
cabin is below 70°F.
AUTO. The hot air valve actuator may run
for a period, but there should be no further
1. Set one engine to at least 80 percent power
actuation of the valve when the MANUAL
when on the ground (if test is carried out in
COOL-HOLD-MANUAL WARM switch
flight, set engines for level flight).
is selected to any position. Leave switch
at HOLD. 2. S e t L E F T a n d R I G H T B L E E D A I R
switches on, MANUAL-OFF-AUTO
5. With the MANUAL-OFF-AUTO switch
switch to AUTO and check that the cabin
still at AUTO, make a selection on the
air control knob, behind the co-pilot seat,
TEMP CONTROL knob, below ambient
is fully down.
temperature and then above ambient
temperature. The hot air valve should be 3. Confirm that temperature output varies
heard running and stop on each selection. appropriately when the TEMP CONTROL
knob is selected to various positions.
6. On aircraft where step 1 is applicable, with
the MANUAL-OFF-AUTO switch still at

21-32 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

4. With engines selected to idle power NOTES

on aircraft fitted with a heat exchanger
package, there should be no heat output
regardless of the selection of the TEMP
CONTROL knob.
5. Select MANUAL-OFF-AUTO switch
and BLEED AIR switches to OFF. Stop
engines.

INSPECTION/CHECK
Inspection of Air Conditioner Inlet
and Exhaust Outlets
Inspect the Air conditioner inlet and exhaust
outlets for condition and security; free of
obstruction. Clean as required.

21 AIR CONDITIONING

FOR TRAINING PURPOSES ONLY 21-33

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

OVERHEAD CONSOLE

FRONT OF CABIN
BEHIND HEADLINER
AUTOMATIC
TEMPERATURE
CONTROLLER
SELECTOR
RHEOSTAT CAUTION LIGHTS
PANEL

WINDSHIELD HEATER OUTLETS

RAM
AIR
O.A.T
SENSOR FOOT WARMERS

RAM AIR
MAIN DUCT FAN VALVE
(MANUAL) DUCT
21 AIR CONDITIONING

EJECTOR OVERHEAT
SWITCH
HOT AIR DUCT
VALVE CHECK TEMP
(MOTORIZED) VALVE SENSOR

PILOT’S CO-PILOT’S
HEATER RECIRCULATED HEATER
OUTLET AIR INTAKE OUTLET

SILENCER

CABIN AIR CONTROL

VALVE (MANUAL)

BLEED AIR CABIN CABIN

CABIN CEILING TEMP CABIN
SUPPLY
BASEBOARD EXHAUST SENSOR LOUVRES
HEATER VENT
OUTLETS FAN
TO HEAT EXCHANGER
(IF FITTED)
LEGEND
PRE MOD BLEED AIR PRE MOD
6/1266 TO TO 6/1266
LH ENGINE ELECTRICAL FLOW RH ENGINE
INTAKE INTAKE
LH DEFLECTOR DEFLECTOR RH
ENGINE ENGINE

BLEED AIR BLEED AIR

SHUT OFF SHUT OFF
VALVE MOD 6/1482 MOD 6/1482 VALVE
OVERBOARD DRAIN VENT CHECK VALVES OVERBOARD DRAIN VENT

Figure 21-47. Air Conditioning - Temperature Control Auto Mode

21-34 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Automatic Temperature Control NOTES

Mode
When the temperature control panel mode
selector switch is set to AUTO, the temperature
selector rheostat is armed for automatic
temperature selection (Figure 21-47).
Positioning the rheostat between COOL and
WARM supplies the desired temperature
demand signal to the controller. At the same
time the temperature controller receives an
ambient temperature signal from the OAT
sensor, an area temperature signal from the
cabin temperature sensor and a temperature
signal from the silencer duct for comparison.

When the temperature controller senses the

signals are out of balance (Wheatstone circuit)
the controller will send a current pulse signal
to the hot air valve to adjust the hot air valve
position either in the open or closed direction.

21 AIR CONDITIONING
The movement of the valve will increase or
decrease the amount of hot air supply depending
on the demand signal sent by the temperature
controller.

If the mode selector switch is in automatic,

with no airflow to the cabin, the temperature
controller will drive the hot air valve either
fully open or closed. To prevent the condition
occurring, the mode selector switch should be
selected off until the engines are operating and
bleed air is available. With constant parameter
monitoring, the hot air valve will continue to
respond to computed signals as long as the
temperature controller is in the automatic mode.

On aircraft equipped with an airframe de-icing,

a dual-pressure switch is required to sense the
regulated air pressure. When the pressure drops
to approximately 25 psi, one contact in the dual
switch closes and inhibits further movement
of the hot-air mixing valve toward the open
position. If pressure continues to decrease
and reaches 20 psi, the second contact in the
switch closes and completes a circuit to close
the hot-air valve. The dual pressure switch
will only control the hot air valve operation
when selected to automatic mode. (See MSM -
“Air-Conditioning Electrical schematic - Auto
Mode” Air Conditioning Electrical Schematic)

FOR TRAINING PURPOSES ONLY 21-35

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

OVERHEAD CONSOLE

FRONT OF CABIN
BEHIND HEADLINER
AUTOMATIC
TEMPERATURE
CONTROLLER

CAUTION LIGHTS
PANEL

WINDSHIELD HEATER OUTLETS

RAM
AIR
O.A.T
SENSOR FOOT WARMERS

RAM AIR
MAIN DUCT FAN VALVE
(MANUAL) DUCT
21 AIR CONDITIONING

EJECTOR OVERHEAT
SWITCH
HOT AIR DUCT
VALVE CHECK TEMP
(MOTORIZED) VALVE SENSOR

PILOT’S CO-PILOT’S
HEATER RECIRCULATED HEATER
OUTLET AIR INTAKE OUTLET

SILENCER

CABIN AIR CONTROL

VALVE (MANUAL)

BLEED AIR CABIN CABIN

CABIN CEILING TEMP CABIN
SUPPLY
BASEBOARD EXHAUST SENSOR LOUVRES
HEATER VENT
OUTLETS FAN
TO HEAT EXCHANGER
(IF FITTED)
LEGEND
PRE MOD BLEED AIR PRE MOD
6/1266 TO TO 6/1266
LH ENGINE ELECTRICAL FLOW RH ENGINE
INTAKE INTAKE
LH DEFLECTOR DEFLECTOR RH
ENGINE ENGINE

BLEED AIR BLEED AIR

SHUT OFF SHUT OFF
VALVE MOD 6/1482 MOD 6/1482 VALVE
OVERBOARD DRAIN VENT CHECK VALVES OVERBOARD DRAIN VENT

Figure 21-48. Air Conditioning - Temperature Control Manual Mode

21-36 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Manual Temperature Control Mode Early model Casco fans had an on-off switch
on the fan body. Later model Casco fans by
When the mode selector switch is set to
mod 6/1238 (TAB 604/2) did not have a switch.
MANUAL (Figure 21-48 & MSM - “Air-
Conditioning Electrical Schematic - Manual
The final model of fan supplied by Caramo was
Mode Operations”), the automatic temperature
equipped with three-position switch on the fan
control system is inhibited, and the manual
body, allowing the pilot to adjust the fan speed
temperature control switch is armed. Holding
or turn the fans off individually. This fan has
this switch to the MANUAL COOL or MANUAL
been approved by mod 6/1897.
WARM position, motors the hot air valve in the
selected direction until the switch is released
to the HOLD position. When using the manual
temperature control switch it should be held in
the MANUAL COOL or MANUAL WARM
positions for only short periods and then released
when the temperature stabilizes. In Manual mode
the valve will motor from one extreme to the
other (open to close) in 30 seconds. Selecting
the mode selector switch to the OFF position will
power Hot Air Valve closed.

21 AIR CONDITIONING
21-55-00 COOLING/
DEMISTING FANS
GENERAL Figure 21-49. Flight Compartment Fans

Two electrically-driven fans are installed in the

flight compartment, approximately 10 inches
either side of the aircraft centerline at station
97.20, and may be used to direct air onto the
pilot and co-pilot windshields for demisting
purposes, or for additional cooling. Both the
fans are 12VDC, but each fan is connected to
the 28VDC bus by a 15 ohm series connected
resistor. Each fan is attached to the aircraft
canopy.

Flight Compartment Fans

Flight compartment fans were installed as
standard equipment by mod 6/1602 beginning
with aircraft serial number 531. Previously, the
fans had been available as an option by S.O.O.
6015. The fans may be adjusted to provide
airflow as required in the flight compartment.
Power is supplied to the fans from the right DC
bus through a combination switch and circuit
breaker labeled FLT COMTP FANS on the
overhead circuit breaker panel.

FOR TRAINING PURPOSES ONLY 21-37

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
21 AIR CONDITIONING

Figure 21-50. Refrigeration System Installation

21-38 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

21-00-00 REFRIGERATION NOTES

SYSTEM
GENERAL
A conventional Freon Air Conditioning system,
optional mod S.O.O. 6109 is available. The
system, including the distribution ducting, is a
self-contained unit.

The system consists of a DC motor-driven

Freon compressor, condenser and fan,
evaporator and fan, receiver dryer, expansion
valve, pressure switch under the cabin floor
and thermostat, control panel, and box section
Y-shaped ducts in the cabin headliner.

Figure 21-50 illustrates a typical installation

in Twin Otter aircraft. A relay panel and

21 AIR CONDITIONING
associated circuit breakers are on the cabin
bulkhead Stn 332 below the cabin floor.

FOR TRAINING PURPOSES ONLY 21-39

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

PART OF PD-K5 PART OF PD-K6

RELAY* RELAY* *RELAYS ARE ENERGIZED WHEN
GENERATORS ARE ON LINE.

FLIGHT K2
EVAPORATOR
OFF FAN

GROUND

POWER
SWITCH

K3
21 AIR CONDITIONING

F4 40A CONDENSOR
NORMAL FAN
F3

40A
FAN ONLY

QUICK COOL
150A
OPERATION K4 LIMITER
SWITCH
TO LEFT DC BUS
F2
F1
120A
5A
COMPRESSOR
MOTOR

K1
TO AUXILIARY
BATTERY BUS
THROUGH
LEFT IGNITION
MODE SWITCH

PART OF
P START
THERMOSTAT SWITCH
SWITCH
HIGH-PRESSURE
SWITCH

K5

Figure 21-51. Refrigeration System Schematic

21-40 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

DESCRIPTION AND OPERATION the compressor motor K1 relay to

stop compressor motor operation.
The control panel labeled AIR CONDITIONING If the K5 relay fails to inhibit the
is located on the overhead console main switch compressor motor operation there
panel. The panel includes two three position would not be sufficient power to
switches. One switch is labeled POWER achieve the engine start.
with selections labeled FLIGHT - OFF- and
GROUND. The FLIGHT selection arms a When both switches are selected to GROUND
circuit (Figure 21-51) through the generator and NORMAL, the K1 compressor motor and
control system preventing operation of the air K4 condenser fan relays are energized for
conditioning unit in flight unless both generators operation. With power to the evaporator fan
are on. The GROUND selection provides for through the condenser fan, now in series mode,
operation using either an external power or both the two fans will operate at moderate speed.
aircraft generators On Line supplying power to
the left DC bus. During flight, with the switch When both switches are selected to GROUND
selected to Flight, both generators must be and QUICK COOL, the K1 compressor motor
on-line for the air condition unit to operate. including K3 evaporator and K4 condenser fan
relays are energized for operation. With power
CAUTION to the evaporator and condenser fans in parallel
mode, both fans operate at a high speed.
During ground mode it is

21 AIR CONDITIONING
recommended that an external CAUTION
power unit be connected and
operating any time that the air When operating on the ground
conditioning system is required with one generator on, do not
to be operational. select QUICK COOL, as the
operating generator may overheat
The other switch is a three position switch because of high current demand.
labeled OPERATION with selections labeled
NORMAL- FAN ONLY- and QUICK COOL. When the switches are set to GROUND and
FAN ONLY, the K1 relay is de-energized
Power is obtained from the left DC bus through to shut down the compressor motor. The K4
a 150 amp current limiter adjacent to the Power. relay is energized to operate the condenser and
Box in the cabin roof area. Four circuit breakers evaporator fans at moderate speed. When the
and five ­­­­­­distribution relays are on a panel attached POWER switch is set to FLIGHT, the direct
to the forward face of bulkhead Stn 332 below ground is replaced with a ground obtained
the cabin floor. The 5-amp circuit breaker labeled through both generator control relays PD-K5
F1 provides protection for the five relays and and PD-K6 providing both generators are ON
associated wiring including switch contacts. The Line and operating. Loss of either generator
120-amp circuit breaker labeled F2 protects the will result in the loss of the refrigeration system
compressor motor and the two 40 amp circuit due to the opening of the generator control
breakers labeled F3 and F4 protect the evaporator relay contact removing the ground.
and condenser fan operations.
As the system requires airflow through the
condenser core to dissipate some of the high-
NOTE pressurized freon liquid temperature, an inlet
Should an engine start be attempted (left) and outlet (right) air grill is installed on
when the air conditioning unit is each side of the fuselage below the floor area.
operational, the K5 relay will Recirculated cabin air is drawn through a grill
receive power through the engine on the cabin floor before passing through the
start switch contacts to de-energize evaporator coils to cool before entering the cabin.

FOR TRAINING PURPOSES ONLY 21-41

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
21 AIR CONDITIONING

Figure 21-52. Refrigeration System Operating Principles

21-42 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

OPERATING PRINCIPLES Compressor

The compressor, adjacent to and forward of the
Figure 21-52 illustrates the operating principles
compressor motor, is attached to a mounting
of a typical refrigeration system. The compressor
bracket secured to the forward pallet. Two
draws in low-pressure freon vapor to discharge
T-connections are provided on the forward face
high-pressure freon vapor after compression.
of the compressor, the upper being the suction
The high temperature pressurized vapor is
connection and the lower the pressure connection.
transformed to high-pressure liquid freon by the
The outboard connection of each tee connects to
condenser unit with an external airflow passing
the system pressure and suction tubing, and the
over the condenser unit. The high-pressure liquid
inboard connections, normally fitted with service
freon flows through a dehydrator removing water
valve stem caps, allow a gage set to be installed
droplets from the freon liquid before passing
for servicing and testing the system. A third
through the expansion valve. The expansion valve
T-connection is secured into the body of the lower
converts the liquid to a high-pressure cold vapor
(pressure) T-connection to permit the pressure
as it passes through the evaporator coils. The
switch to be connected to the system. Due to the
evaporator fan draws cabin recirculated air across
differences with the new V type pulley diameters
the evaporator coils to cool the air. With the heat
increasing the compressor speed, it was necessary
of the air transferred to the freon, the cool air is
to replace the compressor with a new unit (lower
directed into the duct system by the evaporator
displacement per revolution) to maintain the same
fan for distribution in the cabin. The freon gases
horsepower capacity. A later rotary compressor and
in a low-pressure vapor state begin the cycle once

21 AIR CONDITIONING
1000-hour TBO motor became available from the
again as they enter the compressor.
air conditioning manufacturer with Kit No SB-34-1
(S/B 6/414). The change enhanced the overall
Operation is normally controlled by a pressure
operation of compressor output performance.
switch which cycles the compressor on and off
within a predetermined range. If the pressure
switch fails or if other malfunctions occur, a
Condenser
thermostatic switch in the evaporator plenum The air-cooled condenser is mounted to the left
cycles the system within a safe temperature range. of and parallel to the airplane centerline on two
The pressure switch and the thermostatic switch brackets secured to the forward pallet. Two
are in series in the compressor power circuit. Both refrigerant connections are provided at the right
must close before the compressor can operate. rear of the condenser; the lower is the inlet and
the upper the outlet.
COMPONENT DESCRIPTION
Condenser Fan
Compressor Motor The condenser fan, on the left side of the
The 28VDC compressor motor, under the right condenser, is secured at the inboard end to a duct
side of the cabin floor, is secured to the forward attached to the exhaust side of the condenser and
pallet by a swivel front bracket and a rear bracket at the outboard end to the air exhaust duct. A
having two adjustable rods. Two pulleys, attached clamp secures the fan to the pallet. The DC series
to the motor shaft, drive the compressor with fan motor operates at two speeds, depending on
two belts. On aircraft incorporating Mod 6/1684 the control switch selection. When positioned
(S/B 6/382) at 637 the two-belt drive concept to NORMAL or FAN ONLY operation, the
was replaced with a single V type belt to improve condenser and evaporator fans are electrically
pulley retention and eliminate repetitive drive belt connected in series, but when a QUICK COOL
failure problems. A later 1000-hour TBO motor selection is made, the two fans are switched to
became available from the manufacturer by Kit parallel operation.
No SB-33-1 (S/B 6/414).

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 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Evaporator Main Duct

The evaporator is under the right side of the The main duct from the evaporator fan outlet
cabin floor and is secured to the rear pallet by at Stn 321.60 is routed through the cabin floor
two brackets. Two connectors are provided on before passing through the cabin rear bulkhead
the evaporator; an expansion valve connects to Stn 332 to enter the rear baggage compartment
the front connector, and tubing routed to suction where it is vertically positioned before being
on the compressor connects to the rear. attached to a duct in line with the cabin ceiling
distribution ducts. An improved insulated
Receiver-Dryer evaporator cold air duct is available from the
manufacturer by kit SB-032-1 (S/B 6/414) to
The receiver-dryer is secured to the right outboard
reduce heat transfer from the air conditioning
side of the compressor by a clamp. A Tee
cold air duct to an area of the rear baggage
connection at the top of the dryer provides for
compartment to maintain the level of cold air
two connections and incorporates a sight glass.
temperature for passenger comfort.
Tubing from the condenser outlet connects to
the forward connection, and tubing from the rear
connection is routed to the expansion valve. A Distribution Ducts
sight glass inspection hole by mod 6/1521 (TAB
Two distribution air ducts installed in the cabin
652/2) at 460) with a removable snap hose plug in
ceiling run from the forward bulkhead Stn 112
the seat rail immediately above the receiver-dryer
to the rear cabin bulkhead Stn 332 before joining
was adopted to improve accessibility.
21 AIR CONDITIONING

a bifurcated duct passing through the bulkhead

to the main evaporator duct in the rear baggage
Pressure Switch compartment. The distribution ducts are secured
A high-pressure cut-out switch, connected by a to the ceiling panel channel on one side and by
small diameter tube to the compressor pressure velcro pile and hook on the other.
T-connection, is on the left side of the forward
pallet. Air Conditioner Inlet and Exhaust
Thermostat Switch Outlet Inspection
Inspect air conditioner inlet and exhaust outlets
A thermostat switch is secured to the rear side
for condition and security; free of obstruction.
of the evaporator duct.
Clean as required. (Pre Mod 6/2014).
Relays
The five relays identified K1 to K5 are bolted to a
Alternative Air Conditioning Unit
panel installed under the right side of the cabin floor An alternative air conditioning unit, previously
on the forward face of the bulkhead at Stn 332. available from Metro Airlines Texas, is now
supplied by Pace Aviation, Reno, Nevada U.S.A.
Circuit Breakers and Current Limiter with STC approval. The system is lighter by
approximately 51 lbs., has a load rating between
Four circuit breakers identified F1 to F4 secured
95 to 110 amps to cover all aspects of operation
to the fuselage frame Stn 332 are outboard of
and capable of cooling the cabin quickly.
the relay panel below the cabin floor. Access to
the manually reset circuit breakers is obtained
To achieve the level performance, major changes
by removing the right floor panel immediately
were incorporated, such as adopting the latest
forward of the bulkhead. A 150-ampere current
Zee Company compressor and evaporator units
limiter, in the 28V power supply line to the air-
including the relocation of the evaporator unit
conditioning system, is adjacent to the power
from beneath the cabin floor to the rear baggage
distribution and generator control box.
compartment against the forward upper bulkhead
aft face. The relocation of the evaporator prevents

21-44 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

cooling air loss that occurs with the evaporator *The following is an abbreviated description of
beneath the cabin floor area and outlet air duct the maintenance practices and is intended for
into the rear baggage compartment. In addition training purposes only.
two independent dual speed motors each driving
For a more detailed description of the practice,
a squirrel cage fan blowing air through the
refer to the task in the Viking AMM PSM 1-63-2.
evaporator coils directly into the cabin interior
have replaced the evaporator fan. Dual fan is
for quick cabin cooling and single fan operation 21-00-00 MAINTENANCE
when the cabin has cooled sufficiently for
passenger comfort. Ceiling ducts are not used for
PRACTICES
distribution as air is blown directly into the cabin.
The deletion of the condenser fan reduces the SERVICING
electrical load demand. Other changes relocated
the relay and circuit breaker panel from beneath
the cabin floor to the aft baggage compartment
NOTE
adjacent to the evaporator unit to improve access. For the necessary servicing
procedures such as system
Two select switches are on the flight compartment charging, purging, etc., refer
overhead console. One switch is labeled FANS - to the J.B. Systems, Inc., 1000
OFF - AIR CONDITIONER and the other switch Series Maintenance Manual.
is labeled HI - LOW. The system is designed to

21 AIR CONDITIONING
operate with ground power supply and prevent
compressor motor operation during flight should
one generator go off line. With evaporator fans
operational once DC power is available additional
cabin airflow may be obtained from the rear
baggage compartment area to supplement the
normal airflow through passenger gaspers during
flight without the air conditioner unit being
operational. As each evaporator fan is protected
by a 10-amp circuit breaker failure of one fan
will not restrict operation of the other fan.

Compressor motor protection is by a 150-amp

circuit breaker and high-pressure switch.

FOR TRAINING PURPOSES ONLY 21-45

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Aircraft: Revision No. 11 Page:

DE HAVILLAND DHC-6, SERIES 100, 200 & 300 Date: Dec. 23, 2002 21-1 of 1
System & 1. 2. Number Installed
Sequence 3. Number Required for Dispatch
Numbers 4. Remarks or Exceptions

21 AIR CONDITIONING

1 Air Conditioning System D - 0

***
21 AIR CONDITIONING

2 Ventilation Fan C 1 0

3 Flight Compartment Fans C 2 0

4 Individual Cabin Louvres D - 0

***
5 Automatic Temperature C 1 0 Provided manual temperature control is
Control operative.

6 Manual Temperature Control C 1 0 Provided automatic temperature control is

operative.

7 Avionics Cooling Fan (CSI) C 1 0 (M) Provided:

*** a) the avionics installation does not
require avionics cooling fan
operation, and
b) the circuit breaker is secured in the
open (off) position.

Figure 21-53. MMEL - Air Conditioning

21-46 FOR TRAINING PURPOSES ONLY

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

21 AIR CONDITIONING
PAGE INTENTIONALLY LEFT BLANK

FOR TRAINING PURPOSES ONLY 21-47

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 21-54. P
 re-Mod 6/1070 Ram-Air Figure 21-55. P
 re-Mod 6/1070 Cold Air
Scoop (Naca Scoop) Scoop
21 AIR CONDITIONING

TO INSTRUMENTS TO INSTRUMENTS

TEMP CONTROL TEMP CONTROL

MIXING VALVE MIXING VALVE

LH RH
ENGINE TO BLEED-AIR ENGINE
HOT AIR PIPE HOT AIR PIPE
SWITCHES
TO
COLD AIR TO LH ENGINE TO RH ENGINE COLD AIR
CABIN
(FINNED) INTAKE DEFLECTOR INTAKE DEFLECTOR (FINNED)
TEMP TEMP
SENSOR SENSOR
BLEED-AIR SHUTOFF VALVE BLEED-AIR SHUTOFF VALVE

TEMPERATURE INDICATOR LEGEND

CO-PILOT’S SUBPANEL
HOT AIR
TEMPERATURE CONTROLLED
COOLED AIR

MIXED AIR

Figure 21-56. Wing Bleed Air Systems (Series 100/200 Differences)

21-48 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

21-00-00 DIFFERENCES and the bleed air shut-off valve, is connected to a

dual bleed air temperature indicator (left and right
SERIES 100/200 engine) on the co-pilot sub panel.
AIRCRAFT
Early series aircraft differences to pre mod
6/1070 (Ram air scoop) and mod 6/1181
(Ground Fan) before aircraft 136 are shown in
Figure 21-54 and Figure 21-55. Note that the
external air induced through the NACA duct
is connected directly to the jet pump (ejector)
plenum. In this configuration, cold ram air and
cabin recirculated air can both be induced with
bleed air flowing under pressure through the
ejector chamber. The manually operated ram air
valve controls the amount of cold airflow. As
early aircraft do not have the ground operable
duct fan, the aircraft cannot be cooled during
ground mode conditions unless modified with
the freon operated air conditioning unit mod

21 AIR CONDITIONING
S.O.O. 6109. An early attempt to improve on
cabin cooling using larger air scoops with small
fans for ground mode conditions did not produce
the expected level of passenger comfort.

WING BLEED AIR TEMPERATURE

CONTROL MOD 6/1266
Refer to Figure 21-56. Wing Bleed Air Systems
(Series 100/200 Differences).

On aircraft incorporating Mod 6/1266, a temperature

control mixing valve is introduced to limit the
temperature of the bleed air to 350°F, to ensure
that the bleed air temperature will not create a
hazard in the event of a bleed air pipe rupture. The
valve is installed in the system between the engine
compressor and the bleed air shut-off valve. Two
pipes are used to supply bleed air to the temperature
control mixing valve from the engine compressor,
a hot air pipe, and a cold air pipe which is finned
to dissipate heat from the bleed air flowing through
it. The valve cold air inlet port is closed when the
bleed air is below 270°F and the hot air port is open;
when the temperature is between 270°F and 300°F,
the hot air inlet port closes and the cold air inlet
port opens. At temperatures exceeding 350°F both
ports are closed. A temperature sensing bulb, in the
pipe between the temperature control mixing valve

FOR TRAINING PURPOSES ONLY 21-49

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CHAPTER 35
OXYGEN
CONTENTS
Page

35-00-00 OXYGEN..................................................................................................... 35-1

INTRODUCTION................................................................................................ 35-1
GENERAL........................................................................................................... 35-1
35-10-00 CREW OXYGEN SYSTEM......................................................................... 35-3
General................................................................................................................ 35-3
Description and Operation.................................................................................... 35-3
Cylinder........................................................................................................ 35-3
Pressure Gage............................................................................................... 35-3
Charging Valve.............................................................................................. 35-3
Check Valve.................................................................................................. 35-3
Regulators..................................................................................................... 35-5
Operation............................................................................................................. 35-5
35-10-00 MAINTENANCE PRACTICES.................................................................... 35-6

35 OXYGEN
Servicing.............................................................................................................. 35-6
Oxygen System Safety Precautions............................................................... 35-6
Charging Precautions for Oxygen Systems.................................................... 35-6
Charging Crew Oxygen System .................................................................... 35-9
Purging Crew Oxygen System....................................................................... 35-9
Adjustment/Test................................................................................................... 35-9
Test Crew Oxygen System............................................................................. 35-9
35-20-00 PASSENGER OXYGEN SYSTEM S.O.O. 6101........................................ 35-13

FOR TRAINING PURPOSES ONLY 35-i

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page
General.............................................................................................................. 35-13
Description and Operation.................................................................................. 35-13
Cylinders and Valve Assemblies.................................................................. 35-13
Pressure Gages............................................................................................ 35-13
Charging Valve............................................................................................ 35-13
Shut-Off Valves........................................................................................... 35-13
Check Valves............................................................................................... 35-13
Regulator.................................................................................................... 35-14
Passenger Oxygen Masks............................................................................ 35-14
Passenger Cabin Oxygen Outlets................................................................. 35-14
Operation........................................................................................................... 35-14
32-20-00 MAINTENANCE PRACTICES.................................................................. 35-14
Servicing............................................................................................................ 35-14
Charging Passenger Oxygen System............................................................ 35-14
Charging Precautions for Oxygen Systems.................................................. 35-15
Purging Passenger Oxygen System.............................................................. 35-15
Inspection/Check................................................................................................ 35-15
35 OXYGEN

Oxygen System........................................................................................... 35-15

35-00-00 MAINTENANCE PRACTICES.................................................................. 35-17
General Maintenance Practices........................................................................... 35-17
Servicing............................................................................................................ 35-17
Functional Checks.............................................................................................. 35-17
Fault Analysis..................................................................................................... 35-17
Limitations......................................................................................................... 35-17

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 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

ILLUSTRATIONS
Figure Title Page

35-1 Crew Oxygen System Components............................................................35-2

35-2 Smoke Mask and Communication..............................................................35-4
35-3 Oxygen Charging.......................................................................................35-8
35-4 Passenger Oxygen System Components...................................................35-12
35-5 Crew Oxygen System Duration Chart......................................................35-16
35-6 Passenger Oxygen System Duration Chart...............................................35-18

TABLES
Table Title Page
35-1 Average Time of Useful Consciousness....................................................35-16

35 OXYGEN

FOR TRAINING PURPOSES ONLY 35-iii

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CHAPTER 35
OXYGEN

35-00-00 OXYGEN
INTRODUCTION

35 OXYGEN
This chapter covers the oxygen systems (if installed) on the DHC-6 Twin Otter. If both the
crew and passenger oxygen systems are installed, they are interconnected to permit crew
use of passenger oxygen.

GENERAL
The oxygen systems are not standard equipment; pressure gages, charging valves, outlets, check
however, one or both systems may be installed valves, shut-off valves, oxygen outlets, masks,
at customer option. The crew oxygen system regulators, and the necessary plumbing to
is installed by S.O.O. 6044 and the passenger complete the systems.
oxygen system by S.O.O. 6101. A number
of oxygen installations have been adopted to Crew oxygen is a prerequisite for all aircraft
accommodate various geophysical survey and installing a passenger oxygen system.
military aircraft configurations. The systems
consist basically of oxygen cylinders, masks,

FOR TRAINING PURPOSES ONLY 35-1

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
35 OXYGEN

Figure 35-1. Crew Oxygen System Components

35-2 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

35-10-00 CREW OXYGEN Charging Valve

SYSTEM The oxygen-charging valve is adjacent to
the pressure gage. It consists of a cylindrical
housing containing a check valve and a sintered
GENERAL copper filter. A dust cap with retaining chain is
installed on the valve inlet connection.
The crew oxygen system is a diluter-demand
system, providing oxygen for the pilot and
co-pilot through demand regulators.
Check Valve
A tee-type check valve is in the oxygen plumbing
to control direction of flow. Two arrows stamped
DESCRIPTION AND OPERATION on the valve indicate flow direction.
Cylinder
The 650 cubic inch oxygen cylinder (Figure
35-1) is forward of bulkhead Stn 60 beneath
the avionic shelves in the nose baggage
compartment. Access to the bottle is through
the nose baggage compartment door behind the
avionic equipment shields. The oxygen bottle
is painted green for identification. The bottle
cradle support structure was strengthened to
accommodate excessive tightening of retention
clamps.

A self-opening valve in the cylinder outlet

automatically opens when the line is connected
and closes when the line is disconnected. This
feature permits cylinder charging without loss
of contents. The cylinder outlet valve contains
a safety plug and disc to release the oxygen if
excessive thermal expansion occurs. Normal
charge for the cylinder is 1,800 psi. If cylinder
pressure is allowed to drop below 25 psi, the

35 OXYGEN
system may become contaminated, and purging
will be required.

Pressure Gage
A direct-reading bourdon-tube-type gage
labelled OXYGEN CYLINDER PRESSURE
is on the left side of the nose baggage
compartment bottom frame at Stn 44 beneath
the lower shield support. The gage is accessible
through the nose baggage compartment door
(Figure 35-1). The gage is calibrated from 0
to 2,000 psi in increments of 200 psi and reads
pressure in the cylinder.

FOR TRAINING PURPOSES ONLY 35-3

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

KIT ASSY-OXYGEN
HOSE MASEK AND
COMMUNICATION

MASK-FACE
PIECE

MICROPHONE
HOSE AND MASK ASSY-OXYGEN
(WITH MOUNTING PROVISION)

CABLE ASSY-
MICROPHONE

CLAMP PLUG-MICROPHONE
35 OXYGEN

HOSE

CONNECTOR

Figure 35-2. Smoke Mask and Communication

35-4 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Regulators NOTES
An oxygen regulator (Figure 35-1) is on the
lower portion of each instrument panel within
easy reach of each crew member. The regulators
have three switches, a flow indicator, and an
oxygen pressure gage. The SUPPLY switch
colored green has positions labeled “ON” and
“OFF”. The diluter switch colored white is
labeled “100% OXYGEN” and “NORMAL
OXYGEN”. In the 100% OXYGEN position,
undiluted oxygen is supplied to the mask; in
the NORMAL OXYGEN position the amount
of dilution is controlled barometrically at any
given altitude. The switch colored red is labeled
“EMERGENCY”, “NORMAL”, and “TEST
MASK”. In the EMERGENCY position, 100 %
oxygen is supplied to the mask without regard
to the diluter switch (white) position. In the
NORMAL position, supply is controlled by
the switch-colored white. The TEST MASK
position is momentary; when depressed, it
provides oxygen flow to the mask as observed
at the flow indicator (white flag visible). The
system pressure gage, graduated from 0 to 2,000
psi, reads oxygen cylinder pressure.

OPERATION
With the crew masks plugged into the outlets
above the side consoles, low-pressure oxygen
from the regulators is available to both crew
members.

35 OXYGEN

FOR TRAINING PURPOSES ONLY 35-5

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of SYSTEM MAY OCCUR,

the maintenance practices and is intended for AND THE SYSTEM WILL
training purposes only. HAVE TO BE PURGED.
For a more detailed description of the practice,
2. T
 HE OXYGEN CYLINDERS
refer to the task in the Viking AMM PSM 1-63-2.
MUST BE DISCONNECTED
FROM THE SYSTEM,
35-10-00 MAINTENANCE BEFORE ATTEMPTING
PRACTICES TO CARRY OUT A
REPAIR OR COMPONENT
REPLACEMENT.
SERVICING
1. When system lines are disconnected, ensure
Oxygen System Safety that component vents and open ends of
lines are capped.
Precautions
2. When reinstalling oxygen lines, male
NOTE parallel threaded fittings are to be installed
dry. The use of sealing compound is
The following WARNING,
prohibited. Pipe screw threads must be
CAUTION and general rules
sealed using Scotch Brand Tape No. 48;
should be strictly observed when
Pipe Thread Sealant (Minnesota Mining
testing or servicing the oxygen
and Manufacturing Co.) applied to male
system, or replacing oxygen
taper threads only.
system components.
3. O n c o m p l e t i o n o f i n s t a l l a t i o n a n d
reconnection of lines and cylinders, check
WARNING
connections for leaks using leak detecting
compound Specification MIL–L–25567.
ENSURE ALL CLOTHING.
Wipe off with a damp cloth and thoroughly
HANDS, TOOLS, FITTINGS,
dry on completion of test, using a clean
OXYGEN COMPONENTS,
dry cloth.
AND THE WORKING AREA,
ARE FREE FROM OIL AND
GREASE. OIL AND GREASE
Charging Precautions for Oxygen
EXPOSED TO OXYGEN CAN Systems
35 OXYGEN

CAUSE AN EXPLOSION. IF
Before any attempt is made to charge the
AN OIL OR GREASE FILM
cylinders, observe the following precautions
IS FOUND ON OR AROUND
to prevent injury to personnel and damage to
OXYGEN EQUIPMENT,
aircraft by fire and explosion:
WASH CLEAN WITH A
CASTILE SOAP AND WATER
1. All charging and testing operations should
SOLUTION.
be carried out as close to the hangar door
as possible.
CAUTION 2. Only operators familiar with the necessary
safety precautions should be permitted
1. D
O NOT ALLOW
to carry out any operations on oxygen
T H E O X Y G E N
equipment.
SYSTEM PRESSURE
TO FALL BELOW 3. Smoking is prohibited while charging
25 PSI, OTHERWISE operations are being carried out.
CONTAMINATION OF THE

35-6 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

4. Extreme care must be exercised to prevent NOTES

oil or grease from coming in contact with
fittings or the interior of the oxygen system
components and the charging rig.
5. Hands and clothing of operators and all tools
required must be free from oil and grease.

35 OXYGEN

FOR TRAINING PURPOSES ONLY 35-7

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
35 OXYGEN

Figure 35-3. Oxygen Charging

35-8 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Charging Crew Oxygen System 2. Charge crew oxygen system with oxygen.
Refer to Figure 35-3. Oxygen Charging. 3. Position pilot and co-pilot mask outlets
outside flight compartment windows.
Charge cylinder with breathing oxygen to a
4. Set both pilot diluter demand regulator
maximum pressure of 1800 ± 50 psi as follows:
SUPPLY switches to ON, and pressure
supply switches to EMERGENCY.
1. Ensure crew diluter demand regulator
supply switch is OFF. 5. A l l o w s y s t e m t o d e p r e s s u r i z e u n t i l
exhausted.
2. Unscrew crew oxygen system charging
valve dust cap and connect charging rig line 6. Set both pilot diluter demand regulator
to charging valve. SUPPLY switches to OFF, and pressure
supply switches to NORMAL.
3. Turn on charging rig valve and slowly
charge the cylinder with breathing oxygen, 7. Repeat step 2 through 6 at least three times.
at a rate not exceeding a 500 psi pressure
8. Charge crew oxygen system with oxygen.
rise per minute, to 1800 ± 50 psi. On
aircraft with special order oxygen system
to EO 68958 installed, the pressure in each
ADJUSTMENT/TEST
oxygen cylinder should be read from the
gauge on the cylinder and not from the
Test Crew Oxygen System
gauge adjacent to the charging valve. 1. Observe Oxygen System Safety Precautions
2. Ensure that crew oxygen system is charged
NOTE and is free from leaks.
A slow rate of charge is
3. Ensure that flexible breathing hoses are
necessary to avoid overheating
firmly clamped to regulator outlet elbows.
and subsequent danger of fire.
4. Disconnect crew oxygen system line from
4. Turn off charging rig supply valve. crew oxygen cylinder.
5. Slowly loosen rig connection line at system 5. Connect charging rig to crew oxygen
charging valve to allow pressure in line to charging valve, and apply approximately
escape slowly. 25 psi to charging valve from rig. Ensure
oxygen flows from line disconnected from
6. Disconnect charging rig line and screw dust
crew cylinder.

35 OXYGEN
cap on to charging valve connection.
6. Turn off charging rig and disconnect from
7. Check that cylinder and diluter demand
crew charging valve.
regulator gauge pressures coincide.
7. Connect crew oxygen system line to crew
Purging Crew Oxygen System oxygen cylinder.
8. Check diluter demand regulators as follows:
NOTE
A. Check regulator pressure gauge to
The crew oxygen system must
ensure pressure is supplied to regulator.
be purged whenever the system
pressure is less than 25 psi for B. Select regulator SUPPLY switch to ON.
a period of 2 hours or more,
C. Select diluter switch to NORMAL
or when the system has been
OXYGEN.
accidently left open.
D. Depress pressure supply switch to TEST
1. Position aircraft in a well ventilated MASK position for approximately
location and open all doors and windows. 15 seconds. Check that oxygen flows

FOR TRAINING PURPOSES ONLY 35-9

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

freely from outlet hose, that flow NOTES

indicator appears in window during
test, and disappears when flow switch
is released.
E. S e l e c t p r e s s u r e s u p p l y s w i t c h
to EMERGENCY position for
approximately 15 seconds. Check that
100% oxygen flows freely, and flow
indicator appears in window during
test. A change in the sound of the flow
indicates that 100% oxygen is being
supplied.

WARNING

THERE MUST BE NO OIL OR

GREASE IN VICINITY OF
MASK OUTLETS.

F. Return pressure supply switch to

NORMAL position. Inhale a number
of times through the outlet hose, and
check operation of flow indicator each
time an inhalation is taken.
G. S e l e c t d i l u t e r s w i t c h t o 1 0 0 %
OXYGEN. Inhale and check for 100%
oxygen check.
H. S e l e c t d i l u t e r d e m a n d r e g u l a t o r
SUPPLY switch OFF and diluter switch
to NORMAL OXYGEN.
9. Connect charging rig and recharge system
to 1800 ± 50 psi.
35 OXYGEN

35-10 FOR TRAINING PURPOSES ONLY

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

PAGE INTENTIONALLY LEFT BLANK

35 OXYGEN

FOR TRAINING PURPOSES ONLY 35-11

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
35 OXYGEN

Figure 35-4. Passenger Oxygen System Components

35-12 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

35-20-00 PASSENGER calibrated from 0 to 2,000 psi in increments

of 200 psi. Access to the gage is through rear
OXYGEN SYSTEM baggage compartment door.
S.O.O. 6101
Charging Valve
GENERAL The passenger oxygen system charging valve is
located adjacent to the rear oxygen pressure gage
The continuous-flow passenger oxygen system at 376.0 bulkhead frame Stn. It is accessible
is capable of supplying 20 passengers. The through the rear baggage compartment door. The
system is interconnected with the crew oxygen valve is identical to the charging valve for the
system to allow crew member use in the event crew oxygen system.
of an extended flight.
Shut-Off Valves
DESCRIPTION AND OPERATION Three shut-off valves are installed in the
passenger oxygen system. The main shut-
Cylinders and Valve Assemblies off valve is in the center of the rear baggage
Two green colored oxygen cylinders are mounted compartment roof. The other two shut-off
vertically in the rear fuselage. Usually one on valves labeled PASSENGER OXYGEN and
each side of the aircraft located on the aft face PASSENGER TO CREW TRANSFER are
of fuselage bulkhead frame Stn 376.0 (Figure installed on the oxygen regulator panel on
35-4). The cylinders have a total capacity of the forward face of the flight compartment
3,180 cubic inches and are charged to 1,800 ± bulkhead frame Stn 111 behind the right seat.
50 psi. A self-opening valve installed in the neck Each valve has a directional arrow and the
of each cylinder opens automatically when a word “OPEN” on the operating knob.
system pipe union incorporating a nose adapter
is connected to the valve and closes as the pipe
union is disconnected. This permits cylinder
Check Valves
replacement without oxygen loss. Access to both Two check valves installed on the rear face
bottles is through the rear baggage compartment of the bulkhead frame at Stn 376. They are
door and extended baggage shelf. installed adjacent to each oxygen cylinder, to
prevent a reverse flow condition occurring in
A safety disc in the self-opening valve ruptures if the direction to the charging valve and feedback

35 OXYGEN
cylinder pressure becomes excessive due to high between cylinders.
temperature. Each cylinder is secured by a strap
and is lockwired to the strap attachment bracket. A third check valve on the rear face of bulkhead
frame Stn 60.0 forward of the pilot’s oxygen
Cylinder pressure should not be allowed to drop regulator, provides a connection to the crew
below 25 psi; system contamination may result. oxygen system. With the PASSENGER TO
CREW - OXYGEN TRANSFER shut-off valve
open, this check valve prevents an oxygen
Pressure Gages flow from the crew system to the passenger
The oxygen system-charging gage labeled system but will allow an oxygen flow from the
OXYGEN CYLINDER PRESSURE is adjacent passenger system to the crew oxygen regulators.
to the charging valve on the left side of fuselage
bulkhead frame lower web at Stn 376.0. A second All three check valves are tee type with two
oxygen gage labeled OXYGEN CYLINDER arrows to indicate flow direction.
PRESSURE is on the inboard face of the oxygen
regulator panel (Figure 35-4). Each gage is a
direct-reading bourdon-tube-type instrument

FOR TRAINING PURPOSES ONLY 35-13

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Regulator from (or is discharged to) the rebreather bag.

The PASSENGER TO CREW TRANSFER
The continuous-flow, altitude-compensating
shut-off valve can be opened to supply the
passenger oxygen system regulator is enclosed
pilots’ demand regulators from the passenger
by the regulator panel located on the forward
oxygen cylinders. Under these conditions, if the
face of the flight compartment/cabin bulkhead
PASSENGER OXYGEN SHUT OFF valve is
at Stn 111.81, directly behind the co-pilot seat
closed, oxygen to the passenger system is shut
(Figure 35-4). It reduces the high-pressure
off, and only the crew will be supplied.
oxygen from the cylinders to a lower working
pressure and automatically regulates the correct
*The following is an abbreviated description of
amount of oxygen supplied to the passengers
the maintenance practices and is intended for
through the cabin outlets. The regulator does
training purposes only.
not directly regulate oxygen flow but maintains
the required pressure behind each outlet. The For a more detailed description of the practice,
outlet orifice actually meters the flow. refer to the task in the Viking AMM PSM 1-63-2.

Passenger Oxygen Masks 32-20-00 MAINTENANCE

The continuous flow type passenger masks with PRACTICES
a one liter-breathing bag are disposable. They
are connected with plastic tubing to oxygen
outlets. The plastic tube is inserted into the
SERVICING
oxygen outlet for that particular seat.
Charging Passenger Oxygen
Passenger Cabin Oxygen Outlets System
Charge cylinders with breathing oxygen to a
Oxygen outlets are on each side of the cabin
maximum pressure of 1800 ±50 psi as follows:
in the upper region of the cabin walls. Aircraft
with 20 passengers have six outlets on the
1. Ensure oxygen shut-off and oxygen transfer
left side and fourteen on the right side of the
valves, located on passenger oxygen
aircraft.
regulator panel, are closed.
2. Unscrew passenger oxygen system charging
OPERATION valve dust cap and connect charging rig line
to charging valve.
35 OXYGEN

With the main shut-off valve (Figure 35-4)

in the rear baggage compartment open, the 3. Turn on charging rig valve and slowly
passenger oxygen system is controlled by the charge the cylinders at a rate not exceeding
pilot from the regulator panel. When the system a 500 psi pressure rise per minute.
is supplying passengers only, the PASSENGER
TO CREW - OXYGEN TRANSFER valve
is closed, and the PASSENGER - OXYGEN
SHUT OFF valve is open. In this condition,
system pressure is indicated on the pressure
gage, the regulator panel, and oxygen flows at
a regulated pressure to the cabin outlets, where
it is metered to each passenger mask by the
outlet orifice.

In use, 30% of the flow in or out of a

mask originates from (or is discharged to)
atmospheric air, and 70% of the flow originates

35-14 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Charging Precautions for Oxygen 6. Ensure main oxygen shut-off valve in rear
baggage compartment roof is open. Open
Systems PASSENGER TO CREW – OXYGEN
Follow the same precautions that are on page 35-6. TRANSFER shut-off, and PASSENGERS
– OXYGEN SHUT OFF valves on regulator
panel.
Purging Passenger Oxygen
7. Set pilot diluter demand regulator SUPPLY
System switches ON, and pressure supply switches
to EMERGENCY.
NOTE
8. Allow oxygen system to depressurize
1. T
 he passenger oxygen system
until exhausted. Close PASSENGER TO
must be purged whenever the
CREW – OXYGEN TRANSFER shut-off
system pressure is less than
and PASSENGER – OXYGEN SHUT OFF
25 psi for more than 2 hours
valves.
or when the system has been
accidently left open. 9. Repeat step 3 through 8 at least three times.
10. Set the pilot diluter demand regulator
2. I f t h e p a s s e n g e r o x y g e n
SUPPLY switches to OFF, and pressure
system loss of pressure is
supply switches to NORMAL.
caused by a pilot diluter
demand regulator SUPPLY 11. Close PASSENGER TO CREW – OXYGEN
switch being left ON, the crew TRANSFER shut-off and PASSENGERS –
system must also be purged. OXYGEN SHUT OFF valves.
12. Allow system downstream of passenger
1. Position aircraft in a well ventilated
oxygen regulator to depressurize through
location and open all doors and windows.
mask connector extension hose. Remove
2. Disconnect crew system line from crew connector with extension hose.
oxygen cylinder.
13. Connect crew oxygen system line to crew
3. Charge passenger oxygen system with oxygen cylinder.
oxygen.
14. Charge passenger oxygen system.
4. Plug a disposable mask connector with an
extension hose, into a convenient passenger
INSPECTION/CHECK

35 OXYGEN
oxygen outlet, and position open end of
extension hose outside aircraft.
Oxygen System
5. Position both pilot mask outlets outside
Inspect Crew and passenger oxygen systems for
flight compartment windows.
correct pressure.
WARNING
Inspect oxygen indicator and charging valve for
condition and cleanliness.
THERE MUST BE NO OIL OR
GREASE IN THE VICINITY
OF THE EXTENSION HOSE
OUTLET, OR PILOT MASKS.

FOR TRAINING PURPOSES ONLY 35-15

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Altitude Average Time of Useful Consciousness

15,000 to 18,000 ft. 30 minutes or more
22,000 ft. 5 to 10 minutes
25,000 ft. 3 to 5 minutes
28,000 ft. 2 1/2 to 3 minutes
30,000 ft. 1 to 2 minutes

Table 35-1. Average Time of Useful Consciousness

25,000 FT

20,000 FT

15,000 FT

10,000 FT
2,000
MAX SUPPLY PRESSURE
1,800

1,600

1,400
SUPPLY PRESSURE - PSIG

1,200

1,000

800
35 OXYGEN

600

400

200

0
0 1.0 2.0 3.0 4.0 5.0
DURATION - HOURS

Figure 35-5. Crew Oxygen System Duration Chart

35-16 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

35-00-00 MAINTENANCE 35-10-00 and 35-20-00, respectively of the

Maintenance Manual.
PRACTICES
SERVICING
GENERAL MAINTENANCE
Servicing of the oxygen systems is to be
PRACTICES performed only by personnel familiar with
the necessary safety precautions. Charging
The maintenance practices found in the
at a slow rate is required to prevent system
Maintenance Manual usually relate to
overheating and subsequent danger of fire and
adjustment/test procedures. The following
explosion. Complete servicing instructions
maintenance practices are applicable to the
for servicing either the crew or the passenger
oxygen systems in a general sense:
oxygen systems are presented in Chapter/
Section 12-10-35 of the Maintenance Manual.
When oxygen system lines are disconnected,
ensure that component vents and open lines
are capped. FUNCTIONAL CHECKS
When reinstalling oxygen lines, male parallel Functional (operational) checks are normally
threaded fittings must be dry. The use of sealing performed after maintenance has been accomplished
compound is prohibited. Pipe threads must be on a system. They may also be performed when a
sealed with the approved tape, and pipe thread system is suspected of malfunctioning. Functional
sealant applied only to male tapered threads. checks are frequently a part of a maintenance
procedure and are not called out separately. They
On completion of installation and reconnection sometimes include such maintenance tasks as final
of lines and cylinders, connections must adjustment, torquing, and safetying.
be checked for leaks with leak-detecting
compound Specification MIL-L-25567. Wipe Chapter 35 of the Maintenance Manual does
off the excess compound with a damp cloth, not list any functional or operational checks for
and dry thoroughly with a clean, dry cloth. the oxygen systems.

Use only non-sparking tools when performing

maintenance on the oxygen system.
FAULT ANALYSIS
Isolation of a fault or malfunction can be

35 OXYGEN
Do not permit smoking, open flame, or
accomplished by a systematic analysis of the
potential sources of electrical sparks near the
trouble, beginning with the most probable
airplane while maintenance is being performed
cause and progressing to the least probable
on the system. Ensure that all electrical
cause. Any system(s) interfaced with the
power is disconnected and that the airplane is
malfunctioning system should be operating
properly grounded.
properly prior to troubleshooting.
Never attempt to tighten oxygen system fittings
or lines while the system is pressurized. LIMITATIONS
Maintenance personnel must ensure that their Table 35-1 depicts the average time of useful
hands are free of dirt and grease prior to consciousness (time from onset of hypoxia until
performing maintenance on the oxygen systems. loss of effective performance) at various altitudes.

In addition to the above maintenance practices, Figure 35-5 and Figure 35-6 depict oxygen
procedures for purging the crew and passenger duration for the crew oxygen system and the
oxygen systems are found in Chapter/Sections passenger oxygen system, respectively.

FOR TRAINING PURPOSES ONLY 35-17

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

14 PASSENGERS
19 PASSENGERS 8 PASSENGERS

25,000 FT
20,000 FT
15,000 FT

10,000 FT

25,000 FT

20,000 FT

15,000 FT

10,000 FT
MAX PRESS

1,800

1,600

1,400
SUPPLY PRESSURE - PSIG

1,200

1,000

800

600

400

200
35 OXYGEN

0
0 1 2 3 4 5 6
DURATION - HOURS
NOTE

FOR OXYGEN DURATION FOR LESS THAN

PASSENGER CAPACITY. MULTIPLY DURATION
BY PASSENGER CAPACITY
ACTUAL PASSENGERS

EXAMPLE:
IF DURATION FOR 14 PASSENGERS IS
3 HOURS, DURATION FOR 7 PASSENGERS
= 3 X 14 = 6 HOURS
7

Figure 35-6. Passenger Oxygen System Duration Chart

35-18 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CHAPTER 29
HYDRAULICS
CONTENTS
Page
29-00-00 HYDRAULIC POWER................................................................................ 29-1
Introduction......................................................................................................... 29-1
General................................................................................................................ 29-3
29-10-00 HYDRAULIC POWER PACKAGE.............................................................. 29-5
General................................................................................................................ 29-5
Description........................................................................................................... 29-5
Hydraulic Reservoir...................................................................................... 29-5
Damping Accumulator................................................................................... 29-5
Wheel Brakes Accumulator........................................................................... 29-5
Electric Motor-Driven Hydraulic Pump......................................................... 29-6
Hydraulic System Filter................................................................................ 29-6
Pressure Switch............................................................................................. 29-6
Pressure Relief Valve.................................................................................... 29-6
Thermal Relief Valves................................................................................... 29-6
Check Valves........................................................................................................ 29-7
29-30-00 INDICATION.............................................................................................. 29-9
Brake Pressure Indicator...................................................................................... 29-9
System Pressure Indicator.................................................................................. 29-11
Nitrogen Pressure Indicator ........................................................................ 29-11
System Operation............................................................................................... 29-13
29 HYDRAULICS

29-20-00 AUXILIARY HYDRAULIC SYSTEM....................................................... 29-17

General.............................................................................................................. 29-17

FOR TRAINING PURPOSES ONLY 29-i

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page
Hydraulic Handpump.................................................................................. 29-17
Operation........................................................................................................... 29-17
29-20-00 MAINTENANCE PRACTICES.................................................................. 29-17
Servicing............................................................................................................ 29-17
Bleed Hydraulic Handpump........................................................................ 29-17
Inspection/Check................................................................................................ 29-17
Inspection of the Handpump........................................................................ 29-17
29-00-00 MAINTENANCE PRACTICES.................................................................. 29-18
Hydraulic Lines.................................................................................................. 29-18
Hydraulic Pressure Indicators............................................................................. 29-18
Adjustment/Test.......................................................................................... 29-18
Accumulator Air Charging Valve End Caps........................................................ 29-18
Adjustment/Test.......................................................................................... 29-18
Other Maintenance Practices.............................................................................. 29-19
Adjustment/Test................................................................................................. 29-20
Bench Test Hydraulic System Pressure Relief Valve.................................... 29-20
Bench Test Brake Pressure Relief Valve...................................................... 29-20
Bench Test Thermal Relief Valve................................................................. 29-20
Servicing ........................................................................................................... 29-21
Reservoir..................................................................................................... 29-21
Accumulators.............................................................................................. 29-21
Functional Checks.............................................................................................. 29-22
Fault Analysis..................................................................................................... 29-22
29 HYDRAULICS

29-00-00 SERIES 100/200 DIFFERENCES.............................................................. 29-23

Hydraulic Power Package................................................................................... 29-23
System Operation........................................................................................ 29-23

29-ii FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

ILLUSTRATIONS
Figure Title Page
29-1 Hydraulic System Schematic.....................................................................29-2
29-2 Hydraulic Power Package Components......................................................29-4
29-3 
Reservoir Cap Ribbed Expansion...............................................................29-5
29-4 
Nose Wheel Steering Check Valve.............................................................29-7
29-5 
Flap Thermal Relief Check Valves.............................................................29-7
29-6 Hydraulic Indicators..................................................................................29-8
29-7 Accumulator Pressure Indicators...............................................................29-9
29-8 Hydraulic Power Package Components....................................................29-10
29-9 Hydraulic System Schematic...................................................................29-12
29-10 Hydraulic System Electrical Schematic...................................................29-14
29-11 HYD PUMP C/BRK OPEN Caution Light...............................................29-14
29-12 Hydraulic Hand Pump..............................................................................29-16
29-13 
Hydraulic Reservoir and Accumulator Gages...........................................29-21
29-14 Hydraulic Pack Location..........................................................................29-21
29-15 MMEL - Hydraulic Power........................................................................29-22

TABLES
Table Title Page
29-1 Calibration Tolerances.............................................................................29-19
29 HYDRAULICS

FOR TRAINING PURPOSES ONLY 29-iii

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CHAPTER 29
HYDRAULICS

29-00-00 HYDRAULIC POWER

INTRODUCTION
The DHC-6 Twin Otter hydraulic system is pressurized by an electrically driven pump,
backed up by an emergency hand pump. The system provides pressure for four subsystems:
wing flaps, nose wheel steering, wheel brakes and wheel ski operation, if installed. Pressure
indicators monitor the system operation.
29 HYDRAULICS

FOR TRAINING PURPOSES ONLY 29-1

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

LEFT DC BUS HYDRAULIC DAMPING ACCUMULATOR

RESERVOIR PRESSURE INDICATOR
MOTOR AND AIR CHARGE VALVE
PUMP 10

15
5
HYD PUMP
C/BKR OPEN HAND PUMP 0 20
x 1000
CAUTION LIGHT SYSTEM
RELIEF
VALVE ** NOSEWHEEL
HYDRAULIC (1950 PSI) STEERING
SYSTEM ACTUATOR
PRESSURE PRESSURE
INDICATOR SWITCH *
FILTER DAMPING
SERVO
(10 MICRON) ACCUMULATOR
1000

2000
PRESS
PSI

BRAKE ACCUMULATOR PRESSURE

INDICATOR AND AIR CHARGE VALVE

{
BRAKE
10
ACCUMULATOR TO
15
5

SKIS
0 20 BRAKE
x 1000 RELIEF
1000 VALVE
(1.750 PSI) FLAP
2000
PRESS CONTROL
PSI VALVE
BRAKE SYSTEM
PRESSURE
INDICATOR

DH DH
LEGEND
PRESSURE
PARKING THERMAL RELIEF
SUPPLY BRAKE VALVES (1,750 PSI)
RETURN
NITROGEN BRAKE FLAP
MECHANICAL VALVES ACTUATOR

ELECTRICAL

RESTRICTOR
29 HYDRAULICS

BRAKE BRAKE
UNIT UNIT

Figure 29-1. Hydraulic System Schematic

29-2 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

GENERAL NOTES
Refer to Figure 29-1. Hydraulic System
Schematic.

System pressure is supplied by an electric motor-

driven hydraulic pump. Alternative sources of
hydraulic power are: two accumulators, one for
system pressure (damping accumulator), the
other for brakes, and a hydraulic handpump.
Flared pipeline couplings are used throughout
the system.

Fluid from all circuits is returned to the reservoir

through a common return line. The main
components of the hydraulic system are mounted
together on a hydraulic power package below the
flight compartment floor.

Electrical power to operate the pump, is derived

from the contacts of a hydraulic pump relay,
when energized. Because the relay control coil
obtains a ground through the pressure switch
contacts, the relay is energized only when
the system hydraulic pressure falls below the
working pressure value. The circuit breaker,
labeled HYD OIL PUMP, is on the main circuit
breaker panel, and the relay is mounted in
the main distribution box. A hydraulic pump
circuit breaker open caution light is provided.
The caution light, labeled HYD PUMP C/BKR
OPEN and colored amber, is of the press-to-
test type and is located above the clock on
pilot’s side. The relay, mounted behind the pilot
instrument subpanel, adjacent to the caution
light is normally energized when the HYD OIL,
PUMP circuit breaker is engaged. The caution
light relay (K11A) is in the main electrical
distribution box. The caution light is on the pilot
flight instrument panel and electrical power is
supplied through the PLT ENG CONS & TRIM
PNL LT circuit breaker on the main circuit
breaker panel.
29 HYDRAULICS

FOR TRAINING PURPOSES ONLY 29-3

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

LEFT
ACCESS
PANEL
29 HYDRAULICS

Figure 29-2. Hydraulic Power Package Components

29-4 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

29-10-00 HYDRAULIC
POWER PACKAGE
GENERAL
Refer to Figure 29-2. Hydraulic Power Package
Components.

The hydraulic power package supplies hydraulic

pressure to operate the wing flaps, wheel brakes,
nose wheel steering systems, and for lowering
and raising the ski actuators, when fitted.

DESCRIPTION
The hydraulic power package consists of a tray
Figure 29-3. R
 eservoir Cap Ribbed
containing the reservoir, brake and damping
Expansion
accumulators, indicators and charging valves,
electric motor-driven hydraulic pump, system
filter, system pressure relief valve, brake pressure
relief valve, system pressure switch, and associated
hydraulic lines and check valves. The package
Damping Accumulator
is on the fuselage structure beneath the flight The damping accumulator is incorporated in the
compartment floor and is accessible from both hydraulic system to damp out pressure surges in
sides of the fuselage through access doors. the hydraulic system and provide a secondary
source of operating pressure. The accumulator is
secured to the power package tray by two clamps.
Hydraulic Reservoir A combined indicator and air charging valve for
Refer to Figure 29-3. Reservoir Cap Ribbed the damping accumulator is mounted on a bracket
Expansion. on the left side of the power package.

The hydraulic reservoir is a cylindrical tank

containing a filler neck, a filter screen, a cap
Wheel Brakes Accumulator
and dipstick assembly, two suction outlets, a The wheel brakes accumulator provides the
return inlet, and a vent. The reservoir is bolted wheel brakes with supplemental pressure in
to the power package tray. an emergency or during limited periods of
peak demand, and provides pressure when the
The non-pressurized reservoir stores approximately hydraulic system is shut down while parked.
.67 U.S. gallon of MIL-H-5606 hydraulic fluid. The accumulator is secured to the power
package tray by two clamps. A combined
The filler neck, filter screen, and cap and indicator and air charging valve for the wheel
dipstick assembly are accessible through the brakes accumulator is mounted on a bracket on
left access opening. the left side of the power package.
29 HYDRAULICS

Fluid level must be maintained between the

FULL and REFILL marks. The flaps must be
retracted and hydraulic pressure discharged from
both accumulators by repeated operation of the
brakes prior to checking fluid level.

FOR TRAINING PURPOSES ONLY 29-5

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Electric Motor-Driven Hydraulic Thermal Relief Valves

Pump Two additional thermal relief valves are in
the flap system. They are set to relieve any
The electric motor-driven hydraulic pump is
pressure build up in excess of 1750 psi.
on the hydraulic power package tray, and is
the primary source of power for the hydraulic
system. The pump consists of a motor coupled
gear type pump. The motor operates on 28VDC,
supplied from the left DC bus. A 35–ampere
circuit breaker protects the circuit. The motor
is equipped with an internal cooling fan and a
radio noise suppressor.

Hydraulic System Filter

The hydraulic system filter is a tee type
containing a replaceable 10–micron element,
and is installed in the hydraulic power package.
Provision is made to bypass the element should
a blockage or restriction cause a pressure
differential of 50 ± 5 psi maximum between
inlet and outlet.

Pressure Switch
The pressure switch, on the hydraulic power
package tray, electrically controls the motor-
driven hydraulic pump. It regulates the working
pressure of the hydraulic system by switching
off DC power to the motor when the system
pressure reaches 1575 ± 50 psi and switches
on DC power when the pressure drops to 1225
psi minimum.

Pressure Relief Valve

There are two pressure relief valves installed on
the hydraulic power package, one for hydraulic
system, and the other for brake system. Should
a pressure switch malfunction occur resulting
in excessive hydraulic system pressure build
up the relief valve limits system pressure to
1,950 psi. The brake system relief valve is set
to relieve pressure at 1750 psi.
29 HYDRAULICS

29-6 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CHECK VALVES NOTES

Two check valves are on the Hydraulic
package. One check valve prevents reserve
flow through the electric motor pump if the
hand pump is being utilized. The other on the
package is to make the brake accumulator be
a storage device, and the other is a special
purpose check valve in the nose wheel steering
return line with a specific cracking pressure.

Figure 29-4. N
 ose Wheel Steering Check
Valve

29 HYDRAULICS

Figure 29-5. F
 lap Thermal Relief Check
Valves

FOR TRAINING PURPOSES ONLY 29-7

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

BRAKE SYSTEM
PRESSURE PRESSURE
INDICATOR INDICATOR

Figure 29-6. Hydraulic Indicators

29 HYDRAULICS

29-8 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

29-30-00 INDICATION
There are four bourdon tube direct-reading-
type indicators or gages in the system, two of
them in the flight compartment. The hydraulic
system pressure gage on the lower instrument
panel to the right side of the center pedestal
above the pitot selector lever or battery
temperature monitor and the brake pressure
indicator is on the lower instrument panel to
the immediate left side of the center pedestal
(Figure 29-6). The remaining two indicators
are on the hydraulic package tray beside the
reservoir and gas charging valves (Figure 29-7).
Access to both indicators is through the left
access panel beneath the flight compartment.
Figure 29-7. Accumulator Pressure
BRAKE PRESSURE INDICATOR Indicators

The brake pressure indicator shows pressure

available for brake operation. Normally, it
reads the same pressure as the system pressure
indicator. In the event of a hydraulic system
pressure failure, it will indicate the actual
pressure retained in the braking system for
braking.

29 HYDRAULICS

FOR TRAINING PURPOSES ONLY 29-9

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

LEFT
ACCESS
PANEL
29 HYDRAULICS

Figure 29-8. Hydraulic Power Package Components

29-10 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

SYSTEM PRESSURE NOTES

INDICATOR
Refer to Figure 29-8. Hydraulic Power Package
Components.

Calibrated in 100 psi increments up to 2,000

psi, shows hydraulic pressure available for
operation of all subsystems: flaps, nose wheel
steering, wheel brakes and wheel skis if
installed.

Nitrogen Pressure Indicator

Two nitrogen pressure indicators are installed,
one for damping accumulator, and the other for
wheel brakes accumulator. The two indicators
with charging valves are on the hydraulic
power package, and are accessible through the
left-hand access panel on the underside of the
nose section. Each indicator is direct-reading
bourdon tube type, with dials marked X 1000,
and scales marked from 0 to 2.0 in increments
of 0.1.

Pressure indicators will read the gas (nitrogen)

charge of the accumulators when the hydraulic
system pressure has been depleted (Figure
29-7). The nitrogen charge is 750 ± 50 Psi.

29 HYDRAULICS

FOR TRAINING PURPOSES ONLY 29-11

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

LEFT DC BUS HYDRAULIC DAMPING ACCUMULATOR

RESERVOIR PRESSURE INDICATOR
MOTOR AND AIR CHARGE VALVE
PUMP 10

15
5
HYD PUMP
C/BKR OPEN HAND PUMP 0 20
x 1000
CAUTION LIGHT SYSTEM
RELIEF
VALVE ** NOSEWHEEL
HYDRAULIC (1950 PSI) STEERING
SYSTEM ACTUATOR
PRESSURE PRESSURE
INDICATOR SWITCH *
FILTER DAMPING
SERVO
(10 MICRON) ACCUMULATOR
1000

2000
PRESS
PSI

BRAKE ACCUMULATOR PRESSURE

INDICATOR AND AIR CHARGE VALVE

{
BRAKE
10
ACCUMULATOR TO
15
5

SKIS
0 20 BRAKE
x 1000 RELIEF
1000 VALVE
(1.750 PSI) FLAP
2000
PRESS CONTROL
PSI VALVE
BRAKE SYSTEM
PRESSURE
INDICATOR

DH DH
LEGEND
PRESSURE
PARKING THERMAL RELIEF
SUPPLY BRAKE VALVES (1,750 PSI)
RETURN
NITROGEN BRAKE FLAP
MECHANICAL VALVES ACTUATOR

ELECTRICAL

RESTRICTOR
29 HYDRAULICS

BRAKE BRAKE
UNIT UNIT

Figure 29-9. Hydraulic System Schematic

29-12 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

SYSTEM OPERATION A caution light provides a warning that the

hydraulic circuit breaker is not properly
Refer to Figure 29-9. Hydraulic System engaged. The original press to test amber
Schematic. caution light labeled HYD PUMP C/BKR
OPEN was located above the clock on pilot’s
The pressure switch within a predetermined panel. Power for the caution light is taken
range controls the electrically driven pump from the co-pilot panel light dimming control
when DC power is on the aircraft. The hydraulic rheostat labeled COPLT RADIO & VA PNL
system operates at a maximum pressure LTS on the overhead console circuit breaker
between 1225 and 1575 psi with the dead band panel.
range between 150 and 300 psi. Pressurized
fluid is directed through a 10-micron filter with
a replaceable paper element prior to entering
the system.

Refer to Figure 29-10. Hydraulic System

Electrical Schematic.

Power will pass through the K11 hydraulic pump

relay coil in the main distribution box in the cabin
roof to the normally closed contacts of the system
pressure switch providing an immediate ground
to energize the K11 relay coil. With the K11
relay contacts A1 and A2 closed the hydraulic
pump motor will receive power directly from the
35-amp circuit breaker to commence hydraulic
system pressure build up.

When the system pressure switch senses

maximum pressure the switch contacts will open
de energizing the K11 relay coil removing power
to the hydraulic pump motor with the opening the
relay A1 A2 contacts (Figure 29-10). As system
pressure reduces to the lower pressure switch
level the switch contacts will close providing a
ground to energize the K11 relay and recycle the
hydraulic pump operation once again.

With the pressure switch contacts normally

closed the hydraulic pump will commence
operation once the switch is selected to DC
MASTER and the BATTERY/EXTERNAL
switch is selected to any positions.
29 HYDRAULICS

FOR TRAINING PURPOSES ONLY 29-13

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 29-10. Hydraulic System Electrical Schematic

29 HYDRAULICS

Figure 29-11. HYD PUMP C/BRK OPEN Caution Light

29-14 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

When the hydraulic pump circuit breaker driven pump and the hand pump systems are
is disengaged, the caution light relay is inoperative, is severely limited under these
de-energized closing the relay A2 and A3 circumstances. Brake loss is imminent after the
contacts. Power from the dimming control initial application of brakes.
rheostat passing through the caution light relay
closed contacts provides a ground to bring on Figure 29-10 is an electrical schematic of the
the caution light (Figure 29-10). hydraulic system operation.

By crew request the caution light is now

located to a higher position on the flight panel
beside the air speed indicator (Figure 29-11)
to improve visibility for both crew members.
The caution light now receives power from
a 5 amp circuit breaker labeled PLT ENG
CONS & TRIM PNL LT on the main circuit
breaker panel. When the hydraulic pump circuit
breaker is disengaged the caution light relay
is de energized closing the relay A2 and A3
contacts and turns on the caution light.

WARNING

THE HYDRAULIC SYSTEM 35

AMP CIRCUIT BREAKER MUST
BE PROPERLY ENGAGED
AT ALL TIMES TO ENSURE
THAT ELECTRICAL POWER IS
AVAILABLE FOR HYDRAULIC
PUMP OPERATION. THIS IS
ESPECIALLY IMPORTANT
PRIOR TO PRE FLIGHT CHECKS
OR ENGINE OPERATION AS
FORWARD MOVEMENT OF THE
AIRCRAFT WITHOUT BRAKING
PRESSURE COULD RESULT
IN A LIFE THREATENING
SITUATION AND SEVERE
DAMAGE TO THE AIRCRAFT.

Although a hand pump is available to provide

system pressure in the event of the electrically
driven pump failure, a single crew member in
control of the aircraft, during ground mode
and simultaneous operation of the hand pump
for braking, should not be encouraged during
29 HYDRAULICS

engine operation.

Operators should be aware that brake indicated

pressure isolated by system check valves, in the
unlikely situation when both the electrically

FOR TRAINING PURPOSES ONLY 29-15

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 29-12. Hydraulic Hand Pump

29 HYDRAULICS

29-16 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

29-20-00 AUXILIARY *The following is an abbreviated description of

the maintenance practices and is intended for
HYDRAULIC SYSTEM training purposes only.
For a more detailed description of the practice,
GENERAL refer to the task in the Viking AMM PSM 1-63-2.

The auxiliary system consists of a hydraulic

handpump with a suction line from the
29-20-00 MAINTENANCE
reservoir, and a pressure line connected into the PRACTICES
power system on the hydraulic power package.
SERVICING
Hydraulic Handpump
Refer to Figure 29-12. Hydraulic Hand Pump.
Bleed Hydraulic Handpump
1. Ensure electrical power is off.
The hydraulic handpump is of the double acting
2. D i s c h a r g e d a m p i n g a c c u m u l a t o r b y
type, displacing an equal amount of fluid on
operating wing flaps, and wheel brake
each stroke.
accumulator by pumping brake pedals.
The handpump is beneath the flight compartment
3. Slacken off handpump pressure line at tee
floor immediately to the left of the control column
of forward accumulator on hydraulic power
base. A hinged door provides access to the
package. Operate handpump until air has
handpump, and the handle is stowed complete
been expelled from line, and fluid emerges
with withdrawal strap, on the sub-floor, right-
in solid stream. Retighten handpump
hand side below the fire extinguisher.
pressure line at tee. Stow handpump handle.
4. Clean up hydraulic fluid spillage. Check
OPERATION reservoir fluid level.
The double-acting hand pump displaces an
equal quantity of fluid on each stroke. When
INSPECTION/CHECK
operated, the pump pressurizes the entire
system, including the brake system, through
Inspection of the Handpump
a check valve. The system relief valve limits Inspect the emergency handpump handle for
pressure output of the hand pump. Pressure security in stowed position.
buildup to approximately 1,500 psi requires 30
to 40 strokes of the pump handle. When not in
use, the handle must be stowed.

29 HYDRAULICS

FOR TRAINING PURPOSES ONLY 29-17

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of HYDRAULIC PRESSURE

the maintenance practices and is intended for
training purposes only.
INDICATORS
For a more detailed description of the practice, Adjustment/Test
refer to the task in the Viking AMM PSM 1-63-2.
Equipment required:

29-00-00 MAINTENANCE •• Suitable dead weight calibrator or

PRACTICES hydraulic test rig

Bench Test Pressure Indicator

NOTE
T he fol l owi ng mai nt enanc e
NOTE
practices are of a general and This test applies to both
sometimes abbreviated nature. hydraulic system and brake
Complete procedures for specific pressure indicators.
maintenance tasks are found in
Chapter 29 of the Maintenance 1. Discharge hydraulic pressure.
Manual.
2. Remove pressure indicator from aircraft.
Cap pipe.
HYDRAULIC LINES 3. Connect indicator to dead weight calibrator
or hydraulic test rig, using hydraulic fluid
Removal and installation of all hydraulic (MIL–H–5606) as test medium.
lines is self-evident; however, the following
points must be observed: 4. Apply pressure and increase gradually. At
each test pressure point noted in Table 29-1
•• Prior to removing a pressure line, maintain pressure and tap indicator gently
pressure must be depleted by operating to obtain true reading. Check tolerance.
the flaps or brakes, as required.
5. Slowly reduce pressure and check accuracy of
•• Prior to removing an accumulator or indicator at test pressures shown in Table 29-1.
the associated nitrogen lines, indicator,
6. Remove indicator from calibrator or test rig.
or charging valve, discharge the
accumulator nitrogen pre charge by 7. If required for service, reinstall in aircraft.
loosening the charging valve 1/4 turn
and waiting until the nitrogen has ACCUMULATOR AIR
escaped. Failure to do so may result in
serious injury.
CHARGING VALVE END CAPS
•• Cap all disconnected lines. Do not Adjustment/Test
remove caps until immediately prior to
Equipment required:
line reconnection.
•• Torque hydraulic lines and hoses to •• Pressure indicator of proven accuracy.
the values shown in PSM 1-63-2 ATA
12-10-6/03. Ensure there is no preload Bench Test Pressure Indicator
29 HYDRAULICS

or twisting during torquing.

NOTE
This test applies to both damping
and wheel brake accumulators.

29-18 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

1. Discharge hydraulic pressure. OTHER MAINTENANCE

2. Release accumulator air pressure. PRACTICES
3. Remove air charging valve and indicator
•• Lubricate all but the first two threads
assembly from aircraft. Block outlet port.
on straight thread fittings with MIL-H-
4. Connect source of nitrogen or dry air and 5606 prior to installation.
standard indicator of proven accuracy to air
•• Apply amber petrolatum anti seize
charging valve.
compound to tapered thread fittings
5. Apply pressure and increase gradually. At prior to installation.
each test pressure point noted in Table 29-1
•• Do not replace fittings indiscriminately.
maintain pressure and compare assembly
Identify and note position of fittings as they
indicator with standard indicator or proven
are removed to assure that replacement
accuracy. Tap indicator gently to obtain
conforms with original installation.
true reading.
•• Replacement components must be
6. Slowly reduce pressure and check accuracy of
primed with MIL-H-5606 hydraulic
indicator at test pressures shown in Table 29-1.
fluid before installation in the system.
7. Remove block from outlet port, and source
•• After installation of a component,
of pressure. Cap port.
bleed air from the component and
8. If required for service, install assembly in lines by operating the component at
aircraft. low pressure with line connections
loosened until air-free fluid flows, and
Test Pressure PSI Tolerance ± PSI then tighten the connections.
0 30 •• After bleeding, operate the system
through several cycles, and check the
400 40
reservoir quantity. Service the reservoir,
800 50 if required, in accordance with PSM
1-63-2 ATA 12-10-05.
1200 50
•• E n s u r e t h a t c h e c k v a l v e f l o w i s
1600 50
identified and that the check valve is
2000 50 installed in the proper flow direction.
•• Hydraulic components shall be washed
Table 29-1. Calibration Tolerances in cleaning solvent and dried with dry
compressed air. Inspect parts for cracks,
chips, scratches, abnormal wear, and
cleanliness.
•• “Flat” accumulators (nitrogen charge
depleted) cause line hammer when
the system is operated. If the damping
accumulator is flat, the “kick in-kick
out” time interval is greatly reduced,
and the cockpit pressure gage will rise
29 HYDRAULICS

and fall rapidly.

FOR TRAINING PURPOSES ONLY 29-19

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

ADJUSTMENT/TEST Equipment required:

Equipment required: •• Suitable hydraulic test rig

•• Hydraulic test rig using hydraulic fluid Bench Test Brake Pressure Relief
(MIL–H–5606), suitably fitted with
electric motor-driven hydraulic pump
Valve
1. Gain access to hydraulic power package,
•• Shut-off and metering valves
under flight compartment floor, from right
•• Gauge hand access panel.
•• Flowmeter 2. Remove brake pressure relief valve from
hydraulic power package.
•• G r a d u a t e d c y l i n d e r w h i c h , i n
conjunction with the unit being tested, 3. Check for obvious damage.
will reproduce all conditions required
4. Connect relief valve into circuit of suitable
during the test.
test rig.
Bench Test Hydraulic System 5. With outlet port to return, slowly apply
hydraulic pressure to inlet port.
Pressure Relief Valve
6. Check that valve cracks open at 1750 + 50
1. Install valve in test rig.
or – 0 psi pressure.
2. Apply 1750 ± 25 psi hydraulic pressure
7. Slowly release pressure. Check that valve
to valve inlet and check that rated flow
resets at 1600 psi minimum pressure.
through valve is 2.10 Imp (2.5 U.S.) gal/
min. 8. Remove valve from test rig.
3. Reduce inlet pressure to 1680 psi. Check 9. If required for service, reinstall relief valve
that leakage through valve does not exceed in hydraulic power package.
4.5 cc/min.
Bench Test Thermal Relief Valve
NOTE 1. Remove appropriate cabin ceiling panels to
Leakage to be checked during gain access to thermal relief valve.
third minute of a three minute
2. Remove thermal relief valve from aircraft.
waiting period.
3. Check for obvious damage.
4. Slowly reduce inlet pressure. At pressures
4. Connect thermal relief valve into circuit of
of 1460 psi and 975 psi, check that leakage
test rig.
rate through valve does not exceed 1.5 cc/
min. Refer to Note in step 3). 5. With outlet port to return, slowly apply
hydraulic pressure to inlet port.
5. Slowly increase inlet pressure to 1460 psi.
Repeat leakage check given in step 4). 6. Check that valve cracks open at 1750 + 50
or – 0 psi pressure.
6. Increase inlet pressure to 1680 psi. Repeat
leakage check given in step 3). 7. Slowly release pressure. Check that valve
resets at 1575 psi minimum pressure.
7. Reduce inlet pressure to zero and remove
29 HYDRAULICS

valve from test rig. If satisfactory and 8. Remove valve from test rig.
required for service, install in aircraft.
9. If required for service, reinstall relief valve
in aircraft.

29-20 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

SERVICING Accumulators
Release all of the accumulator hydraulic pressure.
Reservoir Charge the accumulator through the charging
Refer to: valve to 750 ± 50 psi with nitrogen or dry air.

•• Figure 29-13. Hydraulic Reservoir and For complete accumulator servicing procedures,
Accumulator Gages. refer to PSM 1-63-2 ATA 12-10-15.
•• Figure 29-14. Hydraulic Pack Location.

Prior to checking the reservoir fluid level,

discharge all hydraulic pressure until the
cockpit pressure indicator reads 0. Reservoir
fluid level is then checked by removing the cap
and reading the scale on the attached dipstick.
Fluid level must be maintained between the
REFILL and FULL marks. If additional fluid is
required, add only the approved hydraulic fluid
specification MIL-H-5606.

For complete reservoir servicing procedures,

refer to PSM 1-63-2 ATA 12-10-15.

Figure 29-13. H
 ydraulic Reservoir and
Accumulator Gages

HYDRAULIC RESERVOIR
29 HYDRAULICS

FILLING AND ACCUMULATOR

CHARGING

Figure 29-14. Hydraulic Pack Location

FOR TRAINING PURPOSES ONLY 29-21

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

FUNCTIONAL CHECKS The cause of trouble in a particular component

can usually be traced to one or more of the
Functional (operational) checks are normally following: leaks (either internal or external),
performed after maintenance has been foreign particles clogging or holding open some
accomplished on a system. They may also part of a component, improper adjustment,
be performed when a system is suspected of mechanical damage, cut or worn O-rings or
malfunctioning. Functional checks are frequently seals, structural failure, or excessive clearance
a part of a maintenance procedure and are not resulting from wear. When the trouble is
called out separately. They sometimes include isolated to a particular component, remove it
such maintenance tasks as final adjustment, for installation of a serviceable component.
torquing, and safetying.
When a hydraulic component has been replaced,
or lines disconnected, pressure-check the
FAULT ANALYSIS system for leaks and proper operation before
returning the airplane to service.
Isolation of a fault or malfunction can be
accomplished by a systematic analysis of the
trouble, beginning with the most probable cause
and proceeding to the least probable cause.

NOTE
Prior to troubleshooting the
hydraulic system, check the
reservoir level and the charge in
the accumulators.

Aircraft: Revision No. 02 Page:

DE HAVILLAND DHC-6, SERIES 100, 200 & 300 Date: Jul. 28, 1993 29-1 of 1
System & 1. 2. Number Installed
Sequence 3. Number Required for Dispatch
Numbers 4. Remarks or Exceptions

29 HYDRAULIC POWER

1 Main Hydraulic System C 1 0 (M) Provided the Brake System Pressure

Pressure Indicator Indicator operates normally.

Figure 29-15. MMEL - Hydraulic Power

29 HYDRAULICS

29-22 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

29-00-00 SERIES 100/200 NOTES

DIFFERENCES
HYDRAULIC POWER PACKAGE
System Operation
Series 100 and 200 including early series 300
aircraft operated with a system maximum
pressure between 1300 and 1550 psi within a
dead band operating range between 150 to 250
psi and a system relief valve setting of 1750
psi. The change which can provide a larger
differential between on and off will reduce the
frequency of pump operation.

29 HYDRAULICS

FOR TRAINING PURPOSES ONLY 29-23

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

32 LANDING GEAR
CHAPTER 32
LANDING GEAR
CONTENTS
Page

32-00-00 LANDING GEAR........................................................................................ 32-1

Introduction......................................................................................................... 32-1
General................................................................................................................ 32-3
32-1­0-00 MAIN GEAR............................................................................................... 32-5
General................................................................................................................ 32-5
32-10-11 MAIN GEAR LEG....................................................................................... 32-7
General................................................................................................................ 32-7
Stay Strut...................................................................................................... 32-7
Upper Platen................................................................................................. 32-8
Shock Absorber........................................................................................... 32-11
32-10-11 MAINTENANCE PRACTICES.................................................................. 32-11
Inspection/Check................................................................................................ 32-11
Shock Absorber Compression Block Bubble Criteria................................... 32-11
MOD 6/1649 Preload Bolts......................................................................... 32-13
Phenolic Filler Plate.................................................................................... 32-13
Fairing......................................................................................................... 32-14
32-20-00 NOSE GEAR............................................................................................. 32-15
General.............................................................................................................. 32-15
32-20-11 NOSE GEAR SHOCK STRUT................................................................... 32-17
General.............................................................................................................. 32-17
32-20-11 MAINTENANCE PRACTICES.................................................................. 32-21

FOR TRAINING PURPOSES ONLY 32-i

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
32 LANDING GEAR

Page

Servicing............................................................................................................ 32-21
Charge Nose Gear Shock Strut.................................................................... 32-21
Lubrication of the Landing Gear................................................................. 32-25
Inspection/Check................................................................................................ 32-25
Inspection of the Nose gear......................................................................... 32-25
32-40-00 WHEELS AND BRAKES.......................................................................... 32-27
General.............................................................................................................. 32-27
32-40-51 MAIN WHEEL.......................................................................................... 32-29
General.............................................................................................................. 32-29
Intermediate Flotation Gear................................................................................ 32-29
Tire Pressures..................................................................................................... 32-29
32-40-61 NOSE WHEEL.......................................................................................... 32-31
General.............................................................................................................. 32-31
32-40-61 MAINTENANCE PRACTICES.................................................................. 32-31
Inspection/Check................................................................................................ 32-31
Inspection of the Nose and Main Wheels..................................................... 32-31
Inspect Main and Nose Wheel Hub............................................................. 32-31
32-40-11 BRAKES.................................................................................................... 32-33
General.............................................................................................................. 32-33
Wheel Brakes System.................................................................................. 32-33
Brake Hydraulic Pressure Indicator............................................................. 32-33
Brake Control Valve.................................................................................... 32-33
Wheel Brake Accumulator........................................................................... 32-35
Brake Assembly.......................................................................................... 32-37
Cleveland Wheel and Brakes....................................................................... 32-39

32-ii FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

32 LANDING GEAR
Page

Operation........................................................................................................... 32-41
32-40-11 MAINTENANCE PRACTICES.................................................................. 32-43
Removal/Installation.......................................................................................... 32-43
Remove Brake Control Valve....................................................................... 32-43
Install Brake Control Valve......................................................................... 32-43
Servicing............................................................................................................ 32-43
Bleed Hydraulic Brake System.................................................................... 32-43
32-40-41 PARKING BRAKE.................................................................................... 32-45
General.............................................................................................................. 32-45
Operation........................................................................................................... 32-45
32-50-00 NOSE WHEEL STEERING SYSTEM....................................................... 32-49
General.............................................................................................................. 32-49
32-50-00 MAINTENANCE PRACTICES.................................................................. 32-53
Servicing............................................................................................................ 32-53
Bleed Nose Wheel Steering System............................................................. 32-53
32-50-11 NOSE WHEEL STEERING ACTUATOR................................................... 32-55
General.............................................................................................................. 32-55
32-50-11 MAINTENANCE PRACTICES.................................................................. 32-55
Servicing............................................................................................................ 32-55
Service Hydraulic Filter.............................................................................. 32-55
32-70-00 SUPPLEMENTARY GEAR........................................................................ 32-57
General.............................................................................................................. 32-57
32-70-00 MAINTENANCE PRACTICES.................................................................. 32-57
Inspection/Check................................................................................................ 32-57
Inspection of the Tail Bumper..................................................................... 32-57

FOR TRAINING PURPOSES ONLY 32-iii

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
32 LANDING GEAR

Page

32-00-00 MAINTENANCE PRACTICES.................................................................. 32-58

General Maintenance Practices........................................................................... 32-58
Tires............................................................................................................ 32-58
Functional Checks.............................................................................................. 32-58
Fault Analysis..................................................................................................... 32-58
Limitations......................................................................................................... 32-59
Good Year Brake Line Wear........................................................................ 32-59
Good Year Disc Guide Lining Wear............................................................. 32-59
32-00-00 LANDING GEAR SPECIAL ORDER OPTIONS....................................... 32-61
Wheel-Skis (Mod S.O.O. 6001).......................................................................... 32-61
Description......................................................................................................... 32-61
32-11-00 Main Wheel-Ski........................................................................... 32-61
32-21-00 Nose Wheel-Ski........................................................................... 32-63
Operation........................................................................................................... 32-65
32-00-00 MAINTENANCE PRACTICES.................................................................. 32-67
Servicing............................................................................................................ 32-67
Main Wheel Ski Lubrication....................................................................... 32-67
Nose Wheel Ski Lubrication........................................................................ 32-69
Adjustment/Test................................................................................................. 32-69
Adjust Sling Actuating Mechanism............................................................. 32-69
Inspection/Check................................................................................................ 32-69
Check Nose Wheel Ski Rigging.................................................................. 32-69
32-61-00 WHEEL-SKI INDICATION ...................................................................... 32-73
General.............................................................................................................. 32-73
32-30-00 EXTENSION AND RETRACTION (WHEEL-SKI)................................... 32-77

32-iv FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

32 LANDING GEAR
Page

General.............................................................................................................. 32-77
32-30-00 MAINTENANCE PRACTICES.................................................................. 32-79
Adjustment/Test................................................................................................. 32-79
Rig Wheel Ski Selector Control.................................................................. 32-79
32-61-00 MAINTENANCE PRACTICES.................................................................. 32-81
Adjustment/Test................................................................................................. 32-81
Adjust Ski Up Limit Switch........................................................................ 32-81
Adjust Ski Down Limit Switch.................................................................... 32-81
32-12-00 MAIN GEAR SPRING SKIS (MOD S.O.O. 6116)..................................... 32-83
General.............................................................................................................. 32-83
32-22-00 NOSEGEAR SPRING SKI......................................................................... 32-87
General.............................................................................................................. 32-87
32-61-00 AIRCRAFT FLOATS................................................................................. 32-89
General.............................................................................................................. 32-89
Standard Floats.................................................................................................. 32-89
Amphibious Floats............................................................................................. 32-91
Jury Strut........................................................................................................... 32-91
Amphibian Landing Gear System....................................................................... 32-93
Emergency Hand Pump............................................................................... 32-93
Float Installation & Removal.............................................................................. 32-94
Removal...................................................................................................... 32-94
Installation.................................................................................................. 32-94
Description and Operation.................................................................................. 32-97
Operation Troubleshooting................................................................................. 32-98
Problem - Power Pack Does not Run After Gear Selection.......................... 32-98

Revision 0.4
FOR TRAINING PURPOSES ONLY 32-v
 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
32 LANDING GEAR

Page
Problem - Power Pack Does not Shut Off After Gear Reaches Position........ 32-98
Problem - Powerpack Shuts off Before Gear Reaches Position.................... 32-98
Problem - Powerpack Cycles on and off After Gear is in Position............... 32-98
Problem - Power Pack Cycles on and off During Gear Cycle....................... 32-98
Problem - Slow Gear Operation Cycle (Considerably Longer than
30 seconds)................................................................................................. 32-99
Problem - Circuit Breaker Pops During Cycle............................................. 32-99
32-00-00 SPECIAL TOOLS....................................................................................32-103

Revision 0.4
32-vi FOR TRAINING PURPOSES ONLY
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32 LANDING GEAR
ILLUSTRATIONS
Figure Title Page

32-1 Twin Otter Standard Landing Gear............................................................32-2

32-2 Main Gear Assembly..................................................................................32-2
32-3 Main Gear..................................................................................................32-4
32-4 Main Gear Leg Installation........................................................................32-6
32-5 Upper Platen Installation - L/H Side of Aircraft.........................................32-8
32-6 Shock Absorber Installation.....................................................................32-10
32-7 Phenolic Filler Plate................................................................................32-12
32-8 Main Landing Gear..................................................................................32-14
32-9 Nose Gear Assembly................................................................................32-16
32-10 Nose Gear Shock Strut Operation............................................................32-17
32-11 Nose Gear Shock Strut Installation..........................................................32-18
32-12 Nose Gear Fork (Typical).........................................................................32-19
32-13 Steering Collar Torque Adjustment..........................................................32-19
32-14 Nose Gear Identification Plate (71-101-11) - Servicing Instructor...........32-20
32-15 Nose Gear Shock Strut............................................................................32-22
32-16 Lubrication Diagram................................................................................32-24
32-17 Main Gear Wheel.....................................................................................32-28
32-18 Main Wheel Installation...........................................................................32-30
32-19 Brake Control Valves...............................................................................32-32
32-20 Wheel Brake Accumulator.......................................................................32-34
32-21 Brake Assembly.......................................................................................32-36
32-22 Cleveland Wheel and Brakes....................................................................32-38
32-23 Parking Brake Handle..............................................................................32-38

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32 LANDING GEAR

Figure Title Page

32-24 Brake System Schematic..........................................................................32-40

32-25 Main Ski..................................................................................................32-41
32-26 Left Main Ski..........................................................................................32-41
32-27 Rear View of Main Ski............................................................................32-42
32-28 Main Ski with Hydraulics........................................................................32-42
32-29 Park Brake Handle Operation..................................................................32-44
32-30 Brake System Schematic..........................................................................32-46
32-31 Nose Wheel Steering System...................................................................32-48
32-33 Nose Wheel Steering Lever......................................................................32-50
32-32 Nose Wheel Steering Cables and Drums .................................................32-50
32-35 Nose Wheel Torque Links and Pip Pin.....................................................32-51
32-34 Nose Wheel Steering...............................................................................32-51
32-36 Nose Wheel Steering Cable Tension Graph..............................................32-52
32-37 Nose Wheel Steering Actuator Installation...............................................32-54
32-38 Test Ports and Schematic Hydraulic Diagram..........................................32-56
32-39 Tail Bumper.............................................................................................32-57
32-40 Brake Wear Limitations...........................................................................32-59
32-41 Main Wheel Ski.......................................................................................32-60
32-42 Main Wheel Skis.....................................................................................32-61
32-43 Nose Wheel Ski.......................................................................................32-62
32-44 Main Wheel Ski - Installation (Sheet 1 of 2)............................................32-64
32-45 Main Wheel Ski - Installation (Sheet 2 of 2)............................................32-66
32-46 Lubrication Diagram - Wheel Skis...........................................................32-67
32-47 Nose Wheel Ski Installation.....................................................................32-68
32-48 Nose Wheel Installation...........................................................................32-70

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32 LANDING GEAR
Figure Title Page
32-49 Ski Selector Panel....................................................................................32-72
32-50 Ski Position Indication Electrical Schematic - Up Position......................32-74
32-51 Ski Position Indication Electrical Schematic - Down Position..................32-75
32-52 Wheel Ski Hydraulic System - Schematic................................................32-76
32-53 Ski Selector - Rigging.............................................................................32-78
32-54 Limit Switch Adjustment.........................................................................32-80
32-55 Main Gear Spring Ski Installation (Sheet 1 of 2).....................................32-82
32-56 Main Gear Spring Ski Installation (Sheet 2 of 2).....................................32-84
32-57 Nose Gear Spring Ski (Mod S.O.O. 6116) (Sheet 1 of 2).........................32-86
32-58 Nose Gear Spring Ski (Sheet 2 of 2)........................................................32-87
32-59 Float Installation......................................................................................32-88
32-60 Aircraft Take-off Dolly (1 of 2)...............................................................32-89
32-61 Aircraft Take-off Dolly (2 of 2)...............................................................32-89
32-62 Amphibious Floats...................................................................................32-90
32-63 Straight Floats.........................................................................................32-91
32-64 Hydraulic Hand Pump..............................................................................32-92
32-65 Hydraulic System Schematic...................................................................32-96

Revision 0.4
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32 LANDING GEAR
CHAPTER 32
LANDING GEAR

32-00-00 LANDING GEAR

INTRODUCTION
The Twin Otter landing gear is non-retractable. Nose wheel steering is mechanically controlled
and hydraulically actuated. Wheel brakes are powered by the main hydraulic system. The user
should consult the Maintenance Manual, applicable AFM supplements and vendor manuals
for additional information on specific manufacturers installations not included in this chapter.

FOR TRAINING PURPOSES ONLY 32-1

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32 LANDING GEAR

Figure 32-1. Twin Otter Standard Landing Gear

Figure 32-2. Main Gear Assembly

32-2 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
GENERAL NOTES
The aircraft is normally fitted with a non-
retractable landing gear which consists of the left
and right main gear assemblies, a steerable nose
gear assembly, and a tail bumper. Each of the
single main wheels incorporates a hydraulically
operated, disc type wheel brake unit.

At the customer’s option, wheel skis or spring

skis may be fitted to the landing gear for operation
on snow or ice. The wheel ski offers the option of
landing either on skis or wheels.

Another customer option is to equip the aircraft

with floats for operation on water. Two types of
float installations are available.

FOR TRAINING PURPOSES ONLY 32-3

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32 LANDING GEAR

Figure 32-3. Main Gear

32-4 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
32-1­0-00 MAIN GEAR NOTES

GENERAL
Refer to Figure 32-3. Main Gear.

Each main gear consists of a leg assembly,

shock absorber, and wheel and brake assembly.
The leg assembly is bolted to two lower
attachment fittings on the fuselage, and these
provide the pivoted point for leg movement.
Restriction and damping of leg movement is
controlled by a shock absorber of the urethane
block type. Each shock absorber is bolted to two
fuselage upper attachment fittings and to metal
pads on the leg assembly. Installed on the axle
is a wheel and a hydraulically-operated brake
assembly. A two piece fairing encloses the leg.

FOR TRAINING PURPOSES ONLY 32-5

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32 LANDING GEAR

Figure 32-4. Main Gear Leg Installation

32-6 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
32-10-11 MAIN GEAR LEG NOTES

GENERAL
Refer to Figure 32-4. Main Gear Leg Installation.

Each main gear leg consists of a Y-shaped tubular

strut, a stay strut and an axle. The Y-shaped strut
comprises a front strut to which is welded a short
rear strut. At the upper ends of both front and rear
struts are pivot fittings which are welded to the
struts. The two pivot fittings are interconnected
by a stay strut. An axle is bolted to the lower
end of the leg. Midway between the axle and the
pivot fittings, a clamp type jacking pad is bonded
to the strut. Four pads each containing a threaded
hole are welded to the leg for attachment of the
shock absorber lower platen. Each main gear leg
is enclosed by front and rear section fairings.

Stay Strut
A stay strut is provided for each main gear leg
and consists of a tube with two flanged end
fittings. Each strut is positioned between the
main gear leg pivot fittings and is shimmed
equally at both ends to maintain the proper
dimension between the pivot fittings. Screws
and special washers secure the strut in position.

FOR TRAINING PURPOSES ONLY 32-7

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32 LANDING GEAR

Upper Platen
The upper platen is the upper mounting surface
for the shock absorber blocks (Figure 32-5).
The upper platen is attached to two fuselage
frames through forward and aft links. The Aft
link mounts the airframe jacking point.

Figure 32-5. Upper Platen Installation - L/H Side of Aircraft

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32 LANDING GEAR
PAGE INTENTIONALLY LEFT BLANK

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32 LANDING GEAR

Figure 32-6. Shock Absorber Installation

32-10 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
Shock Absorber and one half times its diameter from
any vertical surface of the block.
The shock absorber components consist of two
urethane blocks to absorb compression and a d. No bubble shall be separated from
single rebound block to maintain alignment during any adjacent bubble by less than one
landing (Figure 32-6). The two compression inch except as described under cluster
blocks are sandwiched between the upper and limitation exceptions (refer to step 2).
lower platens with a separation plate between the
2. Bubble cluster limitation exceptions are as
blocks. Spigots are installed in both platens and
follows:
separator plate to maintain compression block
alignment. The rebound block with upper and a. Any cluster with a diameter less than
lower end plates is attached to the bottom surface 0.10 inch (which may be considered as a
of the lower platen. A preload bolt attached to the single defect) provided there are not more
rebound block lower end plate is inserted through than three such clusters in the block.
the rebound and both compression blocks before
b. Any cluster formed by not more than
being secured with a preload nut located in the
three bubbles, none of which exceed
upper platen recess area. Tightening the rebound
0.10 inch diameter, provided that the
bolt nut aligns the rebound block and preloads the
separation between the bubbles is greater
compression blocks.
than four times the diameter of the largest
bubble and that there are not more than
The inspection criteria for the blocks is a visual
three such clusters in the block.
check using a strong light and looking through
the block. Part of the criteria is no more than
ten visible bubbles and the diameter of each
bubble must be less than 0.10 inch.

*The following is an abbreviated description of

the maintenance practices and is intended for
training purposes only.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2.

32-10-11 MAINTENANCE
PRACTICES
INSPECTION/CHECK
Shock Absorber Compression
Block Bubble Criteria
1. Limitations for bubbles within a shock
absorber compression block under preload
or free state condition are as follows:
a. There shall not be more than ten visible
bubbles.
b. No single bubble shall be greater than
0.10 inch diameter.
c. No bubble shall be located less than two

FOR TRAINING PURPOSES ONLY 32-11

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32 LANDING GEAR

SPIGOT

0.80

0.62
DIA.

DRILL SIZE “P” (0.323) DIA.

MATERIAL: ALUMINUM ALLOY BAR 2024-T4

SPEC: OOA-200/3
FINISH: ANODIZED (MIL-A-8625)

PACKING PAD

13.50

CL
SYM.
2.62 R
1.50 DIA. HOLE

2.80
(TYP)

DRILL 5/8 DIA.

5.80

MATERIAL: 1/4 INCH THICK PHENOLIC SHEET

SPEC: MIL-P-15035 F.B.M.

Figure 32-7. Phenolic Filler Plate

32-12 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
MOD 6/1649 Preload Bolts alignment of the compression and rebound blocks
is essential to obtain the designed load absorbing
For post mod 6/1649 aircraft the preload bolt
qualities of the main gear installation.
length is shortened. The dimension of the Post
mod bolt is 11.26 to 11.30 inches measured from
the center of the bearing at one end to the threaded
end. Preload bolt is tightened sufficiently to show
a maximum bolt head protrusion of 0.050 inch
above the nut upper face area.

This positions the compression blocks and

provides the rebound block with energy to
expand during landing as the compression
blocks reduce under compression.

With either mods status installed the preload

bolt nuts are cotter pinned once alignment and
precompression has been achieved. Reducing
the bolt length for post mod installations
was a measure taken to prevent damage in
the upper platen recess area. Should a heavy
landing condition occur installations with pre-
mod longer length bolts might contact the top
surface of the upper platen recess area.

Operators who replace compression blocks

should be aware of the dimensional differences
between pre and post mod pre-load rebound
bolt to be assured that the rebound bolt is to the
correct length and compatible with the latest
post mod compression blocks now available.

Phenolic Filler Plate

Refer to Figure 32-7. Phenolic Filler Plate.

Operators experiencing loose compression

blocks, after a period of operation, can insert a
0.25 inch phenolic packing piece between the
lower platen surface and the compression block
as described in TAB 675/11 as a method to firm
up the installation and re-energize the rebound
block (Figure 32-7). The condition is usually
caused by a reduction in compression block width
resulting in loss of rebound block energy and
lower compression block displacement over the
edge of the platen surface. This situation has been
known to occur with aircraft routinely parked
overnight fully loaded for early morning flight.
Although the reported condition will not occur to
all aircraft, operators should be aware that correct

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32 LANDING GEAR

Fairing
A two-piece fiberglass fairing, held together by
screws, covers the leg. The fairing is aligned
on the leg with a rib frame bolted to the top
portion of the leg and at the lower end by a
split seal (Figure 32-8). A blind elastic cord
attached to the lower portion of the leg and
to the fairings holds them together to prevent
water and debris from entering the enclosure
with the leg in motion. As protrusion of the
blind outside the fairing can change the flight
characteristics of the aircraft, in the higher
speed range, the condition of the blind elastic
cord is essential to maintain blind alignment.

LEADING EDGE
TRAILING EDGE

Figure 32-8. Main Landing Gear

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32 LANDING GEAR
32-20-00 NOSE GEAR NOTES

GENERAL
The nose gear consists of a non retractable,
pneudraulic shock strut mounted on the forward
face of the nose compartment bulkhead, and
a single wheel supported on an axle within
the fork of the shock strut. The shock strut is
equipped with a hydraulically operated steering
mechanism to steer the nose wheel. When the
torque links are disconnected (as for towing) the
nose wheel can be rotated 360°. A detachable
cover is provided in the nose compartment to
prevent loose objects obstructing the steering
actuator, which is mounted on the shock strut.

FOR TRAINING PURPOSES ONLY 32-15

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32 LANDING GEAR

CHARGING VALVE

UPPER CYLINDER

STEERING COLLAR

NOSEWHEEL STEERING
ACTUATOR

CENTERING LATCH

QUICK-RELEASE PIN
UPPER TORQUE ARM

LOWER TORQUE ARM

PISTON

FORK

Figure 32-9. Nose Gear Assembly

32-16 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
32-20-11 NOSE GEAR
SHOCK STRUT
GENERAL
Refer to Figure 32-9. Nose Gear Assembly.

The nose gear shock strut consists of an outer

cylinder, piston assembly, nose wheel fork and
a steering collar. The cylinder forms the main
structure of the strut and provides the mounting
for the nose wheel steering actuator (refer
to 32-50-00, Nose Wheel Steering System -
General Data). The nose wheel fork is secured
to the piston and telescopes within the cylinder
which is charged with oil and air, damping
being achieved by restricting the oil and air
flow through orifices in the piston. A valve,
through which the strut may be recharged, is
located on top of the strut.

Figure 32-10. Nose Gear Shock Strut Operation

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32 LANDING GEAR

Figure 2: Nose Gear Shock Strut Installation

Figure 32-11. Nose Gear Shock Strut Installation

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32 LANDING GEAR
Figure 32-12. Nose Gear Fork (Typical)

Figure 32-13. Steering Collar Torque Adjustment

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32 LANDING GEAR

Figure 32-14. Nose Gear Identification Plate (71-101-11) - Servicing Instructor

32-20 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
*The following is an abbreviated description of NOTES
the maintenance practices and is intended for
training purposes only.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2.

32-20-11 MAINTENANCE
PRACTICES
SERVICING
Charge Nose Gear Shock Strut
Refer to:

•• Figure 32-14. Nose Gear Identification

Plate (71-101-11) - Servicing Instructor.
•• Figure 32-15. Nose Gear Shock Strut.

CAUTION
DO NOT FILL SHOCK STRUT
WITH OIL UNDER PRESSURE
AS THIS CAN RESULT IN ITS
FAILURE TO TELESCOPE.
OIL FILLING BY GRAVITY
IS THE ONLY ACCEPTED
PROCEDURE.

The nose gear strut must be serviced in

accordance with the instructions on the nose
gear identification plate.

NOTE
Dry compressed nitrogen is
preferred to compressed air for
charging the shock strut.

Chapter/Section 12-10-20 of the Maintenance

Manual must be consulted during actual
servicing of the strut.

FOR TRAINING PURPOSES ONLY 32-21

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32 LANDING GEAR

Figure 32-15. Nose Gear Shock Strut

32-22 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
PAGE INTENTIONALLY LEFT BLANK

FOR TRAINING PURPOSES ONLY 32-23

32-24 32 LANDING GEAR

SYMBOL TITLE MIL SPEC

A GREASE-LOW 3-GP-683A
AND HIGH OR
TEMPERATURE MIL-G-25760
OR
MIL-G-3278
MIL-PRF-23827
MIL-G-7118
B OIL-GENERAL 3-GP-335A
PURPOSE OR

TWIN OTTER SERIES

PARTS NOMENCLATURE KEY MIL-L-7870A
1. ALL WHEELS, HUBS AND AXLES C SILICONE MIL-S-8660
2. PARKING BRAKE CAM COMPOUND
3. TORQUE ARMS WITH LUBRICATORS
FOR TRAINING PURPOSES ONLY

INSTALLED (4 PLACES)

MAINTENANCE TRAINING MANUAL

APPLICATION SYMBOLS

GREASE GUN OIL CAN HAND

000
FREQUENCY SYMBOL
NOTE 1: USE GREASE MIL-PRF-81322 OR MOBIL SHC 100 TO
PACK WHEEL BEARING ON INSTALLATION.
A A A A MORE FREQUENT LUBRICATION OF WHEEL BEARINGS
500 250 1000 250 MAY BE REQUIRED UNDER SEVERE OFF-RUNWAY
3 1 2 1 CONDITIONS.
(NOTE 2) (NOTE 1) (NOTE 1) NOTE 2: USE GREASE MIL-PRF-23827.

Figure 32-16. Lubrication Diagram

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32 LANDING GEAR
Lubrication of the Landing Gear NOTES
Lubrication points, methods, and frequency of
application are given in Lubrication Diagram.
Extensive use of sealed bearings and dry-
film lubricant, applied during manufacture,
keeps the need for external lubrication
to a minimum. Consequently, daily and
intermediate lubrication is unnecessary, and
periodic application is indicated by a triangle,
the frequency of which is denoted by the figure
on the block at the base of the triangle.

Before applying lubricant, wipe off any dirt

and moisture from the point with a clean, lint-
free cloth, lightly moistened with dry cleaning
solvent. Avoid excessive use of lubricant when
applying by hand to prevent spillage. Carefully
clean up spillage immediately.

INSPECTION/CHECK
Inspection of the Nose gear
Inspect the nose gear for condition and leakage;
clean exposed surface of shock strut piston
with clean cloth.

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32 LANDING GEAR

PAGE INTENTIONALLY LEFT BLANK

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32 LANDING GEAR
32-40-00 WHEELS AND NOTES
BRAKES
GENERAL
The main landing gear wheels are carried on
the axle of each main gear leg, and the nose
wheel is carried on the axle mounted in the fork
of the nose gear. The wheels are of the split
hub type to facilitate removal and installation
of tubeless tires.

FOR TRAINING PURPOSES ONLY 32-27

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32 LANDING GEAR

Figure 32-17. Main Gear Wheel

32-28 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
32-40-51 MAIN WHEEL The Dunlop tire, introduced by mod 6/1526
(TAB 660/1), utilizes the same Goodyear
tube, is heavier than the Goodyear tire, but
GENERAL is a better fit on the wheel rim. The Dunlop
tires were introduced as an alternative tire to
The standard Goodyear main wheels are of split overcome the problem of tire slippage during
configuration manufactured from magnesium. cold weather operations. Goodyear and Dunlop
The main wheel has an 8-ply rating (Pre tires should not be mixed on the main wheels.
Mod 6-M0007) or 10-ply rating (Post Mod
6-M0007), 11.00 x 12 nylon tubeless tire. The
wheels have a static load rating of 6300 lbs.
TIRE PRESSURES
When installing tubeless tires the center seal
Main gear standard 1100 x 12 tire pressure is
or gasket must be installed between both wheel
normally maintained at 38 psi if the ambient
halves to maintain tire pressure (Figure 32-8).
temperature is above 20°F and 34 psi if the
temperature is less. The tires also absorb shocks
A Goodyear aluminum split configuration main
during landing and taxiing, and the pressure
wheel is available by optional mod S.O.O. 6124
change at lower temperatures is necessary to
(TAB 640/5). The inner halves of the main
ensure that the tires meet the necessary energy
wheel include a drive ring to accommodate the
absorbing requirements.
brake gear disc.
For series 300 aircraft the intermediate flotation
When splitting the wheel, it is recommended
main wheel Goodyear and Dunlop tire pressure
that a template be used to keep track of bolt
was increased by mod 6/1574 (TAB 664/8)
position. The reason for tracking this is when
from 27 is 35 psi.
a bolt is cracked it is replaced. When a bolt is
broken the fractured bolt is replaced with the
For series 100/200 aircraft the main wheel tire
bolts on each side.
pressure from 24 to 30 psi as a further means
to prevent tire slippage and pressure loss when
INTERMEDIATE FLOTATION operating in extreme cold temperatures.
GEAR
An intermediate flotation landing gear
configuration by opt mod S.O.O. 6048 is available
for desert and soft field landing strip surfaces.
The main wheel 11.00 x 12 standard tires are
replaced with Goodyear 1500 x 12 type 111 (10
ply) tires, or Dunlop 36 x 1300 x 12 (6 Ply) tires
introduced by mod 6/1526 (TAB 660/1).

In addition, a larger nose wheel fork is installed

on the nose gear leg to accommodate a 11.00 x
12 mainwheel fitted with the larger Goodyear or
Dunlop tire. A longer jury strut is also provided.

Intermediate flotation landing gear installed in

place of conventional landing gear increases
the aircraft empty weight by approximately
111 pounds.

FOR TRAINING PURPOSES ONLY 32-29

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32 LANDING GEAR

Figure 32-18. Main Wheel Installation

Figure 1: Wheel Ski Hydraulic System – Schematic

32-30 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
32-40-61 NOSE WHEEL *The following is an abbreviated description of
the maintenance practices and is intended for
training purposes only.
GENERAL For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2.
The nose wheel is carried on the axle mounted
in the fork of the nose gear, and is of the split
hub type to facilitate removal and installation 32-40-61 MAINTENANCE
of tubeless tires. An O-ring is installed between
the two halves of the hub to provide an airtight
PRACTICES
housing for the tire. The nose wheel has 6-ply
rating 8.90 x 12.50, Type III, low pressure INSPECTION/CHECK
tubeless tire. When modification S.O.O. 6048
is embodied, a main wheel hub fitted with a Inspection of the Nose and Main
15.00 x 12 tire and tube is installed in the nose
wheel position. On aircraft with S.O.O. 6048
Wheels
and Mod 6/1526, a 36 x 13.00 x 12 tire is fitted Inspect the Nose and main wheels for cuts,
to the nose wheel. wear, deterioration and inflation.

Inspect Main and Nose Wheel Hub

1. Clean hub halves with dry cleaning solvent
(P-S-661 or equivalent) and dry with
compressed air or a lint-free cloth.
2. Inspect hub halves for cracks (using
the fluorescent penetrant method or an
approved dye penetrant method), chipping
of rims, damage and corrosion. Replace hub
if cracks or chipping of rims is evident.

NOTE
Small nicks or gages can be
blended out and polished with
fine aluminum oxide cloth. Refer
to Manufacturer’s Overhaul
Manual for definition and
limitation of damage.

3. Inspect drive ring and rivets for looseness.

Inspect drive ring teeth for damage.
4. Inspect bearings and replace if damaged.

FOR TRAINING PURPOSES ONLY 32-31

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32 LANDING GEAR

Figure 32-19. Brake Control Valves

32-32 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
32-40-11 BRAKES On aircraft incorporating Mod 6/1644, a
turnbuckle is installed in each push rod to increase
pressure adjustment for the brake control valve.
GENERAL
The valves are on the flight compartment floor,
The Goodyear wheel brake system brake they function as variable pressure reducers that
assemblies, and brake control valves are meter fluid pressure to the brakes in proportion
hydraulically operated by the pilot or co-pilot to brake pedal deflection. They reduce pressure
rudder pedals. Main hydraulic system or hand to the brake units to a maximum of 1000 ± 50
pump pressure can be utilized for brake application. psi. The valves are made of cast aluminum alloy
If both pressure sources are inoperative, fluid containing a lever-operated floating piston and a
pressure from a check-valve-isolated accumulator spring-loaded poppet valve. They have three ports,
may only be utilized for a limited period. inlet pressure brake pressure, and return and it
provides thermal relief for the brake unit.
Wheel Brakes System A new type of adjustable pushrod was
The wheel brakes system consists of introduced by mod 6/1644 (TAB 670/3) at
hydraulically-operated wheel brake units aircraft 591 to overcome pedal alignment
controlled from the pilot’s and co-pilot’s rudder problems caused by extreme linkage tolerances
pedals. The fluid pressure is metered to the and variations in the cockpit floor structure.
wheel brake units in proportion to brake pedal
depression. When the hydraulic system electric
motor-driven pump or handpump is operated,
fluid is supplied from the main hydraulic
system to the control valves and to charge a
wheel brakes accumulator. Return fluid from
the control valves is fed back to the hydraulic
system common return line. A check valve is
installed upstream of the supply line to the
accumulator. The accumulator supplies brake
pressure when the pumps are not in operation.

Brake Hydraulic Pressure Indicator

The brake hydraulic pressure indicator is
mounted on the pilot’s instrument sub panel.
The indicator is direct-reading bourdon tube
type with dial marked PRESSURE, PSI, and
scales marked from 0 to 2000 psi in increments
of 100 psi.

Brake Control Valve

Two brake control valves are connected through
a system of pushrods, levers, and torsion rods
to the brake pedals at each rudder pedal station
(Figure 32-26). Either pilot or co-pilot can apply
the brakes. Applying toe pressure to either set of
brake pedals rotates the torsion rod actuating the
brake control valves (Figure 32-19).

FOR TRAINING PURPOSES ONLY 32-33

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32 LANDING GEAR

Figure 32-20. Wheel Brake Accumulator

32-34 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
Wheel Brake Accumulator NOTES
Refer to Figure 32-20. Wheel Brake
Accumulator.

The wheel brakes accumulator is on the

hydraulic power package beneath the flight
compartment floor, provides the wheel brakes
with supplemental pressure in an emergency
or during limited periods of peak demand, and
provides pressure when the hydraulic system
is shut down while parked. The accumulator
is connected into the hydraulic system main
pressure supply line to the wheel brakes control
valves, and a combined air pressure gage and air
charging valve is mounted on a bracket on the
left side of the package.

FOR TRAINING PURPOSES ONLY 32-35

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
32 LANDING GEAR

Figure 32-21. Brake Assembly

32-36 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
Brake Assembly NOTES
A brake assembly (Figure 32-21) is bolted to the
torque plate of each main gear axle. The brake
consists of an annular disc geared to a disc drive
ring in the wheel (Figure 32-24), three sets of
brake linings, and three piston assemblies installed
in a brake housing. Metered fluid pressure applied
to the pistons forces the linings against the floating
disc. Springs in each piston provide self-adjustment
by progressively resetting the piston position as the
linings wear. A bleeder screw is located at the top
end of each brake housing assembly to purge air
from the hydraulic fluid.

To gage brake puck wear an adjusting pin extends

from each piston assembly through a threaded
bushing in the cylinder head to indicate lining
wear (Figure 32-19). Although wear indicator
pins recede into the brake housing as wear occurs,
the recommended method of checking puck wear
is to measure the dimension, after applying the
brakes. The distance between the brake disc
and machined face of the main body housing
in the piston housing area as mention in TAB
669/4 to ensure that the maximum .375 inch wear
dimension is not exceeded.

NOTE
E xcess i ve puck wear coul d
result in brake disc scoring and
possible puck dislodgement.
Both conditions limiting effective
braking at critical times.

A brake disc guide is for additional disc support

and prevent miss alignment between the brake
disc and plungers during landing.

Pressure for brake operation is supplied by the

aircraft hydraulic system (electrically driven
pump or the hand pump). If both sources
fail a check valve retains pressure in the
brake accumulator. Pressure left in the brake
accumulator will provide up to 5 applications
brake applications. In this case, do not “pump”
the brakes as brake accumulator pressure will
be rapidly depleted.

FOR TRAINING PURPOSES ONLY 32-37

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
32 LANDING GEAR

Figure 32-22. Cleveland Wheel and Brakes

Figure 32-23. Parking Brake Handle

32-38 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
Cleveland Wheel and Brakes NOTES
An alternative wheel and brake assembly is
available by STC approval from the Parker
Hannifin Corporation (Figure 32-22). This
wheel and brake unit can be installed on all
series Twin Otter aircraft. The Cleveland Wheel
and Brake installation is identified in the aircraft
manufacture publication AEROC 6.6.G.1 for
approved equipment for Twin Otter aircraft.
The wheel is of cast magnesium material and
is suitable for use with all 11.00 x 12 tires. The
wheel will accommodate tire tubes if required.
The brake is a single caliper, four-piston
external disc design with sintered metallic
linings. A flexible wire braided hydraulic hose
replaces the hard steel pipe from the main gear
leg lower flange housing to the brake assembly.

The larger intermediate flotation tires cannot

be installed on the Cleveland wheel and
brake assembly units. The Parker Hannifin
Corporation should be contacted for all details
of the Cleveland wheel and brake installation.

FOR TRAINING PURPOSES ONLY 32-39

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
32 LANDING GEAR

BRAKE ACCUMULATOR PRESSURE

INDICATOR AND AIR CHARGE VALVE

BRAKE
10
ACCUMULATOR
15
5

0 20 BRAKE
x 1000 RELIEF
1000 VALVE
(1,750 PSI)
2000
PRESS
PSI

BRAKE SYSTEM
PRESSURE
INDICATOR

DH DH

LEGEND PARKING
HYDRAULIC SYSTEM PRESSURE BRAKE

METERED BRAKING PRESSURE

RETURN
BRAKE
VALVES
NITROGEN

MECHANICAL

BRAKE BRAKE
UNIT UNIT

Figure 32-24. Brake System Schematic

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32 LANDING GEAR
OPERATION
Refer to:

•• Figure 32-24. Brake System Schematic.

•• Figure 32-25. Main Ski.
•• Figure 32-26. Left Main Ski.

Rotation of the torsion bar by pedal deflection

moves the valve piston, momentarily opening the
pressure inlet valve to the brake port and closing
the return port. Pressure buildup in the brake
assembly reacts against the valve piston, which
moves against torsion bar force to close the inlet Figure 32-25. Main Ski
port. If pedal force is increased, more pressure is
admitted to the brakes. Releasing the brake pedals
returns the valves to the off position, connecting
the brake ports to return.

Pressure metered by the brake control valves

(indirect proportion to force applied to the
pedals) is directed to the brake assemblies,
applying the brakes. Releasing toe pressure on
the pedals relaxes the control valves, allowing
fluid in the brakes to reverse flow to the return
system, releasing the brakes.

Figure 32-26. Left Main Ski

FOR TRAINING PURPOSES ONLY 32-41

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32 LANDING GEAR

Figure 32-27. Rear View of Main Ski Figure 32-28. Main Ski with Hydraulics

32-42 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
*The following is an abbreviated description of mounting bracket and install bolt, washer,
the maintenance practices and is intended for nut and cotter pin.
training purposes only.
3. At brake control valve, connect brake
For a more detailed description of the practice, system indicator hydraulic line (right-hand
refer to the task in the Viking AMM PSM 1-63-2. valve only); connect inlet pressure, brake
pressure and return hydraulic lines.
32-40-11 MAINTENANCE 4. Bleed brake system and function test wheel
PRACTICES brakes.
5. Install access panels in flight compartment.
REMOVAL/INSTALLATION SERVICING
Remove Brake Control Valve Bleed Hydraulic Brake System
1. Discharge brake system accumulator
1. Connect external power to aircraft. Check
hydraulic pressure.
nose wheel steering lever corresponds with
2. Remove access panels in flight compartment. position of nose wheel, and wing flaps
selector with position of wing flaps. Ensure
3. At brake control valve, disconnect brake
parking brake is off.
system indicator hydraulic line (right hand
valve only); disconnect inlet pressure, 2. Set EXTERNAL/BATTERY switch to
brake pressure and return hydraulic lines. EXTERNAL, and DC MASTER switch to on.
Cap open connections and lines. Check electric motor-driven hydraulic pump
charges damping accumulator and wheel
4. Remove cotter pin, nut, washer and bolt
brakes accumulator, and motor cuts out when
attaching brake control linkage levers to
reading on pressure indicators is 1575 ± 50
mounting bracket. Withdraw spring torsion
psi (1550 + 50 or – 0 psi, Pre Mod 6/1570).
rod (short) from brake control valve lever.
3. Depress both left and right brake pedals and
NOTE engage the parking brake.
When removing right-hand brake 4. Place container to receive spillage, and
control valve remove bolt at right- slacken bleeder plug at top rear end of
hand linkage levers and mounting left brake unit. When fluid is clear of air,
bracket. For left-hand valve, retighten bleeder plug.
remove bolt at left-hand linkage
5. Repeat step 4 for right wheel brake unit.
levers and mounting bracket.
6. Release parking brake.
5. Remove four bolts securing brake control
7. Set DC MASTER switch and EXTERNAL/
valve to flight compartment floor, and
BATTERY switch to OFF. Disconnect
move valve outboard to disengage valve
external power.
lever from spring torsion rod (long).
8. Check hydraulic reservoir fluid level.
Install Brake Control Valve
1. Engage valve lever on spring torsion rod
(long), and position brake control valve on
flight compartment floor. Install four bolts
securing valve to floor.
2. Engage spring torsion rod (short) with valve
lever. Position control linkage levers at

FOR TRAINING PURPOSES ONLY 32-43

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32 LANDING GEAR

Figure 32-29. Park Brake Handle Operation

32-44 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
32-40-41 PARKING BRAKE NOTES

GENERAL
A push-pull type parking brake handle is located
on a pedestal between the pilot rudder pedals.
The handle has restricted movement on the
parking brake rod assembly and is spring-loaded
to the off position. The rod assembly is connected
to a forked lever fitted with two adjustable stops.
These stops ride in machined portions of the
brake control valves torsion tube levers, and
when the brake pedals are fully depressed and
the parking brake applied, the forked lever holds
the torsion tube levers and brake control valves
in brakes on position. To release the brakes,
pressure on the brake pedals will allow the spring
loading of the parking handle to push the forked
lever clear of the torque levers.

OPERATION
The brakes can be locked in the applied position
by applying the brakes and then pulling the
PARKING BRAKE handle (Figure 32-29).
This will mechanically lock the brake linkage in
the applied position to hold constant hydraulic
pressure on the brakes. The parking brake may
be released by pressing firmly on the top of the
rudder pedals. The parking brake may only be
released from the left hand pilot position.

NOTE
I t is recommended t hat t he
parking brake handle be held
when the parking brake is
released, to prevent the parking
brake assembly from slamming
back into the off position. Failure
to restrain the handle could
result in damage to the handle
mechanism as it slams forward to
release and potentially damage
the depression in the torque arm
where the linkage locks.

FOR TRAINING PURPOSES ONLY 32-45

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32 LANDING GEAR

Refer to Figure 32-30. Brake System Schematic. Parking brake pressure is limited to 850 psi, which

BRAKE ACCUMULATOR PRESSURE

INDICATOR AND AIR CHARGE VALVE

BRAKE
10
ACCUMULATOR
15
5

0 20 BRAKE
x 1000 RELIEF
1000 VALVE
(1,750 PSI)
2000
PRESS
PSI

BRAKE SYSTEM
PRESSURE
INDICATOR

DH DH

LEGEND PARKING
HYDRAULIC SYSTEM PRESSURE BRAKE

METERED BRAKING PRESSURE

RETURN
BRAKE
VALVES
NITROGEN

MECHANICAL

BRAKE BRAKE
UNIT UNIT

Figure 32-30. Brake System Schematic

32-46 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
is less than the maximum pressure available when NOTES
the brake pedals are fully depressed. The parking
brake is not a substitute for chocks or tiedowns
when the aircraft is parked outdoors.

Pressure for brake operation is normally

supplied by the aircraft hydraulic system
(electrically driven pump or hand pump)
(Figure 32-30). If both sources fail, a check
valve may have retained pressure in the brake
accumulator for brake operation. In this
case, do not “operate” the brakes as brake
accumulator pressure will be rapidly depleted.

FOR TRAINING PURPOSES ONLY 32-47

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
32 LANDING GEAR

Figure 1: Wheel Ski Hydraulic System – Schematic

Figure 32-31. Nose Wheel Steering System

32-48 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
32-50-00 NOSE WHEEL NOTES
STEERING SYSTEM
GENERAL
Refer to Figure 32-31. Nose Wheel Steering
System.

The nose wheel steering system consists of a

hydraulic steering actuator and steering control
system. The steering actuator is mounted on
the nose gear shock strut and is connected by a
link rod to the steering collar and torque brake.
Hydraulic lines from the steering actuator
connect with the main hydraulic system.
Steering control comprises a lever and quadrant,
which operates a cable and pulley system to
rotate a drum and turn a steering valve in the
steering actuator. The steering system control
lever is located on the pilot control column. It
is attached to the left (pilot) yoke and pivots
about the hub of the control lever to transmit
steering inputs through cables to the steering
system. A latch assembly on the upper torque
arm will retain the nose wheel in central position
in event of hydraulic failure during flight. The
latch assembly is disengaged on the ground on
compression of nose gear strut.

Aircraft incorporating Mod 6/1396 embody a

luminous tape nose wheel position indicator
to indicate the nose wheel neutral reference
position. The indicator consists of a luminous
tape strip on the steering control lever hub and
the control column cover.

Aircraft incorporating Mod 6/1478 embody a

nose wheel position indicator light. The light is
secured to the control column cover to illuminate
the indicator. The light is powered from the left
DC bus through the PLT ENG CONS & TRIM
PNL LT circuit breaker and the CONSOLE
FLAP & TRIM PNL LTS dimmer control.

FOR TRAINING PURPOSES ONLY 32-49

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
32 LANDING GEAR

Figure 32-33. Nose Wheel Steering Lever

Figure 32-32. Nose Wheel Steering Cables and Drums

32-50 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
Figure 32-35. Nose Wheel Torque Links and Pip Pin

Figure 32-34. Nose Wheel Steering

FOR TRAINING PURPOSES ONLY 32-51

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32 LANDING GEAR

Figure 32-36. Nose Wheel Steering Cable Tension Graph

Figure 1: Wheel Ski Hydraulic System – Schematic

32-52 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
*The following is an abbreviated description of clamp (jubilee clip). Install quick-release
the maintenance practices and is intended for pin at upper and lower torque arms. Open
training purposes only. nose baggage compartment.
For a more detailed description of the practice,
3. Connect external power to aircraft. Check
refer to the task in the Viking AMM PSM 1-63-2.
wing flaps selector corresponds with
position of wing flaps.
32-50-00 MAINTENANCE 4. Set EXTERNAL/BATTERY switch to
PRACTICES EXTERNAL, DC MASTER switch to
MASTER. Check electric motor-driven
hydraulic pump charges damping
SERVICING accumulator and wheel brakes accumulator,
and that motor cuts out when reading on
Bleed Nose Wheel Steering System pressure indicators is 1575 ± 50 psi (1550
1. Bleeding the nose wheel steering system + 50 or – 0 psi, Pre Mod 6/1570).
need only be accomplished if castoring is
5. Operate nose wheel steering fully left and
not smooth, a hydraulic line or component
right at least five times.
has been replaced, or if the hydraulic
system has been dismantled. 6. Set DC MASTER switch and EXTERNAL/
BATTERY switch to OFF. Disconnect
2. Bleeding can be carried out with the aircraft
external power.
on the ground or in the jacked-up position,
as follows: 7. Check hydraulic reservoir fluid level.
a. Aircraft on ground: 8. If bleeding was carried out with aircraft on
jacks, remove hose damp (jubilee clip) from
Position aircraft nose wheel on greased
latch pin at upper torque arm. Lower aircraft
plates or similar arrangement. Ensure
to ground unless required on jacks for other
parking brake is on, or main wheels are
work. Close nose baggage compartment.
securely chocked. Open nose baggage
compartment. 9. If bleeding was carried out with aircraft on
the ground, remove greased plates. Close
NOTE nose baggage compartment.
Two smooth sheets of steel plate
with grease applied between
them, is a satisfactory platform
for nose wheel steering tests. The
upper plate should turn smoothly
with wheel when steering is
operated without any movement
between the nose wheel tire and
the upper surface of the plate.

b. Aircraft on jacks:
Remove quick-release pin connecting
upper and lower torque arms on shock
strut. Move upper torque arm as required,
push center latch pin against spring so
that latch pin is flush with lower surface
and protrudes the upper surface; secure
latch pin at upper surface using a hose

FOR TRAINING PURPOSES ONLY 32-53

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
32 LANDING GEAR

Figure 32-37. Nose Wheel Steering Actuator Installation

Figure 1: Wheel Ski Hydraulic System – Schematic

32-54 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
32-50-11 NOSE WHEEL NOTE
STEERING ACTUATOR Ultrasonic cleaning equipment
may be used, if available, to
clean filter element.
GENERAL
6. Replace O-ring on filter element is Part No.
The nose wheel steering actuator consists MS28775-010, and O-ring between end cap
of a steering valve, cylinder, piston and rod and housing.
assembly. The actuator is hydraulically operated.
7. Assemble filter element in housing. Install
Movement of the steering control lever in the
and tighten end cap.
flight compartment rotates the drum and changes
the position of the steering valve. This action 8. Replace with new packing. Install packing
directs hydraulic pressure to one side of the and filter to actuator using special tool
piston and rod, moving the piston outboard and T-189. Ensure that direction of flow arrow
rotating the steering linkage and torque links on filter body points towards actuator.
which turn the nose wheel. Internal stops in
9. Install pressure tube assembly.
the steering actuator limit the arc of travel. The
actuator also acts as a shimmy damper during 10. Bleed and function test nose wheel steering
taxiing, take-off, and landing runs. A 10 micron system.
in-line filter at the pressure inlet protects the
internal components of the actuator against
damage from contaminated hydraulic fluid.

*The following is an abbreviated description of

the maintenance practices and is intended for
training purposes only.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2.

32-50-11 MAINTENANCE
PRACTICES
SERVICING
Service Hydraulic Filter
1. Discharge damping accumulator pressure.
2. Place container to receive spillage and
remove pressure tube assembly.
3. Remove filter, using Heroux special tool T-189.
4. Disassemble filter and examine filter
element for damage.
5. Clean filter element by back-flushing with
cleaning solvent and drying with clean
compressed air.

FOR TRAINING PURPOSES ONLY 32-55

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
32 LANDING GEAR

Figure 1: Wheel Ski Hydraulic System – Schematic

Figure 32-38. Test Ports and Schematic Hydraulic Diagram

32-56 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
32-70-00 SUPPLEMENTARY NOTES
GEAR
GENERAL
The supplementary gear consists of a leaf spring
type tail bumper. The tail bumper leaf spring
is installed at the aft end of the rear fuselage,
and is attached to the fuselage structure by
an axle fitting. A rubber bumper pad, bonded
to the center of the spring, together with the
leaf spring, absorbs shock on contact with
the ground. The front end of the leaf spring is
shrouded by a cover.

*The following is an abbreviated description of

the maintenance practices and is intended for
training purposes only.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2.

32-70-00 MAINTENANCE
PRACTICES
INSPECTION/CHECK
Inspection of the Tail Bumper
Inspect the tail bumper for condition and
security.

Figure 32-39. Tail Bumper

FOR TRAINING PURPOSES ONLY 32-57

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32 LANDING GEAR

32-00-00 MAINTENANCE Tires

PRACTICES Inflation pressures for normal operating conditions,
using standard size tires, are 32 psi for nose wheel
tire and 38 psi for main wheel tires. During
GENERAL MAINTENANCE operation at airports where the temperature is
below –20°F (–29°C), inflation pressure for main
PRACTICES wheel tires must be reduced to 34 psi to meet
landing energy absorption requirements.
The maintenance practices found in the
Maintenance Manual usually relate to removal/
Inflation pressures for soft field operating
installation and adjustment/test. The following
conditions, using large size tires (Mod S.O.O.
maintenance practices are applicable to the
6048), are 24 psi for nose wheel tire and 35 psi
landing gear system in a general sense:
for main wheel tires (Mod S.O.O. 6/1574)
•• Do n o t s p i n w h e e l b e a r i n g s w i t h
compressed air. Chapter/Section 12-10-20 of the Maintenance
Manual must be followed for correct inflation.
•• When connecting a hydraulic line to a
fitting, restrain the fitting with a wrench
while torquing the coupling nut. FUNCTIONAL CHECKS
•• Methyl-ethyl-ketone is dangerous. Use
Functional (operational) checks are normally
only in a well-ventilated area, and avoid
performed after maintenance has been
inhalation of fumes.
accomplished on a system. They may also
•• An inflated tire is a potentially dangerous be performed when a system is suspected of
device. Serious injury or death could malfunctioning.
result if a tire is not properly deflated
prior to removal from the airplane. Functional checks are frequently a part of
a maintenance procedure and are not called
•• Wheel and tire assemblies must be
out separately. They sometimes include such
placed in an inflation cage for inflation.
maintenance tasks as final adjustment, torquing,
•• Do not remove the valve core from a and safetying.
tire until the tire has been deflated. The
valve core may eject at high velocity.
FAULT ANALYSIS
•• Do not interchange bolts, washers, and
self-locking nuts between magnesium Isolation of a fault or malfunction can be
and Aluminum alloy wheels. accomplished by a systematic analysis of the
trouble, beginning with the most probable
•• Release the nitrogen precharge from the
cause and progressing to the least probable
nose gear shock strut prior to strut or
cause. Any system(s) interfaced with the
fork removal.
malfunctioning system should be operating
•• Release the nitrogen precharge from the properly prior to troubleshooting.
brake accumulator prior to accumulator
removal.
•• On any wheel found with a cracked
attachment bolt, the bolt on either side
of the failed bolt must also be replaced.
•• On any wheel found with a broken bolt,
replace the bolt. If more than one bolt is
broken all mounting bolts must be replaced.

32-58 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
LIMITATIONS
Good Year Brake Line Wear
With the brake applied, check the distance
between the face of the brake housing and the
brake disc (Figure 32-40).

If the distance exceeds 0.375 inch, the brake

linings must be replaced.

Good Year Disc Guide Lining Wear

The lining on the disc guide must be replaced
when it reaches a thickness of 0.175 inch.

Figure 32-40. Brake Wear Limitations

FOR TRAINING PURPOSES ONLY 32-59

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
32 LANDING GEAR

Figure 1: Main Wheel Ski

Figure 32-41. Main Wheel Ski

32-60 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
32-00-00 LANDING DESCRIPTION
GEAR SPECIAL ORDER 32-11-00 Main Wheel-Ski
OPTIONS Refer to Figure 32-41. Main Wheel Ski.

The main skis are installed on special wheel

WHEEL-SKIS (MOD S.O.O. 6001) axles. Each main gear ski is connected to its
axle by levers, cables, links, and bungee cords,
Wheel skis by optional mod S.O.O. 6001 are
and to three fittings on the lower surface of
manufactured by Bristol Aerospace, nose ski
the wing. Each main wheel ski consists of two
model 3000 and main skis model 5500. The skis,
beams structurally joined at the forward end
controlled by a lever in the flight compartment,
and connected at the rear by the inboard crank
are hydraulically actuated from the aircraft to the
assembly tube.
“ski-landing” or “wheel-landing” positions. The
skis are rigged level in flight and are self trimming
A double acting hydraulic actuator is housed in
in pitch for either the “ski-landing” or “wheel-
the outboard beam of the ski, and is connected
landing” configurations. An indication light that
by its cylinder end to the beam structure. The
shows the ski position is provided, on the same
rod end of the actuator connects to a latch lever
panel incorporating the wheel ski selector lever.
to operate the ski sling, through actuating torque
The skis embody a device for controlling the shaft levers and sling links. A latch pin on the
lateral attitude of the skis, regardless of the amount latch lever penetrates the latch to retain the up
of main gear leg deflection, throughout its range lock when hydraulic power is lost. Hydraulic
of travel from in-flight to maximum landing load lines to the actuator are routed from connections
conditions. This feature applies only when the on the inner side of the ski inboard beam, through
skis are in the “ski-landing” position. A selection the beam and the ski front structure.
to the “wheel-landing” position cancels this
feature and introduces a device for re-trimming Two pedestals, one on each ski beam, form the
the lateral attitude for optimum ground clearance attachment and pivot points for the ski sling. The
during taxiing. Wheel skis installed in place of inboard and outboard crank assemblies, which
conventional wheels increase the aircraft empty connect to the wheel axle inboard and outboard
weight by approximately 660 pounds. lugs, are interconnected within the outboard
beam by two sets of links. The center of the two
When main and nose wheel skis are fitted the skis
can be extended to permit operation on unprepared
snow or ice surfaces, or retracted to permit normal
wheel operation. Although the wheel ski installation
is removable, certain associated components must
remain permanently installed in the aircraft. These
include, the hydraulic piping in the fuselage and
on the main landing gear legs, which provide
hydraulic fluid from the aircraft hydraulic system
to operate the skis, the electrical cables, to provide
ski position indication, a ski position selector panel,
installed in the flight compartment, and special main
wheel axles. At the time of installation, the normal
airspeed indicator and flight limitation placard
must be replaced by an airspeed indicator and flight
limitation placard having markings and limitations
appropriate to a wheel-skiplane.

Figure 32-42. Main Wheel Skis

FOR TRAINING PURPOSES ONLY 32-61

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32 LANDING GEAR

Figure 32-43. Nose Wheel Ski

Fig ure 1: Steering Collar - Removal/Installation

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32 LANDING GEAR
sets of links is operated by a connecting rod and right beams. The rod end of the actuator connects
crankshaft assembly, which in turn is operated by to a latch lever attached to the end of a sling
a shaft connected to the ski sling. This mechanism actuating torque shaft having two arms within
allows freedom of vertical and pitch motion, but the ski tunnel.
controls the lateral angle of the ski, which is
changed from level in the ski down configuration The two arms are connected to either side of a
to an anhedral attitude when retracted. sling by actuating links. A pedestal on each beam
provides the pivot points for the ski sling and
A torsion bar, mounted transversely between the forms the mounting for the pivot blocks which
beams at the rear of the ski, is secured in a socket attach the ski to the nose gear axle splined arms.
plate attached to the inboard ski beam, and pivots
in a pivot bearing attached to the outboard ski Two hydraulic connections are provided, one on
beam. A lever, secured to the outboard end of the inner side of each beam. The right-hand beam
the torsion bar is connected to the upper arm of hydraulic pipe is routed through the right beam
the outboard wheel axle lug by a link assembly and the front section of the ski into the left beam,
and to an eye bolt at the trailing edge of the to the “down” side of the hydraulic actuator.
outboard ski beam by a pretensioning cable. The left-hand hydraulic pipe is routed within
A cable interconnecting the ski sling actuating the left beam to the “up” side of the actuator.
mechanism and the inboard crank assembly Flexible hydraulic hoses are installed between
lever, provides the means of raising the ski for a the connections on each side of the ski to pipe
“wheel-landing”. connections on the respective sides of the nose
wheel shock strut fork. Two trim cables, one
A pitch down limit link and cable is connected connected to each side of the ski sling, are routed
between an eye bolt at the front of the ski and a over pulleys to their respective rear left and right
center fitting on the undersurface of the wing. telescoping shock units. The four telescoping
Two cables, connected together, are connected shock units, two connected to eye bolts at the
between an eyebolt at the rear of the ski, and the front of the ski and two at the rear, connect to
center fitting on the undersurface of the wing, attachment brackets on the nose wheel shock
to limit the pitch up angle of the ski in flight. A strut fork. Each shock strut unit consists of a
cable interconnecting the aft limit cable is routed telescoping unit provided with positive stops to
through a pulley and fairlead to the ski sling. limit the maximum pitch attitudes of the ski, on
which are mounted endless shock cord (bungee)
Two limit microswitches are installed within the rings, to restrain the ski in the neutral position
outboard ski beam, both operated by the latch and to provide a means of retracting the ski when
lever in its extremes of travel, to provide ski the sling is actuated.
position indication.
A ski position electrical cable connector on the
left ski beam and is wired internally to the two
32-21-00 Nose Wheel-Ski limit microswitches operated by the latch lever.
Refer to Figure 32-43. Nose Wheel Ski.

The nose ski is attached to the wheel axle by two

splined arms and to the wheel fork by two pairs of
telescoping shock units. When installing the nose
ski the torque loading on the shimmy damper at
the steering collar must be increased, and when
the skis are removed the torque must be reduced.

The nose wheel ski consists of two beams

structurally joined at the front, and connected at
the rear by a torque tube secured to the left and

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32 LANDING GEAR

Figure 32-44. Main Wheel Ski - Installation (Sheet 1 of 2)

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32 LANDING GEAR
OPERATION crank assembly levers, which are of a different
length, compensate for landing gear and tire
When a “ski-landing” is selected, the actuator deflection.
retracts, withdrawing the latch-pin from the latch
spring, unlocking the latch lever and, through the When the latch lever reaches the limit of its
connecting linkage, moves the sling down under travel, a plunger on the end of the latch lever
the wheel, in doing so the sling impinges on the engages in a stop plate to lock the ski up. The
tire and forces the wheel to roll over the top of two pairs of telescoping shock units each serve
it so moving the ski downward into the extended two purposes.
position.
The forward pair, by means of the shock cords,
Simultaneously, as the sling moves rearward, exert a restoring force to oppose external
the main ski raising cable, connected between pitch down forces arising from aerodynamic
the inboard crank assembly lever and the sling maneuvering or handling loads, and by means of
actuating linkage, is slackened progressively with the internal stops, restrict the pitch down angle
sling movement. On the nose ski the two trim of the ski. The rear pair, by means of the shock
cables slacken off, thus relieving the tension on cords, predetermine (after adjustment) the level
the rear trim units. trim attitude of the ski in fight, and through the
trim cables, allow sufficient pitch up travel to
On selecting “wheel-landing”, the actuator accommodate surface irregularities encountered
extends moving the sling up and forward, on snow or ice when selected to “ski-landing”.
allowing the wheel to roll from it. The main skis
engages the latch, simultaneously the actuating The pitch up angle of the ski is also restricted by
linkage operates the ski raising cable, which lifts the rear shock unit internal stops.
the ski. At the same time the nose ski trim cables,
attached to the sling, tension the rear shock units.

The Main gear skis pitch down angle is restricted

by the limit link and cable. The restoring force
required to counteract external pitch down
loads is provided by the pre-loaded torsion bar,
geometrically linked to the landing gear leg, to
yield equal forces in both the “wheel-landing”
and “ski-landing” configurations, regardless of
landing gear leg deflection.

The pitch up angle of the ski in flight is fixed by

the cables connecting the aft end of the ski to the
center undersurface wing fitting. The restoring
force, required to counteract external pitch up
loads while taxiing on wheels, or skis, is provided
by the spring loaded link connected between
the torsion bar lever and the upper arm of the
outboard wheel axle lug.

To compensate for the change in levels between

the “ski-landing” and “wheel-landing” positions
of the ski, the trim cable interconnecting the
two aft limit cables and the ski sling takes up
slack in the aft limit cables during the transition
between positions. The inboard and outboard

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32 LANDING GEAR

Figure 32-45. Main Wheel Ski - Installation (Sheet 2 of 2)

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32 LANDING GEAR
*The following is an abbreviated description of
the maintenance practices and is intended for
training purposes only.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2.

32-00-00 MAINTENANCE
PRACTICES
SERVICING
Main Wheel Ski Lubrication
NOTE
The lubrication points, method, and
frequency of application are given
in the Main Wheel Ski Lubrication
Diagram (refer to Figure 32-46).

PARTS NOMENCLATURE KEY

1. CABLE PULLEYS
2. SLING ACTUATING TORQUE LINK BEARINGS
3. TORQUE LINK ARM ATTACHMENTS
4. SLING LINK ATTACHMENTS
5. MAIN PEDESTAL BEARINGS
6. LIMITING LINKS
7. SKI TORQUE LINK BEARING BLOCKS
8. AXLE BUSHINGS

Figure 32-46. Lubrication Diagram - Wheel Skis

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32 LANDING GEAR

Fig ure 1: Steering Collar - Removal/Installation

Figure 32-47. Nose Wheel Ski Installation

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32 LANDING GEAR
Nose Wheel Ski Lubrication 6. Adjust tire clearance, if necessary, by removing
or adding shims under base of latch spring
NOTE (removing shims increases tire clearance).
The lubrication points, method
and frequency of application are
NOTE
given in the Nose Wheel Ski Adjusting fork end of hydraulic
Lubrication diagram, contained actuator will not alter tire
in PSM 1-63-2 ATA 12-20-10. clearance and will only lead to
erratic or faulty latch function.
If, however, shims have been
ADJUSTMENT/TEST added or removed, adjustment
of hydraulic actuator is also
NOTE required as in step 7.
The access panels form part of
7. Adjust fork end of hydraulic actuator to
the stressed structure of the ski
give 0.015 inch to 0.060 inch overtravel
and must be reinstalled as soon
after latch engagement.
as adjustments are completed.
8. Check adjustment of Ski Up and Ski Down
limit switches.
Adjust Sling Actuating Mechanism
1. Jack aircraft. INSPECTION/CHECK
2. Remove forward left-hand panel.
Check Nose Wheel Ski Rigging
3. Select ski up position and observe behavior
1. Remove fuselage to leg fairings, jack
of ski and latch mechanism.
aircraft well clear of ground using inboard
4. If latch pin does not engage locking hole of jacking points and nose jacking beam
latch spring, disconnect and adjust fork end SD12538. Level aircraft.
of hydraulic actuator until latch pin engages
2. Switch on electrical power.
locking hole.
3. Ensure SKI POSITION INDICATOR and
5. If latch pin engages locking hole of latch
HYD OIL PUMP circuit breakers are engaged.
spring, release hydraulic pressure and
check that clearance between tire and sling
is between 0.10 inch and 0.60 inch.
NOTE
Observe that motor pump
NOTE charges accumulators and that
motor cuts out when pressure
Tire clearance is adjusted
reaches 1575 ± 50 psi (1550 +
during manufacture and
50 – 0 psi, Pre Mod 6/1570).
assembly, and should not be
affected appreciably in normal
4. Select ski selector lever to UP. Check ski is
service or by the replacement
level with aircraft datum +2° –0°, and that
of related components. Ensure
center bolts at sling pivot are at end of slot
that inadequate tire clearance
in slotted link.
has not been caused by damage
to the latch mechanism or ski 5. S e l e c t s k i s e l e c t o r t o D O W N . M a r k
bottom in area of latch assembly the forward telescoping shock unit
before attempting tire clearance extension rods. Tilt ski fully nose down
adjustment. and measure between the mark and new
position of extension rods. This overtravel
measurement should be 1/2 inch.

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32 LANDING GEAR

Figure 32-48. Nose Wheel Installation

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32 LANDING GEAR
NOTE NOTES
If an adjustment is required to
achieve the 1/2 inch overtravel,
adjust in accordance with ski
installation procedures.

6. Check that neither of the aft telescoping

shock units are pulled from their minimum
length.
7. Select ski selector lever to UP and check
that sling trim cables position ski level with
aircraft datum +2° –0°.
8. At steering collar check, by applying spring
balance to end of torque link, with steering
actuator disconnected, that torque required
to turn steering collar is between 800 and
1000 pound-inches or 70 to 90 pounds
at end of torque link. Reconnect steering
actuator.

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32 LANDING GEAR

Figure 32-49. Ski Selector Panel

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32 LANDING GEAR
32-61-00 WHEEL-SKI NOTES
INDICATION
GENERAL
Refer to:

•• Figure 32-49. Ski Selector Panel.

•• Figure 32-50. Ski Position Indication
Electrical Schematic - Up Position.
•• Figure 32-51. Ski Position Indication
Electrical Schematic - Down Position.

Wheel ski position indication is provided by

a group of six lights mounted on the selector
lever panel which are controlled by limit
switches on each ski assembly. The three upper
lights indicate skis “up” and the lower three
skis “down”. Each pair of lights are located
on the panel in relation to their respective
ski position (for example, the two center
lights provide nose ski indication). Two limit
switches installed on each ski are actuated by
the ski latch lever, one when the ski is “down”
and the other when the ski is “up”. The power
supply for the indicator lights is derived from
the 28 volt left DC busbar, through a 5-ampere
SKI POSITION INDICATION circuit breaker
located on the radio circuit breaker panel, and
a relay operated by the CAUTION LT dimmer
switch. When the skis are operated, the relevant
position limit switches are actuated to provide
a ground and complete the electrical circuit for
their associated lights. When the CAUTION LT
dimmer switch is selected to DIM, the relay
changes over to connect a resistance in series
with the lights, which are connected in parallel.

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32 LANDING GEAR

270 Ω 2W RIGHT SKI

SKI POSITION
INDICATION UP

28V DC 5A B
DIMMING CONTROL
D COMM
RELAY CONTACT
A
DOWN
C
UP NOSE

UP LEFT UP RIGHT
NOSE SKI

C
UP

D COMM

A
DOWN

DN LEFT DN RIGHT

LEFT SKI

DN NOSE

C
UP

D COMM

A
DOWN

NOTE: SWITCHES SHOWN WITH SKI’S IN UP POSITION

Figure 32-50. Ski Position Indication Electrical Schematic - Up Position

Revision 0.4
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32 LANDING GEAR
270 Ω 2W RIGHT SKI
SKI POSITION
INDICATION
UP
28V DC 5A B
DIMMING CONTROL
D COMM
RELAY CONTACT
A
DOWN
C
UP NOSE

UP LEFT UP RIGHT
NOSE SKI

UP
B

D COMM

A
DOWN

DN LEFT DN RIGHT

LEFT SKI

DN NOSE

UP
B

D COMM

A
DOWN

NOTE: SWITCHES SHOWN WITH SKI’S IN DOWN POSITION

Figure 32-51. Ski Position Indication Electrical Schematic - Down Position

Revision 0.4
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32 LANDING GEAR

Figure 32-52. Wheel Ski Hydraulic System - Schematic

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32 LANDING GEAR
32-30-00 EXTENSION NOTES
AND RETRACTION
(WHEEL-SKI)
GENERAL
Refer to Figure 32-52. Wheel Ski Hydraulic
System - Schematic.

Extension and retraction of the wheel skis

is accomplished by the aircraft’s hydraulic
system, to either the “up” or “down” side of
the wheel ski hydraulic actuators. The direction
of system fluid flow, and the return of the
displaced fluid, is determined by the position of
a four-way selector valve. The selector valve,
mounted on a bracket secured to the bulkhead
at fuselage station 60.00, has a lever which is
linked by a push rod assembly to a selector
lever located on a panel secured to the center
pedestal, below the pilot’s instrument panel.
The manually controlled lever, which protrudes
through a slotted plate in the panel, has two
positions marked UP and DOWN.

The bracket which supports the lever and panel

incorporates two adjustable stops to limit the
extent of lever movement in both directions.
The panel also includes indicator lights to
provide ski position indication.

Revision 0.4
FOR TRAINING PURPOSES ONLY 32-77
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32 LANDING GEAR

Figure 1: Wheel Ski Hydraulic System – Schematic

Figure 32-53. Ski Selector - Rigging

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32 LANDING GEAR
*The following is an abbreviated description of 10. Operate selector lever and ensure that valve
the maintenance practices and is intended for travel is limited by the selector lever stops
training purposes only. and not the valve internal stops.
For a more detailed description of the practice, 11. Functionally test wheel ski system.
refer to the task in the Viking AMM PSM 1-63-2.

32-30-00 MAINTENANCE
PRACTICES
ADJUSTMENT/TEST
Rig Wheel Ski Selector Control
Refer to Figure 32-53. Ski Selector - Rigging.

1. Ensure selector control valve is correctly

located with CYL 1 marking on valve body
facing forward.
2. Check valve lever is correctly installed
on valve shaft (that is with lever pointing
upward) and that it is secured by washers and
a screw which is wire-locked to the lever.
3. If connected, disconnect push rod assembly
between valve and selector levers, at
selector lever.
4. Rotate valve lever counter-clockwise (as
viewed from shaft end of valve) until valve
internal stop is contacted.
5. At rear of ski selector panel, screw UP and
DOWN stop screws fully out. Set selector
lever at UP.
6. Adjust push rod assembly and connect
between selector and valve levers. Use
washers and cotter pins to secure flat-
headed pins.
7. Operate lever to UP and DOWN several
times, check control for freedom of
movement and that valve internal stops are
contacted at both extremes of travel.
8. With the selector lever at UP adjust the UP
stop screw to contact selector lever then
screw in one turn.
9. Place selector lever at DOWN, adjust the
DOWN stop screw to contact selector lever,
then screw in one turn.

FOR TRAINING PURPOSES ONLY 32-79

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32 LANDING GEAR

Figure 32-54. Limit Switch Adjustment

Revision 0.4
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32 LANDING GEAR
32-61-00 MAINTENANCE Adjust Ski Down Limit Switch
PRACTICES Refer to Figure 32-54. Limit Switch
Adjustment.

ADJUSTMENT/TEST 1. Remove forward access panel.

2. Loosen microswitch mounting screws.
NOTE 3. Actuate ski to ski down position.
The access panels from part of
4. Move switch and actuator toward latch
the stressed structure of the ski
lever arm until an audible click indicates
and must be reinstalled as soon
that switch is actuated.
as adjustments are completed.
5. Final adjustment of switch must result in
actuation of switch when torque shaft arms
Adjust Ski Up Limit Switch are within 0.2 inch of their stops.
Refer to Figure 32-54. Limit Switch
Adjustment. CAUTION
1. Remove forward access panel. Check that FAILURE TO COMPLY WITH
ski is in ski-up position and that latch pin THE 0.2 INCH ADJUSTMENT
is fully extended and latched. REQUIREMENT COULD
RESULT IN PREMATURE
2. Loosen up-lock microswitch mounting
OPERATION OF SKI
screws.
POSITION INDICATOR LIGHT
3. Move switch toward latch pin until an (BEFORE SKI HAS REACHED
audible click indicates that switch is THE GEOMETRIC LOCK).
actuated. Move switch an additional 1/32
inch approximately (to provide overtravel 6. Function check with continuity tester if skis
for consistent switch actuation in service) are off aircraft. Function check by means
but ensure that latch pin does not come in of indicator lights in flight compartment if
direct contact with switch body. skis are on aircraft.
4. Tighten switch mounting screws. 7. Reinstall access panel.
5. Using continuity tester, function check
switch actuation by operating the ski
hydraulically in and out of lock. If ski is
installed on aircraft, check indicator lights
in flight compartment. Ski UP light should
come on when latch pin engages locking
hole and light should go out when latch pin
comes out of lock.
6. Reinstall access panel.

Revision 0.4
FOR TRAINING PURPOSES ONLY 32-81
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32 LANDING GEAR

Figure 1: Main Gear Spring Ski Installation (Sheet 1 of 2)

Figure 32-55. Main Gear Spring Ski Installation (Sheet 1 of 2)

32-82 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
32-12-00 MAIN GEAR NOTES
SPRING SKIS (MOD
S.O.O. 6116)
GENERAL
Refer to:

•• Figure 32-55. Main Gear Spring Ski

Installation (Sheet 1 of 2).
•• Figure 32-56. Main Gear Spring Ski
Installation (Sheet 2 of 2).

Each main spring ski assembly consists

essentially of a semi-elliptical seven-leaf spring,
a ski, and a harness assembly. The leaf spring,
which is the shock absorbing attachment medium,
is secured to the main landing gear leg wheel axle
by means of a clamp assembly, incorporating a
sleeve and bush assembly. The clamp slides
onto the axle and is secured by two washers
and a ski retaining nut; the retaining nut being
locked to the axle by two bolts with washers and
nuts. The front end of the leaf spring is bolted
through bushes between the upper end of two
pivot shackles; the lower end of the shackles are
bolted through a shackle distance tube located
in the ski main beam. These shackles allow for
spring deflection during landing, and when the
ski is on the ground. The rear end of the leaf
spring is bolted through bushes to a mounting
bracket secured to the ski beam.

Each ski is restrained in flight by front and rear

bungee loaded cables of the harness assembly
which is attached to a fitting on the underside of
its associated wing, and to eyebolts connecting
to the front and rear of the ski. Short check
cables, attached parallel to the front and rear
bungee loops, limit ski movement, and also act
as safety cables in the event of a bungee loop
breaking. To prevent the rear check cable from
chafing the rear bungee loop, the center of the
check cable is secured by plastic lacing to the
center of the bungee loop.

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32 LANDING GEAR

Figure 1: Main Gear Spring Ski Installation (Sheet 1 of 2)

Figure 32-56. Main Gear Spring Ski Installation (Sheet 2 of 2)

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32 LANDING GEAR
PAGE INTENTIONALLY LEFT BLANK

FOR TRAINING PURPOSES ONLY 32-85

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32 LANDING GEAR

Figure 32-57. Nose Gear Spring Ski (Mod S.O.O. 6116) (Sheet 1 of 2)

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32 LANDING GEAR
32-22-00 NOSEGEAR through bushes in the clamp, and with a spacer
positioned on the axle at either side of the clamp,
SPRING SKI the axle is installed as for a normal nose wheel
installation. The front end of the leaf spring is
bolted through bushes between two brackets
GENERAL riveted to the ski base. The rear end of the spring
is bolted, through spacers and bushes, between
Refer to:
the two longitudinal stringers on the ski.
•• Figure 32-57. Nose Gear Spring Ski
The ski retaining harness consists of two rear
(Mod S.O.O. 6116) (Sheet 1 of 2).
cables and two front cables. The rear cables
•• Figure 32-58. Nose Gear Spring Ski are each attached at one end to one side of the
(Sheet 2 of 2). leaf spring rear attachment points, and at the
other end to a bracket on the nose leg fork. The
The nose gear spring ski assembly consists front cables are each attached at one end to the
essentially of a semi-elliptical six-leaf spring, bracket on the nose leg fork, and at the other
a ski, and a harness assembly. As with the main end to ail eyebolt at the front of the ski. The
gear spring skis, the leaf spring is the shock front cables each incorporate a bungee section
absorbing and attachment medium. With the nose in parallel with a check cable.
wheel removed, the nose wheel axle is inserted

Figure 32-58. Nose Gear Spring Ski (Sheet 2 of 2)

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32 LANDING GEAR

Figure 32-59. Float Installation

32-88 FOR TRAINING PURPOSES ONLY

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32 LANDING GEAR
32-61-00 AIRCRAFT Although wing fences are standard equipment
on all Series 300 aircraft, earlier Series 100/200
FLOATS aircraft are required to have wing fences
installed when equipped with floats.
GENERAL Straight CAP floats and associated equipment
installed in place of conventional wheels
As part of the float installation for the DHC-6
increases the aircraft empty weight by
Twin Otter turbo prop aircraft originally
approximately 812 pounds.
manufactured by de Havilland Canada and
currently produced by Viking Air, the following
changes are made to the landplane:

1. The hydraulic landing gear retraction

system components and cockpit controls
are added (Amphibian float models only).
2. T h e l a n d i n g g e a r e m e r g e n c y g e a r
operation hand pump and system are added
(Amphibian float models only).
3. Auxiliary vertical fins are added.

STANDARD FLOATS
Straight floats, manufactured by Canadian
Aircraft Products (CAP), are available for
Series 300 aircraft by opt mod S.O.O. 6082
and by optional mod S.O.O. 6002 for series
Figure 32-60. Aircraft Take-off Dolly (1 of 2)
100/200 aircraft (Figure 32-59). All aircraft with
CAP floats must operate with the short nose
configuration. The main and rear struts between
the fuselage and floats are enclosed in fairings.
The forward and rear spreader bars maintain
float alignment. Beaching gear can be attached
to the floats to remove the aircraft from water.

Many additional changes are required when

floats are installed. All aircraft must have upper
and lower finlets installed on the horizontal
stabilizer surfaces to improve lateral stability of
the aircraft including a feel spring installed in the
elevator control system to provide the required
control characteristics. This spring must be
removed if the aircraft is later reconfigured as
a landplane. Float installation also requires the
addition of an airspeed indicator with different
range markings. An additional stall bar is also
installed on the right wing. Propeller zero pitch
Figure 32-61. Aircraft Take-off Dolly (2 of 2)
blade latches by optional mod S.O.O. 6022 must
be installed for float operation.

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32 LANDING GEAR

Figure 32-62. Amphibious Floats

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32 LANDING GEAR
AMPHIBIOUS FLOATS
A new amphibious float configuration by
Wipaire Inc. has been certified for Series
300 aircraft (Figure 32-62). Wipaire Inc. also
manufactures straight floats for Series 300
aircraft. The long nose baggage compartment
may be retained when Wipline floats are
installed. Both Wipline Amphibious 13000
and Standard float models have STC approval.
Figure 32-63. Straight Floats
Wipline amphibious floats and associated
equipment installed in place of conventional
wheels increases the aircraft empty weight by
approximately 2075 pounds. Wipline straight JURY STRUT
floats increase the aircraft empty weight by
approximately 1400 pounds. A jury strut is provided for use when the aircraft
is being loaded or unloaded. The jury strut clips
The model 13000 seaplane or amphibious on to the lower aft fuselage and will prevent the
float is an all aluminum-constructed float aircraft from settling onto the tail bumper if the
with watertight compartments. The actual center of gravity is temporarily aft of allowable
displacement in fresh water for each float is limits. The jury strut is normally stowed in the
12844 pounds buoyancy for the seaplane and rear baggage compartment when not in use.
12442 pounds buoyancy for the amphibian. The
amphibian float is geometrically the same as the Certain conditions encountered during loading
seaplane except for the addition of landing gear. and unloading may cause the aircraft to tip
backwards. If the cabin and nose baggage
The main landing gear has dual 8:50 x 10 8-ply compartment are empty, the rear baggage
tires and the nose landing gear has one 6.00 x compartment is full, the aft fuel tank is full,
6 8-ply tire. The gear system is hydraulically and heavy freight is then loaded into the cabin
actuated and driven by two hydraulic pumps. through the back door, the aircraft may tip
Brakes are hydraulic and have a caliper on each backwards. To avoid this, unload the rear
main wheel for a total of four brakes. baggage compartment before the nose baggage
compartment if the rear compartment is fully
Steering on land is accomplished by differential loaded, and have the refueller add fuel to the
braking. The nose wheels are full castering. forward tank before filling the aft tank.

Access to the float interior is accomplished by When operating with Intermediate Flotation
removing covers on the top deck and six covers Gear with opt mod S.O.O. 6048 a longer jury
inside the wheel well. When necessary, water strut will be required to accommodate the
inside the float hulls may be removed through increased tail height due to the larger tires
pump-out cups located on the outboard edge of installed on the main wheels.
each float top skin.

FOR TRAINING PURPOSES ONLY 32-91

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
32 LANDING GEAR

LEGEND

1. Hand Pump
2. Support Outside
3. Cover
Rivnut-Attach
Screw-Attach
4. Placard
5. Angle-Lower Support
6. Bolt-Attach
Washer
Plate Nut
7. Reducer-Pressure
“O” Ring
8. Reducer-Return
“O” Ring
9. Assy Hose
10. Support-Inside
11. Bolt-Attach
Washer
Washer-Optional
(May be used in Place of Item 5)

9
10

2
8

11

5
6

Figure 32-64. Hydraulic Hand Pump

Revision 0.4
32-92 FOR TRAINING PURPOSES ONLY
 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

32 LANDING GEAR
AMPHIBIAN LANDING GEAR NOTES
SYSTEM
The landing gear incorporated within the
amphibious floats on this airplane is retractable,
quadricycle type with two swiveling nose (or
bow) wheels and four (4) (two (2) sets of dual)
main wheels. Air-oil shock struts on the two
main landing gear assemblies provide shock
absorption.

The main landing gear has dual 8:50 x 10 8-ply

tires and the nose landing gear has one 6:00 x
6 8-ply tire. The gear system is hydraulically
actuated and driven by two hydraulic pumps.
Brakes are hydraulic and have a caliper on each
main wheel for a total of four brakes.

Steering on land is accomplished by differential

braking. The nose wheels are full castering.

Emergency Hand Pump

Refer to Figure 32-64. Hydraulic Hand Pump.

An emergency hand pump is located on the

floor to the left side of the seat for use in the
event the normal hydraulic system fails. This
hand pump may be used to retract or extend the
land gear. Prior to utilizing the emergency hand
pump, pull the AMPHIB PUMP 1 and 2 circuit
breakers to deactivate the electric hydraulic
pumps. Select UP and DOWN using the normal
landing gear selector handle. Pump the attached
handle back and forth (approximately 400
cycles). When a gear reaches the selected
position, its indicator light will illuminate.
After all four (4) gears are in the selected
position there is a noted increase in resistance
of hand pump operation.

FOR TRAINING PURPOSES ONLY 32-93

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
32 LANDING GEAR

FLOAT INSTALLATION & support the retraction and extension, on

pump per side.
REMOVAL
6. Install flying wires to keep the floats in
Removal their correct position.
This is the removal procedure of the nose and 7. Connect the hydraulic lines to the gear
main landing gear. retraction and extension pressure side of
the pumps.
1. Remove all fairings and plates around nose
8. Bleed both brakes after they have been
and main gear.
connected and checked.
2. Position three jacks, one behind the nose
The regular floats are installed same as the
gear and the others on main jack points on
above amphibious except they use a trailer to
upper platen.
move the floats in position under the plane to
3. Jack all tires clear off the ground by at least be installed.
a foot.
4. Position lifting slings on the four wing
bolts with their pick-up points, use cables
strong enough to support 12,500lbs.
5. Lift aircraft clear of jacks with a crane big
enough to support twice the aircraft gross
weight.
6. Remove nose gear steering and attachment
point from station 60.
7. Remove main gear legs, compression and
rebound blocks from station 239 and 219,
be careful not to let the gear legs drop on
the ground; disconnect brake hydraulic lines
and cap off fittings on the fuselage side.

Installation
The following steps are for installation of the
Wipair 13000 amphibious floats.

1. The floats can be positioned under the

aircraft as they can be moved around on
their tires.
2. The floats are secured together by spread
bars and flying wires.
3. Attach the floats to the front support bracket
of the sloping bulkhead at station 110.
4. Attach main float pick-up struts which are
connected to the fuselage at station 239 using
bolts after they have been lightly greased.
5. Install nose gear bracket in place, where
the two hydraulic pumps are located, they

32-94 FOR TRAINING PURPOSES ONLY

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

32 LANDING GEAR
PAGE INTENTIONALLY LEFT BLANK

FOR TRAINING PURPOSES ONLY 32-95

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
32 LANDING GEAR

Figure 32-65. Hydraulic System Schematic

32-96 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

32 LANDING GEAR
DESCRIPTION AND OPERATION Shock absorption for the main landing gear
is provided by a hydraulically dampened
Retraction and extension of the main and nose air spring. The oil and air share a common
landing gear is effected by a hydraulic actuation chamber. When the oleo is collapsed, the oil is
system shown schematically in Figure 32-65. forced through the main orifice, compressing
the air in the upper cylinder. Extension reverses
The gear system is hydraulically actuated and this process. The extended oleo is initially set
driven by two hydraulic pumps located on the at the factory to 250 psi no load.
fuselage bulkhead 60.00.
The nose gear has an over-center down lock.
A pressure of between 525 and 1250 psi Retraction occurs when pressure is applied to
is maintained in the supply line. When the the forward face of the actuator piston and the
pressure falls below 525 psi, the pressure carriage is drawn along the tracks in the nose
switch activates the pump solenoid, providing box. Gear position light proximity switches are
power to the pump. When the pressure reaches closed when the piston containing the magnetic
1150-1250 psi, the pressure switch deactivates material has reached either end of its travel.
the solenoid and the pump motor stops. Figure
32-65 shows the electrical schematic of the The nose gear consists of composite fiberglass
system. A check valve on the output side of beams that are attached at the bottom to castering
the pump retains pressure in the system while blocks. Inside the block is a castering pin that
the pump is off. The pump has an internal is set into the machined fork assembly. The
relief valve, which directs oil back to the pump castering pin allows the nose wheel to pivot in
reservoir when the line pressure exceeds 1450 a complete circle. The geometry is such that
psi. The system also has an internal relief valve no shimmy dampers are necessary. A spring
to protect against thermal expansion when line loaded ball rides in a groove machined in the
pressure exceeds 2000 psi. castering pin. This groove is a round pocket
on the back face with the result that the cam
A cockpit mounted control valve accomplishes provides retention of the pin the block and self-
the selection of gear up or gear down. Each centering of the wheel. A thrust bearing is on top
float gear has individual indicator lights on the of the castering pin, along with a lower bearing.
control valve allowing the pilot to confirm that
each gear has fully retracted or extended.

An emergency hand pump is provided, in case

of total electric pump failure, or loss of fluid.
The reservoir has additional hydraulic fluid,
available only to the hand pump.

The main gear is mechanically locked in both

up and down positions. Locking and unlocking
is effected utilizing a small amount of lost
motion of the actuator rod. Retraction takes
place when pressure is exerted on the actuator
piston driving the collar along the slide tube.
The lock is tripped when the follower slides
up the contoured track in the actuator. A
reverse process affects extension. Gear position
light proximity switches are closed when the
appropriate hook (containing the magnetic
material) nests over the locking bar.

FOR TRAINING PURPOSES ONLY 32-97

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
32 LANDING GEAR

OPERATION Problem - Powerpack Shuts off

TROUBLESHOOTING Before Gear Reaches Position
Problem - Power Pack Does not Probable Cause
Run After Gear Selection a. Binding or jammed gear retractor,
which causes pressure to build up (and
Probable Cause stay up), and pressure switch shuts off
powerpack.
a. Circuit breaker has failed
b. Pressure switch not pulling in at low Remedy
cut in.
a. Repair retractor.
c. Solenoid switch not pulling in.
d. Faulty pump motor.
Problem - Powerpack Cycles on
e. Motor not properly grounded.
and off After Gear is in Position
Probable Cause
Verification and Remedy
a. Internal hydraulic leak.
a. Reset circuit breaker.
b. External hydraulic leak.
b. Short across pressure switch leads and
see if motor runs. If motor operates,
replace pressure switch.
Remedy
a. Verify leak is not external by checking
c. Short across solenoid pressure switch
fluid level in reservoir and looking at
leads and see if motor runs. If motor
couplings for oil leaks. If no external
operates, replace solenoid pressure
leaks are found, disconnect and cap off the
switch.
hydraulic actuators one at a time and find
d. If c. above does not produce results and the leaky one by process of elimination.
it is verified that voltage was actually If isolating entire system still indicates
applied to motor, it can be assumed internal leak, power pack check valve
motor is bad or not properly grounded. (located in pressure port of pump) is bad
and needs replacement or reseating.
e. Check motor ground.
b. Visually inspect lines, cylinders, and
Problem - Power Pack Does not hoses and replace as necessary.
Shut Off After Gear Reaches Position Problem - Power Pack Cycles on
Probable Cause and off During Gear Cycle
a. Faulty pressure switch.
Probable Cause
b. Faulty or dirty pressure relief valve
a. Binding in retraction unit.
allowing insufficient pressure buildup.
b. Pressure switch cut off limit too low.
Remedy
Remedy
a. Replace pressure switch.
a. Investigate for free operation. Check
b. Clean and check relief valve.
gear that retracts last.
b. Replace pressure switch.

Revision 0.4
32-98 FOR TRAINING PURPOSES ONLY
 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

32 LANDING GEAR
Problem - Slow Gear Operation NOTES
Cycle (Considerably Longer than
30 seconds)
Probable Cause
a. Plugged oil screen.
b. Poor electrical connection to motor.
c. Poor motor.
d. Worn pump gears.

Remedy
a. Clean intake screen located inside
reservoir tank.
b. Connect motor direct to 24 volt source
and note its operation; if good, wire
connection is bad; if operation poor,
motor needs overhaul.
c. Covered in b. above.
d. Replace pump.

Problem - Circuit Breaker Pops

During Cycle
Probable Cause
a. Wire connections bad or corroded.
b. Bad motor brushes.
c. Bad circuit breaker.

Remedy
a. Clean and protect terminal with grease.
b. Overhaul motor.
c. Replace circuit breaker.

Revision 0.4
FOR TRAINING PURPOSES ONLY 32-99
 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
32 LANDING GEAR

HOURS
INSPECTION TIME LIMITS
25 50 100 200
General Placards X

Hulls & Struts Float Installation Float exterior - inspect for

damage, wrinkled metal, X
corrosion, paint loss, etc.

Struts and attach fittings X

Spreader bars X

Float structure (interior) X

Inspection Covers - inspect for

X
condition, security and seal.

Pumper Tube Installation -

inspect for condition, security, X
routing of hoses.

Electrical System Pump and indicator light

wiring - inspect for chafing,
X
broken or loose terminals
and general condition.

Solenoids - inspect for

wiring, mounting and general X
condition.

Pressure Switches - inspect

wiring, mounting and general X
condition.

Pump motors - inspect

wiring, mounting and general X
condition.

Landing Gear Systems Lubricate nose gear tracks X

Nose Gear Box/Block Tracks

measured at slide route for
X
wear, .070 inches or less
wear tolerance.

Nose gear pivot blocks and

forks - inspect for condition, X
lubrication, corrosion, paint.

Nose and Main Wheel

X
Bearing - grease Zerk fittings.

Hydraulic Fluid Level X

Wheels and Tire - inspect for

X
wear, pressure, condition.

32-100 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

32 LANDING GEAR
HOURS
INSPECTION TIME LIMITS
25 50 100 200
Brake Assemblies - inspect
X
for wear, corrosion, leakage.

Hydraulic Fluid Screen - clean

and inspect. Note: If floats sit
for extended periods of time
(I.e. if removed during winter
months), screen should be
cleaned before putting floats X
back into service. Hydraulic
fluid in reservoir should be
checked for moisture or other
contaminates and changes if
necessary.

Landing Gear Systems Main and Nose Gear Actuator,

assemblies - inspect for
X
condition, lubrication, leakage,
corrosion, and cleanliness.

Nose gear springs - scotch-

ply springs, inspect for cracks, X
de-lamination and paint.
Main Gear Drag Link garlock
bushings - inspect for X
condition, lubrication, corrosion.

Main Gear Oleos - inspect for

evidence of leakage, proper
extension, check cylinder for X
corrosion, pitting, cleanliness
and security.

Hydraulic Lines & Fittings -

inspect for leaks, condition X
and security.

Hydraulic manifolds
(if equipped) - inspect for X
condition, security and leaks.

Brake System Plumbing -

inspect for leaks, condition X
and security.

Main Gear Oleos - Service X

Perform reaction test: Inspect main gear up and

down lock hooks for proper X
engagement.

FOR TRAINING PURPOSES ONLY 32-101

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
32 LANDING GEAR

HOURS
INSPECTION TIME LIMITS
25 50 100 200
Inspect nose gear trolley for
X
proper travel.

Inspect nose gear for

excessive side play in the X
down position.

Perform emergency gear

X
extension (if equipped).

Nose and main wheel bearings

X
- disassemble and inspect.

Bolts in Critical Areas – For corrosion, correct torque when installed, or when visual inspection
indicates a need for a torque check.

32-102 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

32 LANDING GEAR
32-00-00 SPECIAL TOOLS
ITEM NO. DESCRIPTION FUNCTION IDENTIFICATION
1 Wrench socket Fits main wheel bearing nut SD5523–1

2 Restricting clamp, nose leg Restricts the vertical movement SD12528–1

of the nose gear, to prevent
excessive movement of aircraft
during maintenance at light load

3 Wrench Used during removal and SD12552–1

installation of nose wheel

4 Wrench Used during removal and SD12553–1

installation of nose wheel

5 Pin To facilitate installation of L/G SD12576–1

mounting bolts to airframe

6 Acorn See item 5 SD12576–3

-7 Tire gauge Used to check air pressure in tires 8106B (Schrader) (or equivalent)

8 Wrench, 3 7/16 Used for removal and SD12577–1

installation of nose wheel on
aircraft fitted with intermediate
flotation gear

9 Wrench, 2 3/4 Used with item 8 SD12577–3

-10 Wheel balancer Used to balance nose wheel WBK–2C (Snap-On)

FOR TRAINING PURPOSES ONLY 32-103

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CHAPTER 27
FLIGHT CONTROLS
CONTENTS
Page

27 FLIGHT CONTROLS
27-00-00 FLIGHT CONTROLS.................................................................................. 27-1
General................................................................................................................ 27-1
27-10-00 AILERON CONTROL SYSTEM.................................................................. 27-5
General................................................................................................................ 27-5
Component Description and Operation................................................................. 27-7
Control Column............................................................................................ 27-7
Aileron Cable Circuits................................................................................... 27-9
Aileron Quadrant.......................................................................................... 27-9
Linkage Quadrant.......................................................................................... 27-9
Aileron Geometric Bellcrank......................................................................... 27-9
Ailerons...................................................................................................... 27-11
27-10-00 MAINTENANCE PRACTICES.................................................................. 27-13
Adjustment/Test................................................................................................. 27-13
Rig Aileron Control System........................................................................ 27-13
Inspection/Check................................................................................................ 27-13
Inspection of the Aileron Skins and Drain Holes......................................... 27-13
27-13-00 AILERON TRIM SYSTEM........................................................................ 27-17
Description......................................................................................................... 27-17
Trim Tab...................................................................................................... 27-17
Position Indicator........................................................................................ 27-17
Operation........................................................................................................... 27-17

FOR TRAINING PURPOSES ONLY 27-i

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page
27-13-00 MAINTENANCE PRACTICES.................................................................. 27-17
Adjustment/Test................................................................................................. 27-17
Rig Aileron Trim Tab System...................................................................... 27-17
27-16-00 AILERON GEARED TAB.......................................................................... 27-19
27 FLIGHT CONTROLS

27-16-00 MAINTENANCE PRACTICES.................................................................. 27-19

Adjustment/Test................................................................................................. 27-19
Rig Aileron Geared Tab............................................................................... 27-19
27-20-00 RUDDER CONTROL SYSTEM................................................................ 27-21
General.............................................................................................................. 27-21
Rudder Torque Tube Assembly.................................................................... 27-25
Rudder Cables............................................................................................. 27-25
Rudder Assembly........................................................................................ 27-27
Rudder Stops............................................................................................... 27-27
27-20-00 MAINTENANCE PRACTICES.................................................................. 27-29
Adjustment/Test................................................................................................. 27-29
Rudder Rigging........................................................................................... 27-29
Rig Rudder Control System......................................................................... 27-29
27-23-00 RUDDER TRIM SYSTEM......................................................................... 27-31
General.............................................................................................................. 27-31
Description......................................................................................................... 27-31
Trim Tab...................................................................................................... 27-31
Trim Tab Screw Jack................................................................................... 27-31
Rudder Trim Tab Handwheel....................................................................... 27-31
Operation........................................................................................................... 27-31
27-23-00 MAINTENANCE PRACTICES.................................................................. 27-32

27-ii FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page
Adjustment/Test................................................................................................. 27-32
Rig Rudder Trim Tab System....................................................................... 27-32
27-26-00 RUDDER GEARED TAB SYSTEM........................................................... 27-35
27-26-00 MAINTENANCE PRACTICES.................................................................. 27-36

27 FLIGHT CONTROLS
Adjustment/Test................................................................................................. 27-36
Rigging Rudder Geared Tab........................................................................ 27-36
27-30-00 ELEVATOR CONTROL SYSTEM............................................................. 27-39
General.............................................................................................................. 27-39
Control Column.......................................................................................... 27-39
Elevator Column Stop Cable....................................................................... 27-39
Elevator Cables........................................................................................... 27-39
Elevator Control Quadrant........................................................................... 27-41
Elevators..................................................................................................... 27-41
Elevator Stops............................................................................................. 27-43
27-30-00 MAINTENANCE PRACTICES.................................................................. 27-45
Adjustment/Test................................................................................................. 27-45
Elevator Rigging......................................................................................... 27-45
Rig Elevator Control System....................................................................... 27-45
Inspection/Check................................................................................................ 27-47
Inspection of Elevator Skins and Drain Holes ............................................ 27-47
27-33-00 ELEVATOR TRIM SYSTEM..................................................................... 27-49
General.............................................................................................................. 27-49
Description......................................................................................................... 27-49
Trim Tab...................................................................................................... 27-49
Trim Tab Handwheel................................................................................... 27-49

FOR TRAINING PURPOSES ONLY 27-iii

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page
Trim Tab Screw Jack................................................................................... 27-49
Operation........................................................................................................... 27-49
27-33-00 MAINTENANCE PRACTICES.................................................................. 27-51
Adjustment/Test................................................................................................. 27-51
27 FLIGHT CONTROLS

Rigging of the Elevator Trim Tab System.................................................... 27-51

27-36-00 FLAP/ELEVATOR INTERCONNECT TRIM SYSTEM............................. 27-53
General.............................................................................................................. 27-53
Description......................................................................................................... 27-53
Ball Screw Jack........................................................................................... 27-53
Trim Tab Screw Jack................................................................................... 27-53
Trim Tab...................................................................................................... 27-53
Operation........................................................................................................... 27-53
27-36-00 MAINTENANCE PRACTICES.................................................................. 27-55
Adjustment/Test................................................................................................. 27-55
Rig Flap/Elevator Interconnect Trim System (Mod 6/1219)......................... 27-55
Rig Flap/Elevator Interconnect Trim System (Mod 6/1775)......................... 27-57
27-38-00 STALL WARNING SYSTEM.................................................................... 27-59
General.............................................................................................................. 27-59
27-38-00 MAINTENANCE PRACTICES.................................................................. 27-61
Inspection/Check................................................................................................ 27-61
Inspection of the Lift Detecting Vanes......................................................... 27-61
Adjustment/Test................................................................................................. 27-61
Adjust Lift Detecting Vanes........................................................................ 27-61
Adjust Upper Vane Circuit Microswitch...................................................... 27-61
Operational Test Stall Warning System........................................................ 27-62

27-iv FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page
27-70-00 GUST LOCKS........................................................................................... 27-65
General.............................................................................................................. 27-65
Rudder Gust Lock....................................................................................... 27-65
27-50-00 WING FLAPS SYSTEM............................................................................ 27-69

27 FLIGHT CONTROLS
General.............................................................................................................. 27-69
Description......................................................................................................... 27-69
Wing Flaps.................................................................................................. 27-71
Wing Flap Hydraulic System....................................................................... 27-71
Flap Actuator.............................................................................................. 27-72
Flap Selector Lever..................................................................................... 27-72
Flap Follow-Up System............................................................................... 27-73
Flap Position Indicator................................................................................ 27-73
Operation .......................................................................................................... 27-73
Flap and Flight Control Magnaformed Pushrods......................................... 27-77
Flap Rigging Plates..................................................................................... 27-79
Wing Flap Clearances................................................................................. 27-81
27-00-00 MAINTENANCE PRACTICES.................................................................. 27-83
General Maintenance Practices........................................................................... 27-83
Control Surface Clearances................................................................................ 27-83
Inspection/Check................................................................................................ 27-83
Inspection of Wing Flap Skins and Drain Holes.......................................... 27-83
Servicing............................................................................................................ 27-85
Bleed Wing Flap System............................................................................. 27-85
Inspection/Check................................................................................................ 27-87
General....................................................................................................... 27-87

FOR TRAINING PURPOSES ONLY 27-v

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page
Cable Inspection.......................................................................................... 27-87
Pulley Inspection......................................................................................... 27-87
Inspection of Control Cables....................................................................... 27-87
General Rigging Instructions....................................................................... 27-89
27 FLIGHT CONTROLS

Tensioning of Control Cables...................................................................... 27-89

Fault Analysis..................................................................................................... 27-91
27-00-00 SPECIAL TOOLS...................................................................................... 27-92

27-vi FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

ILLUSTRATIONS
Figure Title Page

27-1 Control Column and Rudder Pedals...........................................................27-2

27-2 Flight Control Surfaces..............................................................................27-3

27 FLIGHT CONTROLS
27-3 Aileron System..........................................................................................27-4
27-4 Control Column (Sheet 1 of 2)...................................................................27-6
27-5 Aileron Spring Strut..................................................................................27-7
27-6 Control Column (Sheet 2 of 2)...................................................................27-8
27-7 Aileron Installation..................................................................................27-10
27-8 Aileron System - Rigging........................................................................27-12
27-9 Aileron Rigging Marks............................................................................27-14
27-10 Aileron Trim System................................................................................27-16
27-11 Aileron Geared Tab Installation...............................................................27-18
27-12 Rudder Pedal Assembly...........................................................................27-20
27-13 Pedal Rigging Measurements...................................................................27-22
27-14 Rudder Control System............................................................................27-23
27-16 Mid Fuselage Pulley................................................................................27-24
27-15 Rudder Torque Tube Quadrant ................................................................27-24
27-17 Rudder - Fitting Wear Limit Index...........................................................27-26
27-18 Rudder Stops...........................................................................................27-27
27-19 Rudder Quadrant Rigging........................................................................27-28
27-20 Rudder Trim Tab System..........................................................................27-30
27-21 Rudder Trim Tab Handwheel....................................................................27-31
27-22 Rudder Geared Tab .................................................................................27-34
27-23 Rudder Trim and Gear Tab.......................................................................27-35

FOR TRAINING PURPOSES ONLY 27-vii

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure Title Page

27-24 Rudder Gear Tab - Close View.................................................................27-35
27-25 Elevator Control System..........................................................................27-38
27-26 Control Column (Elevator Operation)......................................................27-40
27-27 
Improved Elevator Rear Quadrant............................................................27-41
27 FLIGHT CONTROLS

27-28 Elevator Stops..........................................................................................27-42

27-29 Elevator Torque Tube Flanges..................................................................27-42
27-31 Elevator Rigging......................................................................................27-44
27-30 Approved Rigging Tool............................................................................27-44
27-32 Elevator Control System..........................................................................27-46
27-33 Elevator Trim Tab System........................................................................27-48
27-34 Elevator Trim Tab....................................................................................27-49
27-35 Elevator Trim Tab System - Rigging (Mod 6/1219)..................................27-50
27-36 Flap/Elevator Interconnect Trim System..................................................27-52
27-37 Elevator Trim Screw Jack........................................................................27-53
27-38 Elevator Trim Screw Jack........................................................................27-54
27-39 Flap/Elevator Trim Interconnect System - Rigging (Mod 6-1775)............27-56
27-40 
Stall Warning Light and Lift Detecting Vanes ........................................27-58
27-42 Pitot Heat Switch.....................................................................................27-58
27-41 Flap Actuator Microswitches...................................................................27-58
27-43 Calibration Data for Lift Detector Vane...................................................27-60
27-44 Gust Locks..............................................................................................27-64
27-47 Rudder Gust Lock....................................................................................27-66
27-45 
Original Aileron and Elevator Gust Lock.................................................27-66
27-46 Column Gust Lock...................................................................................27-66
27-48 Wing Flaps..............................................................................................27-68

27-viii FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure Title Page

27-49 Maximum Flap Extension (LH Wing)......................................................27-70
27-50 Maximum Flap Extension (RH Wing)......................................................27-70
27-51 Wing Flap System....................................................................................27-71
27-52 Flap Control and Indication.....................................................................27-72

27 FLIGHT CONTROLS
27-53 Flap Selector (Rotary Cam and Poppet Assembly) ..................................27-74
27-54 Flap Selector (Planetary Gear Assembly).................................................27-75
27-55 Hydraulic System Schematic...................................................................27-76
27-56 
Magnaformed Fittings and Sleeves..........................................................27-77
27-57 Wing Flap Hydraulic System Components...............................................27-78
27-58 
Right Fuselage Flap Rigging Plate...........................................................27-79
27-59 
Left Fuselage Flap Rigging Plate.............................................................27-79
27-60 Wing Flap Clearances..............................................................................27-80
27-61 Fore Flap Rigging Plate...........................................................................27-81
27-62 Rod End Adjustment................................................................................27-82
27-63 Rudder Cable Tension (5/32-Inch Cable)..................................................27-82
27-64 Lubrication Diagram................................................................................27-84
27-65 Cable Inspection......................................................................................27-86
27-66 Access and Inspection Provisions............................................................27-88
27-67 Pulley Wear Patterns................................................................................27-90
27-68 MMEL - Flight Controls..........................................................................27-91

FOR TRAINING PURPOSES ONLY 27-ix

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CHAPTER 27
FLIGHT CONTROLS

27 FLIGHT CONTROLS
27-00-00 FLIGHT CONTROLS
GENERAL
The flight control surfaces are operated in the conventional manner by cables from a dual
control column and dual rudder pedals, allowing the aircraft to be flown from either the
pilot or co-pilot position. The ailerons, elevators and rudder are conventional, but the
ailerons are hinged to arms at the trailing edge of each outboard fore flap. The elevators and
rudders are aerodynamically horn balanced and, together with the ailerons, are internally
mass balanced. Geared tabs are fitted to the ailerons and rudder to provide an aerodynamic
assist to control surface movement. Trim tabs, which are adjustable in flight, are fitted to
the left aileron, left elevator, and rudder. The right elevator incorporates a trim tab which
is interconnected with the wing flap system, and trims the elevators in proportion to wing
flap movement. Each cable circuit is provided with conveniently located turnbuckles, and
push-pull rods which are adjustable by means of screw threaded end adapters. The wing
flaps are interconnected by push-pull rods which are operated by a single hydraulically-
operated actuator mounted in the cabin roof. The user should consult the Maintenance
Manual, applicable AFM supplements and vendor manuals for additional information on
specific manufacturers installations not included in this chapter.

FOR TRAINING PURPOSES ONLY 27-1

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27 FLIGHT CONTROLS

CONTROL COLUMN

RUDDER PEDALS

Figure 27-1. Control Column and Rudder Pedals

27-2 FOR TRAINING PURPOSES ONLY

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Figure 27-2. Flight Control Surfaces 27 FLIGHT CONTROLS

FOR TRAINING PURPOSES ONLY 27-3

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27 FLIGHT CONTROLS

Figure 27-3. Aileron System

27-4 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

27-10-00 AILERON NOTES

CONTROL SYSTEM
GENERAL
Rotary movement of the control column aileron
control wheels, is transmitted by a chain and

27 FLIGHT CONTROLS
sprocket mechanism to a quadrant at the base of
the column. Cables extending from the control
column quadrant, are routed under the flight
compartment floor and up both sides of the
forward face of the flight compartment/cabin
bulkhead, to transmit motion to an aileron
quadrant in the cabin roof aft of the wing front
spar. From the aileron quadrant, a separate cable
circuit is routed through each wing to linkage
quadrants at the left and right second outboard
fore flap hinge arm attachment brackets. A push-
pull rod connects between each linkage quadrant
and a bellcrank lever on the respective fore flap
hinge arm. The ailerons attach to the trailing edge
of the corresponding outboard flap by four aileron
arms protruding from the outboard fore flap hinge
arms. A push-pull rod from each aileron, connects
to the bellcrank lever in the corresponding fore
flap hinge arm.

Because of the method of attachment, the ailerons

droop with the wing flaps, and the geometric
arrangement of the control linkage is such
that the aileron range of movement increases
proportionally as the flaps go down. The ailerons
move differentially at any flap position.

Each aileron carries a geared tab, and the left

aileron carries an electrically-operated trim tab.

FOR TRAINING PURPOSES ONLY 27-5

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27 FLIGHT CONTROLS

Figure 27-4. Control Column (Sheet 1 of 2)

27-6 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

COMPONENT DESCRIPTION
AND OPERATION
Control Column
Refer to:

•• Figure 27-4. Control Column (Sheet 1 of 2).

27 FLIGHT CONTROLS
•• Figure 27-5. Aileron Spring Strut.
•• Figure 27-6. Control Column (Sheet 2 of 2).

The upper section of the control column

consists of a Y-shaped yoke assembly, two
W-type hand wheels, control chains and cables,
and sprockets. Sprockets that are splined to
Figure 27-5. Aileron Spring Strut
the control wheels drive the control chains.
Rotation of either hand wheel transmits control
inputs through chain and cable assemblies to
an aileron quadrant at the base of the column
for aileron control. A spring strut mounted
on the right column is a centering device that
overcomes control circuit friction to return the
control wheels to neutral (Figure 27-5).

Fore and aft movement of the control column

actuates the elevators which are presented in the
Elevator Control System later in this chapter.

The lower quadrant on the control column

mounts the max travel stops for the Aileron
system. It also accepts a rig pin, which when
inserted to positions quadrant, the chain system
and the control wheels to the neutral position.
To position the control wheels to neutral for
rigging purposes, engage the normal aileron
gust lock.

FOR TRAINING PURPOSES ONLY 27-7

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27 FLIGHT CONTROLS

Figure 27-6. Control Column (Sheet 2 of 2)

27-8 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Aileron Cable Circuits Aileron Geometric Bellcrank

The aileron cables proceed rearward under the The geometric bellcrank will vary aileron travel
cockpit floor to pulleys, which direct the cables in relation to flap position. It is non-adjustable
up behind both the pilot and co-pilot seats on and its purpose is to compensate with more
the cockpit side of the sloping bulkhead. It is aileron travel as the flaps lower.
possible for an autopilot servo cables to be
bridge mounted to the aileron cable behind the
co-pilot. The cables then proceed down the

27 FLIGHT CONTROLS
cabin roof to the aileron quadrant in the roof
aft of the front wing spar.

These cables can be tensions in the cabin roof

area.

Aileron Quadrant
This quadrant has three pulleys to transmit pilot
input to the two ailerons. The larger quadrant is
the input quadrant and the smaller quadrants are
out put to the ailerons. The quadrant accepts a
rig pin to put the quadrant in the neutral position.

NOTE
To minimize possibility of
crossed circuits, the left hand
aileron cable has an increased
diameter ball end fitting to mate
with an enlarged hole in the
quadrant located in the wing
center section cabin roof area.

CAUTION
There have been incidents where
the control surfaces have moved
in the appropriate direction and
the cables have been crossed
twice in the wing area. Ensure all
cables are straight and through
the appropriate pulleys, under
the guide pins and not crossed.

Linkage Quadrant
These are in each wing by the center of the
aileron. Each quadrant will accept a rig pin to
put the quadrant in the neutral position. Their
purpose is to transmit cable movement to the
aileron via a fixed linkage (non-adjustable).

FOR TRAINING PURPOSES ONLY 27-9

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27 FLIGHT CONTROLS

Figure 27-7. Aileron Installation

27-10 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Ailerons NOTES
The ailerons are unique. They are attached to
the trailing edge of the outboard fore flaps by
arms protruding from the fore flap hinge arms
(Figure 27-7). A pushrod connects the ailerons
to the geometric bellcrank on the fore flap hinge
arm. Each aileron incorporates a geared tab; the
left aileron mounts a trim tab.

27 FLIGHT CONTROLS
Due to the manner of attachment, the ailerons
extend with the outboard fore flaps. The
geometrical arrangement is such that the degree
of aileron movement increases as the flaps
extend. The ailerons move differentially at any
flap position. The Structural Repair Manual
(1-63-3) describes the limitations and method
of inspecting the aileron balancing and the
procedures required.

FOR TRAINING PURPOSES ONLY 27-11

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27 FLIGHT CONTROLS

Figure 27-8. Aileron System - Rigging

27-12 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of 9. Remove rigging pins and operate controls
the maintenance practices and is intended for over full range of travel with flaps in up
training purposes only. position, then in down position. Adjust stops
at base of control column, if necessary, to
For a more detailed description of the practice,
obtain desired travel.
refer to the task in the Viking AMM PSM 1-63-2.
NOTE
27-10-00 MAINTENANCE Flaps must be in fully up position
PRACTICES

27 FLIGHT CONTROLS
when checking aileron travel.

10. R e c o n n e c t s p r i n g s t r u t t o c o - p i l o t
ADJUSTMENT/TEST handwheel. Adjust, if necessary, to
maintain handwheel neutral position with
Rig Aileron Control System no spring load.
NOTE 11. Check aileron control surface clearances.
Before attempting to rig the aileron 12. Operate ailerons and check for full and free
control system ensure that wing range of movement.
flap system is correctly rigged.
INSPECTION/CHECK
1. Pump flaps to fully up position.
2. Disconnect spring strut from co-pilot
Inspection of the Aileron Skins
handwheel. and Drain Holes
3. Install rigging pin SD10544 (0.25 inch dia) Inspect ailerons for condition and drain holes
in base of control column. for obstruction.
4. Check that axis of each handwheel is
horizontal. If necessary, remove covers
from control column arms and adjust chain
turnbuckles until handwheels are aligned.
Set chain tension to value shown in Figure 2.
5. Check aileron quadrant for neutral setting
by installing rigging pin SD10543 (0.375
inch dia). Adjust cable circuit turnbuckles,
if necessary, to obtain neutral position,
maintaining correct cable tension.
6. Check neutral position of aileron linkage
by inserting rigging pin SD10542 (0.25
inch dia) in linkage pulleys. Adjust circuit
turnbuckles, if necessary, to obtain neutral
position, maintaining correct cable tension.
7. Remove all rigging pins and operate system
through several cycles, then check that all
rigging pins enter rigging holes freely.
8. With rigging pin installed in aileron linkage
pulleys, check that trailing edge of aileron
aligns with trailing edge of flap; adjust
push-pull rod at aileron if necessary.

FOR TRAINING PURPOSES ONLY 27-13

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27 FLIGHT CONTROLS

Figure 27-9. Aileron Rigging Marks

27-14 FOR TRAINING PURPOSES ONLY

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27 FLIGHT CONTROLS
PAGE INTENTIONALLY LEFT BLANK

FOR TRAINING PURPOSES ONLY 27-15

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27 FLIGHT CONTROLS

Figure 27-10. Aileron Trim System

27-16 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

27-13-00 AILERON TRIM *The following is an abbreviated description of

the maintenance practices and is intended for
SYSTEM training purposes only.
For a more detailed description of the practice,
DESCRIPTION refer to the task in the Viking AMM PSM 1-63-2.

Trim Tab 27-13-00 MAINTENANCE

The trim tab is attached to the left inboard end
PRACTICES

27 FLIGHT CONTROLS
of the aileron training edge with a piano hinge.
The actuator should complete a full cycle of
operation within 7 to 13 seconds (PSM 1-63-2 ADJUSTMENT/TEST
ATA 27-10-8 Pg. 3).
Rig Aileron Trim Tab System
Position Indicator 1. Connect external electrical power to aircraft.
The trim tab position indicator is an electrical 2. Operate aileron to neutral position.
unit labeled AIL TRIM. It is on the trim console
3. Operate trim tab until trailing edges of
and displays a visual indication of the trim tab
aileron and trim tab align.
position. The indicator has a graduated scale
with a center 0 position. Scale limits are labeled 4. Check that aileron trim tab indicator reads zero.
LW DN and RW DN. The indicator circuit is
5. If aileron trim tab indicator does not read
powered from the R DC bus and protected by a
zero, disconnect actuator rod from trim
5-amp circuit breaker labeled AIL TRIM IND
tab, operate actuator until zero reading is
on the main circuit breaker panel.
obtained on indicator, adjust actuator rod
length and reconnect to trim tab.
OPERATION
NOTE
When the aileron trim tab switch is pressed to
If a zero reading is not obtained
LW DN the actuator motor rotates to retract the
with the actuator in its mid-travel
actuator rod, which through the push-pull control
position, the trim tab position
rod, moves the trim tab down. The actuator will
transmitter should be readjusted.
continue to retract until the actuator limit switch
operates, or the trim switch is released; in either
6. Operate trim tab through its ranges of
case the actuator will remain in the selected
travel and check for full and free range of
position until a further selection is made. During
movement.
actuator rod movement, the position transmitter
will have relayed the actuator position to the 7. Check trim tab control surface clearances.
position indicator in respect of the amount
8. Remove external power.
of trim tab movement. The same sequence of
events will occur with a RW DN selection,
except that the actuator rod will extend, and the
trim tab will move up.

FOR TRAINING PURPOSES ONLY 27-17

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27 FLIGHT CONTROLS

REAR PIVOT

Figure 27-11. Aileron Geared Tab Installation

27-18 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

27-16-00 AILERON 4. Secure control rod nuts and locknuts where

disturbed, and install cotter pins as necessary.
GEARED TAB 5. Check geared tab control surface clearances.
The aileron-geared tab, attached to the inboard 6. Operate ailerons and check for full and free
trailing edge of each aileron, is “geared” range of movement.
(connected) to the inboard fore flap by forward
and rear control rods and a lever link. A double NOTE
locking feature was adopted between the idler lever

27 FLIGHT CONTROLS
Initial rigging of the gear tab to
link arm and fore flap hinge arm attachment to
neutral is accomplished with the
enhance the structural integrity of the attachment.
flaps in the full down position and
the tab is faired to the aileron.
The mechanical connection is such that the
tab is deflected in a direction opposite the
Refer to TAB 635 regarding
aileron, providing servo action to assist
Aileron Rigging.
aileron movement in flight. The control rods
are adjustable. A new solid connecting rod
was introduced to replace splitting material
tubular rods caused by moisture ingress and
ice expansion during cold weather operations.

*The following is an abbreviated description of

the maintenance practices and is intended for
training purposes only.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2.

27-16-00 MAINTENANCE
PRACTICES
ADJUSTMENT/TEST
Rig Aileron Geared Tab
NOTE
Before attempting to rig the
aileron geared tab, ensure that
aileron and wing flap systems are
correctly rigged.

1. Lower flaps to full down position.

2. With ailerons in neutral, check that lever
link rear pivot is aligned with aileron hinge
center and that tab is in neutral.
3. Adjust forward control rod, as necessary, to
align the pivot, then adjust rear control rod
to align tab.

FOR TRAINING PURPOSES ONLY 27-19

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27 FLIGHT CONTROLS

Figure 27-12. Rudder Pedal Assembly

27-20 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

27-20-00 RUDDER NOTES

CONTROL SYSTEM
GENERAL
The rudder system provides operation of the
rudder from the pilot and co-pilot positions.
It consists of two rudder pedal assemblies, a

27 FLIGHT CONTROLS
torque tube, a rudder control quadrant, rudder
lever, and rudder. The rudder pedals transmit the
movement through the torque tube and rudder
control quadrant under the flight compartment
floor. From the quadrant, two cables convey the
movement to the rudder lever on the rudder. The
rudder cables are routed along the right side of
the aircraft using the same banks of pulleys as the
elevator system, then change direction into the
center of the rear fuselage. The rudder is hinged
to the vertical stabilizer and rear fuselage at three
points. Left and right rudder travel is limited by
rudder stops, but, because left and right travel is
unequal, the stops are of a different size. As an
aid to identification the left stop is colored blue,
and the right stop grey. Two tabs are hinged to
the rudder trailing spar, the upper being the trim
tab, and the lower the geared tab.

Each set of pedals is adjusted fore and aft

for pilot comfort with a knob below each
instrument panel. Pulling the knob allows
spring pressure to move the pedals aft. Pulling
the knob and exerting pressure on both pedals
moves them forward. Releasing the knob locks
the pedals in that position.

FOR TRAINING PURPOSES ONLY 27-21

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

STA 60 IN.

Figure 27-13. Pedal Rigging Measurements

27-22 FOR TRAINING PURPOSES ONLY

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27 FLIGHT CONTROLS

Figure 27-14. Rudder Control System

FOR TRAINING PURPOSES ONLY 27-23

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27 FLIGHT CONTROLS

Figure 27-16. Mid Fuselage Pulley

Figure 27-15. Rudder Torque Tube Quadrant

27-24 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Rudder Torque Tube Assembly NOTES

Refer to:

•• Figure 27-15. Rudder Torque Tube

Quadrant.
•• Figure 27-16. Mid Fuselage Pulley.

The rudder torque tube assembly is below the

27 FLIGHT CONTROLS
cockpit floor and attached to the four rudder
pedals by pushrod assemblies.

Rudder Cables
The cables are on the right side of the aircraft
with an inspection panel by the rear gear fairing.
Under the baggage compartment floor is a pulley
stack to direct the cables back to the centerline
of the aircraft. The cables exit by the rear of the
aircraft where there is handed attachment points
to the lower rudder quadrant.

FOR TRAINING PURPOSES ONLY 27-25

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27 FLIGHT CONTROLS

Figure 27-17. Rudder - Fitting Wear Limit Index

27-26 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Rudder Assembly
Refer to Figure 27-17. Rudder - Fitting Wear
Limit Index.

The rudder is hinged to the rear fuselage at the

lower attachment and to the vertical stabilizer
at the mid and top hinges. The structural
repair manual (1-63-3) describes the method

27 FLIGHT CONTROLS
of inspecting the flight control balance and the
procedures required.

Rudder Stops
Refer to Figure 27-18. Rudder Stops.

The rudder lower quadrant travel is controlled

by the rudder stops on the rear fuselage. On
the 100 series aircraft there were different
colors for different ranges depending on the
landing gear configuration. The 300 series
always uses the same colors no matter what
configuration is installed. See TAB 6/646-1 for
further information.

Figure 27-18. Rudder Stops

FOR TRAINING PURPOSES ONLY 27-27

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-19. Rudder Quadrant Rigging

27-28 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of 9. Repeat step 7 and step 8 with rudder pedals
the maintenance practices and is intended for adjusted to each of the other three positions.
training purposes only.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2.

27-20-00 MAINTENANCE
PRACTICES

27 FLIGHT CONTROLS
ADJUSTMENT/TEST
Rudder Rigging
Refer to Figure 27-19. Rudder Quadrant Rigging.

Using the Index screw located on the left side

of the rear fuselage and a steel ruler adjust to
the measurements as shown in Figure 27-19.

Rig Rudder Control System

1. Set rudder pedals to highest position by
pulling out adjustment catch, and engaging
in bottom hole.
2. Engage rudder gust lock.
3. Using scale SD12568, check rudder neutral
position by measuring 18.73 ± 0.06 inches
between centers of index screw and bolt in
rudder lever (dimension L, Figure 27-19).
4. If measurement is not within limits, adjust
turnbuckles in rear fuselage as necessary to
obtain dimension; tension cables to correct
values.
5. Set rudder pedal catch in highest hole, check
with rudder in neutral, pedals align and angle
of pedals is 73°, with a measurement of 5.40 ±
0.125 inches between bulkhead at station 60.00
and rudder pedal center pivots. Adjust push
rods as necessary. Tighten nuts if disturbed.
6. Disengage rudder gust lock. Lock turnbuckles
using locking clips.
7. Using scale SD12568, check dimension L
(Figure 27-19) for full left and right rudder
positions.
8. Operate rudder and check for full and free
range of movement.

FOR TRAINING PURPOSES ONLY 27-29

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27 FLIGHT CONTROLS

Figure 27-20. Rudder Trim Tab System

27-30 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

27-23-00 RUDDER TRIM OPERATION

SYSTEM Rotation of the trim tab handwheel turns the
cable drum, cables attached to the handwheel
drum and the screw jack drum, move and rotate
GENERAL the screw jack. With rotation the screw jack
either extends or retracts to operate a push-pull
Refer to Figure 27-20. Rudder Trim Tab System.
rod and so move the rudder trim tab. Extension
of the screw jack moves the tab to the right (nose

27 FLIGHT CONTROLS
The rudder trim tab system is hand-operated
left trim), and retraction moves the tab to the left.
from a trim tab handwheel.

DESCRIPTION
Trim Tab
The rudder trim tab is hinged to the trailing edge
of the rudder upper portion and is connected to a
screw jack by an adjustable control rod (detail B).

Trim Tab Screw Jack

The rudder trim tab screw jack converts rotary
movement of the cable drum into linear motion
to operate a push-pull rod. The screw jack
is mounted on the front spar of the rudder
structure in the upper portion of the rudder. The
rudder trim tab screw jack is identical to the
flap/elevator interconnect trim tab screw jack.

Rudder Trim Tab Handwheel

The rudder trim tab handwheel on the trim controls
console at the right of the pilot seat. Rotation of the
handwheel rotates an indicator pointer.

Figure 27-21. Rudder Trim Tab Handwheel

FOR TRAINING PURPOSES ONLY 27-31

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of 8. Check trim tab control surface clearances.
the maintenance practices and is intended for
9. Operate rudder and trim tab system and
training purposes only.
check for full and free range of movement.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2. NOTE
Ensure all cables and components
27-23-00 MAINTENANCE are correctly installed before
PRACTICES adjusting the Rudder trim and
27 FLIGHT CONTROLS

Indication system.

ADJUSTMENT/TEST
Rig Rudder Trim Tab System
Refer to Figure 27-20. Rudder Trim Tab System.
1. Rotate rudder trim tab handwheel until cables
are arranged as in “Detail D”, and cable
terminals are in position shown in “Detail C”.
2. Maintain position as in step 1 and set rudder
trim screw jack to 0.62 inch extension with
cables arranged as shown in “Detail A”.
3. Maintaining positions as in step 1 and step
2, adjust and tension cables in rear fuselage.
For cable tension values refer to 27-00-00.

NOTE
The two cables from turnbuckles
to screw jack are nylon covered,
therefore tensiometer readings
must be taken on the bare cables
forward of the rear turnbuckles.

4. Disconnect connecting rod between screw

jack and trim tab at trim tab attachment.
5. Set trim tab in neutral position by aligning
trim tab and rudder trailing edges.
6. Maintaining position of screw jack as in step 2,
adjust and connect connecting rod to trim tab.
Release nut securing handwheel pointer and align
pointer with neutral position on trim console.
Tighten pointer nut and connecting rod lock nut.
7. Rotate rudder trim handwheel to full nose
left position, and to full nose right position.
Check at extremities of travel that handwheel
and screw jack drums have a minimum of
half a turn of cable on each drum.

27-32 FOR TRAINING PURPOSES ONLY

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27 FLIGHT CONTROLS
PAGE INTENTIONALLY LEFT BLANK

FOR TRAINING PURPOSES ONLY 27-33

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27 FLIGHT CONTROLS

Figure 27-22. Rudder Geared Tab

27-34 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

27-26-00 RUDDER
GEARED TAB SYSTEM
Refer to:

•• Figure 27-22. Rudder Geared Tab.

•• Figure 27-23. Rudder Trim and Gear Tab.

27 FLIGHT CONTROLS
•• Figure 27-24. Rudder Gear Tab - Close
View.

The gear tab (Figure 27-22) is on the lower

portion of the rudder trailing edge.

The system includes a gearbox, levers, control

rods, and the tab. The gearbox is bolted to a
mounting plate on the bottom rudder hinge
bracket of the vertical stabilizer and provides
nonlinear tab deflection during rudder
movement. A link connects the rudder to the
gearbox-operating lever, and the gearbox-
gearing lever is connected to the tab with a
control rod. Rudder motion provides an input
to the gearbox, which changes the tab-to-rudder
movement ratio, moving the tab in the opposite
direction. The rudder gear tab ratio was reduced Figure 27-23. Rudder Trim and Gear Tab
between the tab and the rudder for series 300.
The rudder trim tab angular position of + 25°
remained unchanged.

Figure 27-24. Rudder Gear Tab - Close View

FOR TRAINING PURPOSES ONLY 27-35

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of NOTES

the maintenance practices and is intended for
training purposes only.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2.

27-26-00 MAINTENANCE
PRACTICES
27 FLIGHT CONTROLS

ADJUSTMENT/TEST
Rigging Rudder Geared Tab
Refer to Figure 27-22. Rudder Geared Tab.

1. Check that geared tab is neutral with rudder

neutral.
2. Check that gearbox gearing lever is
horizontal with fork end inboard, and that
rod attachment bolt is vertical.
3. Adjust link to obtain gearing lever position,
then adjust control rod to set tab in neutral.
4. Tighten and wirelock control rod lock nuts
as necessary.
5. Check rudder geared tab control surface
clearances.
6. Operate rudder and check for full and free
range of movement
Make sure that the direction of geared tab
movement is opposite rudder movement.

27-36 FOR TRAINING PURPOSES ONLY

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27 FLIGHT CONTROLS
PAGE INTENTIONALLY LEFT BLANK

FOR TRAINING PURPOSES ONLY 27-37

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27 FLIGHT CONTROLS

Figure 27-25. Elevator Control System

27-38 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

27-30-00 ELEVATOR Elevator Cables

CONTROL SYSTEM The cables also run up the right side of the
aircraft and are visible for inspection under the
panel by the right rear gear fairing. Under the
GENERAL baggage compartment floor is a pulley stack to
direct the cables back to the center line of the
Refer to Figure 27-25. Elevator Control System. aircraft. They attach in the rear fuselage to the
elevator control quadrant.

27 FLIGHT CONTROLS
Elevator deflection is achieved through fore
and aft movement of the control column. A
connecting rod (below the flight compartment
floor) joins the control column to the elevator
control lever which transmits movement to the
elevator control cables. Movement of the elevator
control lever is limited by a cable with swaged
stops which passes through the outboard end of
the control lever; the cable is secured at each
end to elevator control pulley attachment bolts.
From the control lever, movement is transmitted
through cables, routed through pulleys and
fairleads on the right side of the fuselage under
the cabin floor, to an elevator quadrant in the rear
fuselage. An adjustable connecting rod transmits
movement from the quadrant to the elevator
torque tube and elevators. Two hinge attachments
are mounted on the elevator front spar and a third
is located on the flange at the end of the elevator
torque tube. When the elevators are installed,
the torque tube flanges, when aligned and bolted
together, provide the maximum up and down
travel stops. The left elevator carries an elevator
trim tab and the right elevator carries a flap/
elevator interconnect tab.

Control Column
The control column (Figure 27-26), when moved
fore or aft, provides inputs to the elevator
control system. The column pivots about a point,
imparting motion through the elevator control
lever to the elevator control system cables.
When the original elevator gust lock is applied
the column is in the neutral position.

Elevator Column Stop Cable

The stop cable is a secondary stop for elevator
control. Its purpose is to limit travel of the
elevator column. This cable is not tensioned
but can be replaced if excessive wear is noted.

FOR TRAINING PURPOSES ONLY 27-39

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-26. Control Column (Elevator Operation)

27-40 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Elevator Control Quadrant

Refer to Figure 27-27. I mproved Elevator Rear
Quadrant.

Located in the rear fuselage and visible via the

access panels, this unit drives a pushrod to the
right elevator. A new three quarter moon shaped
quadrant by mod 6/1394 at aircraft 331 in lieu of

27 FLIGHT CONTROLS
the figure eight quadrant was adopted. Located in
the aft fuselage bulkhead frame area to preclude
the possibility of the quadrant unporting from
the frame web slot in the bulkhead area. The
new quadrant design eliminates the problem of
distortion and possible contact with the lip of the
bulkhead frame web slot.
Figure 27-27. Improved Elevator Rear
Quadrant
Elevators
These units are hinged in five places to the
horizontal stabilizer. The center hinge is where
the left and right elevators are bolted together.
They are bolted through a torque tube flange
assemble which, when it strikes the center
hinge point acts as the primary stop. A torque
tube from the elevator control quadrant drives
the left elevator. The elevator flexible seals on
the leading edges must be serviceable as their
function is to control airflow in slow flight
conditions. The elevators can be balanced in
the field using the PSM 1-6-3.

FOR TRAINING PURPOSES ONLY 27-41

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-28. Elevator Stops

Figure 27-29. Elevator Torque Tube Flanges

27-42 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Elevator Stops NOTES

Refer to:

•• Figure 27-28. Elevator Stops.

•• Figure 27-29. Elevator Torque Tube
Flanges.
For many aircraft the primary stop blocks

27 FLIGHT CONTROLS
for the elevator were initially bonded to the
horizontal stabilizer/elevator center hinge arm
upper and lower flanges. However, bonding
failures resulting in the displacement of the
stop blocks introduced a more positive method
of retention by mod 6/1798 (S/B 6/432) at
aircraft 805 with the addition of a rivet through
the block and hinge arm flanges.

FOR TRAINING PURPOSES ONLY 27-43

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-30. Approved Rigging Tool

Figure 27-31. Elevator Rigging

27-44 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of and must be completed within

the maintenance practices and is intended for the designated time frame.
training purposes only. The PSM 1-6-7 Part 2 special
inspections describes the checks
For a more detailed description of the practice,
and limitations.
refer to the task in the Viking AMM PSM 1-63-2.

27-30-00 MAINTENANCE Rig Elevator Control System

PRACTICES 1. Lock control column in neutral by installing

27 FLIGHT CONTROLS
gust locks (Pre Mod 6/1676 gust lock
modification). On aircraft with Mod 6/1676
ADJUSTMENT/TEST incorporated, obtain neutral by installing
elevator rigging tool C6GT1047–1 and
Elevator Rigging accompanying rigging pins C6GT1048–1
between control column and plate on
To rig the elevator to neutral the following
instrument panel frame.
conditions apply. The column is secured to
the neutral position and the rig pin is installed 2. On aircraft with Mod 6/1747 incorporated,
in the elevator control quadrant. The upper check stop cable for minimum slackness.
surface of the right elevator horn must be Adjust tension of stop cable by using alternate
within 0.030 inches of the lower edge of the mounting holes in straps to achieve minimum
rigging disk washer on the horizontal stabilizer slackness. Maintaining ease of insertion of
(Figure 27-30 and Figure 27-31). stop cable mounting bolts. End strap hole
pitch may be adjusted between +0.100 and
CAUTION –0.150 inch in increments of 0.050 inches.

The elevator control column NOTE

attaching bellcrank and hand
Inspect elevator quadrant
pump mounting support structure
mounting bracket for cracks,
was reinforced by mod 6/1594
and the quadrant for distortion
(S/B 6/348 Rev C) at aircraft
and loose attachment bolts (refer
508 to overcome structure cracks
to Inspection Requirements
attributed to excessive operation
Manual, Basic and Special
of the hand pump. Transport
Inspections).
Canada and the F.A.A. have both
released Airworthiness Directives
3. Check that top surface of right elevator
CF-77-05R1 and 77-03-07 AMDT
horn and lower edge of rigging disc at
39-3100 relating to the condition.
horizontal stabilizer tip are aligned within
All operators should ensure that
± 0.030 inches (neutral position).
the support structure is free of
cracks regardless of modification.
NOTE
CAUTION If step 3 is satisfactory, ignore
step 4, step 5 and step 6.
Several Service Bulletins have
been issued to re-enforce the 4. Insert rigging pin SD10542 (0.25 inch
need to inspect the elevator and dia.) in elevator control quadrant in rear
rudder components any time the fuselage.
aircraft has been left outside in
a. If rigging pin enters easily, adjust
winds greater than 35 knots.
connecting rod at elevator as necessary
The inspections are mandatory
to comply to step 3.

FOR TRAINING PURPOSES ONLY 27-45

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-32. Elevator Control System

27-46 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

b. If rigging pin does not enter quadrant, NOTES

adjust turnbuckles in rear fuselage until
quadrant setting is obtained; tension
cables.
5. Check rigging disc at right elevator horn
for alignment, as in step 3, and make final
adjustment on connecting rod as necessary.
Safety connecting rod. Lock turnbuckles

27 FLIGHT CONTROLS
using locking clips as necessary.
6. Remove rigging pins.
7. Remove gust locks (Pre Mod 6/1676) or
rigging tool (Mod 6/1676).
8. Check elevator control surface clearances..
9. Operate elevator controls and check for full
and free range of movement.
10. Check rigging of elevator trim tab and flap/
elevator interconnect trim tab systems.

INSPECTION/CHECK
Inspection of Elevator Skins and
Drain Holes
Inspect elevator skins and drain holes for
condition, drain holes for obstruction and wing
tip fairings for condition.

FOR TRAINING PURPOSES ONLY 27-47

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-33. Elevator Trim Tab System

27-48 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

27-33-00 ELEVATOR
TRIM SYSTEM
GENERAL
An elevator trim tab, attached to the left elevator
trailing edge, is operated by a handwheel which

27 FLIGHT CONTROLS
is part of the trim control assembly in the trim
console to the right of the pilot seat.

DESCRIPTION
The elevator trim tab system consists of a
trim tab, trim handwheel, trim screw jack and
trim cables. The cables, connected to a drum Figure 27-34. Elevator Trim Tab
attached to the trim handwheel, are routed
through pulleys and fair leads up the forward
face of the sloping bulkhead, along the cabin
roof to the rear fuselage, to connect to a cable OPERATION
drum attached to the screw jack drive shaft.
Rotary movement of the elevator trim
handwheel turns the cable drum, which through
Trim Tab the cables turns the screw jack cable drum to
The elevator trim tab is hinged to the trailing extend or retract the screw jack. The screw jack
edge of the left elevator. A bracket on the lower operates the connecting rod, to either raise or
surface of the tab provides for the attachment lower the trim tab. The trim tab moves up for
of an adjustable connecting rod which connects a nose down selection, or down for a nose up
to the trim screw jack. Drain holes are provided selection of the trim tab handwheel.
in the tab bottom skin.

Trim Tab Handwheel

The elevator trim tab handwheel is mounted in
the trim console, a cable drum to which the trim
cables connect, is attached to the handwheel.
Mounted on the inboard end of the cable drum is
a spiral grooved plate, which through a pin and
lever arrangement, drives an indicator pointer.

Trim Tab Screw Jack

The elevator trim tab screw jack on the front
spar of the left elevator, converts rotary motion
of the cable drum into linear movement of the
tab connecting rod attachment. The cable drum
is rotated by the trim cables secured to the
drum.

FOR TRAINING PURPOSES ONLY 27-49

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-35. Elevator Trim Tab System - Rigging (Mod 6/1219)

27-50 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of 9. With elevator trim handwheel still in nose
the maintenance practices and is intended for full up position, slacken pointer nut and
training purposes only. position pointer to coincide with nose up
position. Tighten pointer nut.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2. 10. Check elevator trim tab control surface
clearance.
27-33-00 MAINTENANCE 11. Operate trim tab and check for full and free
PRACTICES range of movement. Check at extremities of

27 FLIGHT CONTROLS
travel that handwheel and screw jack drums
have a minimum of half a turn of cable on
ADJUSTMENT/TEST each drum.

Rigging of the Elevator Trim Tab

System
Refer to Figure 27-35. Elevator Trim Tab
System - Rigging (Mod 6/1219).

1. Rotate elevator trim tab handwheel two

turns (approximately) from full nose down
position until cable terminals are located
as shown in, “Detail C”, and cables are
arranged as in “Detail A”.
2. Maintain handwheel and set trim screw jack
in left elevator to 0.690 to 0.720 inches
extended, and arrange cables as shown in,
“Detail D”.
3. Maintaining handwheel and screw jack
cable drum setting, adjust and tension
cables in rear fuselage. For cable tension
values refer to 27-00-00, Lock turnbuckles.
4. Disconnect connecting rod between screw
jack and tab attachment bracket at tab
attachment.
5. Set trim tab in neutral position by aligning
trim tab and elevator trailing edges.
6. Maintaining position of screw jack (as
in step 2), adjust and connect trim tab
connecting rod.
7. Rotate elevator trim handwheel to nose
full up position; ensure that screw jack is
fully retracted. Check that 0.005 inch exists
between trim tab hinge lugs as shown in,
“Detail F”, if not, lengthen connecting rod
to obtain clearance.
8. Tighten and lock connecting rod adjustable end.

FOR TRAINING PURPOSES ONLY 27-51

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-36. Flap/Elevator Interconnect Trim System

27-52 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

27-36-00 FLAP/ELEVATOR Trim Tab

INTERCONNECT TRIM The flap/elevator interconnect trim tab is
hinged to the trailing edge of the right elevator.
SYSTEM A bracket on the lower surface of the tab
provides for the connection of a connecting
rod, from the screw jack slide screw fork end.
GENERAL
The flap/elevator interconnect trim system OPERATION

27 FLIGHT CONTROLS
is a remote mechanical operation in which
wing flap actuator travel is used to apply During flap operation, movement of the right
compensating nose down trim proportional to inboard operating bellcrank causes the ball
the amount of flap extension. screw jack spindle and cable drum to rotate.
The cables attached to the ball screw jack
drum transmits this movement to turn the
DESCRIPTION interconnect trim screw jack cable drum, which
extends or retracts the screw jack slide screw
The system comprises a ball screw jack, a cable
and operates the interconnect trim tab through
system, trim tab screw jack, connecting rod and
the connecting rod. For a flaps down selection,
trim tab.
the interconnect trim tab moves up, to provide a
nose down trim proportional to flap movement.
Ball Screw Jack
The flap/elevator interconnect ball screw jack,
mounted in the cabin roof structure and connected
to the flap operating mechanism, converts flap
system linear motion into rotary motion. The ball
screw jack comprises a ball screw assembly, a
spindle and an end support. The ball screw assembly
consists of a threaded shaft, which engages ball
bearings enclosed in a tube integral with a ball
cage. The ball cage is keyed into a spindle which
has an integral cable drum, and the spindle rotates
on a bearing mounted on the end support bracket.
The end support bracket is bolted to the cabin roof
structure and a fork end installed on the screw shaft
connects to the flap operating mechanism. Linear
movement of the screw shaft causes the ball cage
and spindle to rotate and drive the cable drum.

Trim Tab Screw Jack Figure 27-37. Elevator Trim Screw Jack
The flap/elevator interconnect trim tab screw
jack on the front spar of the right elevator,
converts rotary motion into linear motion. The
screw jack comprises a cable drum spindle
which rotates in bearings in a stop cover, and
a slide screw assembly. The cable drum is
internally threaded and engages with external
threads on the slide screw. A fork end is
threaded and riveted into the slide screw.

FOR TRAINING PURPOSES ONLY 27-53

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-38. Elevator Trim Screw Jack

27-54 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of 8. Pump flaps fully down, ensure that at
the maintenance practices and is intended for extremities of travel, ball screw jack and trim
training purposes only. screw jack cable drums have a minimum of
half a turn of cable left on drum.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2. 9. Check flap/elevator interconnect trim tab
control surface clearances.
27-36-00 MAINTENANCE 10. Operate flaps up and down and check
PRACTICES interconnect system for full and free range

27 FLIGHT CONTROLS
of movement.

ADJUSTMENT/TEST
Rig Flap/Elevator Interconnect
Trim System (Mod 6/1219)
NOTE
Before attempting to rig the
flap/elevator interconnect trim
system, ensure that the wing flap
system is correctly rigged.

1. Ensure wing flaps are pumped fully up.

2. Set cables on interconnect ball screw jack
cable drum as in Figure 27-38, “Detail A”,
with inboard cable terminal uppermost.
3. Set trim screw jack extension to between
0.030 and 0.070 inch and arrange cables on
screw jack drum as shown in Figure 27-38,
“Detail B”.
4. Maintaining positions of ball screw jack and
screw jack as in step 2 and step 3, adjust
and tension trim cables at turnbuckles in
rear fuselage. For cable tension values refer
to 27-00-00.
5. Disconnect connecting rod between screw
jack and tab attachment bracket, at tab
attachment.
6. Set the trim tab 12° down. Measure angular
difference along upper skin, or measure
0.85 ± 0.05 inch from elevator trailing edge
to trim tab trailing edge with tab down.
7. With trim screw jack set as in step 3 and tab
set as in step 6, adjust and connect trim tab
connecting rod. Lock connecting rod lock nut.

FOR TRAINING PURPOSES ONLY 27-55

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-39. Flap/Elevator Trim Interconnect System - Rigging (Mod 6-1775)

27-56 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Rig Flap/Elevator Interconnect 10. If clearance is between 0.001 and 0.029 inch:
Trim System (Mod 6/1775) a. Dismount ball screw jack (“Detail A”,
and rotate 180° clockwise (looking
Refer to Figure 27-39. Flap/Elevator Trim
outboard). Reinstall 0.100 inch thick
Interconnect System - Rigging (Mod 6-1775).
packing on outboard side of structure.
b. Dismount trim tab screw jack (“Detail
NOTE B”), and rotate 180° counterclockwise
Before attempting to rig the (looking aft).

27 FLIGHT CONTROLS
flap/elevator interconnect trim
c. Reinstall both jacks.
system, ensure that the wing flap
system is correctly rigged. 11. Adjust turnbuckles to obtain correct cable
tension values. For cable tension values,
1. Ensure wing flaps are pumped fully up. refer to 27-00-00, Maintain trim jack setting
obtained in step 9 or step 10).
2. Set cables on interconnect ball screw jack
cable drum as in “Detail A”, with inboard 12. Lock both turnbuckles.
cable terminal uppermost.
13. Set the trim tab 12° down. Measure angular
3. Disconnect connecting rod between screw difference along upper skin, or measure
jack and tab attachment bracket at tab 0.85 ± 0.05 inch from elevator trailing edge
attachment. to trim tab trailing edge with tab down.
4. Set trim screw jack extension to 0.050 inch 14. With trim screw jack set as in step 9 or
and arrange cables on screw jack drum as step 10 and tab set as in step 13, adjust
shown in “Detail B”. and connect trim tab connecting rod. Lock
connecting rod lock nut.
5. C o n n e c t t u r n b u c k l e s a n d t i g h t e n t o
approximately equal lengths. 15. Pump flaps fully down, ensure that at
extremities of travel, ball screw jack and trim
6. Check there is clearance at trim tab screw
screw jack cable drums have a minimum of
jack as shown in, “Detail B”, of 0.030/0.070
half a turn of cable left on drum.
inch. If faces touch, dismount jack, rotate
its mounting flange 180° and reinstall jack. 16. Check flap/elevator interconnect trim tab
control surface clearance.
7. Measure clearance at trim tab screw jack as
shown in “Detail B”. 17. Operate flaps up and down and check
interconnect system for full and free range
8. If clearance is between 0.030 and 0.070
of movement.
inch, omit step 9 or step 10.
9. If clearance is between 0.071 and 0.084 inch:
a. Dismount ball screw jack (“Detail A”),
and rotate 180° clockwise (looking
outboard). Transfer the 0.100 inch thick
packing from outboard to inboard side
of structure, to place it under jack.
b. Dismount trim tab screw jack (“Detail
B”), and rotate 180° counterclockwise
(looking aft).
c. Reinstall both jacks.

FOR TRAINING PURPOSES ONLY 27-57

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-40. S
 tall Warning Light and Lift Figure 27-41. Flap Actuator Microswitches
Detecting Vanes

Figure 27-42. Pitot Heat Switch

27-58 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

27-38-00 STALL NOTES

WARNING SYSTEM
GENERAL
The stall warning system warns of impending
stalls by bringing on an amber STALL light on

27 FLIGHT CONTROLS
the pilot instrument panel (Figure 27-40 and
Figure 27-41).

Two lift detecting vanes are mounted in the

leading edge of the left wing. The lower vane is
operative over the full range of flap operation; the
upper vane is operative from 12° of flap travel,
and the stall warning micro switch activates it
in the wing flap hydraulic system . As a stall
condition approaches, the stagnation point moves
behind the affected vane, causing it to deflect and
actuate a switch to bring on the STALL light
between 4 to 9 knots above the stall speed. An
audible warning horn sounds simultaneously
with the STALL light.

The safe flight lift detector switches include

a heating element to remove condensation
from collecting in the switch contact area.
When operating in extreme cold wet conditions
the heating effect is marginal limiting the
performance period of the lift detector.

Power to the heating elements for both detector

switches is from the L DC BUS through the
left pitot heater 7.5 amp circuit breaker labeled
PITOT HTR L on the main circuit breaker panel
to the pitot heat selected switch labeled PITOT
HEAT on the overhead console switch panel
(Figure 27-42). When the pitot heat switch is
selected to ON, power is also available to the
heating elements of both lift detectors Figure
27-42.

The STALL warning light on the pilot flight

panel and the audible warning horn on the
forward face of the flight compartment
bulkhead Stn 111 above the pilot are tested
by placing the CAUTION LT switch on the
overhead console in the TEST position.

FOR TRAINING PURPOSES ONLY 27-59

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-43. Calibration Data for Lift Detector Vane

27-60 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of 2. C a r e f u l l y w i t h d r a w m o u n t i n g p l a t e

the maintenance practices and is intended for assembly away from leading edge
training purposes only. sufficiently to gain access to rear of
mounting plate.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2. 3. Using suitable marking instrument, mark rear
of mounting plate and flanges of vent to be
27-38-00 MAINTENANCE moved to show original installation position.

PRACTICES 4. Using data given in Figure 27-43, change

27 FLIGHT CONTROLS
position of appropriate vane in direction
required to produce warning at specified
INSPECTION/CHECK air speed.

Inspection of the Lift Detecting NOTE

Vanes The specified airspeed at which
stall warning is given (STALL
Inspect the stall warning lift detector vanes for
light on) is 4 to 9 knots above
condition, free movement and security.
stalling speeds. For stalling
speeds at specified aircraft
ADJUSTMENT/TEST gross weight with various flap
settings, refer to Flight Manual
Refer to Figure 27-43. Calibration Data for Lift PSM 1-63-1A.
Detector Vane.
5. Secure detector vane mounting hardware and
reinstall mounting plate to wing leading edge.
Adjust Lift Detecting Vanes
NOTE Adjust Upper Vane Circuit
The positions of the lift detecting
Microswitch
vanes on the mounting plate
assembly determine the airspeed
NOTE
at which stall warning is given. Before attempting the following
Therefore, whenever a vane is adjustments, ensure that the
installed, an air test is required wing flaps are correctly rigged.
to determine the correct position
of the vane on the mounting 1. Connect external power source to aircraft.
plate. The lower vane determines
2. Using handpump, pump wing flaps to fully
the airspeed at which warning is
up position.
given in the range of 0° to 40°
flaps down, and the upper vane 3. Set BUS TIE switch to NORMAL and
determines the airspeed at which ensure that STALL WARN circuit breaker
warning is given in the range of is energized. Operate CAUTION LT switch
13° to 40° flaps down. The data and check that STALL light comes on and
in Figure 27-43 gives the amount that warning horn sounds (if installed).
of change in airspeed produced
4. Pump flaps to 12° down as indicated by
by 1/8-inch movement of vane
flap position indicator.
position for various flap settings.
5. Release cabin roof upholstery sufficiently to
1. Remove lift detector vane mounting plate gain access to upper vane circuit microswitch
assembly from wing leading edge. positioned adjacent to flap actuator.

FOR TRAINING PURPOSES ONLY 27-61

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

6. With second operator actuating upper vane, NOTE

adjust switch so that STALL light comes
Two persons are required to
on, then back off switch until light goes out.
perform steps 5 through 9.
Secure adjusting screws.
5. Actuate lower vane gently to complete
CAUTION circuit and hold vane in that position. Pump
flaps from fully up to fully down and check
ENSURE THAT PITOT HEAT
that STALL light remains on.
SWITCH IS OFF BEFORE
27 FLIGHT CONTROLS

TOUCHING STALL WARNING

VANE, OTHERWISE A BURN CAUTION
COULD RESULT.
ENSURE THAT PITOT HEAT
SWITCH IS OFF BEFORE
NOTE TOUCHING STALL WARNING
VANE, OTHERWISE A BURN
On aircraft with warning horn
COULD RESULT.
installed, check that horn sounds
whenever STALL light comes on.
NOTE
7. Return flaps to fully up position and with
On aircraft with warning horn
upper vane actuated, lower flaps smoothly
installed, check that horn sounds
and slowly until STALL light comes on,
whenever STALL light comes on.
then stop pumping.
8. Check that flap position indicator reads 6. Release lower vane and return flaps to fully
12° ± 2°. up position.
9. If STALL light does not come on within 7. Actuate upper vane gently and check that
range specified in step 8, repeat adjustment STALL light does not come on.
procedure as necessary until light does
8. With upper vane actuated, pump flaps
come on correctly.
slowly and smoothly down until STALL
10. Replace upholstery released in step 5 and light comes on, then stop pumping. Check
disconnect external power from aircraft. that flap position indicator reads 12° ± 2°.

Operational Test Stall Warning

System
1. Ensure that wing flap system is operating
properly and that indicating system is accurate.
2. Connect external power to aircraft.
3. Using handpump, pump wing flaps to fully
up position.
4. Set BUS TIE switch to NORMAL and ensure
STALLWARN circuit breaker is energized.
Operate CAUTION LT TEST switch and
check that STALL light comes on, and that
warning horn sounds (when installed).

27-62 FOR TRAINING PURPOSES ONLY

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

27 FLIGHT CONTROLS
PAGE INTENTIONALLY LEFT BLANK

FOR TRAINING PURPOSES ONLY 27-63

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-44. Gust Locks

27-64 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

27-70-00 GUST LOCKS NOTES

GENERAL
Refer to Figure 27-44. Gust Locks.

Gust locks are provided to secure the main flight

control surfaces (ailerons, rudder and elevator)

27 FLIGHT CONTROLS
while the aircraft is parked. The rudder system is
provided with a built-in system controlled by an
operating lever in the flight compartment floor in
front of the pilot seat. Both aileron and elevator
gust locks are removable struts (part of flyaway
kit) which engage fittings on the instrument panel
structure, control column and floor. The elevator
gust lock strut locks the control column in a
forward position. The rudder gust lock must be
engaged and the handle held up until the vertical
strut is installed. A pivoted flag provides visual
warning that the gust lock is installed.

Rudder Gust Lock

The rudder gust lock is a permanently
installed mechanical linkage beneath the flight
compartment floor which secures the rudder in the
neutral position. The gust lock linkage consists of
a hinged lever, connecting rods, a bellcrank and
a spring-loaded lever. The hinged lever is flush
with the flight compartment floor and when lifted
to select rudder gust lock, operates the connecting
rods to engage the spring-loaded lever in a slot in
the rudder control quadrant. A spring connected
to the spring-loaded lever, ensures that the lock
cannot be engaged unless selected.

FOR TRAINING PURPOSES ONLY 27-65

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-45. O
 riginal Aileron and Figure 27-46. Column Gust Lock
Elevator Gust Lock

Figure 27-47. Rudder Gust Lock

27-66 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Refer to: NOTES

•• Figure 27-45. O
 riginal Aileron and
Elevator Gust Lock.
•• Figure 27-46. Column Gust Lock.
•• Figure 27-47. Rudder Gust Lock.

A device that braces the pilot control wheel and

27 FLIGHT CONTROLS
control column arm to the instrument panel locks
the ailerons and elevators. Two prongs engaged
with the right yoke of the control wheel secure
the control wheel, and the column is braced
between two lugs on the column and one lug on
the instrument panel. The aileron and elevator
control lock of the gust lock assay is stowed under
the pilot seat and the rudder vertical locking strut
is stowed behind the pilot seat.

Aircraft with gust locks secure the control

column with the elevator in a tail down
position. The change, which shortened the
aileron and elevator gust hook assembly and
reshaped the rudder vertical strut to engage
with the instrument panel attachment lug, was
required to prevent the aircraft from becoming
airborne with the gust lock still engaged.

Aircraft with gust locks incorporate a red

warning flag attached to the gust lock to limit
crew observation of the flight instruments
as an ultimate method of warning the crew
that the gust lock is still engaged. The gust
lock and warning flag are mandatory and
fulfill the requirements of Transport Canada
Airworthiness Directive CF-90-01 for all series
aircraft.

CAUTION
If aircraft are left outside in winds
greater than 35 Knots the PSM
1-6-7 Part 2 Special inspections
advise certain primary flight
controls and the cables, bellcrank
and cables must be checked for
security and serviceability.

FOR TRAINING PURPOSES ONLY 27-67

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-48. Wing Flaps

27-68 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

27-50-00 WING FLAPS NOTES

SYSTEM
GENERAL
Refer to Figure 27-48 through Figure 27-54.

27 FLIGHT CONTROLS
The wing flaps system consists of inboard and
outboard flaps hinged to each wing; each outboard
flap has an aileron hinged at its trailing edge and
each inboard flap has a trailing flap hinged at
its trailing edge. A selector lever in the flight
compartment controls a hydraulic selector valve,
which determines the position of a hydraulic
actuator. The actuator, through linkage, bellcranks
and push-pull rods, connects to and operates
the wing flaps. An indicator system is provided
to register the position of the flaps. Lights are
provided on each side of the flap selector lever and
inside the flap position indicator pointer.

DESCRIPTION
The inboard (fore and trailing) and outboard
flaps, hinged to each wing, are all interconnected
by push-pull rods, idler levers and bellcranks,
which are connected to a single hydraulic
actuator piston rod by links. The flap selector
lever in the flight compartment, is connected by
cables to a selector pulley assembly mounted on
the selector valve operating shaft; the selector
valve being attached to the flap actuator. The
selector pulley assembly comprises of two
pulleys mounted side by side, either of which
operates the selector valve independently.

The cables from the flap selector lever are

connected to the outboard pulley, and a
follow-up cable system from the flap actuator
piston rod, connects to the inboard pulley.
Hydraulic lines from the aircraft main
hydraulic system connect to the flap selector
valve. A mechanically-operated flap position
indicator is on the windshield center post. To
maintain aircraft trim during flap operation,
an interconnecting system to the right elevator
trim tab is installed.

FOR TRAINING PURPOSES ONLY 27-69

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-49. Maximum Flap Extension (LH Wing)

Figure 27-50. Maximum Flap Extension (RH Wing)

27-70 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Wing Flaps Wing Flap Hydraulic System

Refer to: The wing flap hydraulic circuit lines are tapped
into the main hydraulic system lines at the
•• Figure 27-49. Maximum Flap Extension hydraulic power package beneath the flight
(LH Wing). compartment floor, and are routed to the flap
selector valve on the flap actuator. Restrictions
•• Figure 27-50. Maximum Flap Extension
are provided in the flaps up and flaps down
(RH Wing).
ports of the actuator and a check valve is

27 FLIGHT CONTROLS
fitted in the pressure supply line at the flap
The inboard and outboard flap hinge arms,
selector valve. Both the flap up and down lines,
are hinged to the flap hinge arm attachment
incorporate a thermal relief valve.
brackets, which are bolted to the wing structure.
A common flap hinge arm attachment bracket is
located at station 172.50 and accommodates the
end hinge arms of the inboard fore and outboard
flaps. Fore flap hinge arms at stations 97.50
and 247.15 (approximately) are connected to
the flap system push-pull rods. The inboard
trailing flap hinge arms are hinged to those of
the fore flap, and a connecting rod connects
both flap hinge arms at station 97.50 to control
the travel of the trailing flap.

Figure 27-51. Wing Flap System

FOR TRAINING PURPOSES ONLY 27-71

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Flap Actuator the flaps. Cables connected to the quadrant are

routed over pulleys to connect to the outboard
Refer to Figure 27-57. Wing Flap Hydraulic
pulley on the flap selector valve spindle.
System Components.

The wing flap actuator, with the selector NOTE

valve attached, is mounted in a frame located
The flap selector and the flap
centrally in the cabin roof. The actuator piston
position indicator are calibrated
rod is connected by links to the left and right
from 0 to 40°, but total travel is
27 FLIGHT CONTROLS

operating bellcranks.
37.5°.

Flap Selector Lever NOTE

The flap selector lever assembly, mounted in the
On aircraft with Airframe De-Ice
flight compartment overhead console, comprises
boots, the mod for automatic
a selector lever, quadrant, pulley and cable
inflation of the Horizontal
system, and serrated segment. The selector
Stabilizers deicer boots. There
lever, located behind the engine control levers,
will be a micro switch and cam
is connected by a connecting rod to the quadrant
assembly installed on the cable
immediately forward of the lever. A pawl on the
and input pulley.
lever, engages the serrated segment about which
the lever pivots, for intermediate settings of

Figure 27-52. Flap Control and Indication

27-72 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

NOTE indicator pointer. When the piston rod extends,

the pointer is pulled downwards against
Rigging procedures for the Flap
the tension of the indicator spring. During
Selector Valve are found in PSM
retraction of the piston rod, the tension of
1-63-2 ATA 27.
the spring returns the pointer to the flaps up
position; the spring maintains the cable tension.
Flap Follow-Up System
Restrictors in the flap actuator up and down ports
A cable connected to a bracket on the actuator
limit the rate of fluid flow to provide a smooth

27 FLIGHT CONTROLS
piston rod, is routed to the inboard pulley on
steady movement of the flap surface during
the flap selector valve spindle.
extension and retraction in in-flight conditions.
On the ground the down selection will be faster
Flap Position Indicator than the up selection. The check valve in the
pressure line isolates the flap circuit in the event
The flap position indicator system comprises
of a failure in the hydraulic pressure supply.
of a pointer/fixed scale assembly and a cable
assembly. The pointer/fixed scale assembly is
A flap/elevator trim interconnect ball screw
mounted to the windshield center post and the
jack, attached to the airframe structure and the
cable assembly connects the pointer and flap
right inboard bellcrank, operates during flap
actuator piston head. Pointer flap indication
movement to transmit movement to the flap/
(reading) is accomplished when the actuator
elevator trim interconnect circuit to maintain
piston moves the cable, thereby moving the
aircraft elevator trim during flap operation.
pointer proportionately and, by a spring
attached to the pointer inside the indicator.
A hydraulic schematic of the flap system is
shown in Figure 27-55.
The cable assembly is routed over pulleys and
consists of a cord and carbon steel forward
cable and a carbon steel rear cable. NOTE
It must be remembered that
OPERATION the flaps are held in place by a
hydraulic lock. Before opening
The flap selector lever (Figure 27-52) moves any hydraulic line in the flap
in a slot labeled “FLAPS” with approximate system the flaps must be in the
position settings in 10° increments from 0 to full down position or supported
40° The lever incorporates a lock that must be by a trestle to ensure the flaps
depressed to for movement. do not fall.

Selecting a flap position moves the cables

to position the selector valve that directs
hydraulic pressure to the flap actuator up
or down port. As the flap drive mechanism
moves, a follow-up cable on the bellcrank will
reposition the selector valve to the null position
stopping the flaps at the desired position.

The angle of the inboard trailing flap position

is shown on the flap position indicator scale.

The flap indicator mechanism registers the

flap position by the cable which is connected
between the actuator piston rod head and the

FOR TRAINING PURPOSES ONLY 27-73

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-53. Flap Selector (Rotary Cam and Poppet Assembly)

27-74 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

27 FLIGHT CONTROLS
Figure 27-54. Flap Selector (Planetary Gear Assembly)

FOR TRAINING PURPOSES ONLY 27-75

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

LEFT DC BUS HYDRAULIC DAMPING ACCUMULATOR

RESERVOIR PRESSURE INDICATOR
MOTOR AND AIR CHARGE VALVE
PUMP 10

15
5
HYD PUMP
C/BKR OPEN HAND PUMP 0 20
x 1000
CAUTION LIGHT SYSTEM
RELIEF
27 FLIGHT CONTROLS

VALVE ** NOSEWHEEL
HYDRAULIC (1950 PSI) STEERING
SYSTEM ACTUATOR
PRESSURE PRESSURE
INDICATOR SWITCH *
FILTER DAMPING
SERVO
(10 MICRON) ACCUMULATOR
1000

2000
PRESS
PSI

BRAKE ACCUMULATOR PRESSURE

INDICATOR AND AIR CHARGE VALVE
BRAKE

{
10
ACCUMULATOR TO
15
5

SKIS
0 20 BRAKE
x 1000 RELIEF
1000 VALVE
(1.750 PSI) FLAP
2000
PRESS CONTROL
PSI VALVE
BRAKE SYSTEM
PRESSURE
INDICATOR

DH DH
LEGEND
PRESSURE
PARKING THERMAL RELIEF
SUPPLY BRAKE VALVES (1,750 PSI)
RETURN
NITROGEN BRAKE FLAP
MECHANICAL VALVES ACTUATOR

ELECTRICAL

RESTRICTOR

BRAKE BRAKE
UNIT UNIT

Figure 27-55. Hydraulic System Schematic

27-76 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Flap and Flight Control

Magnaformed Pushrods
For all Series of aircraft the flap and flight
control pushrods incorporate end-fitting
magnaformed in position. Since their
introduction a number of changes have occurred
which are well documented with regards to
Airworthiness Directives, Service Bulletins,

27 FLIGHT CONTROLS
modifications and special inspections. Several
pushrod issues are clarified as follows:

Present pushrods are now manufactured from

6061 material with sleeves also magnaformed
in position over the formed tubular end fitting
(Figure 27-56). The new flap and flight control
pushrods, which are identified in service
bulletin 6/502, are authorized by mods 6/1781,
1785, 1791, and 1802. As the new pushrods are
Figure 27-56. M
 agnaformed Fittings and
cost effective, requiring a 2400 flight hours
Sleeves
or 2 years whichever comes first inspection
interval in lieu of the 800 flight hours or 1
year whichever comes first inspection cycle
for all remaining 2024T81 material pushrods,
the change is beneficial to the overall operation
of the aircraft. All existing 2024T3 flight
control pushrods, that were allowed to remain
in service with sleeves installed, are now time
limited and should be removed from service
within the specified time frame permitted
by their applicable Airworthiness Directive
requirements.

Transport Canada has now revised and released

both Airworthiness Directive CF 79-22R2
and CF80-03R4 pertains to the flap and flight
control pushrods respectively for all series
aircraft under their jurisdiction. The Directive
identifies the latest 6061 material pushrods now
available from the aircraft manufacturer and the
mandatory replacement of all 2024T3 material
flight control pushrods with sleeves installed
that may still exist throughout the fleet.

FOR TRAINING PURPOSES ONLY 27-77

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-57. Wing Flap Hydraulic System Components

27-78 FOR TRAINING PURPOSES ONLY
 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Flap Rigging Plates

Refer to:

•• Figure 27-58. R
 ight Fuselage Flap
Rigging Plate.
•• Figure 27-59. L
 eft Fuselage Flap
Rigging Plate.

27 FLIGHT CONTROLS
Located on each side of the fuselage are rigging
plates for the full up position. Located on the
fore flaps will be plates to be used to position
the fore flaps in the full up position.

Figure 27-58. R
 ight Fuselage Flap Rigging
Plate

Figure 27-59. L
 eft Fuselage Flap Rigging
Plate

FOR TRAINING PURPOSES ONLY 27-79

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-60. Wing Flap Clearances

27-80 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Wing Flap Clearances NOTES

Refer to:

•• Figure 27-60. Wing Flap Clearances.

•• Figure 27-61. Fore Flap Rigging Plate.

These measurements are critical and must be

maintained to ensure full STOL capabilities.

27 FLIGHT CONTROLS
The rubber stops on the flap arms will minimize
between the flap arms.

Figure 27-61. Fore Flap Rigging Plate

FOR TRAINING PURPOSES ONLY 27-81

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-62. Rod End Adjustment

Figure 27-63. Rudder Cable Tension (5/32-Inch Cable)

27-82 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of holes in the rod ends so that the original
the maintenance practices and is intended for distance is maintained.
training purposes only.
When a pushrod is replaced, carefully measure
For a more detailed description of the practice,
the distance between rod end centers in the
refer to the task in the Viking AMM PSM 1-63-2.
old pushrod and then position rod ends of
the replacement pushrod so that the original
27-00-00 MAINTENANCE distance is maintained.
PRACTICES

27 FLIGHT CONTROLS
Ensure that all rod ends are within witness
hole limits after adjustments are made.
GENERAL MAINTENANCE (Figure 27-62).
PRACTICES
CONTROL SURFACE
The maintenance practices found in Chapter
27 of the Maintenance Manual usually relate
CLEARANCES
to removal/installation and inspection/check
Maximum and minimum clearances of control
procedures. The following maintenance
surfaces are provided in Chapter/Section 27-00-
practices are applicable to the flight control
00 of the Maintenance Manual.
system in a general sense.

If an aileron or tab, or elevator or tab, has been INSPECTION/CHECK

repaired or has had additional paint applied, the
assembly must be balanced as described in the Inspection of Wing Flap Skins and
Structural Repair Manual, PSM 1-6-3.
Drain Holes
Inspect wing flap skins and drain holes for
NOTE condition, drain holes for obstruction and wing
The operator can check the tip fairings for condition.
rudder balance, but if a rudder
is found to be out of balance as
per the instructions shown in the
PSM 1-6-3 Repair Manual, the
manufacturer must be consulted.

Exercise speci al care when perform ing

maintenance on the flight control system, with
emphasis on proper use and placement of
attaching hardware and the possibility of fouling
the controls with mislaid tools and parts.

When performing maintenance, do not

disturb pushrod end fittings unless absolutely
necessary. If repositioning is required, the
exact position of each rod end fitting should be
recorded so that it can be returned to its original
location when maintenance is completed.

When necessary to replace a rod end fitting,

carefully measure between centers of attaching

FOR TRAINING PURPOSES ONLY 27-83

27-84 27 FLIGHT CONTROLS

SYMBOL TITLE MIL SPEC

A GREASE-LOW 3-GP-683A A
AND HIGH OR 500
TEMPERATURE MIL-G-25760
OR 1
MIL-G-3278 (NOTE 1)
C
MIL-PRF-23827 1500
MIL-G-7118 4 B
PARTS NOMENCLATURE KEY B OIL-GENERAL 3-GP-335A 500
1. FLAP/ELEVATOR TRIM, ELEVATOR TRIM, PURPOSE OR 5
RUDDER TRIM, SCREW JACKS MIL-L-7870A (NOTE 2)

TWIN OTTER SERIES

2. FLAP SELECTOR GEAR MECHANISM C SILICONE MIL-S-8660
3. CONTROL COLUMN CHAIN AND SPROCKETS COMPOUND
4. ALL PIANO TYPE HINGES C
5. ELEVATOR TRIM PULLEY BRACKET BUSHES 1500
6. FLAP/ELEVATOR BALL SCREW JACK A 4
FOR TRAINING PURPOSES ONLY

7. WING ROOT RIB/FLAP PUSHROD ROLLERS: 500

INSIDE SURFACE OF SPACER AND BOLT. 6
(NOTE 1)

A
1500
2 A

MAINTENANCE TRAINING MANUAL

500
1
A (NOTE 1)
1000
3

APPLICATION SYMBOLS

A GREASE GUN OIL CAN HAND

1000
7

000
FREQUENCY SYMBOL
NOTE 1: SHAFT EXTERNALLY, JACK FULLY EXTENDED
NOTE 2: USE GREASE (A) ON ASSEMBLY ONLY, REFER TO
MAINTENANCE MANUAL.

Figure 27-64. Lubrication Diagram

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

SERVICING NOTES
Figure 27-64 shows part of the lubrication
diagram for flight control system components.
Refer to Chapter/Section 27-20-l0 of the
Maintenance Manual for the complete diagram.

Bleed Wing Flap System

27 FLIGHT CONTROLS
NOTE
Bleeding the wing flap system
need only be accomplished if
the flap operation is not smooth,
a hydraulic line or component
has been replaced, or the main
hydraulic system has been
dismantled.

1. Check that hydraulic reservoir is full.

2. Switch on electrical power by setting
EXTERNAL–BATTERY switch to
EXTERNAL (for use with external power
source) or BATTERY, DC MASTER switch
to on, and check that BUS TIE switch is set
to NORMAL.
3. Set HYD OIL PUMP circuit breaker.

NOTE
Observe that motor pump
charges accumulators and that
motor cuts out when pressure
reaches 1550 +50 –0 psi or 1575
±50 psi (Mod 6/1570).

4. Check that flap area is clear of equipment

and personnel.
5. Select flaps down and allow them to travel
to full down position, select flaps up and
allow them to travel to full up position.
Repeat for ten full selections to ensure
complete bleeding of system.
6. Check flap system for smooth operation.
7. Switch off electrical power.

FOR TRAINING PURPOSES ONLY 27-85

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-65. Cable Inspection

27-86 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

INSPECTION/CHECK broken wires is by running a lint-free rag along

the cable. The rag will catch on any projections.
General If worn, frayed or corroded cables are suspected,
slacken turnbuckles to allow careful twisting
Inspection/check procedures are provided in
of cables to open strands for a more detailed
the Maintenance Practices paragraph of each
inspection. Replace cables if worn or corroded and
chapter/section of the Maintenance Manual,
if wear limits in 20-60-01, Inspection, Cleaning
when required.
and Lubrication of 7x19 and 7x7 Control Cables -
General Data are exceeded. Always replace cables

27 FLIGHT CONTROLS
Cable Inspection where serviceability is marginal.
Control cables are subjected to a variety
Remove corrosion and or zinc dust from the
of environmental conditions and forms of
external surfaces in accordance with 20-60-01,
deterioration that result in wire/strand breakage
Inspection.
or corrosion. Broken wires can be detected by
passing a cloth along the length of the cable
(Figure 27-65).

The absence of snags is not positive proof that

broken wires do not exist. If in doubt, remove
the cables and bend into a loop as shown.
Broken wires will then be readily apparent.

Pulley Inspection
Cable pulleys should be periodically rotated
to provide a new bearing surface for the cable,
since it sometimes operates in a small arc.
Various cable system malfunctions may be
analyzed by observing pulley wear patterns.
(Figure 27-67) shows common wear patterns
related to particular malfunctions.

Inspection of Control Cables

Inspect control cables at prescribed inspection
intervals to ensure their serviceability. Before
inspecting cables installed in an aircraft, ensure
that all accessible portions of the cable run are
clean. Dust and metal particles will readily adhere
to grease and impair smooth operation of the
controls. Cables in the cabin area of fuselage,
though not accessible along their entire run, can
be cleaned and inspected at access points where
they pass over pulleys or through fairleads where
broken wires and wear are most likely to occur.
Move flight controls through their full range
of travel to allow for maximum cable length
inspection. A control tending to foul momentarily
is often an indication of a frayed cable fouling on
a fairlead or pulley. A simple method of detecting

FOR TRAINING PURPOSES ONLY 27-87

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 3: Access and Inspection Provisions

Figure 27-66. Access and Inspection Provisions

27-88 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

General Rigging Instructions NOTES

The rigging of each control system of the flight
controls is detailed in the MAINTENANCE
PRACTICES of the individual systems.
For each procedure it is assumed that all
components are correctly installed, that the
cables are fitted but not tensioned, and that all
jam nuts on push-pull rods and turnbuckles are

27 FLIGHT CONTROLS
locked on the completion of rigging. It is also
assumed that access panels and upholstery are
removed, as required, for accomplishing these
procedures and are replaced on completion of
the operation. The hydraulic system must be
fully serviceable before wing flap, aileron, or
flap/elevator interconnect trim rigging is started.

Tensioning of Control Cables

Whenever flight control system cables are
slackened or replaced, they must be retensioned
to the value shown in the AMM in chapter 27
under graphs. Where possible, tensiometer
readings are to be taken on bare cable. For cables
fully covered in nylon, nominal values 15% lower
than those shown in the AMM in the graphs
section are to be used. When checking cable
tensions, observe the following precautions:

1. Ensure that tensiometer is of correct type

for cable.
2. Vibrate cables to prevent an incorrect
reading due to friction in circuit.
3. Position tensiometer on center portion of
cable and not close to a quadrant, bellcrank,
turnbuckle, etc.
4. Recheck cable tensions after locking
turnbuckles.

FOR TRAINING PURPOSES ONLY 27-89

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
27 FLIGHT CONTROLS

Figure 27-67. Pulley Wear Patterns

27-90 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

FAULT ANALYSIS
Isolation of a fault or malfunction can be
accomplished by a systematic analysis of the
trouble, beginning with the most probable cause
and progressing to the least probable cause.
Any system(s) interfaced with the troubled
system should be operating properly prior to
troubleshooting.

27 FLIGHT CONTROLS
Aircraft: Revision No. 10 Page:
DE HAVILLAND DHC-6, SERIES 100, 200 & 300 Date: Jun. 14, 2002 27-1 of 1
System & 1. 2. Number Installed
Sequence 3. Number Required for Dispatch
Numbers 4. Remarks or Exceptions

27 FLIGHT CONTROLS

1 Aileron Trim Tab Indicator C 1 0 (O) Provided the aileron trim tab is visually
checked for full and free movement, and is
confirmed neutral prior to each flight.

2 Aileron Trim Control C 1 0 (M) Provided:

a) the aileron trim tab is confirmed
neutral prior to each flight; and,
b) the aileron trim circuit breaker is
pulled.

3 Rudder Trim Tab Indicator C 1 0 (O) Provided the rudder trim tab is visually
checked for full and free movement, and is
confirmed neutral prior to each flight.

4 Stall Warning Light C 1 0 (O) May be inoperative provided a stall

warning horn is installed and operative.

Figure 27-68. MMEL - Flight Controls

FOR TRAINING PURPOSES ONLY 27-91

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

27-00-00 SPECIAL TOOLS

ITEM NO. DESCRIPTION FUNCTION IDENTIFICATION
-1 Airframe rigging kit Used to rig flight controls comprising SD12572–1
SD12567–1, SD12568–1 and
SD12571–1

-2 Rigging pin kit See item 1 comprising pins SD10542–1, SD12567–1

27 FLIGHT CONTROLS

SD10543–1 and SD10544–1

3 Rigging pin, aileron elevator Rigging pin (included in kit SD12567–1) SD10542–1

4 Rigging pin, aileron Rigging pin (included in kit SD12567–1) SD10543–1

5 Rigging pin, aileron Rigging pin (included in kit SD12567–1) SD10544–1

6 Wrench, control column To attain access to control column SD12575–1

locknut
7 Rudder travel scale To measure travel of rudder (included in SD12568–1
kit SD12572–1)

-8 Tool roll Holds loose tools (included in kit SD12571–1

AD12572–1)

9 Rigging tool, elevator Used with gust lock SD12510–7 C6GT1047–1

10 Rigging pin Attached to and used with item 9 C6GT1048–1

27-92 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CHAPTER 30
ICE AND RAIN PROTECTION
CONTENTS
Page

30-00-00 ICE AND RAIN PROTECTION................................................................... 30-1

Introduction......................................................................................................... 30-1
General................................................................................................................ 30-1
Aircraft Deicing Certification Modifications................................................ 30-3
30-10-00 WING AND TAIL DEICING SYSTEM........................................................ 30-3
General................................................................................................................ 30-3

30 ICE AND RAIN PROTECTION

Description........................................................................................................... 30-5
Bleed Air Pressure......................................................................................... 30-5
Distributor Valves.......................................................................................... 30-5
Distributor Valve Heater Jacket..................................................................... 30-6
Ejector.......................................................................................................... 30-6
Electronic Timer............................................................................................ 30-6
Water Separators........................................................................................... 30-6
Deicing Boots............................................................................................... 30-6
Horizontal Stabilizer Deicer Boot Indicator Lights........................................ 30-9
Horizontal Stabilizer Deicing Pressure Switches........................................... 30-9
Airframe Deicing System Control................................................................. 30-9
Horizontal Stabilizer Automatic Deicer Boot Operation.......................................................30-11
Control............................................................................................................... 30-11
Automatic System Check................................................................................... 30-13
30-10-21 ELECTRONIC TIMER.............................................................................. 30-15

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Page
General.............................................................................................................. 30-15
Servicing............................................................................................................ 30-15
30-10-21 MAINTENANCE PRACTICES.................................................................. 30-16
Adjustment/Test................................................................................................. 30-16
Operational Test.......................................................................................... 30-16
Functionally Test Wing and Tail De-icing System........................................ 30-16
Inspection/Check................................................................................................ 30-17
Inspection of the Wing and Tail De-icer Boots............................................ 30-17
Inspection of the Stall Bar........................................................................... 30-17
30 ICE AND RAIN PROTECTION

30-20-00 ENGINE AIR INTAKE ANTI-ICING SYSTEM........................................ 30-19

General.............................................................................................................. 30-19
Control............................................................................................................... 30-19
Operation........................................................................................................... 30-21
30-20-00 MAINTENANCE PRACTICES.................................................................. 30-21
Adjustment/Test................................................................................................. 30-21
Operational Test Engine Air Intake Anti-icing System................................. 30-21
30-30-00 PITOT HEATING SYSTEM....................................................................... 30-23
General.............................................................................................................. 30-23
Control............................................................................................................... 30-23
Operation........................................................................................................... 30-25
30-30-00 MAINTENANCE PRACTICES.................................................................. 30-26
Inspection/Check................................................................................................ 30-26
Inspection of the Pitot Heads and Static Vents............................................. 30-26
Operational Test................................................................................................. 30-26
PITOT HTR L and R................................................................................... 30-26

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Page
30-40-00 WINDSHIELD HEATING SYSTEM......................................................... 30-29
General.............................................................................................................. 30-29
Control............................................................................................................... 30-29
Operation........................................................................................................... 30-31
30-40-11 WINDSHIELD HEAT RELAY BOX.......................................................... 30-33
Description......................................................................................................... 30-33
30-40-11 MAINTENANCE PRACTICES.................................................................. 30-33
Inspection/Check................................................................................................ 30-33
Inspection of the Wing Windshield.............................................................. 30-33

30 ICE AND RAIN PROTECTION

30-45-01 WINDSHIELD WIPER SYSTEMS............................................................ 30-35
General.............................................................................................................. 30-35
Control............................................................................................................... 30-35
Operation........................................................................................................... 30-37
30-45-01 MAINTENANCE PRACTICES.................................................................. 30-37
Adjustment/Test................................................................................................. 30-37
Operational Test Windshield Wiper System................................................. 30-37
30-00-00 WINDSHIELD WASHER SYSTEM.......................................................... 30-38
Control............................................................................................................... 30-38
Operation........................................................................................................... 30-39
30-60-00 PROPELLER DEICING SYSTEM............................................................. 30-41
General.............................................................................................................. 30-41
30-60-12 BRUSH MODULE UNITS............................................................................ 30-43
Brush Block Slip Ring Assembly....................................................................... 30-43
Oil Pressure Switches......................................................................................... 30-43
30-60-12 MAINTENANCE PRACTICES.................................................................. 30-45

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Page
Removal/Installation.......................................................................................... 30-45
Remove Brush Module Assembly................................................................ 30-45
Install Brush Module Assembly................................................................... 30-45
30-60-21 CONTROL BOX........................................................................................ 30-46
30-60-31 TIMER....................................................................................................... 30-47
Control............................................................................................................... 30-47
Indication........................................................................................................... 30-47
Operation........................................................................................................... 30-48
Surface Protection.............................................................................................. 30-48
30 ICE AND RAIN PROTECTION

30-00-00 MAINTENANCE PRACTICES.................................................................. 30-48

General Maintenance Practices........................................................................... 30-49
Deicer Boot Modification.................................................................................. 30-49
Servicing............................................................................................................ 30-49

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 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

ILLUSTRATIONS
Figure Title Page
30-1 Wing Boot Deicing System........................................................................30-2
30-2 Wing Boots................................................................................................30-3
30-3 Tail Boots..................................................................................................30-3
30-4 Wing and Horizontal Stabilizer Deicer Boots Operation............................30-4
30-5 
Low Pressure Bleed System Components...................................................30-5
30-6 Wing Distributor Valve..............................................................................30-5
30-7 Tail Distributor Valve.................................................................................30-5
30-8 MMEL - Wing Deice System.....................................................................30-7

30 ICE AND RAIN PROTECTION

30-9 Anti-icing and Deicing Controls and Indicators.........................................30-8
30-10 Wing and Tail Deicing System Electrical Schematic (S.O.O. 6004).........30-10
30-11 Wing Flap Selector and Indication...........................................................30-12
30-12 STAB DEICE PRESS Indicator Lights....................................................30-13
30-13 Transistorized Sequential Timer...............................................................30-14
30-14 Transistorized Sequential Timer - Electrical Schematic...........................30-14
30-15 Deice Panel..............................................................................................30-15
30-16 Engine Air Intake Anti-Icing Boot...........................................................30-18
30-17 Engine Air-Intake Anti-Icing Electrical Schematic...................................30-18
30-18 MMEL - Engine Inlet Deicing Boots.......................................................30-20
30-19 Intake Anti-Ice Switch Panel....................................................................30-20
30-20 Pitot Probe...............................................................................................30-22
30-21 Stall Vanes...............................................................................................30-22
30-22 PITOT HEAT Control Switch..................................................................30-22
30-23 Engine Air-Intake Anti-Icing Electrical Schematic...................................30-24

FOR TRAINING PURPOSES ONLY 30-v

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure Title Page

30-24 MMEL - Pitot Heaters.............................................................................30-26
30-25 Windshield Control Panel........................................................................30-28
30-26 Windshield Heating Electrical Controllers...............................................30-28
30-27 
Overhead Circuit Breaker Panel...............................................................30-29
30-28 Windshield Heat Operation......................................................................30-30
30-29 Windshield Heat Relay Box.....................................................................30-32
30-30 Windshield Wiper System Installation.....................................................30-34
30-31 Windshield Wiper Control.......................................................................30-35
30-32 Pilot Preferred Wiper Position.................................................................30-35
30 ICE AND RAIN PROTECTION

30-33 Windshield Wiper Operation....................................................................30-36

30-34 Windshield Washer Reservoir..................................................................30-38
30-35 Washer Control........................................................................................30-38
30-36 MMEL - Windshield Wipers....................................................................30-39
30-37 Propeller Deicing Boots...........................................................................30-40
30-38 Prop DEICE Switch.................................................................................30-40
30-39 Modular Brush Block Assembly..............................................................30-42
30-40 
Prop Slip Ring on Back of Prop Bulk Head.............................................30-43
30-41 Oil Pressure Switch.................................................................................30-43
30-42 Brush Module Assembly - Checking Brush Wear.....................................30-44
30-43 Propeller Deicing Control Box.................................................................30-46
30-44 Prop Deicing Timer..................................................................................30-47
30-45 Prop Deicing Timer..................................................................................30-48
30-46 MMEL - Propeller Deicing Systems........................................................30-48

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TABLES
Table Title Page
30-1 De-Icer Boots Timing Sequence...............................................................30-16

30 ICE AND RAIN PROTECTION

FOR TRAINING PURPOSES ONLY 30-vii

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CHAPTER 30
ICE AND RAIN PROTECTION

30 ICE AND RAIN PROTECTION

30-00-00 ICE AND RAIN PROTECTION
INTRODUCTION
This chapter describes the ice and rain protection systems installed (or available as options)
on DHC-6 Twin Otter airplanes. All values given, such as for temperature, time, or pressure,
may not be current information. Actual values may be obtained from the manuals and
publications issued by or on behalf of the airplane manufacturer.

GENERAL
The customer-option deicing and anti-icing Electrical heating elements integral within the
systems provide for wing, tail, propeller, and windshield, deice the windshield. Fuselage
windshield deicing, and engine air intake anti- side skin protection from propeller blade ice
icing. Wing and tail deicing is accomplished is presently achieved with metal ice shields in
with pneumatically operated boots. Propeller lieu of the celastic material previously used.
deicing and engine air intake anti-icing are
both accomplished with electrically heated
boots, each system operating independently.

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 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

DISTRIBUTOR
LEFT AND RIGHT VALVE
STABILIZER BOOT
PRESSURE EJECTOR DEICER
SWITCHES BOOTS
(MOD 6/1393)

HEATED JACKET

DISTRIBUTOR WATER
VALVE SEPARATOR
WATER SEPARATOR
30 ICE AND RAIN PROTECTION

ELECTRONIC
TIMER

WATER PRESSURE
SEPARATOR SUPPLY
DEICER (PRE MOD 6/1440) REFER TO
BOOT PNUEMATIC FS 480 0 DISTRIBUTOR
SYSTEM FOR VALVE
SWITCH DETAILS EXHAUST
PANEL OF THIS AREA
PRESSURE
SWITCH

WATER
SEPARATOR MOD 6/1440

WATER SEPARATOR
SUCTION
TO SUPPLY
DEICER
LOW PRESSURE BOOT
WARNING LIGHT HEATED JACKET

WS 260 0
PRESSURE SUCTION
TEST POINT TEST POINT
DEICER
BOOT

Figure 30-1. Wing Boot Deicing System

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 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Aircraft Deicing Certification Metal stall bars, which were riveted to the wing
leading edges on non deiced aircraft, are removed
Modifications and replaced with neoprene material stall bars
To fulfill aircraft certification requirements bonded in position to the leading edge surface
for all series aircraft to operate in known of the deicing boot. The horizontal stabilizer
icing conditions, the following optional leading edge surfaces must be protected from
modifications are required: damage caused by ice thrown off by the wing and
propeller ice deice systems by a nylon cap bonded
Airframe deicing.......................... S.O.O. 6004 to the metal leading edge surface. The horizontal
stabilizer deicing boots are then installed over the
Propeller deicing.......................... S.O.O. 6005 bonded nylon cap surface.

Wing inspection lights................. S.O.O. 6006 The wing and tail boots (Figure 30-2 & Figure
30-3) are pneumatically operated and electrically
Heated windscreens...................... S.O.O. 6006 controlled. Manual control is selective,
permitting the pilot to select either the inner or
Replaced by.................. S.O.O. 6187 (C.A.A.) outer wing boots or the left or right tail boots.

Windscreen wipers...................S.O.O. 6009 or

30 ICE AND RAIN PROTECTION

Windscreen wipers....................... S.O.O. 6157
now basic by 6/1607

Optional modifications for all series aircraft

that may be installed but are not mandatory to
operate in known icing conditions.

Engine air intake deicing.............. S.O.O. 6062

Fuselage ice guards...................... S.O.O. 6080

(Celastic) replaced by.......S.O.O. 6168 (metal)

Figure 30-2. Wing Boots
30-10-00 WING AND
TAIL DEICING SYSTEM
GENERAL
The wing and tail deicing system (Figure 30-1) is
electrically controlled and utilizes bleed air from
the engine compressors to actuate rubber boots on
the leading edges of both wings and the horizontal
stabilizer. Inflation/deflation pulses flex both wing
boot inner and outer sections and the left and right
horizontal stabilizer boots in sequence to break
up ice formations. Wing inspection lights are
available for determining ice thickness on the wing
leading edges during night operations.
Figure 30-3. Tail Boots

FOR TRAINING PURPOSES ONLY 30-3

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
Figure 30-4. Wing and Horizontal Stabilizer Deicer Boots Operation
30 ICE AND RAIN PROTECTION 30-4 FOR TRAINING PURPOSES ONLY
 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

DESCRIPTION Electrical heater jackets are bonded to each

distributor valve to prevent moisture and ice
Bleed Air Pressure from inhibiting its operation. Power for the
heater jackets is from the right DC bus through
Pneumatic operating pressure for deicing boot
a 7.5 amp circuit breaker labeled AFR DE ICE
inflation is obtained from the bleed air system,
MAN on the main circuit breaker panel and
which first passes through a heat exchanger located
controlled by the VALVE HTR switch on the
in the cabin roof to reduce the bleed air temperature
overhead console. (Figure 30-9).
(Figure 30-4). The cool air is then filtered by
a mesh wire element to remove contamination
before passing through a regulator which reduces
the input pressure to a working pressure of 18 WATER SEPARATOR DISTRIBUTOR
psi. The regulator is set to relieve pressure at 25 VALVE
psi should the regulator fail. The air is passed
through water separators into the distributor valves
before entering the deicer boot segments. The
deicing system includes an ejector operating on the HEATER
venturi principle to create a vacuum to purge air JACKET
pressure from within the boots during the deflation
operation of the distributor valves.

30 ICE AND RAIN PROTECTION

Distributor Valves
There are three distributor valves: one in each Figure 30-6. Wing Distributor Valve
wing at Stn 260.00 and one in the fuselage aft
section at Stn 486.00 (Figure 30-4 & Figure
30-5). Each distributor valve has a pressure
inlet port, suction outlet port, two ports to the DISTRIBUTOR
deicer boots, an exhaust port vented overboard VALVE
in a low-pressure area, and two control RIGHT AND LEFT
STABILIZER BOOT
solenoids that are energized in a sequence PRESSURE
EJECTOR
determined by the electronic timer. SWITCHES
MOD 6/1393

PRESSURE HEATER
REGULATOR JACKET

ELECTRICAL
WATER CABLE
SEPARATOR

Figure 30-7. Tail Distributor Valve

HEAT
ELECTRONIC EXCHANGER
TIMER

Figure 30-5. L
 ow Pressure Bleed System
Components

FOR TRAINING PURPOSES ONLY 30-5

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Distributor Valve Heater Jacket system is inoperative, such as a disengaged

circuit breaker, the other type of operation will
Electrical heater jackets are bonded to each
function, assuming the major components are
distributor valve to prevent moisture and ice
operational. If the auto selection is selected
from inhibiting its operation. Power for the
to off, the timer will continue to run until the
heater jackets is from the right DC bus through
cycle is completed.
a 7.5 amp circuit breaker labeled AFR DE ICE
MAN on the main circuit breaker panel and
controlled by the VALVE HTR switch on the Water Separators
overhead console (Figure 30-9).
Water separators with filter were are to
overcome the problems of water condensation
WARNING from bleed air entering the distributor valves
and deicing boots. There are three water
Separation of the heater jacket separators, one in each wing and one in the
from the valve body will cause rear fuselage before the distributor valves.
an overheat condition which may Water separators are outboard in the wing
result in heater jacket burning and beside the distributor valves. With the wing
loss of body heat to dissipate water water separators closer to the distributor valve,
ingress from within the valve. the moisture content through the valve would
30 ICE AND RAIN PROTECTION

be negligible and easily dissipated by heater

jacket heat.
Ejector
Refer to Figure 30-7. Tail Distributor Valve.
Deicing Boots
An ejector is in the rear fuselage section, aft of Refer to;
the horizontal stabilizer distributor valve. The
ejector operates on the venturi principle to create •• Figure 30-2. Wing Boots.
a vacuum for deicer boot deflation. The ejector
•• Figure 30-3. Tail Boots.
will operate continuously once engine operation
has commenced providing the bleed air valve is
Inflatable deicing boots are bonded to the
selected open for that particular engine.
leading edge of each wing from a point just
outboard of the wing landing light to the
Electronic Timer wingtips. The wing boots include inward and
outward sections for separate pressurization
Refer to Figure 30-5. L
 ow Pressure Bleed
as controlled by the timer unit. The tail boots
System Components.
are supplied in one single section covering the
length of the leading edge skin. A nylon cap is
The electronic timer, in the cabin roof, provides
bonded over the horizontal stabilizer leading
cycling of the distributor valve solenoids. The
edge skin to protect the metal surface from ice
timer consists of a metal box with an external
damage. The reinforcement caps are mandatory
connector receptacle, a rotary circuit selector,
for all airframe-deiced aircraft. Aircraft with
and component board and associated wiring.
the airframe deicing system removed can still
Timer location is shown in Figure 30-1. The
retain the bonded reinforcement caps on the
timer receives power from the left DC bus
horizontal stabilizer leading edge skins for
through the 5 amp circuit breaker labeled AFR
normal operations. The deicer boots are bonded
DEICE AUTO on the main circuit breaker
to each leading edge and connected to their
panel. In the manual mode, power is obtained
appropriate distributor valve with hoses.
from the right DC bus through a 5 amp circuit
breaker labeled AFR DEICE MAN also on
Operators should be aware that deicer boot
the main circuit breaker panel. Both power
deterioration caused by puncture holes on the
sources operate the electric timer. If one

30-6 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

outer surfaces is the outcome of static electrical

build up on the boot surface discharging to
atmosphere or the metal surface beneath the
boot. To minimize the puncture condition an
application of conductive cement should be
applied to the edge of the deicer boot and
surrounding structure. For ice removal it is
essential that the boots are fully operational
at all times, as perforated boots will not
completely pressurize.

When the wing deicing boots are installed,

a neoprene material stall bar is bonded in
position to the leading edge of the boot in lieu
of the metal riveted stall bar.

Aircraft: Revision No. 11 Page:

DE HAVILLAND DHC-6, SERIES 100, 200 & 300 Date: Dec. 23, 2002 30-3 of 3

30 ICE AND RAIN PROTECTION

System & 1. 2. Number Installed
Sequence 3. Number Required for Dispatch
Numbers 4. Remarks or Exceptions

30 ICE AND RAIN PROTECTION

8 Automatic Surface Deicing C 1 0 Provided the manual function is confirmed

*** System Function (SOO 6004) operative prior to each flight.

9 Stabilizer Deicing B - 0 One or both may be inoperative provided

*** Pressure Indicator Lights flight is not conducted in known or forecast
icing conditions.

Figure 30-8. MMEL - Wing Deice System

FOR TRAINING PURPOSES ONLY 30-7

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
30 ICE AND RAIN PROTECTION

Figure 30-9. Anti-icing and Deicing Controls and Indicators

30-8 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Horizontal Stabilizer Deicer Boot Manual momentary selection of wing deicer

boots is made with the switch labeled WING
Indicator Lights INNER and WING OUTER. Adjacent to this
Two blue press to test indicator lights labeled switch is another manual mode switch for
“stab deice press” identified left and right are momentary inflation of the horizontal stabilizer
by SFAR 23 requirement on the rear face of the boots. It is labeled LEFT STAB and RIGHT
overhead console (Figure 30-9). Their purpose STAB. A two position switch labeled VALVE
is to indicate pressure in the pressure lines HTR controls the power to the heater jackets
from the distributor valve to the horizontal on the three distributor valves.
stabilizer deicer boots. Each light is operated
by a pressure switch in the left or right pressure The system is normally operated in the
line to the left and right stabilizer deicer boot to automatic mode. By selecting the mode selector
bring on the light when pressure is metered to to AUTO, the cycle selector switch is set for
that particular boot. The lights come on every the desired cycle, fast or slow (fast for heavy
inflation cycle whether in the automatic or the ice buildup, slow for light ice buildup). The
manual mode. The lights are powered through electronic timer operates to inflate the inner
a 5 amp or 7.5 amp circuit breaker labeled portion of the wing boots for five seconds, the
AFR DEICE MAN on the main circuit breaker outer portion of the wing boot for five seconds,
panel. The rim of each light can be rotated for the left stabilizer boot for three seconds,

30 ICE AND RAIN PROTECTION

dimming. and finally the right stabilizer boot for three
seconds. Then the timer enters a dwell mode of
either 44 seconds (fast cycle) or two minutes
Horizontal Stabilizer Deicing and 44 seconds (slow cycle), as determined by
Pressure Switches system selection.
Two pressure switches are by SFAR 23 at the
outlet ports of the horizontal stabilizer distributor
valve, they turn on the indicator lights when the
horizontal stabilizer deicer boot is pressurized.
The normally open contacts of the switch close
with increasing pressure of 15 psi to provide a
current path to bring on the light.

Airframe Deicing System Control

The system is controlled by switches on the
overhead console deice switch panel under the
broad heading DEICER BOOTS (Figure 30-9).
From the left to right the switches are:

•• A u t o m a t i c o r M a n u a l m o d e o f
operation is selected with a three
position mode selector switch labeled
MANUAL-OFF-AUTO.
•• Electronic timer cycling speed is selected
with a two-position cycle selector switch
labeled FAST and SLOW.

FOR TRAINING PURPOSES ONLY 30-9

30-10 30 ICE AND RAIN PROTECTION

TWIN OTTER SERIES

FOR TRAINING PURPOSES ONLY

MAINTENANCE TRAINING MANUAL

Figure 30-10. Wing and Tail Deicing System Electrical Schematic (S.O.O. 6004)
 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

HORIZONTAL STABILIZER The K1 and K2 relays have a three second

time delay operation and K3 relay has a twelve
AUTOMATIC DEICER BOOT second time delay time operation before the
OPERATION relay coils are energized. All four relay A2
to A3 contacts are normally closed when the
Ice accumulation on the horizontal stabilizer relays are de-energized.
surface when flaps are extended is a condition,
which can cause loss of control. Several
aircraft incidents are believed to have occurred
with the aircraft in this condition. To prevent
similar incidents occurring an automatic
operation of the horizontal stabilizer deicer
boots whenever the airframe deicing system
is not operating and the flaps are extended
during approach. The bleed air valves must
always be open for the system to operate, as
air pressure is necessary.

The automatic inflation concept is

30 ICE AND RAIN PROTECTION

recommended by the aircraft manufacture
for all aircraft certified for ice operations
to combat the condition of undetected ice
adhering to the horizontal stabilizer surfaces
during flight.

CONTROL
Automatic operation of the horizontal
stabilizer deicer boots will occur during flap
extension when airframe deicing AUTO/
MANUAL switch is selected OFF and bleed
air is available. A microswitch operated by
cam rotation, which responds to flap extension,
controls power application for tail distributor
valve operation. The contoured cam will
close the microswitch contacts when flaps
are initially extended beyond 5 degrees. The
cam is contoured to open the switch contacts
between 10 to 12 degrees to deactivate the
system before closing again should the flaps be
further extended to between 15 and 17 degrees
to full flap deployment. A relay panel labeled
HORIX STAB DEICE BOOTS 10° FLAP
CONT in the cabin roof adjacent to terminal
block TB25 has four relays, three of which
K1 to K3 are time delayed relays to control
the pressure sequence of distributor valve and
deicer boot operation.

FOR TRAINING PURPOSES ONLY 30-11

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
Figure 30-11. Wing Flap Selector and Indication
30 ICE AND RAIN PROTECTION 30-12 FOR TRAINING PURPOSES ONLY
 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

AUTOMATIC SYSTEM CHECK

Refer to:

•• Figure 30-11. Wing Flap Selector and

Indication.
•• Figure 30-12. STAB DEICE PRESS
Indicator Lights.

The system is checked with the airframe deicing

system AUTO/MANUAL select switch in the
OFF position and both engine bleed air valves
in the open position for bleed air pressure
supply. With flap extended to 5 degrees, both
horizontal stabilizer boots will cycle two
times. The initial deicer boots operation will
occur within the 10.5 degree flap extension
range. When the flaps are beyond 12 degrees,
the electrical signal will be disabled. Further

30 ICE AND RAIN PROTECTION

flap extension between 15 and 17 degrees to
full flap extension will allow the stabilizer
deicer boots to cycle again two times. With
the airframe deicing system switch selected to
MANUAL or AUTO the automatic operation of
the stabilizer deicer boots will not occur when
flaps are fully extended.

Figure 30-12. STAB DEICE PRESS Indicator Lights

FOR TRAINING PURPOSES ONLY 30-13

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 30-13. Transistorized Sequential Timer

30 ICE AND RAIN PROTECTION

Figure 30-14. Transistorized Sequential Timer - Electrical Schematic

30-14 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

30-10-21 ELECTRONIC unijunction transistor as shown in the electrical

schematic diagram (Figure 30-14). The box
TIMER has a cover which is sealed with silicon rubber
and secured with four screws, it also has two
flanges which are drilled to facilitate mounting
GENERAL the complete assembly.
The Bendix transistorized sequential timer
(Part No. 42E13–3B), provides the energizing SERVICING
voltage to the wing and tail de-icing distributor
valves, in the system. The timer, in the left Since the transistorized sequential timer is an
side of the cabin roof between fuselage station electronic assembly, complete disassembly
frames 200.36 and 219.32, provides a five (and subsequent overhaul and reassembly) is
second “on” period for the two solenoids in each impractical and undesirable, therefore, only
wing distributor valve, and a three second “on” parts which have been established by testing
period for both solenoids in the tail distributor and inspection as being damaged should be
valve. Cycling can be controlled at a fast or replaced. If it is necessary to remove the timer
slow rate depending on the selection of the cover, remove screws. The timer cover is
FAST or SLOW switch when the wing and tail sealed with silicon rubber, and removal may
de-icing AUTO–OFF–MANUAL switch is set be difficult. The component board is treated

30 ICE AND RAIN PROTECTION

to AUTO. The total cycling time (including and sealed during manufacture with a fungus-
dwell time) is 60 seconds when selected to resistant varnish, so if components on the
FAST, and 180 seconds at SLOW. The timer component board are replaced, that area of the
(Figure 30-13), consists of a metal box with an board where the component is replaced must
external connector receptacle, which houses a be revarnished with a fungus-resistant varnish
rotary circuit selector, component board and (Grade 3 Salicyl Anilide – Fungicide, Maas
the necessary electrical wiring. The component Waldstein Company, Newark, N. J.) Refer
board contains the circuit resistors, capacitors, to Equipment Overhaul Manual for details of
a silicon controlled rectifier, and a silicon component replacement or wiring renewal.

Figure 30-15. Deice Panel

FOR TRAINING PURPOSES ONLY 30-15

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of as long as switch is held in position, and

the maintenance practices and is intended for deflate when switch is selected to OFF or
training purposes only. another position.
For a more detailed description of the practice, 8. Check that when LEFT STAB is selected the
refer to the task in the Viking AMM PSM 1-63-2. LEFT STAB DEICE PRESS indicator light
comes on within 2 seconds, and that when
30-10-21 MAINTENANCE RIGHT STAB is selected the RIGHT STAB
DEICE PRESS indicator light comes on
PRACTICES within 2 seconds. Ensure that the indicator
lights do not, at any time, come on together.
Select DEICER BOOTS switch to OFF.
ADJUSTMENT/TEST
9. Shut down engine and check LOW PRESS
Operational Test caution light comes on before switching off
DC MASTER switch.
1. Start engine.
2. Ensure AFR DEICE AUTO and AFR Functionally Test Wing and Tail
DEICE MAN circuit breakers are engaged
and LOW PRESS caution light is off.
De-icing System
30 ICE AND RAIN PROTECTION

Equipment required:
3. Select DEICER BOOTS system selector
switch to AUTO and cycle selector switch
•• Pressure rig and gauge required for
to FAST.
testing the de-icing system, is as
4. Have second operator outside aircraft to specified for the pneumatic and air
observe boot inflation sequence as listed conditioning systems.
in Table 30-1. Check horizontal stabilizer
•• Suction gauge capable of measuring 4 to
de-icing boots indicator light for operation.
10 inches Hg, with the necessary adapter
5. Select cycle selector switch to SLOW and to connect gauge to the suction test point.
repeat step 4.
1. Remove cabin roof upholstery in location
6. Select system selector switch to MANUAL.
of station 177.00 to provide access to heat
7. Select each distributor valve switch in turn, exchanger.
to WING INNER, WING OUTER and OFF,
2. Disconnect bleed air line at inlet to heat
LEFT STAB, RIGHT STAB and OFF. The
exchanger and connect pressure rig.
appropriate boot section must inflate in time
specified in Table 30-1, remain inflated

SEQUENCE OF TIME TAKEN FOR DWELL TIME AT INFLATION SELECTION

INFLATION INFLATION
FAST SLOW
Wing Inner 5 seconds

Wing Outer 5 seconds

44 ± 10 seconds 2 min 44 ± 20 seconds
Left Stabilizer 3 seconds

Right Stabilizer 3 seconds

Table 30-1. De-Icer Boots Timing Sequence

30-16 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

3. Remove cabin roof upholstery as required 13. Check that when LEFT STAB is selected
and connect suction gauge to the tee fitting the LEFT STAB DEICE PRESS indicator
in suction line forward of station 200.36. light comes on within 2 seconds, and
that when RIGHT STAB is selected the
4. Connect external electrical power supply.
RIGHT STAB DEICE PRESS indicator
5. With de-icer circuit breakers (AFR DEICE light comes on within 2 seconds. Ensure
AUTO and AFR DEICE MAN) engage, that the indicator lights do not, at any time,
check that low pressure warning light is on. come on together.
6. Start pressure rig and pressurize system to 14. Release pressure and note that low pressure
maintain 18 ± 2 psi on rig pressure gauge, warning light comes on at 15 ± 2 psi.
and 4 to 10 inches Hg suction. Check that Ensure that both STAB DEICE PRESS
low pressure warning light goes out. indicator lights are off.
15. Select VALVE HTR, switch on and check
NOTE that heater jackets on wing and fuselage
If a leak in the system is distributor valves are warm. Set VALVE
apparent, leak test the system in HTR switch to OFF.
accordance with PSM 21-40-00.
16. Set DEICER BOOTS switch to OFF.

30 ICE AND RAIN PROTECTION

7. Select system selector switch on de-icer 17. Reconnect bleed air line at inlet to heat
panel to AUTO and cycle selector switch exchanger.
to FAST. A temporary loss of pressure and
suction is allowed.
INSPECTION/CHECK
8. Check that sequence of boot inflation,
correct inflation time and dwell time are Inspection of the Wing and Tail
in accordance with Table 30-1. Check that
the left and right horizontal stabilizer boot
De-icer Boots
indicator lights (STAB DEICE PRESS) Inspect the wing and tail de-icer boots for
each come on for 3 seconds in each 60 condition.
second cycle. Ensure that the indicator
lights do not, at any time, come on together.
Inspection of the Stall Bar
9. Ensure that each boot section deflates fully
Inspect the wing deiced bonded stall bar for
in 20 seconds.
condition and proper adhesion.
10. Select cycle selector switch to SLOW and
check that sequence of inflation, correct
inflation time and dwell time are in
accordance with Table 30-1. Check that
indicator lights operate as in step 8.
11. Select system selector switch to MANUAL.
12. Select each distributor valve switch, in turn,
to WING INNER, WING OUTER and OFF,
LEFT STAB, RIGHT STAB and OFF. The
appropriate boot section must inflate in time
specified in Table 30-1, remain inflated as
long as switch is held in position, and deflate
when switch is selected to OFF or another
position.

FOR TRAINING PURPOSES ONLY 30-17

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 30-16. Engine Air Intake Anti-Icing Boot

30 ICE AND RAIN PROTECTION

Figure 30-17. Engine Air-Intake Anti-Icing Electrical Schematic

30-18 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

30-20-00 ENGINE AIR and heating element, if not detected

early. Also, dislodgement of the
INTAKE ANTI-ICING thermal temperature sensor from
SYSTEM against the boot has been known to
cause a similar overheat condition,
several of which resulting in
GENERAL structure damage.

An optional electrically heated neoprene rubber

boot is available for installation on the nacelle
air inlet lip (Figure 30-16).

The optional nacelle air inlet anti-icing system

prevents formation of ice on the inlet lip of the
engine air intake. This is achieved by utilizing
an electrically heated boot which is bonded to
the leading edge of the engine lower cowling. A
thermostatic sensing element protrudes through
the lip structure, making contact with the inside

30 ICE AND RAIN PROTECTION

surface of the boot to sense boot temperature.

The nacelle air inlet anti-icing installation

is not a mandatory requirement for aircraft
certified for ice operations.

CONTROL
The nacelle inlet anti-icing for both engines
is controlled by a two-position switch on the
overhead panel (Figure 30-10). The switch is
labeled “INTAKE ANTI-ICE.” When the switch
is moved to the INTAKE ANTI-ICE position, DC
power is applied to the heating element through
a thermostatic switch. Power is supplied from
the left and right DC bus through 25 amp circuit
breakers labeled INT ANTI ICE L and INT ANTI
ICE R on the main circuit breaker panel for both
engine lower cowling air intakes.

WARNING

The intake anti-ice system must

be selected off when the aircraft is
static and the engine is shut down.
Switching the system on for testing
is permissible providing caution
is exercised by ground crew.
Absence of cooling air to dissipate
boot heat will rapidly increase boot
temperature, damaging the boot

FOR TRAINING PURPOSES ONLY 30-19

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
30 ICE AND RAIN PROTECTION

Figure 30-19. Intake Anti-Ice Switch Panel

Aircraft: Revision No. 11 Page:

DE HAVILLAND DHC-6, SERIES 100, 200 & 300 Date: Dec. 23, 2002 30-3 of 3
System & 1. 2. Number Installed
Sequence 3. Number Required for Dispatch
Numbers 4. Remarks or Exceptions

30 ICE AND RAIN PROTECTION

11 Engine Inlet Deicing C - 0 One or both may be inoperative provided:

*** Boots

a) flight is not conducted in known or

forecast icing conditions; and,

b) the associated intake deflector is

operative.

Figure 30-18. MMEL - Engine Inlet Deicing Boots

30-20 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

OPERATION A. Set INTAKE ANTI-ICE switch to ON

for not more than five seconds.
Refer to Figure 30-17. Engine Air-Intake Anti-
B. Check by hand to see that the anti-icing
Icing Electrical Schematic.
boots heat up.
With 28 volts DC power applied through the 25 C. Repeat if necessary to heat up anti-icing
amp circuit breakers and the system INTAKE boots, but observe an interval of three
ANTI-ICE switch selected on, power will pass seconds between each on selection of
through both K12 and K13 relay coils, located the INTAKE ANTI-ICE switch.
in the main distribution box, to thermal sensor
D. Switch off INTAKE ANTI-ICE switch
switches installed beneath the deicer boot in the
and external power source.
lower cowling air intake lip skin.
2. If a generator is used, proceed as follows:
The boot will begin to heat up rapidly when
A. Set IND SELECT switch to L or R
power is supplied. The heat sensing switch
GEN, depending on which generator
contacts will open when the boot temperature
is running.
reaches 190°F. This will de-energize the
system control relays, removing power to the B. Set INTAKE ANTI-ICE switch to on
heating elements. When the boot temperature for not more than 5 seconds and check

30 ICE AND RAIN PROTECTION

decreases to approximately 160°F (71±4°C), that DC loadmeter increases.
the heat sensing switch contacts will close
C. Alternately pull INT ANTI ICE L
again, energizing the relays and power the
and R circuit breakers and check that
heating elements. The system will continue to
loadmeter decreases.
cycle within this temperature range until the
switch is moved to the OFF position. D. Switch off INTAKE ANTI-ICE switch
and shut down electrical power.
*The following is an abbreviated description of
the maintenance practices and is intended for
training purposes only.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2.

30-20-00 MAINTENANCE
PRACTICES
ADJUSTMENT/TEST
Operational Test Engine Air
Intake Anti-icing System
NOTE
An operational test can be
carried out using an external
power source, or with an engine
running and generator operating.

1. When an external power source is used,

proceed as follows:

FOR TRAINING PURPOSES ONLY 30-21

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 30-20. Pitot Probe Figure 30-21. Stall Vanes

30 ICE AND RAIN PROTECTION

Figure 30-22. PITOT HEAT Control Switch

30-22 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

30-30-00 PITOT NOTES

HEATING SYSTEM
GENERAL
Refer to:

•• Figure 30-20. Pitot Probe.

•• Figure 30-21. Stall Vanes.

Pitot head installations are on the two sides of

the forward fuselage.

Each pitot probe contains a heating element to

prevent ice formation. The static ports are not
heated.

30 ICE AND RAIN PROTECTION

CONTROL
Refer to Figure 30-22. PITOT HEAT Control
Switch.

The pitot anti-ice system is controlled by a

two-position switch labeled PITOT HEAT on
the overhead console main switch panel.

FOR TRAINING PURPOSES ONLY 30-23

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

PITOT HEATERS *
30 ICE AND RAIN PROTECTION

STALL
VANE
HEATERS

* IF INSTALLED
ON

OFF
PITOT HEAT

L DC BUS R DC BUS

Figure 30-23. Engine Air-Intake Anti-Icing Electrical Schematic

30-24 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

OPERATION NOTES
When selected to HEAT (forward), power is
supplied to the pitot tube heating elements
from the left and right DC buses, through two
7.5 amp circuit breakers labeled PITOT HEAT
L and PITOT HEAT R on the main circuit
breaker panel.

The wing lift detectors (stall warning sensor)

are also fitted with a heating element, and this
is turned on automatically whenever the PITOT
HEAT switch is selected on. Power to heat the
lift detectors is supplied on the same circuit as
the left hand pitot tube.

CAUTION
The pitot heat system should

30 ICE AND RAIN PROTECTION

not be left switched on for
prolonged periods when the
airplane is on the ground. If the
pitot heat is selected on when
the aircraft is stationary, or not
turned off until the aircraft has
been parked following flight,
serious injury will result if
ground personnel inadvertently
touch the pitot tube. To avoid
the risk of injury, ensure that the
pitot heat switch is off before
turning the DC MASTER switch
on, and shut off the pitot heat
as soon as practical, following
landing. Ground operation
check should be limited to one
minute.

Figure 30-23 illustrates operation of the pitot

anti-ice system.

FOR TRAINING PURPOSES ONLY 30-25

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of

the maintenance practices and is intended for
training purposes only.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2.

30-30-00 MAINTENANCE
PRACTICES
INSPECTION/CHECK
Inspection of the Pitot Heads and
Static Vents
Inspect the pitot heads and static vents.

OPERATIONAL TEST
30 ICE AND RAIN PROTECTION

PITOT HTR L and R

1. Ensure PITOT HTR L and R circuit
breakers are engaged.
2. Switch on pitot heater and check that pitot
head warms up.

CAUTION
DO NOT LEAVE PITOT
HEATER SWITCHED ON FOR
MORE THAN ONE MINUTE.

Aircraft: Revision No. 07 Page:

DE HAVILLAND DHC-6, SERIES 100, 200 & 300 Date: Sep. 18, 1998 30-2 of 3
System & 1. 2. Number Installed
Sequence 3. Number Required for Dispatch
Numbers 4. Remarks or Exceptions

30 ICE AND RAIN PROTECTION

7 Pitot Heaters B 2 1 One may be inoperative for day VMC flight
provided:
a) there is no visible moisture; and,
b) flight is not conducted in known or
forecast icing conditions.

Figure 30-24. MMEL - Pitot Heaters

30-26 FOR TRAINING PURPOSES ONLY

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

30 ICE AND RAIN PROTECTION

PAGE INTENTIONALLY LEFT BLANK

FOR TRAINING PURPOSES ONLY 30-27

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

WIPER
30 ICE AND RAIN PROTECTION

Figure 30-25. Windshield Control Panel

Figure 30-26. Windshield Heating Electrical Controllers

30-28 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

30-40-00 WINDSHIELD Distribution and Generator Control box in the

right hand cabin roof area. The 30 amp circuit
HEATING SYSTEM breakers are not accessible in flight, as roof
panels need to be removed to gain access.
GENERAL
The pilot and co-pilot windshields are of
laminated structure consisting of inner and
outer glass panes and a vinyl sandwich. Fine
wire conductors are imbedded in the vinyl.
Except for a common control switch, the pilot
and co-pilot windshield heat constitutes two
separate systems.

Each system includes a controller, a heat

sensor, and a control relay. The sensor transmits
temperature signals to the controller, which then
cycles the system on and off in a temperature
range which will prevent ice formation.

30 ICE AND RAIN PROTECTION

The later version of the heated windscreen
fulfills bird impact requirements for the
British C.A.A. by adopting retaining
angles manufactured from thicker material
and continuous windshield heat operation
throughout all seasons.

CONTROL
A two-position switch on the WINDSHIELD
control panel (Figure 30-25) controls the
windshield anti-ice system. The switch has
positions labeled “OFF” and “HEAT”. The
single switch controls power to the left and
right windshield.
Figure 30-27. O
 verhead Circuit Breaker
Panel
Four circuit breakers supply power to operate
the system (Figure 30-26). Control voltage to
operate the relays and temperature sensors is
from the left and right DC buses and protected
by two 5 amp circuit breakers labeled W/S
HEAT L and W/S HEAT R on the overhead
console circuit breaker panel (Figure 30-27).
Two relays labeled LEFT HEAT RELAY and
RIGHT HEAT RELAY in the windshield heat
relay box in the cabin roof area.

High amperage power to heat the left and right

windshield is provided by two 30 amp circuit
breakers identified as CB2 and CB1 in the Power

FOR TRAINING PURPOSES ONLY 30-29

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

HEATER

SENSOR

RIGHT
CONTROLLER
30 ICE AND RAIN PROTECTION

LEFT
CONTROLLER

ON
WINDSHIELD
HEAT
SWITCH
OFF
CONTROL
RELAYS

LEGEND
30A
30A
5A
5A

CONTROL CIRCUIT

SENSING CIRCUIT

HEATING CIRCUIT
L DC R DC
BUS BUS L DC BUS R DC BUS

Figure 30-28. Windshield Heat Operation

30-30 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

OPERATION NOTES
Figure 30-28 illustrates operation of the
windshield heat system.

When the windshield switch is selected to

HEAT, power from both DC buses is directed
through the 5 amp circuit breaker labeled
W/S HEAT L and W/S HEAT R including the
select switch to the left and right temperature
controller. With the relays energized and the
contacts closed, power is supplied from the
left and right DC buses to pass through the 30
amp circuit breakers labeled CB2 and CB1 to
the left and right windshield heating elements.
When the windshield sensors record maximum
temperature, the temperature controller power
output to the heat relay will be disconnected.

30 ICE AND RAIN PROTECTION

De-energizing the relay coil will open the relay
contacts eliminating power to the windshield
heating elements. Both windshields operate
independently from each other.

FOR TRAINING PURPOSES ONLY 30-31

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
30 ICE AND RAIN PROTECTION

Figure 1: Windshield Heat Relay Box

Figure 30-29. Windshield Heat Relay Box

30-32 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

30-40-11 WINDSHIELD NOTES

HEAT RELAY BOX
DESCRIPTION
The relay box (Figure 30-29) consists of an
aluminum alloy box which houses the two
control relays (K1 and K5) associated with the
windshield heating system. The box, is closed
by a cover secured by two screws. The relay
box is in the cabin roof.

*The following is an abbreviated description of

the maintenance practices and is intended for
training purposes only.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2.

30 ICE AND RAIN PROTECTION

30-40-11 MAINTENANCE
PRACTICES
INSPECTION/CHECK
Inspection of the Wing Windshield
Inspect the heated windshields for condition,
discoloration, delamination and security.

FOR TRAINING PURPOSES ONLY 30-33

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
30 ICE AND RAIN PROTECTION

Fig ure 1: Windshield Wiper System Installation

Figure 30-30. Windshield Wiper System Installation

30-34 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

30-45-01 WINDSHIELD
WIPER SYSTEMS
GENERAL
WIPER
The first systems were supplied by Marquette,
and then Alco was later adopted as basic
aircraft equipment. The two systems operate by
a single 28VDC motor through converter units
and flexible cable drives, each with a 12 inch
wiper blade (Figure 30-25). The wipers operate
at two speeds, fast or slow. When selected to
PARK the two wipers should stop adjacent to
and parallel with the windshield center post.

When operated, the wiper blades move toward

and away from each other. The Marquette wiper
blades could be parked to operate in a similar

30 ICE AND RAIN PROTECTION

alignment to the Alco system, if required.
Figure 30-31. Windshield Wiper Control
CONTROL
Two switches, a mode selector, and a speed selector
control the windshield wiper system. They are on
the control panel labeled WINDSHIELD above the
right windshield (Figure 30-31). The mode selector
switch has three positions labeled PARK, OFF,
and ON. The switch is spring-loaded from PARK
to the OFF position. The speed selector switch is
labeled SLOW and FAST. Power is supplied to the
windshield wipers from the left DC bus, through
a 10 amp circuit breaker labeled W/S WIPER
WASHER.

Figure 30-32. Pilot Preferred Wiper Position

FOR TRAINING PURPOSES ONLY 30-35

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
30 ICE AND RAIN PROTECTION

W/S WIPER
ON
C
DC BUS 10A B
WIPER
SWITCH OFF A

W/S WIPER W/S WIPER

CONVERTER MOTOR
PARK L SIDE

FAST

1.5 SLOW

Figure 30-33. Windshield Wiper Operation

30-36 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

OPERATION 1. Connect an external power to the aircraft.

2. Engage the W/S WIPER circuit breaker and
Figure 30-33 illustrates the electrical schematic
select the left WIPER switch to ON.
of the windshield wiper blade operation for both
systems. If the mode selector switch is positioned 3. Ensure that each blade wipes the windshield
to OFF, the wiper blades stop immediately. clean of water, that wiper arms traverse 90°,
Momentarily holding the switch at PARK will and that both blades operate to between
drive the blades to their park position. There is 1.8 and 2.0 inches from the center of the
no limit speed for windshield wiper operation. windshield center post.
However air loads imposed when the aircraft is
4. Select the right WIPER switch between
travelling over 100 KIAS make it difficult for the
FAST and SLOW and check that speed of
wipers to function, and may make it impossible to
wipers varies accordingly.
move the wipers to the parked position. The circuit
breaker may disengage if the wipers are turned on 5. Select the left WIPER switch to PARK then
for a prolonged period of time above 100 KIAS. release to OFF position. Ensure both wiper
blades parks 1.8 to 2.0 inches from center
of the windshield center post.
CAUTION
6. Disengage W/S WIPER circuit breaker.
The windshield wipers

30 ICE AND RAIN PROTECTION

7. Disconnect external Power.
should never be operated on
a dry windshield, because the
windshield will be scratched. It is
recommended that the wipers speed
switch be left in the fast position.
This minimizes the chances of the
motor being overloaded if run on a
dry windshield.

*The following is an abbreviated description of

the maintenance practices and is intended for
training purposes only.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2.

30-45-01 MAINTENANCE
PRACTICES
ADJUSTMENT/TEST
Operational Test Windshield
Wiper System
NOTE
During the following test, maintain
an adequate supply of water to the
windshields, to prevent the blades
operating on a dry surface.

FOR TRAINING PURPOSES ONLY 30-37

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

30-00-00 WINDSHIELD To eliminate the risk of isopropyl alcohol ever

being used in the field. Many operators have
WASHER SYSTEM removed the washer system as recommended
and a small round blanking cap now covers the
Refer to: switch position.
For aircraft retaining the washing system the
•• Figure 30-34. Windshield Washer
windshield washer installation consists of a
Reservoir.
reservoir with a capacity of 1.5 US gallons,
•• Figure 30-35. Washer Control. a pump, and two jet nozzles forward of the
windshields. The reservoir is in the lower left
The windshield washer was installed as basic
corner of the flight compartment, under the
aircraft equipment. The system was later deleted
main circuit breaker panel.
from production aircraft as the system was later
considered as a possible fire hazard if operated
in the windscreen deicing mode using isopropyl CONTROL
alcohol as a deicing agent.
The windshield washer is controlled by
Investigation of a serious in flight fire determined a momentary two-position switch on the
that the fire was caused by ignition of the highly windshield wiper control panel (Figure 30-35)
30 ICE AND RAIN PROTECTION

flammable isopropyl alcohol as a result of a leak above the right windshield.

under the cabin floor. The leak was caused by
The switch positions are labeled OFF and
deterioration of the fluid lines, and a service
WASHER, and the switch is spring loaded
bulletin was sent to all operators forbidding the
to the OFF position. Power is supplied to the
use of isopropyl alcohol, and requiring that a
windshield washer pump from the left DC bus,
new label be installed on the reservoir. The new
through a 10 amp circuit breaker labeled W/S
label by Transport Canada’s AD CF82-20 Dated
WIPER WASHER.
July 7th 1982, prohibited the use of isopropyl
alcohol, and identified two fluids, Kilfrost or
Pace, for windshield washing only.

WIPER

Figure 30-34. Windshield Washer Reservoir Figure 30-35. Washer Control

30-38 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

OPERATION
When the switch is held at the WASHER
position, a circuit is completed which supplies
power to the pump. Fluid is sprayed onto
the windshields by the nozzles on the nose
structure forward of each windshield.

Aircraft: Revision No. 07 Page:

DE HAVILLAND DHC-6, SERIES 100, 200 & 300 Date: Sep. 18, 1998 30-2 of 3
System & 1. 2. Number Installed
Sequence 3. Number Required for Dispatch
Numbers 4. Remarks or Exceptions
30 ICE AND RAIN PROTECTION

6 Windshield Wipers C - 0 One or both may be inoperative provided

*** flight is not conducted in precipitation
within five nautical miles of the airport of

30 ICE AND RAIN PROTECTION

take-off or intended landing.

Modes C 2 1 The fast or slow mode may be inoperative.

Figure 30-36. MMEL - Windshield Wipers

FOR TRAINING PURPOSES ONLY 30-39

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

INNER

OUTER
30 ICE AND RAIN PROTECTION

Figure 30-37. Propeller Deicing Boots

Figure 30-38. Prop DEICE Switch

30-40 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

30-60-00 PROPELLER NOTES

DEICING SYSTEM
GENERAL
Refer to Figure 30-37. Propeller Deicing Boots.

Deicer boots are bonded to the leading edge of

each propeller blade. Each boot consists of two
separate heating elements, an inner and outer
portion embedded in the boots. The elements
are divided into four groups consisting of
left propeller inner, left propeller outer, right
propeller inner and right propeller outer.
System power is from the 28VDC bus through
a 20 amp PROP DEICE circuit breaker. The
brush block assembly has now been replaced
by a brush module unit.

30 ICE AND RAIN PROTECTION

FOR TRAINING PURPOSES ONLY 30-41

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

BRUSH RETAINER
MOUNTING SCREW

BRACKET

3 O
2 N
0
BRUSH

4 SY

6
S
E
A
RETAINER
ASSEMBLY

CONNECTOR
30 ICE AND RAIN PROTECTION

FLAT WASHER

LOCK WASHER

CONNECTOR WASHER NUT NUT

MOUNTING
SCREW

BRACKET PIN IDENT

BRUSH MODULE A

BRUSH MODULE B
A

CB
KEY BRUSH MODULE C

CONNECTOR

Figure 30-39. Modular Brush Block Assembly

30-42 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

30-60-12 BRUSH MODULE OIL PRESSURE SWITCHES

UNITS Refer to Figure 30-41. Oil Pressure Switch.

Three-brush module units are attached to the An oil pressure switch is on the engine
engine reduction gearbox in place of the brush accessory gearbox of each engine. With an
block assembly. Each module consists of a engine operating, its oil pressure switch
plastic housing with an internal brush and spring. contacts are closed, providing a ground for the
Each module also has a wire connected to the associated propeller deicing relay operating
brush with the three wires leading to a connector coil circuit. This allows the relay to energize
for connection to the propeller deicing system. when the system is switched on. The switch
also creates a circuit to the applicable ENGINE
OIL PRESSURE caution light.
BRUSH BLOCK SLIP RING
ASSEMBLY
Refer to:

•• Figure 30-39. Modular Brush Block

30 ICE AND RAIN PROTECTION

Assembly.
•• Figure 30-40. Prop Slip Ring on Back
of Prop Bulk Head.

Mounted on the rear face of the spinner

bulkhead is a three track slip ring assembly. It
is used to conduct current from the brush block
assemblies to blade heaters.

Figure 30-41. Oil Pressure Switch

Figure 30-40. P
 rop Slip Ring on Back of
Prop Bulk Head

FOR TRAINING PURPOSES ONLY 30-43

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
30 ICE AND RAIN PROTECTION

Figure 30-42. Brush Module Assembly - Checking Brush Wear

30-44 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of mounting bracket. A minimum distance of

the maintenance practices and is intended for 1/32–inch and a maximum of 3/32–inch
training purposes only. must be maintained between brush module
assembly and slip ring surface.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2. 4. B r u s h m o d u l e a s s e m b l i e s m u s t b e
positioned so that brushes contact slip rings
30-60-12 MAINTENANCE at an angle of approximately 2°, to prevent
chattering that could cause brushes to break
PRACTICES up (refer to FFigure 30-42). The angle is
measured toward the direction of rotation
of slip rings.
REMOVAL/INSTALLATION
Remove Brush Module Assembly NOTE
On all installations, allow a
1. Ensure that PROP DEICE circuit breaker
run-in period of at least one hour
is disengaged.
before operating the propeller
2. Disconnect electrical connector from brush de-icing system.
module assembly.

30 ICE AND RAIN PROTECTION

5. Connect electrical connector to brush
3. Remove and retain two nuts, four washers
module assembly.
and two bolts then remove brush module
and shim (if shim is installed). Retain shim.

Install Brush Module Assembly

CAUTION
FAILURE TO EXERCISE
EXTREME CARE WHEN
HANDLING, OR IMPOSING
SIDE LOADS ON BRUSHES
DURING INSTALLATION
COULD RESULT IN BRUSH
DAMAGE.

1. Position brush module assembly on engine

mounting bracket and insert bolts with
washers under bolt heads through module
and bracket. Do not install nuts and washers
at this time.
2. Check that brushes are aligned with
propeller slip rings so that entire brush
face is in contact with copper rings. Check
alignment through entire 360 degrees of
slip ring rotation. If not aligned install one
or more shims as required to align brushes
to approximate center of slip rings.
3. Install nuts and washers, and tighten to
secure brush module assembly to engine

FOR TRAINING PURPOSES ONLY 30-45

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

30-60-21 CONTROL BOX

The propeller control box (Figure 30-43) in the
cabin roof, is a metal box housing two relays
and a 5 amp circuit breaker. Each relay controls
one propeller deicing system. When energized,
it completes two circuits: one from the timer to
the outer heating elements in the blade boots of
the associated propeller and the other to the inner
elements of the boots on the same propeller. The
circuit breaker is connected in series with the coils
of both relays for protection of both coils.
30 ICE AND RAIN PROTECTION

K16 K17
5

CIRCUIT BREAKER

Figure 30-43. Propeller Deicing Control Box

30-46 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

30-60-31 TIMER elements and then to the outer heating elements.

Power to heat the elements is obtained from the
left DC Bus through a 20 amp circuit breaker
Refer to Figure 30-44. Prop Deicing Timer.
labeled PROP DEICE on the main circuit
breaker panel.
A timer in the cabin roof provides four
sequenced power outputs, two to each
Control power to operate the prop deice timer
propeller deicing system. It supplies heat in
and relays is obtained from a 5 amp circuit
approximately 30 second intervals to each
breaker labeled PROP DEICE installed in the
heating element. The timer does not have a
prop deicing control box. The 5 amp circuit
nulling function. If the system is switched off,
breaker is not accessible in flight, as cabin roof
and if the timer is a BF Goodrich 3E1150-3, the
panels need to be removed to gain access to it.
timer ceases operation, heating power is shut
off. When the system is turned on again, the
Voltage supplied to the prop deice relays must
timer will resume at the point where it finished,
travel in series through the engine low oil
completing the heating interval on the element
pressure sensor (Figure 30-41). If the engine
last in operation. If a BF Goodrich 3E1150-10
oil pressure is less than 40 psi, power will not
is installed, the timer will step ahead to the next
be supplied to the boots on that propeller. This
segment if the switch is turned off.
ensures that the propeller deice heating elements

30 ICE AND RAIN PROTECTION

will not function if the engine is not running.
CONTROL
The propeller deicing system is electrically INDICATION
controlled for both propellers by a single two-
position switch labeled PROP DEICE on the Although current fluctuations can be observed
overhead deice switch panel (Figure 30-38). on the load mater an ammeter by EO 68411
It applies power to the timer, which in turn is available as a factory installed option to
controls four sequenced outputs, two to each enable the pilot to observe current draw of the
propeller, applying power first to the inner propeller deice system.

Figure 30-44. Prop Deicing Timer

FOR TRAINING PURPOSES ONLY 30-47

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

OPERATION
See MSM for Operations details (MSM 30-9).

SURFACE PROTECTION
Ice is thrown off the propellers at high speed
and with considerable force. This can cause
chipping of paint on the side of the fuselage
and dents in the fuselage skin.

The metal ice guard shields are attached

to the fuselage structure by nylock screws
in conjunction with rivnuts. A sponge seal
positioned between the shield and the fuselage
skin dampens ice impact. A moisture drain
hole, at the bottom, will allow moisture behind
the shield to escape.
30 ICE AND RAIN PROTECTION

Figure 30-45. Prop Deicing Timer

Aircraft: Revision No. 07 Page:

DE HAVILLAND DHC-6, SERIES 100, 200 & 300 Date: Sep. 18, 1998 30-2 of 3
System & 1. 2. Number Installed
Sequence 3. Number Required for Dispatch
Numbers 4. Remarks or Exceptions

30 ICE AND RAIN PROTECTION

4 Propeller Deicing Systems C 2 0 One or both may be inoperative provided

flight is not conducted in known or forecast
icing conditions.

Figure 30-46. MMEL - Propeller Deicing Systems

30-00-00 MAINTENANCE
PRACTICES

30-48 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

GENERAL MAINTENANCE Ensure pitot probe covers are removed prior to

energizing the probe heating system.
PRACTICES
Operating the windshield wipers on a
The maintenance practices found in the
windshield covered with dust or sand will
Maintenance Manual usually relate to removal/
cause damage to the surface. During preflight
installation, adjustment test, and servicing
or ground operation, lift the wiper blades off
procedures. The following maintenance
the surface, remove dust, sand, or dirt from the
practices are applicable to the ice and rain
blade, and apply water to the windshield.
protection systems in a general sense:

Solvents are toxic. When working with solvents, SERVICING

adequate ventilation must be provided and the
use of protective clothing strictly adhered to. PSM 12-20-05 provides instructions for cleaning
windshields and cabin windows. The procedure
When installing deicer boots, do not apply calls for large amounts of soap and water applied
adhesive if relative humidity exceeds 90%. with a sponge or rags. Any rubbing necessary to
remove stubborn spots must be done with bare
Removal and installation of deicer boots is an hands to prevent generation of static electricity,
involved and lengthy procedure. which attracts dust. After cleaning, polishing

30 ICE AND RAIN PROTECTION

with an antistatic plastic cleaner and wiping to
DEICER BOOT MODIFICATION a high gloss is required.

Wing and stabilizer boots manufactured Some servicing procedures are located within
by Kleber were seriously considered as chapter/sections of the Maintenance Manual
replacement boots as the wing boots were pertaining to maintenance practices of some
available in two separate sections. components. Typical of these is the servicing
of the propeller deicing brush block assembly
Although the section boot had merit with regard in Chapter/Section 30-60-11.
to inner boot replacement, when deteriorated
from engine exhaust heat, the Kleber boots did
not appear to provide the expected service life
that would justify a change from the present
post mod 6/1579 B.F.Goodrich deicing boots.

Refer to the applicable chapter/section of the

Maintenance Manual for detailed instructions,
including illustrations.

The anti-icing and deicing systems are designed

for use in a cold and wet in-flight environment.
Operating these systems at other times in less
demanding situations may rapidly damage
the components. When necessary to test or
troubleshoot one of these systems, it should be
isolated, if possible. Be aware of which units
are thermostatically controlled and which units
are operated with common controls. Always
keep operation of any anti-icing or deicing
system within the specified limits provided by
the Maintenance Manual.

FOR TRAINING PURPOSES ONLY 30-49

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CHAPTER 33
LIGHTING
CONTENTS
Page

33-00-00 LIGHTING.................................................................................................. 33-1

General................................................................................................................ 33-1
33-10-00 FLIGHT COMPARTMENT LIGHTING....................................................... 33-3
General................................................................................................................ 33-3
Flight Instruments, Engine Instruments, and Emergency Panel Lights.................. 33-3
Co-Pilot Flight Instrument and Right Radio Panel Lights.............................. 33-3
Panel Lighting............................................................................................... 33-3
Caution Lights..................................................................................................... 33-5
General......................................................................................................... 33-5
Caution Lights Dimming/Test Switch............................................................ 33-7
Caution Lights Dimming Control Box........................................................... 33-9
Power Supply................................................................................................ 33-9
Cockpit Utility Lights........................................................................................ 33-11

33 LIGHTING
Cockpit Dome Light........................................................................................... 33-11
33-10-00 MAINTENANCE PRACTICES.................................................................. 33-11
Adjustment/Test................................................................................................. 33-11
Operational Test Panel and Instrument Lights............................................. 33-11
Operational Test Caution Lights.................................................................. 33-11
33-20-00 PASSENGER COMPARTMENT LIGHTS................................................. 33-13
General.............................................................................................................. 33-13
Description......................................................................................................... 33-13

FOR TRAINING PURPOSES ONLY 33-i

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page
Cabin Lighting............................................................................................ 33-13
Passenger Entrance Lights........................................................................... 33-13
Toilet Light................................................................................................. 33-13
Cabin Signs (Mod S.O.O. 6110).................................................................. 33-13
Passenger Reading Lights............................................................................ 33-15
33-50-00 EMERGENCY LIGHTING - OPTIONAL.................................................. 33-17
33-50-00 MAINTENANCE PRACTICES.................................................................. 33-17
Adjustment/Test................................................................................................. 33-17
Operational Test Emergency Lights (S.O.O. Mod 6179).............................. 33-17
33-30-00 CARGO AND SERVICE COMPARTMENT LIGHTS................................ 33-19
General.............................................................................................................. 33-19
Forward Baggage Compartment Light......................................................... 33-19
Rear Baggage Compartment Lights............................................................. 33-19
33-40-00 EXTERIOR LIGHTING............................................................................. 33-21
General.............................................................................................................. 33-21
Position Lights................................................................................................... 33-21
Landing Lights................................................................................................... 33-23
33 LIGHTING

Anti-Collision Lights......................................................................................... 33-23

Taxi Light.......................................................................................................... 33-25
Wing Inspection Lights...................................................................................... 33-27
Logo Lights........................................................................................................ 33-27
33-40-00 MAINTENANCE PRACTICES.................................................................. 33-28
Adjustment/Test................................................................................................. 33-28
Operational Test Landing Lights.................................................................. 33-28
Operational Test Position Lights.................................................................. 33-28

33-ii FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page
Operational Test Anti-Collision Lights........................................................ 33-28
Operational Test Taxi Light......................................................................... 33-28
Operational Test Wing Inspection Lights..................................................... 33-28
33-00-00 OPTIONAL SYSTEMS.............................................................................. 33-31
S.O.O. 6142 Dual 400 Cycle System.................................................................. 33-31
Door Warning System........................................................................................ 33-31
Propeller Autofeather Advisory Lights............................................................... 33-31
Beta System Advisory Lights............................................................................. 33-31
Power Lever Test Switch..................................................................................... 33-33
Stall Warning Light and Horn............................................................................ 33-33
Fire Warning Lights........................................................................................... 33-33
Instrument Lighting........................................................................................... 33-34
Anticollision Light Brush Wear Check............................................................... 33-34

33 LIGHTING

FOR TRAINING PURPOSES ONLY 33-iii

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

ILLUSTRATIONS
Figure Title Page

33-1 
Instrument Post Lights (Typical)................................................................33-2
33-2 Instrument Lights......................................................................................33-2
33-3 General Panel Lighting..............................................................................33-2
33-4 DC Master, Caution and Emergency Panels...............................................33-6
33-5 Caution LT Switch.....................................................................................33-7
33-6 Caution Lights Dimming Control Box.......................................................33-8
33-7 Cockpit Utility Lights..............................................................................33-10
33-8 Cockpit Dome Light................................................................................33-10
33-9 Cabin Light Switch..................................................................................33-12
33-10 Entrance Boarding Lights and Control.....................................................33-12
33-11 Cabin Signs.............................................................................................33-13
33-12 Reading Lights and Control.....................................................................33-14
33-13 Passenger Reading Lights........................................................................33-15
33-14 Emergency Light and Control..................................................................33-16
33-15 Forward Baggage Compartment Light.....................................................33-18

33 LIGHTING
33-16 Baggage Comp Light Switch...................................................................33-18
33-17 Exterior Lighting Locations.....................................................................33-20
33-18 Position Lights and Control.....................................................................33-21
33-19 Landing Lights and Control.....................................................................33-22
33-20 
Anticollision and Beacon Lights and Controls.........................................33-23
33-21 Taxi Light and Control.............................................................................33-24
33-22 Wing Inspection Light and Control..........................................................33-26
33-23 Logo Lights.............................................................................................33-27

FOR TRAINING PURPOSES ONLY 33-v

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure Title Page

33-24 Caution Light Panel - 400 CYCLE..........................................................33-30
33-25 Caution Light Panel - PNEUMATIC PRESS............................................33-30
33-26 Caution Light Panel - DOORS UNLOCKED...........................................33-30
33-27 
Propeller Autofeather Advisory Lights.....................................................33-30
33-28 Beta System Advisory Lights...................................................................33-30
33-29 Power Lever Test Switch..........................................................................33-32
33-30 Stall Warning Light.................................................................................33-32
33-31 Fire Warning Lights.................................................................................33-32
33-32 Typical Instrument Post Lights.................................................................33-34
33-33 MMEL - Lights (Sheet 1 of 3).................................................................33-36
33-34 MMEL - Lights (Sheet 2 of 3).................................................................33-37
33-35 MMEL - Lights (Sheet 3 of 3).................................................................33-38

TABLES
Table Title Page
33 LIGHTING

33-1 Caution Lights...........................................................................................33-4

33-vi FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CHAPTER 33
LIGHTING

33-00-00 LIGHTING
GENERAL

33 LIGHTING
The aircraft lighting system includes interior and exterior illumination and a caution light system.
The interior lighting consists of six general cabin lights, two cockpit utility lights, a cockpit dome
light, passenger reading lights, an entrance light, forward and aft baggage compartment lights, and
cabin sign lights. An airstair door light on the fuselage exterior is in circuit with the entrance light.

All lights operate from the aircraft 28 VDC power supply system. Individual circuits are protected
by thermal push-pull circuit breakers.

FOR TRAINING PURPOSES ONLY 33-1

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 33-1. Instrument Post Lights Figure 33-2. Instrument Lights

(Typical)
33 LIGHTING

Figure 33-3. General Panel Lighting

33-2 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

33-10-00 FLIGHT Co-Pilot Flight Instrument and

COMPARTMENT Right Radio Panel Lights
LIGHTING The co-pilot flight instrument panel, voltmeter/
loadmeter, right-hand radio panel, co-pilot
oxygen gages, hydraulic system gage, DC
GENERAL volt and load meters including de-icing gages
(when installed) are illuminated by lights
The flight compartment lights include pilot powered from the right DC bus. These lights
and co-pilot utility lights, panel lighting, and are controlled by a dimmer rheostat labeled
the caution lights. As a customer option (Mod COPLT RADIO & VA PNL LTS positioned
S.O.O. 6071) a dome light can be fitted on the on the overhead console circuit breaker panel.
overhead console. The circuits are protected by a 5-amp circuit
breaker labeled COPLT RAD & VA PNL LT
The panel and instrument lighting is controlled on the main circuit breaker panel.
and supplied as follows: The brilliance of the
lights is selected by rheostat dimmers from
BRT to DIM.
Panel Lighting
Refer to Figure 33-3. General Panel Lighting.
FLIGHT INSTRUMENTS, White panel lighting is provided by integral
ENGINE INSTRUMENTS, AND lights in each panel for the flight instruments
and engine instrument panels, trim controls,
EMERGENCY PANEL LIGHTS overhead console, emergency panel, and DC
meter panel.
The pilot flight instrument panel, engine
instrument panel, emergency panel, left-hand
These are miniature lamp assemblies embedded
radio panel, the brake hydraulic panel, the
in the control panels to illuminate the panel
pilot oxygen gages, and the magnetic standby
lettering. The integral light assemblies consist
compass are illuminated by lights powered
of a printed circuit on a glass base epoxy resin
from the left DC bus and controlled with a
material with a miniature lamp cemented to it.
dimmer control unit. The PLT ENG INST &
The lamp wires are soft soldered to the printed
EMER PNL LTS is on the overhead console
circuit. The light assemblies are installed into
below the AC fuse panel (Figure 33-2). The
panel cut-outs and secured by screws, which are
circuit is protected by a 5-amp circuit breaker

33 LIGHTING
covered with PVC insulation tape. To replace
labeled PLT ENG CONS & TRIM PNL LT on
lamps the facing (lighting) panel must be removed
the main circuit-breaker panel.
from the associated backing panel to gain access.
The trim console panel, overhead console
On later aircraft, a small + symbol on each panel
switch panels, intake deflector and flap
indicates where the electrical connection providing
position indicator lights and (Post Mod
power to the lights is on the backside of the panel.
6/1478) nose wheel position indicator light
are also supplied from the left DC bus through
Post lights for instrument lighting were
the PLT ENG CONS & TRIM PNL LT circuit
introduced beginning with aircraft serial number
breaker, but are dimmed through the dimmer
95 to provide a standard panel configuration
control marked CONSOLE FLAPS & TRIM
more adaptable to customer requirements. The
PNL LTS. As a customer option (Mod S.O.O.
post lamps are bolted to the panel through the
6161) flap selector and position indicator
instrument itself, thereby replacing one of the
lights can be fitted.
instrument attaching bolts. The lamps used
are midget flange base type 327, with colored
filters installed in the lamp caps.

FOR TRAINING PURPOSES ONLY 33-3

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Indication Cause Immediate Action Remarks

Indicated generator has Attempt maximum of two Remaining generator carries load;
overvoltage or undervoltage or resets; if light is still on, shut bus tie is normal.
has failed. generator off.

Indicated generator has Shut off generator. Do not If light is still on after three minutes,
overheated. attempt to reset. shut down engine and feather
propeller. Monitor fire detection.

Deicing and autopilot bleed-air Check bleed-air switches,

pressure is below 14 psi. increase NG to 70% or more.
Engine or bleed-air pipe has Shut off engine bleed supply
failed. of inoperative engine. Shut off
heating system if necessary.

Propeller levers are not at full Advance propeller levers to full

increase when power levers increase if on final approach.
are set below 75%.

DUCT OVERHEAT
Light illuminates at 300 oF. Open ram-air valve. Set
Indicating high temperature temperature controls to cooler
in plenum area under cockpit temperature. Also cabin air
floor. control valve may be opened.

Cockpit, airstair, right cabin, or Close and secure open door. If airborne, turn seat belt sign on,
either baggage door is open. and land at nearest suitable airport.

Indicated engine oil pressure is Shut down engine.

less than 40 psi.

Selected inverter has failed. Select other inverter.

Only 75 usable pounds remains Check forward fuel gauge. See AFM para. 3.4.2 through 3.4.4.
in forward collector tank.
33 LIGHTING

Only 110 usable pounds Check aft fuel gauge. See AFM para. 3.4.2 through 3.4.4.
remaining in aft collector tank.

Pull circuit breaker of failed Automatic changeover activates

pump. No.2 boost pump. If this does not
occur, turn on standby boost pump
emergency switch.
If both pumps for the same tank
have failed, move the fuel selector to
BOTH ON FWD or AFT as applicable
and monitor fuel quantity indicator.

Propeller levers are not at full Advance propeller levers to full

increase when power levers increase if on final approach.
are set below 75%.

Table 33-1. Caution Lights

33-4 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

The post lights can be easily replaced or has an amber inscription on a black background
changed. Loose post lamp assemblies should be and, when ON, indicates a malfunction (or
serviced promptly, because of the possibility of system selected to the OFF position) of
a short circuit developing in the post lamp base, the aircraft systems and can comprise the
which may result in a loss from service of all following:
of the lights fed from the same circuit breaker.
•• L GENERATOR,
Edge lights illuminate the engine instrument •• R GENERATOR,
panel, the hydraulic pressure gages, overhead
•• L ENGINE OIL PRESSURE,
console, the emergency panel, trim controls,
DC meter panel, and flap indicator. •• R ENGINE OIL PRESSURE,
•• AFT FUEL LOW LEVEL,
CAUTION LIGHTS •• FWD FUEL LOW LEVEL,
General •• BOOST PUMP 1 AFT PRESS,
The accompanying Table 33-1 shows the •• BOOST PUMP 2 AFT PRESS,
caution lights, inscriptions, reasons for coming
•• BOOST PUMP 1 FWD PRESS,
on, and immediate action to be taken for each
caution light on the annunciator panel. It may •• BOOST PUMP 2 FWD PRESS,
be folded out as reference when reading each
•• DUCT OVERHEAT,
chapter.
•• L 400 CYCLE,
NOTE •• R 400 CYCLE,
There are two possible •• RESET PROPS,
configurations for Autofeather
•• LOW PRESS,
system and Beta Back Up system
annunciator lights, only one •• L GENERATOR OVERHEAT,
configuration would be installed
•• R GENERATOR OVERHEAT and
on the aircraft.
•• DOORS UNLOCKED caution lights are
The warning system for Twin Otter aircraft available as customer options.
provides a caution light annunciation to the crew

33 LIGHTING
of airplane equipment malfunctions, indications The light assemblies comprising either two or
of unsafe operating conditions, which require three dual-lamp units are on a caution lights
immediate attention, or an indication that a panel. In the case of a duct overheat, low
particular system is in operation. Beginning fuel level or low oil pressure condition, or
with aircraft serial number 311 an SFAR mod if a generator or boost pump fails, a circuit
6/1277 (S/B 6/209 Rev C), a horn was added to is completed in the affected system which
sound with the caution light for stall warning, provides a ground to bring on the applicable
and a bell was added to ring in the event of an caution light. Also, if an operating condition
engine fire. arises where the propellers should be reset to
maintain correct flight characteristics, a caution
An equivalent warning system was available by light comes on.
S.O.O. 6033 on earlier serial number aircraft.
The LOW PRESS light (if fitted) is controlled
Two panels of nine caution lights are installed by a pressure switch in the aircraft pneumatic
above the fire emergency panel. The lights system and indicates a fault condition which
are arranged in three rows on each side of the could affect the aircraft flight instruments or
magnetic standby compass. Each caution light autopilot system.

FOR TRAINING PURPOSES ONLY 33-5

33-6 33 LIGHTING

* CUSTOMER OPTION

TWIN OTTER SERIES

FOR TRAINING PURPOSES ONLY

* * * *
RESET PROPS

L 400 CYCLE R 400 CYCLE

MAINTENANCE TRAINING MANUAL

Figure 33-4. DC Master, Caution and Emergency Panels
 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

The L or R GENERATOR OVERHEAT lights The spring loaded momentary TEST position
(if fitted) indicate a fault condition (overload (Figure 33-5), a ground is completed to the
or bearing failure) in the appropriate DC caution light dimming control box. This checks
generator. It is accomplished by temperature all eighteen individual lamps including the
sensors in each generator. battery temperature light and the Beta backup
power lever microswitch test light if installed.
The DOORS UNLOCKED light (if fitted) Unused lights should come on with a horizontal
will come on if either the airstair door, the bar showing the full width of the light.
right cabin door, the front or rear baggage
compartment doors are not locked. When Mod In the BRT position, the caution lights operate
6/1268 or 6/1239 is installed, a relay and an at full brilliance. It is recommended that the
airstair door lock switch are added. switch be left in the BRIGHT position at all
times unless adjusted, as required, by the crew.
A STALL WARN light on the pilot instrument
panel is operated by either the upper or lower In the DIM position, relays in the caution lights
stall warning transmitter vane on the wing dimming control box are energized, switching a
leading edge.. resistor in series with each caution light.

Caution Lights Dimming/Test The DIM and BRT positions provide alternative
degrees of lighting brilliance for all caution
Switch lights, Beta range lights, Beta backup disarmed
Refer to Figure 33-4. DC Master, Caution and light, autofeather indicator lights, stall warning
Emergency Panels. light, and, if applicable, wheel ski position
indicator lights.
A three-position switch on the overhead
console switch panel is marked CAUTION The TEST and DIM positions control the
LT with DIM, BRT and TEST positions. The autofeather lights through the caution lights
stall warning horn will sound when the caution dimming control box and the beta backup
lights are tested. The autopilot annunciators, if lights, through the beta backup control box.
installed, battery temperature warning light and Only the TEST position operates the STALL
engine FIRE PULL lights are not considered WARN light.
part of the caution light system and are tested
separately using other switches.

33 LIGHTING

Figure 33-5. Caution LT Switch

FOR TRAINING PURPOSES ONLY 33-7

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
33 LIGHTING

Figu re 1: Caution Lights Dimming Control Box

Figure 33-6. Caution Lights Dimming Control Box

33-8 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Caution Lights Dimming Control There are two bulbs in each caution light
assembly. The bulbs are Grimes 327 or
Box MS25237Ð327 and are similar to the post
The caution lights dimming control box (Figure light lamps.
33-6) forms part of the master caution system
and consists of an aluminum alloy box which The test and intensity circuit receives power
houses a number of control relays, resistors, from the right bus and is protected by a 5-amp
diodes and a capacitor. Electrical connection circuit breaker labeled CAUT LT DIM on the
to the box is via two multipin connectors. A main circuit breaker panel.
wiring diagram of the box assembly is shown in
the Wiring Diagram Manual, PSM 1–63–2W.

Power Supply
The caution lights are powered from the
28V left or right DC buses, through circuit
breakers for the individual systems circuits.
There are no AC powered caution lights. DC
powered oil pressure and low fuel caution
lights provide a backup for the AC powered
gages normally used to monitor these systems.
When both left and right caution lights are
displayed for a system, power for the lights
is obtained from the opposite bus than the
system protected by the light, to ensure that
should an electrical failure occur in any one
bus system, the caution light will receive
power from the other bus and illuminate to
indicate the failure. Loss of the power from
one bus would extinguish all the caution lights
powered from that bus if the bus tie switch is
not in the normal position. It is important that
the pilot be aware that if one bus is without
power, the caution lights will not accurately

33 LIGHTING
reflect the state of the aircraft systems.

The left bus provides power to the R

GENERATOR, L ENGINE OIL PRESS,
400 CYCLE, AFT FUEL LOW LEVEL,
PNEUMATIC PRESS , BOOST PUMP 1
FWD PRESS, BOOST PUMP 1 AFT PRESS,
GENERATOR OVERHEAT R2 DUCT
OVERHEAT, and if installed ANTISKID.

The right bus provides power to the L

GENERATOR, FWD FUEL LOW LEVEL,
DOORS UNLOCKED, R ENGINE OIL PRESS,
BOOST PUMP 2 FWD PRESS, BOOST PUMP
2 AFT PRESS, GENERATOR OVERHEAT,
PROP RESET.

FOR TRAINING PURPOSES ONLY 33-9

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 33-7. Cockpit Utility Lights

33 LIGHTING

Figure 33-8. Cockpit Dome Light

33-10 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

COCKPIT UTILITY LIGHTS regulator panel, brake hydraulic gauge, and

compass lights come on at full brightness.
Two cockpit utility lights are mounted in quick
2. Turn rheostat control to DIM and check that
release clips, one above each flight compartment
lights are dimmed.
door (Figure 33-7). Alternatively they can be
mounted on the bulkhead behind the pilot and 3. Operate CONSOLE FLAP & PNL LTS
co-pilot seats. The lights are controlled by a dimmer control to BRT and check that
two-position On / Off switch labeled FLIGHT overhead console switch panels, intake
COMP LT on the overhead console. They deflector and flap position indicator lights
may be dimmed by rotating a rheostat switch come on at full brightness. Switch on
on the case of each light. The lights have the aileron trim switch and check that trim
capability of illumination in either red or white panel light comes on.
light controlled with an integral rotating sleeve
4. Turn rheostat control to DIM and check that
at the front of the light. The circuit is powered
lights are dimmed.
from the right DC bus and is protected by a
5-amp circuit breaker labeled FLT COMP LT 5. Operate COPLT RADIO & VA PNL LTS
on the main circuit breaker panel. dimmer control to BRT and check that
meter panel, right hand radio panel, system
hydraulic gauge, co-pilot instrument panel
COCKPIT DOME LIGHT and co-pilot oxygen regulator panel lights
come on at full brightness.
A dome light is installed on the flight compartment
roof to the right of the overhead console (Figure 6. Turn rheostat control to DIM and check that
33-8). The light is powered from the right DC bus lights are dimmed.
through the 5 amp circuit breaker labeled FLT
COMP LT on the main circuit-breaker panel. Operational Test Caution Lights
1. With power on buses, operate CAUTION
Light control is accomplished with a built in
LT DIM–BRT–TEST switch to TEST and
two-position ON/OFF pushbutton switch.
check that all caution lights come on at full
brightness.
*The following is an abbreviated description of
the maintenance practices and is intended for 2. Hold switch to TEST and check that beta
training purposes only. backup system BETA RANGE and BETA
BACKUP DISARMED lights, autofeather
For a more detailed description of the practice,
SEL and ARM lights, and STALL WARN

33 LIGHTING
refer to the task in the Viking AMM PSM 1-63-2.
light (and horn if installed) come on.
Release switch.
33-10-00 MAINTENANCE 3. Set switch to DIM and check that all lights
PRACTICES except STALL WARN are dimmed. Set
switch to BRT and check that lights return
to bright.
ADJUSTMENT/TEST
Operational Test Panel and
Instrument Lights
1. With power on buses, operate PLT ENG
INST & EMER PNL LTS rheostat control to
BRT and check that pilot flight instrument
panel, engine instrument panel, emergency
panel, left hand radio panel, pilot oxygen

FOR TRAINING PURPOSES ONLY 33-11

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 33-9. Cabin Light Switch

33 LIGHTING

Figure 33-10. Entrance Boarding Lights and Control

33-12 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

33-20-00 PASSENGER The passenger entrance lights are powered from

the main battery bus, through a 5 amp circuit
COMPARTMENT LIGHTS breaker labeled COMP LTS on the rear cabin
bulkhead circuit-breaker panel above the right
hand seat.
GENERAL
The passenger compartment lights include Toilet Light
cabin lighting (basic or commuter interior),
The toilet light (when installed) derives its
passenger reading lights, toilet light, and
supply from the main battery bus through the
warning sign lights.
COMP LTS circuit breaker, and is controlled
by a switch marked TOILET LT installed on
DESCRIPTION the rear face of bulkhead 332.00.

Cabin Lighting Cabin Signs (Mod S.O.O. 6110)

Refer to Figure 33-9. Cabin Light Switch.
Refer to Figure 33-11. Cabin Signs.
The cabin lighting system consists of six
Two cabin signs, NO SMOKING and FASTEN
flush-mounted dome lights installed down the
SEAT BELTS, may be installed in the cabin
center-line of the cabin roof. The lights are
to the right of the cabin/flight compartment
controlled by one of a group of three switches
doorway at station 111.00 (Figure 33-11). They
marked LIGHTING on the flight compartment
are illuminated by a number of small bulbs
overhead console switch panel, and the circuit
behind translucent panels. The cabin signs are
is protected by the circuit breakers identified
controlled by separate on-off switches labeled
GENERAL on the overhead circuit breaker
NO SMOKING and FASTEN SEAT BELT
panel, and powered from the right DC bus. The
which are on the overhead console switch panel.
cabin lights switch is marked GENERAL DIM–
OFF–BRIGHT, and when selected to the DIM
The circuits are powered from the right DC
position connects a 5 ohm dimming resistor in
bus through a 5-amp circuit breaker labeled
series with the supply to the cabin lights.
FLT COMP LT on the main circuit breaker
panel. Cabin signs are available in numerous
Passenger Entrance Lights languages to accommodate the needs of
passengers throughout the world.
Refer to Figure 33-10. Entrance Boarding Lights

33 LIGHTING
and Control.

The entrance lights consist of a threshold floodlight

recessed in the cabin roof above the airstair door
and an airstair door floodlight on the fuselage
exterior skin forward of the door entrance.

The lights are operated by either a switch marked

BOARDING LT forward of the left cabin door or
a switch marked LIGHTING ENTRANCE–ON–
ON on the overhead console panel in the flight
compartment. (Figure 33-10). On installations
incorporating S.O.O. Mod 6175, the switch
marked BOARDING LT is on the rear face of
the commissary, forward of the left cabin door.
Figure 33-11. Cabin Signs

FOR TRAINING PURPOSES ONLY 33-13

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

EMER

DISARM

TEST

ARM
33 LIGHTING

Figure 33-12. Reading Lights and Control

33-14 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Passenger Reading Lights

Refer to Figure 33-12. Reading Lights and Control.

Series 300 aircraft by mod 6/1225 are fitted with

twenty passenger reading lights, six on the left
and fourteen on the right, installed in the cove
ventilation duct along the top of the cabin walls
(Figure 33-12). The lights are controlled with a
two-position switch labeled READING on the
overhead console switch panel. A pushbutton
switch adjacent to each light provides passenger
control over individual lights providing the pilot
has selected the READING light switch to ON.

The circuits are powered from the left and

right DC bus and are protected by two circuit Figure 33-13. Passenger Reading Lights
breakers. One 5 amp circuit breaker labeled
CABIN LTS READING receiving power
from the left bus for the left side of the cabin
and the other 7.5 amp circuit breaker labeled
READING receiving power from the right bus
for the right side of the cabin. Both circuit
breakers are on the overhead console circuit
breaker panel.

33 LIGHTING

FOR TRAINING PURPOSES ONLY 33-15

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

EMER

DISARM

TEST

ARM
33 LIGHTING

Figure 33-14. Emergency Light and Control

33-16 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

33-50-00 EMERGENCY batteries, and lamp. Selecting TEST the DC

power is disrupted and the emergency lights
LIGHTING - OPTIONAL will come on.

The DISARM position (momentary-on) prevents

GENERAL the power control circuit from energizing the
lamp by battery power when DC power is
Emergency lighting is a requirement for removed from the aircraft and should be selected
all aircraft operating under British CAA before switching off the DC master switch.
regulations, and is optional on other aircraft When the switch is selected to DISARM the
(Figure 33-14). Two configurations were battery and DC power are disconnected.
available S.O.O. 6098 and 6179.
*The following is an abbreviated description of
The initial emergency lighting, S.O.O. 6098, the maintenance practices and is intended for
installed two removable dry cell units in the training purposes only.
cabin roof. An integral switch labeled PUSH ON;
PUSH & PULL TO RESET controlled each unit For a more detailed description of the practice,
(Figure 33-14). The lights were designed to come refer to the task in the Viking AMM PSM 1-63-2.
on automatically should the aircraft encounter an
abnormal landing or incident condition with the
tripping of the switch. The switch is activated
33-50-00 MAINTENANCE
with a positive or negative acceleration between 2 PRACTICES
to 4 Gs. The inertia switch is retained in position
by a ball click mechanism and a bayonet catch
in a circular retaining ring. The unit can be
ADJUSTMENT/TEST
withdrawn from the container to be handheld.
Operational Test Emergency
The later configuration by S.O.O. 6179 provided Lights (S.O.O. Mod 6179)
two lights in the cabin center ceiling panel at
1. Connect external electrical power to
stations 164 and 295 to provide illumination
aircraft.
in the event of a DC power failure. Each light
is powered by two integral nickel cadmium 2. Set emergency light switch in flight
batteries, which are on continuous trickle charge compartment to ARM, and check that both
when the DC system is active. The emergency charging indicator lights on each unit come on.
lights are controlled by a three-position switch

33 LIGHTING
3. Set switch to TEST. Check charging
ARM, TEST, and DISARM, on the overhead
indicator lights go out and both emergency
console switch panel labeled EMER. The lights
lights come on.
receive power from the right DC bus to a 5
amp circuit breaker labeled EMER LTS on the 4. Set switch to DISARM. All lights should
overhead circuit breaker panel. be out.
5. Disconnect external electrical power.
The ARM position (lever locked) is normally
selected for flight it arms the lights for automatic
operation should a power failure occur. With DC
power available and the control switch in the
ARM position, the internal batteries will receive
a trickle charge. Charging indicator lights on the
emergency light will come on.

The TEST position interrupts the airplane

input voltage, testing the monitoring circuit,

FOR TRAINING PURPOSES ONLY 33-17

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 33-15. Forward Baggage Compartment Light

33 LIGHTING

Figure 33-16. Baggage Comp Light Switch

33-18 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

33-30-00 CARGO NOTES

AND SERVICE
COMPARTMENT LIGHTS
GENERAL
The cargo and service compartment lights comprise
front and rear baggage compartment lights. The
front and rear baggage compartment lights are
powered from the main battery bus through a 5
amp circuit breaker labeled COMP LTS, which is
on the cabin rear bulkhead circuit breaker panel.

Forward Baggage Compartment

Light
The long nose baggage compartment was installed
on all aircraft except floatplanes beginning
at aircraft serial number 116 by mod 6/1077.
The light is operated with a pushbutton switch
installed on the light assembly, similar to the
flight compartment dome light (Figure 33-15).

For short nose aircraft the baggage compartment

light is operated by the opening of the hinged
nose cap that activates a microswitch.

Rear Baggage Compartment

Lights
Two dome lights illuminate the rear baggage

33 LIGHTING
compartment. Two rear baggage compartment
lights are energized by either a limit switch
whenever the side access door is opened, or by
a switch marked BAGGAGE COMP LT in the
baggage compartment on the bulkhead at station
332.00 (Figure 33-16). The switch may be operated
by hand when the compartment is entered from the
cabin during flight. The switch is adjacent to the
door between the baggage compartment and cabin,
on the aft face of the bulkhead between the cabin
and baggage compartment.

NOTE
Ensure that the lights are not
inadvertently left on overnight,
as it will deplete the battery.

FOR TRAINING PURPOSES ONLY 33-19

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

POSITION LIGHT BEACON LIGHT

LOGO LIGHTS

ANTICOLLISION LIGHT

WING INSPECTION LIGHT

POSITION LIGHT

BEACON LIGHT

LANDING LIGHT LANDING LIGHT

33 LIGHTING

TAXI LIGHT

Figure 33-17. Exterior Lighting Locations

33-20 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

33-40-00 EXTERIOR POSITION LIGHTS

LIGHTING The conventional position (navigation) lights
consist of red (left) and green (right) wingtip
lights and a white taillight. A plastic vertical
GENERAL reflector plate was installed at aircraft serial
number 15 by mod 6/1024 adjacent to the wing
The exterior lighting system comprises landing
position lights, to allow clear observation of
lights, an upper anti-collision light and position
the light from the flight compartment (Figure
lights or combined strobe and position lights if
33-13).
Mod 6/1513 is incorporated. Customer option
equipment can include a lower anti-collision
The lights are controlled with a switch labeled
light, wing inspection lights and a taxi light.
POSN on the overhead console (Figure 33-14).
The circuit is powered from the left DC bus and
A lower beacon light and strobe lights can also
is protected by a 5 amp circuit breaker labeled
be installed. All exterior lights, except for the
POSN LT on the main circuit breaker panel.
boarding (entrance) light, are powered from the
left or right DC bus.

Exterior lighting locations are shown in (Figure

33-12).

VERTICAL
REFLECTOR
PLATE

33 LIGHTING

Figure 33-18. Position Lights and Control

FOR TRAINING PURPOSES ONLY 33-21

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
33 LIGHTING

Figure 33-19. Landing Lights and Control

33-22 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

LANDING LIGHTS The white lens replaced the red lens for the
upper and lower beacon light locations.
Two 250-watt landing lights are installed, one
in each wing leading edge outboard of the White strobe lights are standard equipment for
engine. The lights are controlled with two all series. An integral strobe and position light
(LEFT and RIGHT) LANDING LT switches replaced the existing wing position lights with
(Figure 33-19) on the overhead console. a lightning protection horn on each wing tip
(Figure 33-20).
The switches have two-positions, LEFT and OFF,
and RIGHT and OFF. The circuits are powered A flasher unit was installed above the cabin
from the left and right DC buses through 10 amp roof at station 210 and power supply units were
circuit breakers labeled LDG LT L and LDG LT on each wing tip rib.
R on the main circuit breaker panel.

The wing landing light glass cover and bonded

seal prevents engine exhaust gas distortion of
the early polycarbonate material covers. The
covers are essential to prevent corrosive engine
exhaust gases from entering the wing.

ANTI-COLLISION LIGHTS
Early aircraft were initially equipped with a dual
filament red beacon light rotated by a 28VDC
motor contained within the light assembly housing,
mounted on the tip of the horizontal stabilizer.
For aircraft before strobe navigation lights the
beacon light and motor were powered from the left
DC bus through a 5 amp circuit breaker labeled
BEACON LT on the main circuit breaker panel,
and controlled by a single position ON OFF switch
labeled ANTI COLL LT OR BEACON LT on the
overhead console lighting panel.

33 LIGHTING
For aircraft with strobe lighting, the upper
beacon light is powered through a 5 amp circuit
breaker labeled BEACON LT in the overhead
circuit breaker panel and controlled with a
single position switch labeled BEACON LT on
the overhead console lighting panel.

If required, a second beacon light could be

installed on the lower fuselage by S.O.O. 6125.
This light receives power from the right DC bus
through a separate 5 amp circuit breaker labeled
BEACON LT LOWER or ANTI COLL LT
LOWER, on the overhead circuit breaker panel,
to a two-position switch labeled ANTI COLL
LIGHTS on the overhead console lighting panel. Figure 33-20. A
 nticollision and Beacon
Lights and Controls

FOR TRAINING PURPOSES ONLY 33-23

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
33 LIGHTING

Figure 33-21. Taxi Light and Control

33-24 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

TAXI LIGHT NOTES

Refer to Figure 33-21. Taxi Light and Control.

The taxi light on the nosewheel fork left side

by optional mod S.O.O. 6012 was later adopted
as standard aircraft equipment by mod 6/1600
beginning at aircraft 531 (Figure 33-21). The 100
watt light receives power from the right DC bus
through a 5-amp circuit breaker labeled TAXI LT
on the main circuit breaker panel, and controlled
by the on off switch labeled TAXI LT, on the
overhead console switch panel.

WARNING

Ground handlers must exercise

care when towing the aircraft not to
damage the power supply cable to
the nosewheel mounted taxi light.
The cable is long enough to allow
free movement of the nosewheel
60° either side of center when it is
connected to the steering system,
but the nosewheel can rotate freely
through 360° when the torque link
pip pin is removed.

33 LIGHTING

FOR TRAINING PURPOSES ONLY 33-25

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
33 LIGHTING

Figure 33-22. Wing Inspection Light and Control

33-26 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

WING INSPECTION LIGHTS NOTES

Refer to Figure 33-22. Wing Inspection Light and
Control.

Aircraft certified for icing operation must have

wing inspection lights installed by mod S.O.O.
6006 to allow the pilot to monitor the condition
of the wing surface and deicing boot operation. A
40-watt light is on the outboard side of each engine
nacelle to illuminate the outer wing leading edge
surface. The lights are powered from the right bus
through a 5 amp circuit breaker labeled WING
INSP LT on the main circuit breaker panel and
controlled by a two-position on off switch (Figure
33-22) labeled WING INSP LT on the overhead
console deicing panel.

LOGO LIGHTS
Refer to Figure 33-23. Logo Lights.

An optional installation by EO 68849 is available

to locate Devore brand lights in the horizontal
stabilizer structure to illuminate a company logo
on the vertical stabilizer surface. The lights will
illuminate both sides of the vertical stabilizer.
Power is obtained from the right DC bus through
a 5 amp circuit breaker labeled LOGO LT, in the
main circuit breaker panel, to an on off switch in
the overhead console.

33 LIGHTING

Figure 33-23. Logo Lights

FOR TRAINING PURPOSES ONLY 33-27

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of Operational Test Wing Inspection

the maintenance practices and is intended for
training purposes only.
Lights
1. Switch on the WING INSP LT switch and
For a more detailed description of the practice,
check that right and left wing inspection
refer to the task in the Viking AMM PSM 1-63-2.
lights come on.

33-40-00 MAINTENANCE 2. Switch off WING INSP LT switch.

PRACTICES
ADJUSTMENT/TEST
Operational Test Landing Lights
1. Connect external power source to aircraft buses.
2. Switch on LANDING LT LEFT switch
and check that left landing light comes on;
switch OFF left landing light.
3. Switch on LANDING LT RIGHT switch
and check that right landing light comes on;
switch OFF right landing light.

Operational Test Position Lights

1. Switch on the POSN switch and check
that the left and right wing, and tail lights
come on.
2. Switch off the POSN switch.

Operational Test Anti-Collision

Lights
1. Switch on the ANTI COLL or BEACON
33 LIGHTING

(Mod 6/1513) switch and check that

the upper and lower (if installed) anti-
collision lights come on and rotate quietly
at 45 ± 5 rpm.
2. Switch off ANTI COLL or BEACON
switch.

Operational Test Taxi Light

1. Switch on the TAXI LT switch and check
that taxi light comes on.
2. Switch off TAXI LT switch.

33-28 FOR TRAINING PURPOSES ONLY

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

PAGE INTENTIONALLY LEFT BLANK

33 LIGHTING

FOR TRAINING PURPOSES ONLY 33-29

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

RESET PROPS

Figure 33-24. Caution Light Panel - 400 CYCLE

RESET PROPS

Figure 33-25. Caution Light Panel - PNEUMATIC PRESS

RESET PROPS
33 LIGHTING

Figure 33-26. Caution Light Panel - DOORS UNLOCKED

Figure 33-27. Propeller Autofeather Figure 33-28. Beta System Advisory Lights
Advisory Lights

33-30 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

33-00-00 OPTIONAL PROPELLER AUTOFEATHER

SYSTEMS ADVISORY LIGHTS
Refer to Figure 33-27. P
 ropeller Autofeather
S.O.O. 6142 DUAL 400 CYCLE Advisory Lights.
SYSTEM Early Series 300 aircraft had two autofeather
advisory lights manufactured by Honeywell
Refer to:
installed on the instrument panel. These are
powered from the left bus through a 5-amp
•• Figure 33-24. Caution Light Panel - 400
circuit breaker labeled PROP AUTO FEATH
CYCLE.
on the main circuit breaker panel.
•• Figure 33-25. Caution Light Panel -
PNEUMATIC PRESS.
BETA SYSTEM ADVISORY
British CAA certification requirements called LIGHTS
for the isolation of 400 cycle buses. On aircraft
equipped with this modification, both inverters Figure 33-28. Beta System Advisory Lights.
operate simultaneously. Two caution lights,
labeled L 400 CYCLE and R 400 CYCLE, are Two Beta Range advisory lights labeled L and
on each side of the annunciator caution light R and a single BACK UP DISARMED light,
panel. The L 400 CYCLE light is powered from all manufactured by Dialco, were installed on
the right DC bus and the R 400 CYCLE light is the instrument panel. These lights are powered
powered from the left DC bus. from the right bus through a 5-amp circuit
breaker labeled BETA SYS on the main circuit
Although all aircraft are provided with a caution breaker panel.
light labeled PNEUMATIC PRESS, the light
is only functional in aircraft equipped with A modular type switch and light assembly
airframe deicing. The PNEUMATIC PRESS manufactured by Aerospace Optics was
caution light warns of a pressure reduction in the adopted to replace the early autofeather and
operating pressure available for either system. beta warning lights, including the initial Licon
switch and light assembly. The modular switch
is installed in many higher serial number Series
DOOR WARNING SYSTEM 300 instrument panels.

33 LIGHTING
Refer to Figure 33-26. Caution Light Panel -
DOORS UNLOCKED.

The door warning system, which operates the

DOORS UNLOCKED light, was installed as
standard equipment beginning at aircraft serial
number 311. Previously, a door warning system
had been available as S.O.O. 6061.

FOR TRAINING PURPOSES ONLY 33-31

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 33-29. Power Lever Test Switch Figure 33-30. Stall Warning Light
33 LIGHTING

Figure 33-31. Fire Warning Lights

33-32 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

POWER LEVER TEST SWITCH The bulbs used in the FIRE PULL handles
are 327 lamps, the same as a post lamp. If
Refer to Figure 33-29. Power Lever Test necessary, an inoperative fire handle warning
Switch. light could be replaced in the field with a
spare post lamp by simply screwing out the
A power lever test switch with an integral lamp assembly from the side of the pull handle
indicator light to ground test the correct operation and replacing the bulb. See ATA 26 Fire
of the power lever operated microswitch. The Protection for further information.
switch, marked “PWR LEV TEST”, is adjacent
to the BETA RANGE TEST switch.

STALL WARNING LIGHT AND

HORN
Refer to Figure 33-30. Stall Warning Light.

The red stall warning light on the pilots

instrument panel and audible stall horn in the
flight compartment are powered from the left bus
through a 5-amp circuit breaker labeled STALL
WARN on the main circuit breaker panel.

NOTE
When selecting the CAUTION
LT TEST switch, the STALL
warning light will come on to
indicate a good test, but will not
go to the Dim selection.

FIRE WARNING LIGHTS

Refer to Figure 33-31. Fire Warning Lights.

33 LIGHTING
Fire warning lights are on the emergency
panel above the instrument panel responding
to the left and right nacelle engine positions.
The lights in the FIRE PULL handle will
come on when the heat sensor probes in the
engine nacelle area sense an overtemperature
condition. The left bus supplies power to the
LEFT and RIGHT pull handle light circuits
through 5 amp circuit breakers labeled FIRE
DET L and FIRE DET R, both of which are on
the main circuit breaker panel. A fire-warning
bell, which was previously an option, became
standard equipment beginning with aircraft
serial number 311.

FOR TRAINING PURPOSES ONLY 33-33

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of NOTES

the maintenance practices and is intended for
training purposes only.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2.

33-00-00 MAINTENANCE
PRACTICES
INSTRUMENT LIGHTING
An instrument lighting post light (eyebrow
light) and light mount replaces one instrument
mounting bolt (Figure 33-32). The lights used
are midget flange base devices with integral
color filters installed in the light cap. Light
replacement is accomplished by pulling the
lamp cap straight out of its retainer.

ANTICOLLISION LIGHT BRUSH

WEAR CHECK
The lens must be removed from the light
assembly and brush visibility checked through
the inspection hole in the side of the brush
holder. If the brush is not visible, the brush is
worn beyond tolerance and must be replaced.
33 LIGHTING

Figure 33-32. Typical Instrument Post Lights

33-34 FOR TRAINING PURPOSES ONLY

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

PAGE INTENTIONALLY LEFT BLANK

33 LIGHTING

FOR TRAINING PURPOSES ONLY 33-35

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Aircraft: Revision No. 05 Page:

DE HAVILLAND DHC-6, SERIES 100, 200 & 300 Date: Mar. 17, 1995 33-1 of 3
System & 1. 2. Number Installed
Sequence 3. Number Required for Dispatch
Numbers 4. Remarks or Exceptions

33 LIGHTS

1 Cockpit and Instrument Light C - - Individual lights may be inoperative

Systems provided the remaining lights are:

a) sufficient to clearly illuminate all

instruments and switches;

b) positioned so that direct rays are

shielded from flight crew member's
eyes; and,

c) of controllable intensity, unless it is

proven that this feature is
unnecessary.

2 Cabin Lights C - - Individual lights may be inoperative

provided:

a) the adjacent light is operative;

b) inoperative lights do not exceed 50

percent of the total number installed;
and,

c) there is sufficient lighting for crew

members to perform their required
duties.
33 LIGHTING

Figure 33-33. MMEL - Lights (Sheet 1 of 3)

33-36 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Aircraft: Revision No. 11 Page:

DE HAVILLAND DHC-6, SERIES 100, 200 & 300 Date: Dec. 23, 2002 33-2 of 3
System & 1. 2. Number Installed
Sequence 3. Number Required for Dispatch
Numbers 4. Remarks or Exceptions

33 LIGHTS

3 Wing Inspection Lights C 2 0 May be inoperative for daylight operations.

*** (SOO 6006)

SINGLE PILOT OPERATION

C 2 1 Left light required for night flight in known

or forecast icing conditions.

TWO PILOT OPERATION

C 2 0 May be inoperative provided a portable

lamp/light of adequate capacity for wing
and or control surface inspection is
available for night operations in icing
conditions.

4 Landing Lights C 2 0 One or both may be inoperative for

daylight operations.

C 2 1 One may be inoperative for night

operations provided the taxi light operates
normally.

5 Taxi Light C 1 0

6 Position Lights C 3 0 One or more may be inoperative for

daylight operations.

33 LIGHTING

Figure 33-34. MMEL - Lights (Sheet 2 of 3)

FOR TRAINING PURPOSES ONLY 33-37

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Aircraft: Revision No. 11 Page:

DE HAVILLAND DHC-6, SERIES 100, 200 & 300 Date: Dec. 23, 2002 33-3 of 3
System & 1. 2. Number Installed
Sequence 3. Number Required for Dispatch
Numbers 4. Remarks or Exceptions

33 LIGHTS

7 Anti-Collision Beacon Light C 1 0 May be inoperative for day light

System operations.

C 1 0 May be inoperative for night operations

provided the strobe light system operates
normally.

NOTE:
In both cases adequate precautions must
be taken to clear the area prior to engine
start and while engines running.

8 Strobe Lights (CSI) C 2 0

***

8 Cabin Emergency Lighting C 1 1 Individual light bulbs may be inoperative

*** System (SOO 6034, 6098, provided compliance is shown with
6179) minimum acceptable lighting levels
specified in certification documents.

NOTE:
Not required for all cargo operations
provided the flight deck crew are the only
occupants of the aircraft.

10 Passenger Notice "No C 1 0 May be inoperative provided procedures

Smoking/Fasten Seat Belt" are established and used to alert cabin
33 LIGHTING

System crew and notify passengers when seat

belts should be fastened and smoking
prohibited.

NOTE:
Not required for all cargo operations
provided the flight deck crew are the only
occupants of the aircraft.

Figure 33-35. MMEL - Lights (Sheet 3 of 3)

33-38 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CHAPTER 31
INDICATING AND RECORDING SYSTEMS
CONTENTS
Page

31-00-00 INDICATING AND RECORDING SYSTEMS............................................ 31-1

Introduction......................................................................................................... 31-1
31-10-00 INSTRUMENT AND CONTROL PANELS.................................................. 31-3
Description........................................................................................................... 31-3
Pilot Flight Instrument Panel......................................................................... 31-4
Co-pilot Flight Instrument Panel................................................................... 31-5
Co-pilot Flight Instrument Panel (Mods S.O.O. 6075 or 6/1604)................... 31-5
Engine Instrument Panel............................................................................... 31-7
Emergency Panel........................................................................................... 31-7
DC Meter Panel............................................................................................. 31-7
31-10-01 INSTRUMENT AND CONTROL PANELS (MOD 6/1475).......................... 31-9
Description........................................................................................................... 31-9
Pilot Flight Instrument Panel....................................................................... 31-10
Co-pilot Flight Instrument Panel................................................................. 31-11
Co-pilot Flight Instrument Panel (Mod S.O.O. 6075 or 6/1604).................. 31-11
Engine and Fuel Instrument Panel............................................................... 31-13
Emergency Panel......................................................................................... 31-13
DC Meter Panel........................................................................................... 31-13
RECORDING SYSTEMS

31-20-11 CLOCK...................................................................................................... 31-15

31 INDICATING AND

General.............................................................................................................. 31-15

FOR TRAINING PURPOSES ONLY 31-i

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

ILLUSTRATIONS
Figure Title Page

31-1 Instrument and Control Panels...................................................................31-2

31-2 Pilot Flight Instrument Panel.....................................................................31-4
31-3 Co-pilot Flight Instrument Panel................................................................31-5
31-4 Engine Instrument Panel............................................................................31-6
31-5 Instrument and Control Panels (Mod 6/1475)............................................31-8
31-6 Pilot Flight Instrument Panel (Mod 6/1475).............................................31-10
31-7 Co-pilot Flight Instrument Panel (Mod S.O.O. 6075)...............................31-11
31-8 Engine and Fuel Instrument Panel (Mod 6/1475).....................................31-12
31-9 Pilot Flight Instrument Panel...................................................................31-14
31-10 Co-pilot Flight Instrument Panel (Mod S.O.O. 6075)...............................31-15
31-11 MMEL - Indicating and Recording Systems............................................31-16

RECORDING SYSTEMS
31 INDICATING AND

FOR TRAINING PURPOSES ONLY 31-iii

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CHAPTER 31
INDICATING AND RECORDING
SYSTEMS

31-00-00 INDICATING AND RECORDING SYSTEMS

INTRODUCTION
The aircraft instruments comprise pilot flight instruments, co-pilot flight instruments
supplied as customer option items and engine instruments, one set for each engine;
navigation instruments installed to customer requirements, and miscellaneous instruments.
This chapter contains information on the instrument and control panels and the clock (which
comes in the miscellaneous category). The instruments are covered in the appropriate
chapter for the system in which they are used. RECORDING SYSTEMS
31 INDICATING AND

FOR TRAINING PURPOSES ONLY 31-1

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
Figure 31-1. Instrument and Control Panels
RECORDING SYSTEMS
31 INDICATING AND 31-2 FOR TRAINING PURPOSES ONLY
 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

31-10-00 INSTRUMENT NOTES

AND CONTROL PANELS
DESCRIPTION
Refer to Figure 31-1. Instrument and Control
Panels.

The instrument and control panels are

comprised of the pilot and co-pilot sub panels,
the engine instrument panel, the emergency
panel, and the DC meter panel. The panels,
colored gray with white markings, are located
across the front of the flight compartment below
the windshield and secured to a frame which is
shock-mounted to the airframe structure. On
aircraft incorporating Mod 6/1445, provision is
made in the frame and structure for the addition
and/or relocation of shockmounts if required
for custom instrument installation. The area
between the engine instrument panel and the
co-pilot flight instrument panel is reserved for
customer option radio equipment.

CAUTION
CHANGES TO PANEL
INSTRUMENT INSTALLATION
COULD RESULT IN A
REQUIRED CHANGE OF
SHOCKMOUNT TYPE AND/OR
LOCATION. FOR AIRCRAFT
INCORPORATING MOD
6/1445, REFER TO VIKING AIR
TECHNICAL SUPPORT, FOR
CORRECT SELECTION AND
LOCATION OF INSTRUMENT
FRAME SHOCKMOUNTS.
RECORDING SYSTEMS
31 INDICATING AND

FOR TRAINING PURPOSES ONLY 31-3

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Pilot Flight Instrument Panel The flight and navigation instruments are as
follows:
Refer to Figure 31-2. Pilot Flight Instrument
Panel.
•• Airspeed indicator.
The pilot flight instrument panel is furnished •• Attitude indicator.
with the pilot flight and navigation instruments,
•• Altimeter.
the fuel control switches and fuel quantity
indicators, the standby booster pump switches, •• Vertical speed indicator.
the propeller autofeather control switch and
•• Directional indicator.
indicator, the beta range and beta back-up
disarmed indicator lights, the marker beacon •• Turn and slip indicator.
indicator lights, the stall caution light and the
aircraft clock. The instruments are illuminated In addition, provision is made for the installation
by post lights and the intensity of the lighting is of the gyro compass connector, slaving switch and
controlled by rheostat dimmer controls. calibration card (Mod S.O.O. 6081), fuel crossfeed
valve position indicator (Mod S.O.O. 6035), and
three additional customer option instruments.
Unused positions are covered by blanking plates.
RECORDING SYSTEMS
31 INDICATING AND

Figure 31-2. Pilot Flight Instrument Panel

31-4 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Co-pilot Flight Instrument Panel basic arrangement for flight instruments is

illustrated in Figure 31-3 which also shows
Refer to Figure 31-3. Co-pilot Flight Instrument
an alternative arrangement for an additional
Panel.
navigation instrument. On aircraft with Mod
6/1635 incorporated, the instrument panel
On basic aircraft the co-pilot flight instrument
mounting frame provides increased space for
panel is not installed, and the position is
custom avionics. A radio call label can also
covered with a blanking plate.
be installed on the panel. Unused positions are
covered by blanking plates. The instruments
Co-pilot Flight Instrument Panel are illuminated by post lights. The intensity of
the lighting is controlled by rheostat dimmer
(Mods S.O.O. 6075 or 6/1604) controls.
On aircraft with the co-pilot panel installed,
the furnishings are varied according to the
operator’s individual requirements. The

RECORDING SYSTEMS
31 INDICATING AND

Figure 31-3. Co-pilot Flight Instrument Panel

FOR TRAINING PURPOSES ONLY 31-5

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
RECORDING SYSTEMS
31 INDICATING AND

Figure 4: Engine Instrument Panel

Figure 31-4. Engine Instrument Panel

31-6 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Engine Instrument Panel NOTES

Refer to Figure 31-4. Engine Instrument Panel.

The engine instrument panel contains two sets

of engine instruments (one for each engine).
The instruments on the panel are illuminated
by edge lights which are installed in a plastic
facing panel. The facing panel is secured by
screws to a metal backing panel. The intensity
of the lighting is controlled by rheostat dimmer
controls.

The engine instruments are as follows:

•• Torque pressure indicators.

•• Propeller tachometer indicators.
•• Gas generator tachometer indicators.
•• Fuel flow indicators.
•• Turbine temperature indicators.
•• Oil temperature indicators.
•• Oil pressure indicators.
•• Intake deflector indicators.

Emergency Panel
The emergency panel is above the engine
instrument panel and contains the left and
right fuel emergency shut-off switches, the fire
detection switch, and left and right engine fire
extinguisher control handles.

DC Meter Panel
The DC meter panel above the radio equipment
panel, contains a DC voltmeter, DC loadmeter
and a meter select switch.
RECORDING SYSTEMS
31 INDICATING AND

FOR TRAINING PURPOSES ONLY 31-7

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
RECORDING SYSTEMS
31 INDICATING AND

Figure 31-5. Instrument and Control Panels (Mod 6/1475)

31-8 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

31-10-01 INSTRUMENT NOTES

AND CONTROL PANELS
(MOD 6/1475)
DESCRIPTION
Refer to Figure 31-5. Instrument and Control
Panels (Mod 6/1475).

The instrument and control panels are comprised

of the pilot and co-pilot sub panels, the engine
and fuel instrument panel, the emergency panel,
and the DC meter panel. The panels, colored
gray with white markings, are located across
the front of the flight compartment below the
windshield and secured to a frame which is
shock-mounted to the airframe structure. On
aircraft incorporating Mod 6/1445, provision is
made in the frame and structure for the addition
and/or relocation of shockmounts if required
for custom instrument installation. The area
between the engine instrument panel and the
co-pilot flight instrument panel is reserved for
customer option radio equipment.

CAUTION
CHANGES TO PANEL
INSTRUMENT INSTALLATION
COULD RESULT IN A
REQUIRED CHANGE OF
SHOCKMOUNT TYPE AND/OR
LOCATION. FOR AIRCRAFT
INCORPORATING MOD
6/1445, REFER TO VIKING AIR
TECHNICAL SUPPORT, FOR
CORRECT SELECTION AND
LOCATION OF INSTRUMENT
FRAME SHOCKMOUNTS.
RECORDING SYSTEMS
31 INDICATING AND

FOR TRAINING PURPOSES ONLY 31-9

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Pilot Flight Instrument Panel The flight and navigation instruments are as
follows:
Refer to Figure 31-6. Pilot Flight Instrument
Panel (Mod 6/1475).
•• Airspeed indicator.
The pilot flight instrument panel is furnished •• Attitude indicator.
with the pilot flight and navigation instruments,
•• Altimeter.
the propeller autofeather control switch and
indicator, the beta range and beta back-up •• Vertical speed indicator.
disarmed indicator lights, the marker beacon
•• Directional indicator.
indicator lights, the stall caution light and the
aircraft clock. The instruments are illuminated •• Turn and slip indicator.
by post lights and the intensity of the lighting
In addition, provision is made for the installation
is controlled by rheostat dimmer controls.
of the gyro compass annunciator, slaving switch
and calibration card (Mod S.O.O. 6081), and
three additional customer option instruments.
Unused positions are covered by blanking plates.
RECORDING SYSTEMS
31 INDICATING AND

Figure 31-6. Pilot Flight Instrument Panel (Mod 6/1475)

31-10 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Co-pilot Flight Instrument Panel mounting frame provides increased space for
custom avionics. A radio call label can also
On basic aircraft the co-pilot flight instrument
be installed on the panel. Unused positions are
panel is not installed, and the position is
covered by blanking plates. The instruments
covered with a blanking plate.
are illuminated by post lights. The intensity of
the lighting is controlled by rheostat dimmer
Co-pilot Flight Instrument Panel controls.
(Mod S.O.O. 6075 or 6/1604)
On aircraft with the co-pilot panel installed
the furnishings are varied according to the
operator’s individual requirements. The
basic arrangement for flight instruments is
illustrated in Figure 3 which also shows an
alternative arrangement for an additional
navigation instrument. On aircraft with Mod
6/1635 incorporated, the instrument panel

RECORDING SYSTEMS
31 INDICATING AND

Figure 31-7. Co-pilot Flight Instrument Panel (Mod S.O.O. 6075)

FOR TRAINING PURPOSES ONLY 31-11

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
RECORDING SYSTEMS
31 INDICATING AND

Figure 4: Engine Instrument Panel

Figure 31-8. Engine and Fuel Instrument Panel (Mod 6/1475)

31-12 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Engine and Fuel Instrument Panel NOTES

Refer to Figure 31-8. Engine and Fuel
Instrument Panel (Mod 6/1475).

The engine and fuel instrument panel contains

two sets of engine instruments (one for each
engine), the fuel control switches, fuel quantity
indicators and the standby booster pump
switches. The instruments are illuminated by
post lights and the intensity of the lighting is
controlled by rheostat dimmer controls.

The engine instruments are as follows:

•• Torque pressure indicators.

•• Propeller tachometer indicators.
•• Gas generator tachometer indicators.
•• Fuel flow indicators.
•• Turbine temperature indicators.
•• Oil temperature indicators.
•• Oil pressure indicators.
•• Intake deflector indicators.
In addition, provision is made for the
installation of fuel crossfeed valve position
indicator (Mod S.O.O. 6035).

Emergency Panel
The emergency panel is above the engine and
fuel instrument panel and contains the left and
right fuel emergency shut-off switches, the
fire detection switch, and left and right engine
fire extinguisher control handles. The panel is
illuminated by edge lights which are installed
in a plastic facing panel. The facing panel is
secured by screws to the top face of the engine
and fuel instrument panel. The intensity of
the lighting is controlled by rheostat dimmer
controls.
RECORDING SYSTEMS
31 INDICATING AND

DC Meter Panel
The DC meter panel above the radio equipment
panel, contains a DC voltmeter, DC loadmeter
and a meter select switch.

FOR TRAINING PURPOSES ONLY 31-13

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
Figure 31-9. Pilot Flight Instrument Panel
RECORDING SYSTEMS
31 INDICATING AND 31-14 FOR TRAINING PURPOSES ONLY
 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

31-20-11 CLOCK
GENERAL
On basic aircraft the clock is on the pilot flight
instrument panel (refer to Figure 31-9). The
clock is a spring-driven, eight-day instrument.
Provision for a similar clock is provided on the
co-pilot flight instrument panel (refer to Figure
31-10). Elapsed time clocks can be installed as
alternatives.

RECORDING SYSTEMS
31 INDICATING AND

Figure 31-10. Co-pilot Flight Instrument Panel (Mod S.O.O. 6075)

FOR TRAINING PURPOSES ONLY 31-15

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Aircraft: Revision No. 06 Page:

DE HAVILLAND DHC-6, SERIES 100, 200 & 300 Date: Dec. 05, 1997 31-1 of 1
System & 1. 2. Number Installed
Sequence 3. Number Required for Dispatch
Numbers 4. Remarks or Exceptions

31 INDICATING/RECORDING
SYSTEMS

1 Clock with sweep second C 1 - (O) As required by regulations.

hand or electric digital clock

2 Flight Hour Recorder C - 0 (O)

***

3 Engine Hour Recorder C - 0 (O)

***

4 Flight Data Recorder D - 0 As required by regulations.

*** (FDR)

Figure 31-11. MMEL - Indicating and Recording Systems

RECORDING SYSTEMS
31 INDICATING AND

31-16 FOR TRAINING PURPOSES ONLY

 23 COMMUNICATIONS
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CHAPTER 23
COMMUNICATIONS
CONTENTS
Page

23-00-00 COMMUNICATIONS.................................................................................. 23-1

Introduction......................................................................................................... 23-1
General................................................................................................................ 23-3
23-10-00 HIGH FREQUENCY RADIOS.................................................................... 23-3
General................................................................................................................ 23-3
Description........................................................................................................... 23-4
HF Transceiver.............................................................................................. 23-4
Coupler......................................................................................................... 23-4
Antenna......................................................................................................... 23-5
Control.......................................................................................................... 23-5
23-10-00 KHF 950...................................................................................................... 23-7
System Description.............................................................................................. 23-7
23-10-00 KCU 951 CONTROL DISPLAY UNIT........................................................ 23-7
Antenna Tuning.................................................................................................... 23-9
Operation............................................................................................................. 23-9
Fault Indication.................................................................................................... 23-9
23-10-00 MAINTENANCE PRACTICES.................................................................. 23-11
Removal/Installation.......................................................................................... 23-11
KHF 950 System......................................................................................... 23-11
HF Antenna - Removal................................................................................ 23-11
HF Antenna - Installation............................................................................ 23-11

FOR TRAINING PURPOSES ONLY 23-i

23 COMMUNICATIONS

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page
23-20-00 SELECTIVE CALLING............................................................................. 23-15
Introduction....................................................................................................... 23-15
General.............................................................................................................. 23-15
System Description............................................................................................ 23-15
Control Panel.............................................................................................. 23-15
Decoder....................................................................................................... 23-15
Self-Test...................................................................................................... 23-15
23-21-00 VHF COMMUNICATION......................................................................... 23-16
General.............................................................................................................. 23-16
Description......................................................................................................... 23-16
Operation........................................................................................................... 23-16
Current Regulations.................................................................................... 23-16
GNS 430 Keys and Buttons......................................................................... 23-19
To Power on the GNS 430........................................................................... 23-20
23-50-00 AUDIO INTEGRATING SYSTEM............................................................ 23-25
Introduction....................................................................................................... 23-25
General.............................................................................................................. 23-25
System Description............................................................................................ 23-26
23-60-00 STATIC DISCHARGING SYSTEM........................................................... 23-30
Introduction....................................................................................................... 23-30
General.............................................................................................................. 23-30
System Description............................................................................................ 23-30
Operation........................................................................................................... 23-31
Static Discharge Maintenance Practices...................................................... 23-31
23-60-00 MAINTENANCE PRACTICES.................................................................. 23-31

23-ii FOR TRAINING PURPOSES ONLY

 23 COMMUNICATIONS
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page

Servicing............................................................................................................ 23-31
Service Wicks............................................................................................. 23-31
Removal/Installation.......................................................................................... 23-31
Remove Static Discharge Wick.................................................................... 23-31
Install Static Discharge Wick...................................................................... 23-31

FOR TRAINING PURPOSES ONLY 23-iii

 23 COMMUNICATIONS
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

ILLUSTRATIONS
Figure Title Page

23-1 Radio Navigation and Communications Equipment and Antennas.............23-2

23-2 KTR 952 Amp/Antenna and KTR 953 Receiver/Exciter.............................23-6
23-3 Control Display Unit..................................................................................23-7
23-4 KHF 950 Interconnect Schematic..............................................................23-8
23-5 HF Antenna - Removal/Installation..........................................................23-10
23-6 SELCAL..................................................................................................23-14
23-7 GNS 430 Panel........................................................................................23-18
23-8 GNS 430 No.1 Interconnect Schematic (Sheet 1 of 2).............................23-21
23-9 GNS 430 No.1 Interconnect Schematic (Sheet 2 of 2).............................23-22
23-10 GNS 430 No.2 Interconnect Schematic (Sheet 1 of 2).............................23-23
23-11 GNS 430 No.2 Interconnect Schematic (Sheet 2 of 2).............................23-24
23-12 MMEL - Communication Systems...........................................................23-25
23-13 Audio Selector Panel...............................................................................23-26
23-14 Audio Panel KMA 24H (Sheet 1 of 3).....................................................23-27
23-15 Audio Panel KMA 24H (Sheet 2 of 3).....................................................23-28
23-16 Audio Panel KMA 24H (Sheet 3 of 3).....................................................23-29
23-17 Static Discharge Wick..............................................................................23-30
23-18 MMEL - Communications.......................................................................23-32

TABLES
Table Title Page

23-1 Frequency Assignments...........................................................................23-17

FOR TRAINING PURPOSES ONLY 23-v

 23 COMMUNICATIONS
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CHAPTER 23
COMMUNICATIONS

23-00-00 COMMUNICATIONS
INTRODUCTION
The DHC-6 Twin Otter avionics covered in this chapter are the following systems:
•• HF/SELCAL Communications
•• VHF Communications
•• Audio Integrating Systems
•• Static Wicks.
It is not inclusive of all the optional avionics items available for installation. The user
should consult the Maintenance Manual, applicable supplements in the AFM, and vendor
manuals for additional information and information on specific systems not included in
this chapter.

FOR TRAINING PURPOSES ONLY 23-1

23 COMMUNICATIONS

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 23-1. Radio Navigation and Communications Equipment and Antennas

23-2 FOR TRAINING PURPOSES ONLY

 23 COMMUNICATIONS
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

GENERAL are strictly line of sight. This refers to radio line of

sight and not optical one of sight, which has to do
The first communications equipment developed with vision. This means that radio signals can travel
for aircraft use operated in the high frequency beyond the optical horizon. Radio signals passing
(HF) spectrum, which covers from 2 to 30 MHz, through the atmosphere of the earth are bent by
and which has some significant drawbacks for diffraction more than are visible light waves. This
aircraft use. allows the radio signals to travel around the earth,
which is curved. There is very little attenuation of
The wavelength and the length of the antenna radio signals traveling through air, and the major
for these HF communications was relatively loss of radio power is the spreading of the radio
long. Some of these early transmitters used a signal over an increasing area.
trailing wire antenna that was reeled from the
rear of the aircraft in flight. Of course, this VHF communications usually take place on only
antenna was of no use on the ground, and if one frequency or simplex. This means that the
the pilot forgot to reel in the antenna before airborne equipment and the ground equipment
landing, it could prove to be a hazard. transmit and receive on exactly the same
frequency. There are exceptions to this, such as
The HF band is susceptible to atmospheric when an airborne station receives a reply through
propagation. The signal can carry many a navigation facility, which is called half duplex.
thousands of miles depending on the
propagation, which is somewhat unpredictable. The communications band is divided into
Although atmospheric propagation is necessary segments assigned to the differing types of
for long-range communications it is an services used in aircraft communications. The
annoyance for short-range communications. band from 118.00 MHz to 121.4 MHz is used
for air traffic control, which primarily involves
Early communications systems used amplitude control towers at airports. 121.5 MHz is a
modulation, which is susceptible to ignition noise universal emergency frequency for all aircraft.
and other static. In fact, one of the more important This frequency is monitored by most aviation
tasks of the early developers of the aircraft facilities and any calls made on this frequency
communications systems was the development will result in a quick answer and aid. Emergency
of effective ignition shielding systems. locator transmitters also operate on this channel.

As technology advanced, aircraft communications

systems moved to the VHF portion of the spectrum,
23-10-00 HIGH
which requires antennas only a few feet long and FREQUENCY RADIOS
for which there is no significant atmospheric
propagation. Aviation communications systems GENERAL
were standardized before FM was developed and,
to this day, aircraft communications systems must For reliable communications beyond the range
operate in spite of the problems of engine static of VHF communications, that is about 200
and the use of amplitude modulation. miles, aircraft rely on high frequency (HF)
single sideband radio. There is no place within
The present-day aircraft communications band the continental United States where VHF
extends from 118.0 MHz to 135.975 MHz, communications are not available and thus all
with 25 kHz channel spacing. There is a 8.33 air traffic control is conducted by VHF radio.
kHz channel spacing in the United States and In sparsely populated areas, such as Alaska, the
mandated in Europe, which opens up 3 times northern portions of Canada, and some other
more channels for use. foreign countries, and particularly in trans-
oceanic flights, the HF radio or SATCOM will
Because VHF signals are not affected by be the only available communications.
atmospheric propagation, aircraft communications

FOR TRAINING PURPOSES ONLY 23-3

23 COMMUNICATIONS

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Early airborne HF radio equipment was very HF Transceiver

large and complicated. It consisted of mostly
Although there are minor differences, the
vacuum tubes, and required tremendous amounts
typical synthesized HF RT is a dual-conversion
of power. In today’s world of technology,
up-converting transceiver with a frequency
vacuum tubes have been replaced with solid state
range from 2MHz to 29.9999MHz in 100HZ
technology that is smaller and more efficient.
steps, for a total of 279,999 channels.

DESCRIPTION The radio frequency (RF) is passed through

broadly tuned front-end filters that are switched
The HF portion of the frequency spectrum is with PIN diodes under control of the MHz
officially defined as a frequency range of 2.0 to and 10s-MHz switches of the HF RT control
30.0 MHz. This frequency range is characterized head. This reduces the amount of energy within
by atmospheric propagation of the radio signals, the RF amplifier and reduces the amount of
where signals are refracted by the upper layers intermodulation generated in the RF amplifier
of the atmosphere. In this fashion, signals are and mixer. In addition, the bandpass filter will
not transmitted out into space, but are returned reduce spurious responses from the transmitter.
to earth a great distance from the point of origin. The received signal is fed to a high-pass filter
that has a cutoff frequency slightly below
Signals in the VHF portion of the spectrum 2.0MHz. The purpose of this filter is to remove
are usually capable of penetrating the energy from the standard broadcast band from
ionosphere, so that any signals aimed towards 550 kHz to 1600 kHz. The HF transceiver
the ionosphere simply break through the layers does not cover this range, and there are many
of ionized gases and escape into space. Lower broadcast stations operating at rather high
frequencies are not capable of penetration and power, which can cause serious interference.
are reflected or refracted toward the earth.

The frequency at which radio signals just begin

Coupler
to penetrate the ionosphere and escape into space Because the antenna used in an aircraft
is called the maximum usable frequency (MUF). installation will not be resonant at all operating
The MUF changes from hour to hour and day to frequencies, an antenna-tuning unit is required
day, as well as over a period of eleven years (due to match the impedance and tune the antenna to
to the cyclic variation of sunspots). The number resonance. The nominal output impedance of the
of sunspots has a strong effect on the ionosphere RF power amplifier is 50 Ω. The antenna tuner
and the MUF can rise above 50 MHz, making adjusts the impedance of the antenna so that it
normally line-of-sight VHF signals travel very presents a 50 Ω impedance load to the amplifier.
long distances. During periods of low sunspot To a radio frequency source, every length of
activity, the MUF can drop to as low as 5 MHz wire used as an antenna presents impedance.
at certain times of the day. The HF transceiver The angle associated with impedance is the
must have a wide frequency range to adjust for phase difference between the voltage and current
the variations in the MUF. flowing in the antenna. The antenna- tuner unit
adjusts the magnitude of the antenna impedance
The HF transceiver covers the frequency range to 50 Ω and the phase to zero degrees, which is
from 2MHz to 30MHz in 100Hz steps. The equivalent to a 50 Ω load.
transceiver is capable of operation on upper
sideband (USB), lower sideband (LSB), and in Because most antennas used in an aircraft
some cases amplitude modulation (AM). installation have a low impedance due to their
relatively short lengths, an impedance matching
A typical HF system consists of a transceiver (RT), is used to lower the 50 Ω output impedance
coupler, conformal antenna, and a control head. from the network comprised of inductors and
capacitors used to give an impedance of 50 Ω.

23-4 FOR TRAINING PURPOSES ONLY

 23 COMMUNICATIONS
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Antenna NOTES
Unlike the VHF antenna that can be installed in
practically any convenient location the HF antenna
can offer unique installation problems, the longer
the antenna, the better. Eight feet is considered to
be the absolute minimum desired. The antenna
should be as close to the longitudinal axis of the
aircraft as possible. One end of the antenna must
enter the aircraft within six to eight inches of the
antenna tuning unit. This can be extended by the
use of a shielded lead in wire. The far end of the
antenna can be either insulated or grounded to the
aircraft. The radiation resistance of short antennas
is very low and all connections must be very
secure and offer a low impedance to RF currents.
This is also true of any grounding straps and other
connections made to the antenna tuning unit.

A spring tensioning unit should be provided

to allow for fuselage flexure and to prevent
unnecessary breakage of the antenna wire.
The rear mount must have a lower breaking
point than the front mounting so that, should
breakage occur, the rear, rather than the front,
will break loose. Having the rear attached while
the front is free is a very dangerous situation,
as the free wire can become entangled in the
vertical stabilizer or the rudder. A jumper is
used around the spring unit rather than relying
on the spring to supply electrical continuity.

Control
The control contains the necessary switches
and knobs to tune the radio, switch the modes
and adjust the volume.

CAUTION
Because HF may pose a shock
hazard during transmission.
Situational awareness should be
exercised when working around
the aircraft during HF testing.

FOR TRAINING PURPOSES ONLY 23-5

23 COMMUNICATIONS

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*KCU 951 Control Display Unit Aircraft

HF Antenna

KA 161 Antenna
External Capacitor Unit

KTR 953
Receiver/Exciter

KAC 952
Power Amplifier/
Antenna Coupler

Figure 23-2. KTR 952 Amp/Antenna and KTR 953 Receiver/Exciter

23-6 FOR TRAINING PURPOSES ONLY

 23 COMMUNICATIONS
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

23-10-00 KHF 950 MHz range. It provides semi-duplex capability

through the 99 programmable channels to interface
with maritime radiotelephone networks. A Dzus
SYSTEM DESCRIPTION rail-mounted control display unit, the KCU 951,
uses electronic gas discharge readouts to display
Refer to Figure 23-2. KTR 952 Amp/Antenna frequency, channel and mode of operation. All
and KTR 953 Receiver/Exciter. necessary controls for operation of the KHF 950
system, including programming of all preset
The KHF 950 is a solid-state HF single sideband channels, are on the KCU 951. The 99 channels can
transceiver system. The KHF 950 system can be be easily programmed by the pilot on the ground or
controlled by either a KCU 1051 Dzus rail-mounted in the air, and the nonvolatile memory stores this
control display unit, a KCU 951 Dzus rail-mounted information even when the system is turned off.
control display unit. All the control units work
in conjunction with a KAC 952 power amplifier/
antenna coupler and a KTR 953 receiver/exciter. CAUTION
When performing a radio check
23-10-00 KCU 951 on the ground, make certain that
CONTROL DISPLAY UNIT all personnel are clear of the
HF antenna before transmitting.
Serious RF burns can result from
The KCU 951 (Figure 23-3) provides the pilot
direct contact with the antenna
access to 99 programmable channels plus a full
or antenna terminal when the
280,000 operating frequencies in the 2.0 to 29.9999
system is transmitting.

Gas discharge Smaller gas discharge Photocell dims display EMISSION MODE switch
readouts display all characters display automatically. selects lower sideband
frequencies and preset emission mode, transmit (LSB where approved),
channel numbers. indicator and program upper sideband (USB) or
mode indication. AM modes.

FREQ/CHAN
BENDIX/KING (frequency/channel)
HF switch selects either
direct tuning or
123456.6 99 preset channel
LSB AM USB TX PGM operation.
MODE FREQ CHAN
FREQ KHZ CHANNEL
PULL
PGM (program)
switch permits pilot
OFF to change
CLARIFIER SQUELCH VOLUME frequency and
STO PGM emission mode of
preset channel.
CLARIFIER knob adjusts
receive frequency to
improve speech quality in a SQUELCH knob helps OFF/VOLUME knob STO (store) switch Concentric
single sideband operating cut out background turns system on and stores displayed Frequency/Channel
mode. Use of this control is noise when not adjusts audio volume. frequency and emission knobs set
only required when receiving a signal. mode in memory. frequency or select
station-to-station frequency preset channel.
difference is significant.

Figure 23-3. Control Display Unit

FOR TRAINING PURPOSES ONLY 23-7

23 COMMUNICATIONS

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 23-4. KHF 950 Interconnect Schematic

23-8 FOR TRAINING PURPOSES ONLY FOR TRAINING PURPOSES ONLY

 23 COMMUNICATIONS
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

ANTENNA TUNING 9. Verify that during transmit, TX is indicated

on the display.
When using a KCU 951 the antenna coupler
retunes the antenna under the following FAULT INDICATION
conditions:
Simply key the mic and the automatic antenna
1. When the system is first turned on (and coupler begins a new tuning cycle to clear the
warmed up) and the mic key is pressed. fault.
2. When a new channel or frequency is
selected and the mic key is pressed to
transmit, the system initiates a tuning
sequence for ideal impedance match. The
antenna coupler is always utilized when
transmitting (regardless of mode) and while
receiving in the simplex mode (after mic
key is pressed for tuning). The antenna
coupler is bypassed during receive when
operating in semi-duplex or receive-only
channelized operation.
The system is then functioning as a receiver
connected directly to the HF antenna.

OPERATION
The operation of the HF system is as follows:

1. Turn the unit on.

2. Allow a one to three minute warm-up
period. During this period, the display will
be dashed out. Select the Frequency mode,
move the cursor through each digit, and
verify that frequency change is provided.
3. Verify that emission mode selection is
possible. The available emission modes
are LSB, USB, and AM.
4. Tune the HF Control unit to a known
ground station.
5. Select HF on the MIC SELECT rotary
knob.
6. Select HF (phone and speakers) on the
audio control panel.
7. Key the mic and allow tuning of receiver.
8. Carry out a transmission and verify that
message is sent and reply is received from
ground station.

FOR TRAINING PURPOSES ONLY 23-9

23 COMMUNICATIONS

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 23-5. HF Antenna - Removal/Installation

23-10 FOR TRAINING PURPOSES ONLY

 23 COMMUNICATIONS
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of in strain insulator, tension unit, and mast
the maintenance practices and is intended for assembly. Tap wire retriever tool lightly
training purposes only. with a non-metallic hammer to release
chuck jaws, and pull out wire.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2. 6. Unscrew three end caps of “Tee” connector
and remove. Split apart the sleeve assembly
23-10-00 MAINTENANCE and remove wire.

PRACTICES 7. To remove antenna feed-through, remove

aircraft upholstery where necessary,
unscrew locknut at base of feed-through.
REMOVAL/INSTALLATION 8. Unscrew two feed-through assembly mounting
screws and remove complete with washers.
KHF 950 System
9. Remove base assembly of feed-through
Refer to Figure 23-5. HF Antenna - Removal/
assembly.
Installation.
10. R e m o v e u p p e r p a r t o f f e e d - t h r o u g h
Removal and installation of the system assembly from outer skin of aircraft.
components is straightforward and consists of
disconnecting electrical connectors and releasing HF Antenna - Installation
hold-down fasteners and attachment screws. The
Install the HF antenna and proceed as follows:
radio equipment shelves are grounded to the rack
structure, and ground jumpers are connected
1. Cut antenna to approximate length, allow a few
between the shelves and equipment mountings.
extra inches for later cutting to correct length.
It is essential that good electrical contact is
maintained between ground points. 2. Insert the end of the wire into wire retriever
tool (MS25119–1) and push firmly until the
wire bottoms.
HF Antenna - Removal
3. Place a sharp clean knife in the slot on the
Remove the HF antenna and proceed as follows:
retriever tool and rotate the wire until the
insulation is scored all around by the blade.
1. Cut through antenna attachment cable about 4
inches from tension unit on vertical stabilizer. 4. Pull the wire from the retriever tool and
remove 3/8–inch insulation. Be careful not
2. Remove cotter pin, flat head pin and tension
to nick or scratch the wire.
unit from eyebolt on vertical stabilizer.
5. Clean the end of the wire thoroughly.
3. Cut through antenna attachment cable and
antenna cable about 4 inches from each 6. Unscrew and remove the mast sleeve from
end of strain insulator; cut through antenna the deadend mast assembly and thread the
cable about 4 inches from mast assembly wire through the sleeve.
and connector tee.
7. Unscrew and remove the mast end plug and
4. Unscrew and remove end caps from strain the large gasket.
insulator and connector tee; unscrew and
8. R e m o v e t h e s m a l l g a s k e t , a n d m a s t
remove support sleeves from tension unit,
adjustable assembly, and the mast seal.
feed through insulator and aft support
sleeve from mast assembly. 9. Insert screwdriver into the slot in the mast
adjustable assembly and turn the screw
5. Remove insulation from ends of wire, slide
clockwise as far as it will go.
wire retriever tool (MS25119–1) over bare
wire until it touches jaws of chuck assembly 10. Clean the jaws of the chuck.

FOR TRAINING PURPOSES ONLY 23-11

23 COMMUNICATIONS

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

11. Insert the mast seal into the mast assembly. 26. Connect wire end to a second strain
Do not wipe the silicone compound out of insulator using procedure laid down in step
the mast opening. 19 and step 21.
12. Insert the mast adjustable assembly into the 27. Bare 0.5 inches of the antenna wire 2.5
mast so that the keyway engages properly inches from the strain insulator installed in
and the adjustable mast assembly fits step 26. Ensure wire is clean.
snugly against the mast seal.
28. Prepare one end of a length of antenna wire
13. Fit the small gasket, the large gasket, and (100.00 ± 0.5 inches) as detailed in step 2
the mast end plug. Do not screw the mast through step 5.
end as far as it will go.
29. Feed one end through the third end cap
14. Thrust the wire firmly through the opening of the “Tee” connector and wrap around
in the end of the mast assembly, as far as it bared wire core of antenna using three to
will go, to permit chuck to engage the bared four turns.
end of wire.
30. Solder the connection.
15. Give the wire a sharp tug to ensure a firm
31. Apply sealant to joint and adjacent antenna
connection. Wire insulation should pass
wire.
through the mast seal into the counterbore
of the chuck housing. 32. Place the two halves of the “Tee” connector
over the junction and screw up the three
16. Tighten the mast sleeve.
end caps finger tight, allowing a bead of
17. Cut wire to correct length (6.5 ± 0.5 inches). sealant to form all around between “Tee”
connector and end caps.
18. Prepare the other end of the wire as
described in step 2 through step 5. 33. Prepare both ends of a length of antenna
wire (132.00 ± 0.5 inches) as described in
19. Feed wire through strain insulator end
step 2 through step 5.
cap and insert into jaws of strain insulator
chuck assembly. 34. Feed one end of wire through the end cap of
the strain insulator installed in step 26 and
20. Give wire a sharp tug to ensure a firm
insert into jaws of chuck assembly. Give
connection.
wire a sharp tug to ensure a firm connection.
21. Apply sealant to threads of strain insulator
35. Apply sealant to threads of strain insulator
and end cap, replace end cap finger tight,
and end cap, replace end cap finger tight
allowing bead of sealant to form all around
allowing bead of sealant to form all around
between strain insulator and end cap.
between strain insulator and end cap.
22. Prepare both ends of a length of antenna
36. Feed the other end of wire prepared in step
wire (approximately 312.0 ± 0.5 inches) as
33 through the end cap of the tension unit
described in step 2 through step 5.
and insert into the jaws of the tension unit
23. Feed one end through strain insulator end chuck assembly. Give wire a sharp tug to
cap and insert in jaws of chuck assembly. ensure a firm connection.
Give wire a sharp tug to ensure a firm
37. Apply sealant and replace end cap as in
connection.
step 35.
24. Apply sealant, replace end cap as described
38. Attach connector to tension unit using the
in step 21.
flat head pin, cotter pin and washer removed
25. Insert other end of wire through the two end in step 2 in HF Antenna - Removal.
caps of the “Tee” connector. Ensure that
39. Attach other end of connector to eyebolt
one end cap faces left and that the other
on the vertical stabilizer using the flat head
faces right.

23-12 FOR TRAINING PURPOSES ONLY

 23 COMMUNICATIONS
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

pin, cotter pin and washer removed in step NOTES

2 in HF Antenna - Removal.
40. Feed the other end of the antenna lead-in
wire attached in step 28 through the antenna
lead-in insulator sleeve.
41. Strip back 4.0 inches of insulation from
lead-in wire and feed wire core into lead-in
insulator connector bolt until wire emerges
from slot in end of bolt and wind wire
around bolt for three or four turns.
42. Secure feed-through assembly in position
using mounting hardware detailed in step 7
through step 10 in HF Antenna - Removal.
43. Insert a screwdriver into the hole in the
dead end mast adjustable assembly to
engage in the slot in the tension screw
and turn it counterclockwise until correct
tension is obtained (between 30 and 40
pounds).
44. Replace the mast end plug.
45. Wipe off surplus silicone compound from
the mast end plugs.

NOTE
The wire lengths and feed-
through insulator position
will vary according to type of
equipment installed, but assembly
details will be the same.

FOR TRAINING PURPOSES ONLY 23-13

23 COMMUNICATIONS

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

SONALERT ALARM CHANNEL 1 INDICATOR LIGHT

- Causes a steady tone to alert the - FLASHING - cell has been
flight crew an incoming call received from the base station
- Steady tone may be adjusted by sending a coded signal
maintenance personnel to sound
a long or short “ping” tone. TEST PUSHBUTTON (momentary action)
PUSH - light comes on showing system test
- Audio tone sounds
- All channel indicators flash
- RESET light comes on
- System test is stopped when pushbutton is
released.

SELCAL
1 TEST
2

3 RESET
4 5

CHANNEL 2-5 INDICATOR LIGHTS RESET PUSHBUTTON (momentary action)

- Currently not operational. - call has been received from the base
station sending the coded signal (channel 1
indicator light also comes on)

PUSH - aural and visual alerts stop

- Use audio control panel to answer call by
pushing HF pushbutton on, selecting HF on
MIC switch and pushing related PTT, XMIT
or hand mic switch to transmit.

Figure 23-6. SELCAL

23-14 FOR TRAINING PURPOSES ONLY

 23 COMMUNICATIONS
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

23-20-00 SELECTIVE Decoder

CALLING The front panel of the decoder unit has four
sixteen-position thumbwheel switches, with
each indent identified with an alphabetical
INTRODUCTION letter. The four switches are set to the code
given to the aircraft.
The Selcal interfaces with an HF or VHF system
to provide an audible and visual indication
to the flight crew of an incoming radio call,
Self-Test
eliminating the need for continuous aural When the TEST button on the control panel
monitoring of the communicating frequency. is pushed, an internal check of the SELCAL
circuit begins and the following should occur:
GENERAL •• The ten-second audio tone is heard in
the audio integrating system
The selective calling (SELCAL) system operates
with the HF communications system to give •• The Sonalert alarm makes a sound
audible and visual indications to the flight
•• All five channel indicator lights flash
crew of an incoming radio call, this removes
on and off
the need to continuously monitor the aural
communication frequency. These indications •• The RESET and TEST pushbuttons
are supplied through a Sonalert alarm and a light flash on and off.
that flashes installed on the SELCAL control
panel. The system operates only to signals
from a SELCAL equipped ground station that
transmits coded audio tone signals which are
given to the aircraft. Normal operation of the HF
system is not affected by the SELCAL system.

The system is connected to the HF receiver, but

can monitor as many as five receivers.

SYSTEM DESCRIPTION
Refer to Figure 23-6. SELCAL.

Control Panel
The control panel contains the functions that
follow:

•• Sonalert alarm
•• Five channel indicator lights
•• Self-test pushbutton
•• Reset pushbutton.

The unit has an integrally lit front panel and

all electrical connections are supplied with a
multi-pin connector on the back of the unit.

FOR TRAINING PURPOSES ONLY 23-15

23 COMMUNICATIONS

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

23-21-00 VHF •• T R A N S M I T T E R S E L E C T I O N -
Connect the microphone to the selected
COMMUNICATION receiver by means of the audio amp.
•• VOLUME ADJUSTMENT - If no
GENERAL signal is received, squelch will mute the
receiver, and if you disable the squelch
Communications equipment consists an audible background noise can be
mostly of voice radio transmitting and heard. Adjust VOL control to obtain a
receiving equipment. There are some data satisfactory noise level then quiet the
communications equipment being installed into receiver by activating the squelch again.
aircraft. However the vast majority of aircraft
communications takes place via voice.
Current Regulations
Being able to communicate from aircraft to air Currently the spacing of frequencies in the
traffic controller is extremely important for the United States is set at 25kHz. In Europe the
safety of the crew and the passengers. frequency spacing is set at 8.33kHz. In order
to make intercontinental flights, the radio
frequency spacing must be able to switch
DESCRIPTION from 8.33 to 25kHz and 25 to 8.33kHz. This
is accomplished manually by selecting either
VHF Comm radios transmit on a frequency
narrow band or wide band on the VHF control
range of 118.00 to 151.975 MHz in 25kHz
head. The different types of radios accomplish
increments. It is considered to be a line of
this task in a different manner, but the end
sight radio and is typically able to transmit and
result is the same.
receive at a range up to 100 miles. Depending
on atmospheric conditions, this range may
Table 23-1 give specific frequency assignments.
extend upwards of 500 miles.

The typical VHF comm radio system consists 118.00 - 121.40 Air Traffic Control
of a receiver/transmitter (RT), a control head,
121.5 Emergency
and lower and/or upper blade antenna.
121.6-121.9 Airport Ground Control

OPERATION 121.95 Flight Schools

121.975 Private Aircraft Advisory

Assuming that all VHF circuit breakers are in,
radio master switch is on, and DC power is 122.0 - 122.675 Flight Service Station
available, the VHF Comm system will function
122.7 Unicom
as follows:
122.725 Unicom for Private Airports
•• EN ERGIZATION - Typi c a lly the
122.75 Air to Air
volume knob turns the system on or off.
122.8 Unicom Uncontrolled Airports
•• FREQUENCY SELECTION - Adjust
the MHz dial to the desired frequency 122.85 Multicom
by rotating inner and outer selectors.
122.9 Multicom
Frequency channeling is instantaneous.
122.925 Multicom
•• RECEIVER SELECTION - Connect
the headsets or speakers to the selected 122.95 Unicom Controlled Airports
receiver by means of the audio amp.
122.975 Unicom

23-16 FOR TRAINING PURPOSES ONLY

 23 COMMUNICATIONS
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

123.0 Unicom Uncontrolled Airports NOTES

123.05 Unicom Heliports

123.075 Unicom Heliports

123.1 Search and Rescue

123.15 - 123.575 Flight Test

123.3 Flight School

123.5 Flight School

123.6 - 123.65 FSS or Air Traffic Control

123.675 - 128.8 Air Traffic Control

128.825 - 132.0 En Route

132.05 - 135.95 Air Traffic Control

Table 23-1. Frequency Assignments

FOR TRAINING PURPOSES ONLY 23-17

23 COMMUNICATIONS

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

COM Power/Volume COM Flip-Flop CLR (clear) RNG (map range) ENT (enter)

VLOC Volume VLOC Flip-Flop Direct-to MENU

Small Left Knob CDI MSG (message) PROC (procedures) Small Right
Knob

Large Left Knob OBS FPL (flight plan) Large Right Knob

Figure 23-7. GNS 430 Panel

23-18 FOR TRAINING PURPOSES ONLY

 23 COMMUNICATIONS
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

GNS 430 Keys and Buttons The Direct-to Key provides access to the
direct-to function, which allows the pilot to
Refer to:
enter a destination waypoint and establishes a
direct course to the selected destination.
•• Figure 23-7. GNS 430 Panel.
•• Figure 23-8. GNS 430 No.1 Interconnect The RNG Key allows the pilot to select the
Schematic (Sheet 1 of 2). desired map range. Use the up arrow of the key
to zoom out to a larger area, or the down arrow
•• Figure 23-9. GNS 430 No.1 Interconnect
to zoom in to a smaller area.
Schematic (Sheet 2 of 2).
•• Figure 23-10. GNS 430 No.2 Interconnect The MENU Key displays a context-sensitive list of
Schematic (Sheet 1 of 2). options. This options list allows the pilot to access
additional features or make settings changes, which
•• Figure 23-11. GNS 430 No.2 Interconnect
relate to the currently displayed page.
Schematic (Sheet 2 of 2).
The ENT Key is used to approve an operation
Left Hand Keys and Knobs or complete data entry. It is also used to
The COM Power/Volume Knob controls unit confirm information, such as during power on.
power and communications radio volume. Press
momentarily to disable automatic squelch control. The large right knob is used to select between
the various page groups: NAV, WPT, AUX, or
The VLOC Volume Knob controls audio volume NRST. With the on-screen cursor enabled, the
for the selected VOR/Localizer frequency. Press large right knob allows the pilot to move the
momentarily to enable/disable the ident tone. cursor about the page. The large right knob is
also used to move the target pointer right (turn
The COM Flip-flop Key is used to swap the clockwise) or left (counterclockwise) when the
active and standby COM frequencies. Press and map panning function is active.
hold to select emergency channel (121.500 MHz).
The small right knob is used to select between
The VLOC Flip-flop Key is used to swap the the various pages within one of the groups
active and standby VLOC frequencies (i.e., listed above. Press this knob momentarily to
make the selected standby frequency active). display the on-screen cursor. The cursor allows
the pilot to enter data and/or make a selection
The small left knob is used to tune the kilohertz from a list of options. The small right knob is
(kHz) value of the standby frequency for the COM also used to move the target pointer up (turn
transceiver or the VLOC receiver, whichever is clockwise) or down (counterclockwise) when
currently selected by the tuning cursor. Press this the map panning function is active.
knob momentarily to toggle the tuning cursor
between the COM and VLOC frequency fields.
Bottom Row Keys
The large left knob is used to tune the megahertz The CDI Key is used to toggle which navigation
(MHz) value of the standby frequency for the source (GPS or VLOC) provides output to an
COM transceiver or the VLOC receiver, whichever external HSI or CDI.
is currently selected by the tuning cursor.
The OBS Key is used to select manual or
automatic sequencing of waypoints. Pressing the
Right Hand Keys and Knobs OBS Key selects OBS mode, which retains the
The CLR Key is used to erase information, current “active to” waypoint as the navigation
remove map detail, or to cancel an entry. Press reference even after passing the waypoint (i.e.,
and hold the CLR key to immediately display prevents sequencing to the next waypoint).
the Default NAV Page. Pressing the OBS Key again returns the unit to

FOR TRAINING PURPOSES ONLY 23-19

23 COMMUNICATIONS

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

normal operation, with automatic sequencing of NOTES

waypoints. When OBS mode is selected, the pilot
may set the desired course to/from a waypoint
using the “Select OBS course” pop-up window,
or an external OBS selector on the HSI or CDI.

The MSG Key is used to view system messages

and to alert the pilot to important warnings and
requirements.

The FPL Key allows the pilot to create, edit,

activate, and invert flight plans, as well as
access approaches, departures, and arrivals.
A closest point to flight plan feature is also
available from the FPL Key.

The PROC Key allows the pilot to select and

remove approaches, departures, and arrivals
from the flight plan. When using a flight plan,
available procedures for the departure and/
or arrival airport are offered automatically.
Otherwise, the pilot may select the desired
airport, then the desired procedure.

To Power on the GNS 430

Turn the COM Power/Volume Knob clockwise
to turn the unit power on and set the desired
radio volume.

A welcome page appears briefly while the unit

performs a self-test, followed sequentially by
the Unit Type Page and the Software Versions
Page. Then (depending on configuration) the
Weather Page, the Traffic Page, the Aviation
Data Page, the Land/Terrain/Obstacles
Database Page, and the Situational Awareness
Page are sequentially displayed.

Once the self-test concludes, the Database

Confirmation Page is displayed, showing the
effective and expiration dates of the Jeppesen
database on the NavData® card. Press the ENT
Key to acknowledge the Database Page and
proceed to the Instrument Panel Self-test Page.

23-20 FOR TRAINING PURPOSES ONLY

 23 COMMUNICATIONS
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 23-8. GNS 430 No.1 Interconnect Schematic (Sheet 1 of 2)

FOR TRAINING PURPOSES ONLY FOR TRAINING PURPOSES ONLY 23-21

23 COMMUNICATIONS

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 23-9. GNS 430 No.1 Interconnect Schematic (Sheet 2 of 2)

23-22 FOR TRAINING PURPOSES ONLY FOR TRAINING PURPOSES ONLY

 23 COMMUNICATIONS
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 23-10. GNS 430 No.2 Interconnect Schematic (Sheet 1 of 2)

FOR TRAINING PURPOSES ONLY FOR TRAINING PURPOSES ONLY 23-23

23 COMMUNICATIONS

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 23-11. GNS 430 No.2 Interconnect Schematic (Sheet 2 of 2)

23-24 FOR TRAINING PURPOSES ONLY FOR TRAINING PURPOSES ONLY

 23 COMMUNICATIONS
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Aircraft: Revision No. 07 Page:

DE HAVILLAND DHC-6, SERIES 100, 200 & 300 Date: Sep. 18, 1998 23-1 of 2
System & 1. 2. Number Installed
Sequence 3. Number Required for Dispatch
Numbers 4. Remarks or Exceptions

23 COMMUNICATIONS

1 Communications Systems D - - Any in excess of those required by

(VHF, HF, UHF) regulation and not powered by a standby
or emergency bus may be inoperative.

Figure 23-12. MMEL - Communication Systems

23-50-00 AUDIO
INTEGRATING SYSTEM
INTRODUCTION
The audio integration system provides an
interface between pilot and co-pilot. It
also provides an interface with navigation
receivers, radio communication transmission
and reception by pilot and co-pilot, passenger
address and external (ramp hailer).

GENERAL
The audio integrating system gives the
functions that follow:

1. Control of the aircraft radio communications

transmission and reception by the pilot and
the co-pilot.
2. Monitors the aircraft navigation radio
receivers by the pilot and the co-pilot.
3. Interphone communications between the
pilot and the co-pilot.
4. Interphone communications between the
flight crew.
5. Passenger address.
6. External ramp hailer.

FOR TRAINING PURPOSES ONLY 23-25

23 COMMUNICATIONS

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

SYSTEM DESCRIPTION Separate auto pushbuttons are provided for

speaker and headphones.
Refer to:
Speaker outputs are provided for the cockpit
•• Figure 23-13. Audio Selector Panel. P.A. (passenger address) and external (ramp
hail) speakers.
•• Figure 23-14. Audio Panel KMA 24H
(Sheet 1 of 3).
The INT VOL (Intercom Volume) control on
•• Figure 23-15. Audio Panel KMA 24H the front panel controls the volume of the crew
(Sheet 2 of 3). members intercommunications only. It does not
affect the volume of the 500-ohm audio inputs
•• Figure 23-16. Audio Panel KMA 24H
that are selected for the headphones.
(Sheet 3 of 3).
The KMA 24H is a solid state, TSO’d
audio selector Panel with separate isolation
amplifiers for speakers and head phones. The
unit provides interphone communication for
up to five intercom systems. It is designed to
for use in both single and dual audio panel
installations. Front panel controls consist of
input selector switches for both speaker and
headphone amplifiers, a microphone selector
switch, and an interphone volume control.

The KMA 24H has switched inputs for three

transceivers (either HF or TEL, COM 1 and
COM 2) and up to six receivers (NAV 1 & 2,
DME, MARKER BEACON, & ADF 1 & 2) The
AUTO function may be substituted for ADF 2.

The auto function on some versions

automatically selects the audio of the
transceiver in use according to the position of
the MIC SELECT switch.

BEDIX/KING SPEAKER COM 1 COM 2

INT VOL HF PA
HF 1 COM 2 1 NAV 2 DME MKR ADF AUTO OFF EXT

KMA 24W TSO

PHONE MIC SELECT

Figure 23-13. Audio Selector Panel

23-26 FOR TRAINING PURPOSES ONLY

 23 COMMUNICATIONS
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 23-14. Audio Panel KMA 24H (Sheet 1 of 3)

FOR TRAINING PURPOSES ONLY FOR TRAINING PURPOSES ONLY 23-27

23 COMMUNICATIONS

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 23-15. Audio Panel KMA 24H (Sheet 2 of 3)

23-28 FOR TRAINING PURPOSES ONLY FOR TRAINING PURPOSES ONLY

 23 COMMUNICATIONS
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 23-16. Audio Panel KMA 24H (Sheet 3 of 3)

FOR TRAINING PURPOSES ONLY FOR TRAINING PURPOSES ONLY 23-29

23 COMMUNICATIONS

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

23-60-00 STATIC SYSTEM DESCRIPTION

DISCHARGING SYSTEM Each static discharger has a semi-flexible
resistive element covered with shrink tubing,
a rope floss of fine fibers about 13 inches
INTRODUCTION long. Protruding from the trailing end will be
about 1 inch of rope floss and the other end
The static discharge system is installed on
will have an electrical lug for attachment to
board aircraft to dissipate static build up on
the airframe.
the airframe. P (precipitation) static discharges
through the static wick.
The Static wicks are installed as follows: one
on each aileron, one on each flap, two on
GENERAL the right hand elevator, one on the left hand
elevator and three on the rudder
The static discharging system has dischargers
which dissipate electrostatic charges from the The MMEL states that one may be missing
aircraft structure to the adjacent air, to keep from the rudder and one from the right elevator.
to a minimum radio interference caused by
these charges. Each static discharger consists of a resistive
coated fiberglass rod with a pellet of composite
carbon-based material at the tip. The pellet is
fitted into an aluminium alloy sleeve, which
in turn is fitted onto the discharger rod by
means of conductive epoxy cement. A resistant
sheathing is heatshrunk over the entire length
of the discharger rod and tip assembly, leaving
approximately 1/8 inches (0.32 cm) of carbon
tip material exposed.

Figure 23-17. Static Discharge Wick

23-30 FOR TRAINING PURPOSES ONLY

 23 COMMUNICATIONS
TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

OPERATION and remove the wick.

2. If necessary, prepare the end of the new
As the aircraft moves through the air, the
static discharge wick.
aerodynamic end cap at the trailing end of each
discharger makes an area of low pressure around
the small protruding wire tips. This decrease
Install Static Discharge Wick
in pressure increases discharge efficiency and 1. Secure new wick to airframe with screws
permits ionization to occur in the slipstream, and lock washers, making sure that metal
because of the impressed potential between the to metal contact is maintained.
electrified aircraft and the adjacent air dielectric.
2. E n s u r e p a r a l l e l a l i g n m e n t w i t h
line-of-flight.
Static Discharge Maintenance
Practices
The maintenance procedure for removal/
installation, adjustment, and testing is as per
AMM 23-60-01.

*The following is an abbreviated description of

the maintenance practices and is intended for
training purposes only.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2.

23-60-00 MAINTENANCE
PRACTICES
SERVICING
Service Wicks
1. Cut off ragged edges from wick. Clean if
necessary with lint-free cloth.
2. Measure 1 inch from exposed end of rope
floss, and mark plastic sheath.
3. Carefully cut back and remove plastic
sheathing position marked in step (2), and
remove the cotton covering from around
the rope floss.
4. Ensure that the overall length of serviced
wick exceeds 6 inches.

REMOVAL/INSTALLATION
Remove Static Discharge Wick
1. Remove two screws and lock washers
securing static discharge wick to airframe,

FOR TRAINING PURPOSES ONLY 23-31

23 COMMUNICATIONS

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Aircraft: Revision No. 07 Page:

DE HAVILLAND DHC-6, SERIES 100, 200 & 300 Date: Sep. 18, 1998 23-1 of 2
System & 1. 2. Number Installed
Sequence 3. Number Required for Dispatch
Numbers 4. Remarks or Exceptions

23 COMMUNICATIONS

1 Communications Systems D - - Any in excess of those required by

(VHF, HF, UHF) regulation and not powered by a standby
or emergency bus may be inoperative.

2 Passenger Address System B 1 0 (O) May be inoperative provided:

a) alternate procedures are established
b) required safety briefings are given to
passengers using a means that will
and used, and ensure the briefings
are audible to each passenger.
All cargo operation. D 1 0

3 Static Discharge Wicks C - - One may be missing from the rudder and
one from the right elevator.

4 Cockpit Speakers D - 0 May be inoperative provided headsets are

*** installed and used by each person on
flight deck duty.

5 Headsets and Microphones D - - Only those in excess of those required for

each flight deck crew member may be
inoperative.

6 Pre-recorded Passenger D - 0 (O) Provided alternate procedures are

Announcement System established and used.
***
All cargo operation. D 1 0

7 Audio Selector Panels D 2 - Co-pilot's panel may be inoperative for

single pilot operation provided the pilot's
use of all avionics equipment is not
affected.

Figure 23-18. MMEL - Communications

23-32 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CHAPTER 34
NAVIGATION
CONTENTS
Page

34-00-00 NAVIGATION............................................................................................. 34-1

34 NAVIGATION
Introduction......................................................................................................... 34-1
General................................................................................................................ 34-3
Removal/Installation..................................................................................... 34-3
34-10-00 FLIGHT ENVIRONMENTAL DATA........................................................... 34-3
General................................................................................................................ 34-3
Fuel/Air Data Computer....................................................................................... 34-3
34-11-01 PITOT-STATIC SYSTEM............................................................................. 34-5
Basic System........................................................................................................ 34-5
Dual System......................................................................................................... 34-7
Pitot Tubes.................................................................................................. 34-11
Static Ports.................................................................................................. 34-11
Static Valve................................................................................................. 34-11
34-11-01 MAINTENANCE PRACTICES.................................................................. 34-11
System Maintenance.......................................................................................... 34-11
System Servicing............................................................................................... 34-11
Removal/Installation.......................................................................................... 34-11
Adjustment/Test................................................................................................. 34-12
Leak Test Pitot Pressure Line...................................................................... 34-12
Leak Test Static Pressure Lines................................................................... 34-12
Function Test Pitot Heater........................................................................... 34-12

FOR TRAINING PURPOSES ONLY 34-i

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page
Cleaning/Painting............................................................................................... 34-12
34-00-00 FLIGHT INSTRUMENTS.......................................................................... 34-13
General.............................................................................................................. 34-13
34-13-00 AIRSPEED INDICATOR........................................................................... 34-15
34 NAVIGATION

General.............................................................................................................. 34-15
34-13-00 MAINTENANCE PRACTICES.................................................................. 34-15
Adjustment/Test................................................................................................. 34-15
Test Airspeed Indicator................................................................................ 34-15
34-14-00 ALTIMETER.............................................................................................. 34-16
General.............................................................................................................. 34-16
34-14-00 MAINTENANCE PRACTICES.................................................................. 34-17
Adjustment/Test................................................................................................. 34-17
Adjust Altimeter.......................................................................................... 34-17
Test Altimeter.............................................................................................. 34-17
34-12-00 VERTICAL SPEED INDICATOR.............................................................. 34-18
General.............................................................................................................. 34-18
34-12-00 MAINTENANCE PRACTICES.................................................................. 34-18
Adjustment/Test................................................................................................. 34-18
34-21-00 TURN AND SLIP INDICATOR................................................................. 34-19
General.............................................................................................................. 34-19
Customer Options....................................................................................... 34-19
34-21-00 MAINTENANCE PRACTICES.................................................................. 34-19
Adjustment/Test................................................................................................. 34-19
34-00-00 KCS-55A SYSTEM................................................................................... 34-21
General.............................................................................................................. 34-21

34-ii FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page
Pictorial Navigation Indicator (KI-525A)........................................................... 34-21
Compass Card............................................................................................. 34-21
Lubber Line................................................................................................ 34-21
Aircraft Symbol.......................................................................................... 34-21

34 NAVIGATION
Selected Course Pointer............................................................................... 34-21
Course Select Knob..................................................................................... 34-21
VOR/LOC/RNAV Deviation Bar................................................................. 34-21
Deviation Scale........................................................................................... 34-21
Heading Select Bug..................................................................................... 34-22
Heading Select Knob................................................................................... 34-22
To/From Indicator....................................................................................... 34-22
Dual Glideslope Pointers............................................................................. 34-22
Glideslope Deviation Scale......................................................................... 34-22
Compass Warning Flag................................................................................ 34-22
NAV Warning Flag...................................................................................... 34-22
Directional Gyro (KG-102A)............................................................................. 34-23
Magnetic Azimuth Transmitter (Flux Valve) KMT-112....................................... 34-23
Autopilot Adapters KA-52 or KA-57.................................................................. 34-24
Slaving Control and Compensating Unit KA-51B.............................................. 34-24
Slaving Meter.............................................................................................. 34-25
Slave and Free Gyro Locking Switch.......................................................... 34-25
Clockwise Adjustment................................................................................. 34-25
Counter-clockwise Adjustment.................................................................... 34-25
Operation.................................................................................................... 34-25
Instructions For Continued Airworthiness................................................... 34-25

FOR TRAINING PURPOSES ONLY 34-iii

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page
34-22-00 ATTITUDE DIRECTOR INDICATOR....................................................... 34-27
Introduction....................................................................................................... 34-27
General.............................................................................................................. 34-27
Vertical Gyro............................................................................................... 34-27
34 NAVIGATION

34-00-00 C-14A COMPASS SYSTEM...................................................................... 34-29

General.............................................................................................................. 34-29
34-25-00 STANDBY MAGNETIC COMPASS.......................................................... 34-31
General.............................................................................................................. 34-31
34-25-00 MAINTENANCE PRACTICES.................................................................. 34-33
Adjustment/Test................................................................................................. 34-33
Compass Swing........................................................................................... 34-33
34-22-00 ATTITUDE INDICATOR........................................................................... 34-35
General.............................................................................................................. 34-35
34-24-11 RADIO MAGNETIC INDICATOR............................................................ 34-37
General.............................................................................................................. 34-37
Outside Air Temperature Gage.................................................................... 34-37
34-00-00 NAVIGATION........................................................................................... 34-39
General.............................................................................................................. 34-39
34-00-00 WEATHER RADAR SYSTEM (ART 2000) - OPTIONAL......................... 34-41
Introduction....................................................................................................... 34-41
General.............................................................................................................. 34-41
System Description............................................................................................ 34-41
Antenna Receiver/Transmitter..................................................................... 34-43
Weather Radar Indicator/Controller............................................................. 34-43
Operation........................................................................................................... 34-45

34-iv FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page
34-00-00 MAINTENANCE PRACTICES.................................................................. 34-47
Removal/Installation.......................................................................................... 34-47
Antenna Receiver/Transmitter..................................................................... 34-47
34-00-00 STORMSCOPE.......................................................................................... 34-49

34 NAVIGATION
General.............................................................................................................. 34-49
System Description............................................................................................ 34-49
Stormscope (WX-500)................................................................................ 34-49
34-00-00 RADIO ALTIMETER................................................................................ 34-51
General.............................................................................................................. 34-51
System Description............................................................................................ 34-51
Antennas..................................................................................................... 34-51
Operation........................................................................................................... 34-53
34-00-00 ENHANCED GROUND PROXIMITY WARNING SYSTEM
(EGPWS) - OPTIONAL............................................................................................ 34-55
Introduction....................................................................................................... 34-55
General.............................................................................................................. 34-55
System Description............................................................................................ 34-55
Runway Database........................................................................................ 34-55
Operation........................................................................................................... 34-57
Mode 1 - Excessive Descent Rate................................................................ 34-57
Mode 2A/2B - Terrain Closure Rate............................................................ 34-57
Mode 3 - Descent After Take-Off................................................................ 34-59
Mode 4A/4B/4C - Unsafe Terrain Clearance............................................... 34-61
Mode 5 - Descent Below Glideslope........................................................... 34-63
Mode 6 - Advisory Callouts........................................................................ 34-63

FOR TRAINING PURPOSES ONLY 34-v

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page
Excessive Bank Angle Callout..................................................................... 34-63
Terrain Clearance Floor............................................................................... 34-65
PULL UP and BELOW G/S Annunciator Switches...................................... 34-65
Operational Test of the EGPWS.................................................................. 34-65
34 NAVIGATION

Loading of the EGPWS Terrain Database.................................................... 34-66

34-00-00 TRAFFIC ALERT AND COLLISION AVOIDANCE SYSTEM
(TCAS II) - OPTIONAL........................................................................................... 34-69
Introduction....................................................................................................... 34-69
General.............................................................................................................. 34-69
System Description............................................................................................ 34-69
Component Details............................................................................................. 34-69
Control Unit................................................................................................ 34-71
Transmitter/Receiver Unit........................................................................... 34-73
Vertical Speed/Traffic Advisory/Resolution Advisory (VSI/TRA)
Indicator...................................................................................................... 34-75
Antennas..................................................................................................... 34-77
Operation........................................................................................................... 34-77
Operating Modes................................................................................................ 34-77
STBY.......................................................................................................... 34-77
AUTO......................................................................................................... 34-77
Symbology......................................................................................................... 34-79
Colors......................................................................................................... 34-79
Traffic Advisory.......................................................................................... 34-79
Resolution Advisory.................................................................................... 34-79
Proximate Traffic........................................................................................ 34-79
Other Traffic............................................................................................... 34-79

34-vi FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page
Self-Test............................................................................................................. 34-79
34-00-00 TRAFFIC ALERT AND COLLISION AVOIDANCE SYSTEM II
(TCAS II) (MOD S.O.O. 6219)................................................................................. 34-80
General.............................................................................................................. 34-80
System Description............................................................................................ 34-80

34 NAVIGATION
34-00-00 MAINTENANCE PRACTICES.................................................................. 34-83
Adjustment/Test................................................................................................. 34-83
Function Test............................................................................................... 34-83
Scheduled Maintenance Requirements........................................................ 34-83
34-51-00 DISTANCE MEASURING EQUIPMENT (DME)...................................... 34-85
Introduction....................................................................................................... 34-85
General.............................................................................................................. 34-85
System Description............................................................................................ 34-85
Transceiver.................................................................................................. 34-89
Indicator...................................................................................................... 34-89
Antenna....................................................................................................... 34-89
Operation........................................................................................................... 34-89
34-52-00 AIR TRAFFIC CONTROL (ATC).............................................................. 34-92
Introduction....................................................................................................... 34-92
General.............................................................................................................. 34-92
System Description............................................................................................ 34-92
Mode Selection Keys................................................................................... 34-93
Code Selection............................................................................................ 34-93
Keys for Other GTX 330 Functions............................................................. 34-93
Altitude Trend Indicator.............................................................................. 34-94

FOR TRAINING PURPOSES ONLY 34-vii

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page
Automatic ALT/GND Mode Switching........................................................ 34-94
Failure Annunciation................................................................................... 34-94
34-53-00 ADF........................................................................................................... 34-97
General.............................................................................................................. 34-97
34 NAVIGATION

Receiver............................................................................................................. 34-97
Turn-On...................................................................................................... 34-97
Frequency Selection.................................................................................... 34-97
Operating Modes......................................................................................... 34-97
ADF Test (PRE-FLIGHT or IN-FLIGHT)................................................... 34-98
Operating the Timers................................................................................... 34-98
Erroneous ADF Bearings Due to Radio Frequency Phenomena
Station Overlap........................................................................................... 34-99
Electrical Storms......................................................................................... 34-99
Night Effect................................................................................................ 34-99
Mountain Effect.......................................................................................... 34-99
34-55-00 (NAV) VOR..............................................................................................34-103
Operation.........................................................................................................34-103
34-55-00 (NAV) ILS...............................................................................................34-104
Operation.........................................................................................................34-104
Localizer..........................................................................................................34-104
Glide Slope......................................................................................................34-104
Marker Beacon.................................................................................................34-105
GNS 430 Keys and Buttons..............................................................................34-105
GPS..................................................................................................................34-105
Acronyms.........................................................................................................34-105
34-00-00 SERIES 100 AND 200 DIFFERENCES...................................................34-109

34-viii FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

ILLUSTRATIONS
Figure Title Page

34-1 F/ADC 2000..............................................................................................34-2

34-2 Basic System.............................................................................................34-4
34-3 Dual System..............................................................................................34-6

34 NAVIGATION
34-4 Dual System Schematic.............................................................................34-8
34-5 Pitot Tube................................................................................................34-10
34-6 Static Ports..............................................................................................34-10
34-7 Pilot Static Emergency Selector...............................................................34-10
34-8 Airspeed Indicator...................................................................................34-14
34-9 Altimeter.................................................................................................34-16
34-10 Vertical Speed Indicator...........................................................................34-18
34-11 Turn and Slip Indicator............................................................................34-19
34-12 Pictorial Navigation Indicator (KI-525A).................................................34-20
34-13 Directional Gyro (KG-102A)...................................................................34-23
34-14 
Magnetic Azimuth Transmitter (Flux Valve) KMT-112.............................34-23
34-15 
Autopilot Adapters KA-52 or KA-57.......................................................34-24
34-16 
Slaving Control and Compensating Unit KA-51B....................................34-24
34-17 Attitude Director Indicator.......................................................................34-26
34-18 C-14A Compass System..........................................................................34-28
34-19 Magnetic Standby Compass.....................................................................34-30
34-20 Compass Swing (Sheet 1 of 2).................................................................34-32
34-21 Compass Swing (Sheet 2 of 2).................................................................34-32
34-22 Attitude Indicator....................................................................................34-34
34-23 Radio Magnetic Indicator........................................................................34-36

FOR TRAINING PURPOSES ONLY 34-ix

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure Title Page

34-24 Outside Air Temperature Gage and Probe................................................34-36

34-25 Navigation General..................................................................................34-38
34-26 Radar Transmitting/Receiving Speed.......................................................34-40
34-27 Antenna Receiver/Transmitter..................................................................34-42
34 NAVIGATION

34-28 Weather Radar Indicator/Controller.........................................................34-42

34-29 Radar Indicator/Controller Test Pattern....................................................34-44
34-30 MMEL - Weather Radar - Thunderstorm Detection..................................34-47
34-31 Transmitter/Receiver................................................................................34-50
34-32 Antenna...................................................................................................34-50
34-33 KNI 415 Indicator....................................................................................34-50
34-34 ADI.........................................................................................................34-52
34-35 Enhanced Ground Proximity Warning System..........................................34-54
34-36 GNS 430..................................................................................................34-54
34-37 Mode 1 - Excessive Descent Rate............................................................34-56
34-38 Mode 2A/2B - Terrain Closure Rate.........................................................34-56
34-39 Mode 3 - Descent After Take-Off.............................................................34-58
34-40 Mode 4A/4B/4C - Aural Alerts................................................................34-60
34-41 Mode 5 - Descent Below Glideslope........................................................34-62
34-42 Excessive Bank Angle Callout.................................................................34-62
34-43 Terrain Clearance Floor...........................................................................34-64
34-44 Pull UP and BELOW G/S Annunciator Switches.....................................34-64
34-45 TCAS Intruder Caution and Warning Areas.............................................34-68
34-46 TCAS Control Unit (Collins - 92T).........................................................34-70
34-47 TCAS Transmitter/Receiver Unit.............................................................34-72
34-48 VSI/TRA Indicator Display Mode (1 of 3)...............................................34-74

34-x FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure Title Page

34-49 VSI/TRA Indicator Display Mode (2 of 3)...............................................34-74

34-50 VSI/TRA Indicator Display Mode (3 of 3)...............................................34-75
34-51 TCAS Antennas.......................................................................................34-76
34-52 MMEL - VSI and Transponder/Altitude Reporting...................................34-78

34 NAVIGATION
34-53 MMEL - TCAS........................................................................................34-82
34-54 KN 63......................................................................................................34-84
34-55 KN 63 - Description................................................................................34-86
34-56 Transceiver..............................................................................................34-88
34-57 Indicator..................................................................................................34-88
34-58 Antenna...................................................................................................34-88
34-59 MMEL - DME.........................................................................................34-90
34-60 Transponder GARMIN GTX330..............................................................34-92
34-61 MMEL - Transponders.............................................................................34-94
34-62 XPDR GTX330 Schematic......................................................................34-95
34-63 Receiver...................................................................................................34-96
34-64 ADF No.1 KR 87 - Interconnect Schematic.......................................... 34-100
34-65 MMEL -ADF........................................................................................ 34-101
34-66 GNS 430............................................................................................... 34-104
34-67 GNS 430 No.1 - Interconnect Schematic (Sheet 1 of 2)........................ 34-106
34-68 GNS 430 No.1 - Interconnect Schematic (Sheet 2 of 2)........................ 34-107
34-69 GNS 430 No. 2 - Interconnect Schematic............................................. 34-108
34-70 MMEL - Marker Beacon System and Navigation Equipment................ 34-109

FOR TRAINING PURPOSES ONLY 34-xi

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CHAPTER 34
NAVIGATION

34 NAVIGATION
34-00-00 NAVIGATION
INTRODUCTION
The DHC-6 Twin Otter avionics covered in this chapter include the navigational information
required for aircraft operation provided by the following systems: flight environmental data, attitude
and direction, landing and taxiing aids and independent position determining. It is not inclusive of
all the optional avionics systems available for installation. The user should consult the Maintenance
Manual, applicable supplements and vendor manuals for additional information and information
on specific manufacturers and systems not included in this chapter.

FOR TRAINING PURPOSES ONLY 34-1

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

PITOT

STATIC

NAV
OAT RECEIVER

MAG. HEADING FUEL/AIRDATA

COMPUTER

L. ENGINE F/F
STANDARD
ARINC 429
R. ENGINE F/F
OUTPUT TO
EFIS/FLIGHT
MANAGEMENT
SYSTEM

BARO POT
(Optional)

Figure 34-1. F/ADC 2000

34-2 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

GENERAL Standard flight instruments consist of:

Removal/Installation •• Airspeed indicator.

The removal/installation procedures related to •• Altimeter.
the navigation equipment is self-evident and
•• Vertical speed indicator.
should conform to aircraft standard practices.
•• Turn and slip indicator.
To gain access to the electrical cable runs
•• Directional gyro.
however, it will be necessary to remove

34 NAVIGATION
the interior panels by peeling back the roof •• Magnetic standby compass.
upholstery, removing attachment screws
•• Attitude indicator.
holding panels to the fuselage formers, and
allowing the panel to fall free of their Velcro •• Outside air temperature.
tape, exposing electrical wiring.
FUEL/AIR DATA COMPUTER
To replace upholstery, reposition the screws
through existing holes in the panels and fit the Refer to Figure 34-1. F/ADC 2000.
screws into the original holes in the formers.
Hold the bottom edge of the panel away from The system incorporated F/ADC 2000 (fuel/
the Velcro tape on the aircraft side until the air data computer) interfaces with the aircraft
screws are tightened sufficiently to hold the systems and calculates fuel used, PAS, TAT
panel in its original position. and OAT Barometric pressure corrections.

Carefully align the bottom edge of the panel The F/ADC pressure altitude data is used by
with the tape on the aircraft side, position the GPS to substitute for the 4th satellite if
the panel then press the bottom edge of the it is not available to provide 3D positioning.
panel firmly to the tape, check for correct Many optional flight instrument installations
position. To replace the roof upholstery, install are available, which includes vertical speed/
the screws holding the panel to the formers, traffic advisory systems.
press upholstery to tape and check for correct
position. The fuel/air data system is a remote mounted
unit which is connected to the GPS receiver
34-10-00 FLIGHT through serial data and standard ARINC 429
outputs. It is also connected to the pitot and
ENVIRONMENTAL DATA static line, OAT probe, fuel flow sensors and
the aircraft heading source. In addition, optional
barometric information may be received from
GENERAL the aircraft altimeter, when available.
The standard flight environmental system
installation is made up of the following:

•• Pilot pitot tube.

•• Co-pilot pitot tube.
•• Four static vent ports.
•• Static source emergency selector valve.

FOR TRAINING PURPOSES ONLY 34-3

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

Figure 34-2. Basic System

34-4 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-11-01 PITOT-STATIC NOTES

SYSTEM
BASIC SYSTEM
Figure 34-2. Basic System.

The basic pitot-static system is comprised of

34 NAVIGATION
a single pitot system and two independent
balanced static systems to operate the pilot
airspeed indicator, altimeter and vertical speed
indicator. The pitot system consists of an
electrically-heated pitot head, installed on a
mast projecting from the left side of the aircraft
nose section, connected to the pilot airspeed
indicator by a flexible hose. A moisture trap is
incorporated in the lowest part of the pitot line,
adjacent to the mast, to provide for drainage
and can also be used for the connection of test
equipment.

The static system consists of four static vents,

two on each side of the aircraft nose section
one above the other, to supply static pressure
through metal lines and hoses, a static manifold
and a static valve to the pilot airspeed indicator,
altimeter and vertical speed indicator. The lines
from each static vent are routed to the static
manifold, from where two lines are routed
to the inlet ports of the static valve. From
the valve, a line is routed to the pilot flight
instruments. Installed in the lowest point of
this line, immediately below the static valve,
is a moisture trap which can also be used as a
connection point for test equipment.

The static valve has two operating positions

marked NORM and EMER. When set to
NORM, static pressure from the lower vents
is routed to the pilot instruments and static
pressure from the upper vents is blanked off
by the valve. When the static valve is set to
EMER, static pressure from the upper vents is
routed to the pilot instruments, and pressure
from the lower vents is blanked off.

FOR TRAINING PURPOSES ONLY 34-5

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

Figure 34-3. Dual System

34-6 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

DUAL SYSTEM NOTES

Refer to:

•• Figure 34-3. Dual System.

•• Figure 34-4. Dual System Schematic.
The customer option pitot-static system (Mod
S.O.O. 6075) is comprised of two independent
pitot systems and two independent balanced

34 NAVIGATION
static systems to operate the pilot and co-pilot
airspeed indicators, altimeters, and vertical
speed indicators. The pitot systems consist
of two electrically-heated pitot heads, each
installed on a mast projecting from the left
and right side of the aircraft nose section
respectively. The left side serves the pilot
airspeed indicator and the right side serves
the co-pilot airspeed indicator. Each pitot
head is connected to its respective airspeed
indicator by a flexible hose and a moisture
trap is installed in the lowest part of each line
adjacent to the mast.

The static systems consist of four static vents,

two on each side of the aircraft nose section,
one above the other. The lines from each static
vent are routed to the static manifold where the
lines from the upper vents interconnect and the
lines from the lower vents interconnect. Static
pressure from the lower vents is routed from
the manifold to the static valve by a single
line where it is switched to the pilot flight
instruments with the valve set to NORM. Static
pressure from the upper vents is routed by one
line directly to the co-pilot flight instruments
and by a second line to the static valve.

When the static valve is set to EMER, static

pressure to the pilot flight instruments is
switched from the lower vents to the upper
vents and the static pressure from the lower
vents is blanked off.

FOR TRAINING PURPOSES ONLY 34-7

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

IAS ALT VSI IAS ALT VSI

34 NAVIGATION

Figure 34-4. Dual System Schematic

34-8 FOR TRAINING PURPOSES ONLY

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34 NAVIGATION
PAGE INTENTIONALLY LEFT BLANK

FOR TRAINING PURPOSES ONLY 34-9

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

Co-pilot Static Port

Pilot Static Port

Figure 34-5. Pitot Tube Figure 34-6. Static Ports

Figure 34-7. Pilot Static Emergency Selector

34-10 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Pitot Tubes *The following is an abbreviated description of

the maintenance practices and is intended for
Refer to Figure 34-5. Pitot Tube.
training purposes only.
The pilot pitot tube is mounted on the left For a more detailed description of the practice,
side of the fuselage nose. The co-pilot pitot refer to the task in the Viking AMM PSM 1-63-2.
tube (when installed) is mounted on the right
side. They provide independent supplies as
shown in Figure 34-2. Both pitot tubes are
34-11-01 MAINTENANCE
electrically heated. Control is by the PITOT PRACTICES

34 NAVIGATION
HEAT switch from the left and right DC buses.
The circuits are protected by the PITOT HTR
L and R circuit breakers on the main circuit-
SYSTEM MAINTENANCE
breaker panel (see Chapter 30, “Ice and Rain
To properly maintain the system a pitot-static
Protection”, for additional information).
test set, (SD12561-1), or equivalent is required.

Static Ports SYSTEM SERVICING

Refer to Figure 34-6. Static Ports.
•• Keep pitot heads and static ports clean
Two static air ports are installed on each side and free from foreign matter. Do not
of the fuselage nose. In normal operation the use polishing compounds.
lower port on each side supplies the pilot
•• Ensure that drain holes in pitot heads
instruments. The two upper ports are connected
are kept open.
to the PILOT STATIC emergency selector
valve. When dual instruments are installed, •• Function test pitot heaters.
the two upper ports are also connected to the
co-pilot instruments. Dual static ports are CAUTION
provided to the pilot and co-pilot instruments to
minimize static pressure errors to the airspeed DO NOT LEAVE PITOT
indicator, altimeter, and VSI due to sideslip HEATERS SWITCHED ON
effects. The static pressure ports are not heated. FOR LONGER THAN ONE
MINUTE. DO NOT BLOW
INTO PITOT HEADS WHILE
Static Valve THEY ARE CONNECTED TO
Refer to Figure 34-7. Pilot Static Emergency INSTRUMENTS.
Selector.

The static valve installed on the pylon, or,

REMOVAL/INSTALLATION
on aircraft incorporating Mod 6/1700, on the
When components of the pitot-static system
co-pilot sub panel, is provided to transfer static
are disconnected or removed, observe the
pressure for the pilot flight instruments from
following precautions:
the lower static vents to the upper static vents
in the event of an emergency.
•• Ensure that pitot and static lines are
correctly identified.
The valve has two operating positions marked
NORM and EMER. The operating position •• Cap lines and component ports.
is indicated by the valve handle tip which is
•• Carry out a leak test on reconnection of
painted white and acts as a pointer. Operation
lines to components.
of the valve is described above under Dual
System.

FOR TRAINING PURPOSES ONLY 34-11

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

ADJUSTMENT/TEST 8. Set static valve to EMER (on Pre Mod

6/1506, remove witness wire).
Leak Test Pitot Pressure Line 9. Repeat step 4 to 6.
1. Seal all holes in pitot head.
10. Disconnect test set and reinstall blanking
2. Remove blanking cap from moisture cap.
trap and connect a pitot-static test set
11. Remove seals from upper static vents.
(SD12561-1).
12. Set static valve to NORM (on Pre Mod
3. Apply pressure slowly until the airspeed
6/1506, install witness wire MS20995CU20
34 NAVIGATION

indicator reads 150 knots. Tap panel lightly

on handle).
when applying pressure to ensure freedom
of pointer movement.
Function Test Pitot Heater
4. Seal off pressure and check that airspeed
1. Connect external power source to aircraft.
indicator reading does not drop more than
2 knots in one minute. 2. Set BATTERY-OFF-EXTERNAL switch to
EXTERNAL and switch on DC MASTER
5. Release pressure slowly until airspeed
switch.
indicator reads 20 knots, then seal off
pressure. 3. Switch on pitot heater and check that pitot
head warms up.
6. Remove seal from drain hole in pitot head,
and check that airspeed indicator pointer 4. Switch OFF DC MASTER switch and
drops back to 0. BATTERY-OFF-EXTERNAL switch.
Disconnect external power source.
7. Remove remaining seals from pitot head,
disconnect test set and reinstall blanking
cap. CAUTION
DO NOT LEAVE PITOT
Leak Test Static Pressure Lines HEATER SWITCHED ON FOR
1. Seal both lower static vents. MORE THAN ONE MINUTE.
2. Disconnect blanking cap from moisture trap
adjacent to static valve and connect pitot- CLEANING/PAINTING
static test set.
1. Disconnect pitot and static lines from
3. Ensure static valve is set to NORM.
instruments and accessories. Cap instrument
4. With instruments properly connected and and accessory port.
altimeter set to 0, apply suction slowly until
2. Remove clean and dry all moisture traps.
altimeter reads 1000 feet above elevation at
which aircraft is standing. Tap panel lightly 3. Using pressure source of 25 psi nitrogen
when applying suction to ensure freedom of or clean dry air, blow through each pitot
pointer movement. and static line (from inside of aircraft to
outside) until clear of dust and moisture.
5. Seal off suction and check that altimeter
pointer does not drop more than 50 feet in 4. Reinstall moisture traps and reconnect lines
one minute. to instruments.
6. Release suction slowly to avoid damage to 5. Carry out pitot and static leak test.
vertical speed indicator.
7. Remove seals from lower static vents, and
seal both upper static vents.

34-12 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-00-00 FLIGHT NOTES

INSTRUMENTS
GENERAL
Flight instruments are covered in Chapter 34
of the Maintenance Manual. They are installed
in a shock-mounted instrument panel. Pilot

34 NAVIGATION
instruments only are standard in most aircraft,
but on later models the co-pilot instruments
became standard equipment.

The Maintenance Manual allows adjustment

and test of most of the flight instruments.
Typical procedures include such tests as:

•• Diaphragm leak test.

•• Case leak test.
•• Pointer position error tests.
•• Friction error tests.
•• Scale error tests.
•• Lag tests.

The following tools/equipment are typically

required to perform bench tests on the aircraft
flight instruments:

•• Mercury manometer.
•• Barometer.
•• Bell jar.
•• Altimeter.
•• Stopwatch.
•• Controlled vacuum source.
•• Controlled pressure source.

NOTE
The instruments should be
vibrated or gently tapped
during all tests unless otherwise
specified.

FOR TRAINING PURPOSES ONLY 34-13

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

White Arc: 58-96 Knots

Red Radial:
66 Knots for Land,
67 Knots for Floats/Skis

Blue Radial:
82 Knots for Land,
89 Knots for Floats/Skis

Red Radial:
170 Knots for Land,
160 Knots for Floats/Skis Green Arc:
74-170 Knots for Land,
74-160 Knots for Floats/Skis

Figure 34-8. Airspeed Indicator

34-14 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-13-00 AIRSPEED 4. Seal off the suction and check that the
pointer does not decrease by more than five
INDICATOR knots in one minute. A pointer drop of more
than 5 knots in one minute indicates a case
leak, and the indicator should be replaced.
GENERAL
5. Slowly release the suction to allow the
Refer to Figure 34-8. Airspeed Indicator. pointer to return to zero slowly then
disconnect the test set.
The airspeed indicator is calibrated in knots

34 NAVIGATION
with the scale reading from 0 to 250 in 5-knot CAUTION
increments starting at 30. Large numerals denote
20-knot increments, from 40 through 200. DO NOT APPLY SUCTION
Airspeed and flap operating limits are marked on AT PITOT CONNECTION
the face of the instrument and vary, depending on AS THIS WILL DAMAGE
whether the airplane is a land aircraft or equipped MECHANISM.
with floats or skis. The instrument converts pitot
and static pressures into an airspeed indication 6. Connect the test set to the pitot connection
by a differential-pressure-measuring mechanism. on the airspeed indicator and leave the
static connection open to atmosphere.
*The following is an abbreviated description of
7. Slowly apply pressure until the indicator
the maintenance practices and is intended for
pointer reads 250 knots.
training purposes only.
8. Seal off the pressure and check that the
For a more detailed description of the practice,
pointer reading does not change. (No
refer to the task in the Viking AMM PSM 1-63-2.
leakage).

34-13-00 MAINTENANCE 9. Release the pressure slowly until the pointer

reads 0, then disconnect the test set.
PRACTICES 10. Reconnect the pitot and static lines to the
airspeed indicator.
ADJUSTMENT/TEST
Test Airspeed Indicator
1. Disconnect lines from the pitot and static
connections on the airspeed indicator and
cap the lines.

CAUTION
DO NOT APPLY PRESSURE
AT STATIC CONNECTION
AS THIS WILL DAMAGE
MECHANISM.

2. Connect the pitot-static test set to the static

connection on the indicator and leave the
pitot connection open to atmosphere.
3. Slowly apply suction until indicator pointer
reads 100 knots.

FOR TRAINING PURPOSES ONLY 34-15

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-14-00 ALTIMETER
GENERAL
Refer to Figure 34-9. Altimeter.

The altimeter is connected to the static pressure

systems. The instrument has three pointers
which indicate altitude in hundreds, thousands
34 NAVIGATION

and tens of thousands of feet respectively.

A barometric scale, calibrated in inches of
mercury (or millibars if Mod S.O.O. 6122 is
installed), and seen through an aperture in
the instrument dial, can be adjusted to read
sea level atmospheric pressure or the local
prevailing atmospheric pressure, when the
pointers are set to indicate either the elevation
of the airfield, or 0 ft., as desired.

On aircraft with Mod 6/1589 incorporated, the Figure 34-9. Altimeter

altimeter has two barometric scales, one in
inches of mercury and one in millibars.

Each scale can be seen through its own aperture

in the instrument dial. A knob at the bottom of
the instrument rotates the barometric scales,
and by a gear system, simultaneously rotates
the pointers to obtain corrected readings at
various altitudes.

34-16 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of 5. Slowly release the suction and disconnect
the maintenance practices and is intended for the test set.
training purposes only.
6. Reconnect the static pressure line to the
For a more detailed description of the practice, altimeter.
refer to the task in the Viking AMM PSM 1-63-2.

34-14-00 MAINTENANCE
PRACTICES

34 NAVIGATION
ADJUSTMENT/TEST
Adjust Altimeter
Set the altimeter pointers to airfield elevation
or zero required, lightly tapping panel to
overcome pointer friction. If the barometric
scale does not indicate prevailing barometric
pressure, adjust instrument as follows:

1. Ensure that the pointers read airfield

elevation or zero as required.
2. Loosen the lock-screw adjacent to the knob
at the bottom of the instrument sufficiently
to slide the screw to the left along its slot.
3. Pull the knob outwards to disengage it from
the pointers, taking care not to disturb the
pointer setting, and rotate the knob to set
the barometric scale to read the prevailing
barometric pressure.
4. Push the knob in to re-engage it with the
pointers, taking care not to disturb the
settings then return the lock-screw to its
original position and tighten in place.

Test Altimeter
1. Disconnect the static pressure line at the
altimeter and cap the line.
2. Connect a pitot-static test set to the
altimeter static port.
3. Slowly apply suction until 1000 feet is
indicated, lightly tapping instrument panel
to overcome pointer friction.
4. Seal off the suction and check that pointer
drop does not exceed 20 feet in ten
seconds.

FOR TRAINING PURPOSES ONLY 34-17

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-12-00 VERTICAL
SPEED INDICATOR
GENERAL
Refer to Figure 34-10. Vertical Speed Indicator.

The vertical speed indicator (VSI) operates from

34 NAVIGATION

static pressure. It has a range of +3,000 feet to

−3,000 feet per minute with the first 1,000 feet
each side of zero marked in 100-foot increments.
The indicator embodies a differential pressure
mechanism which is actuated by momentary
pressure changes between the inside of the
capsule and the instrument case as the aircraft
climbs or descends. An adjusting screw at the
bottom left of the bezel is provided to reset the
pointer to zero.
Figure 34-10. Vertical Speed Indicator
*The following is an abbreviated description of
the maintenance practices and is intended for
training purposes only.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2.

34-12-00 MAINTENANCE
PRACTICES
ADJUSTMENT/TEST
Currently there are NO on aircraft adjustments
or tests with regards to the Vertical Speed
indicator. The AMM does include bench tests
for this and all other indicators contained in the
pitot/static system.

34-18 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-21-00 TURN AND

SLIP INDICATOR
GENERAL
Refer to Figure 34-11. Turn and Slip Indicator.

The turn and slip indicator displays the turning

34 NAVIGATION
rate, and the slip or skid of the aircraft when
performing a coordinated maneuver. The
instrument is a combination unit, consisting of
a spring-controlled direct-reading, electrically-
driven gyroscope to indicate turns, and a
fluid-damped, ball-type inclinometer to indicate
a slip or skid.

Power to operate the indicator is from the 28

VDC system and routed through the TURN
& SLIP PLT 5-ampere circuit breaker on the Figure 34-11. Turn and Slip Indicator
circuit breaker panel in the flight compartment.

Customer Options
When the customer option co-pilot turn and slip
indicator is installed, power is routed through the
5-ampere TURN & SLIP COPLT circuit breaker.

When the customer option autopilot is installed,

the basic turn and slip indicator must be
replaced by one which can provide an input to
the autopilot system.

*The following is an abbreviated description of

the maintenance practices and is intended for
training purposes only.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2.

34-21-00 MAINTENANCE
PRACTICES
ADJUSTMENT/TEST
Currently there are NO on aircraft adjustments
or tests with regards to the Turn and Slip
Indicator. The AMM does include bench tests
for this instrument.

FOR TRAINING PURPOSES ONLY 34-19

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

Nav Warning Flag Lubber Line Compass Warning Flag

Heading Marker

Course Select
Pointer

To-from Indicator

Dual Glideslope Dual Glideslope

Pointer Pointer
Vor/localizer
Deviation
Aircraft Symbol

Course Select Heading Select

Knob Knob

Compass Cord

Figure 34-12. Pictorial Navigation Indicator (KI-525A)

34-20 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-00-00 KCS-55A Aircraft Symbol

SYSTEM The aircraft symbol is a fixed representation of
the actual aircraft. This symbol always points
toward the top of the display and the lubber line.
GENERAL
The KCS-55A is a remote mounted compass
Selected Course Pointer
system consisting of the following units: On this two-part arrow, the “head” indicates the
desired VOR or localizer course and the “tail”

34 NAVIGATION
•• P i c t o r i a l n a v i g a t i o n i n d i c a t o r indicates the reciprocal. This pointer is set by
(KI-525A). rotating the course select knob.
•• Directional gyro (KG-102A).
•• M a g n e t i c a z i m u t h t r a n s m i t t e r
Course Select Knob
(KMT-112). This knob is rotated to turn the selected course
pointer to the desired course on the compass
•• Autopilot adapters (KA-52 & KA-57).
card. (This knob corresponds to the omni
•• Slaving accessory unit KA-51 A/B). bearing selector on standard NAV indicators).

PICTORIAL NAVIGATION VOR/LOC/RNAV Deviation Bar

INDICATOR (KI-525A) This bar corresponds to the “left/right” needle
on standard course deviation indicators. When
Refer to Figure 34-12. Pictorial Navigation
the aircraft is precisely on the VOR radial or
Indicator (KI-525A).
Localizer course, it forms the center section
of the selected course pointer and will be
The KI-525A provides a pictorial display
positioned under the aircraft symbol. When
of the horizontal navigation situation and
off course or approaching an new course, it will
also provides manual controls for course
move to one side or the other. Since the entire
and heading datum selection. The KI-525A
VOR and localizer display rotates with the
provides outputs for autopilot or flight director,
compass card, the angular relationship between
VOR receivers, and additional compass loads.
the deviation bar and the symbolic aircraft
Magnetic heading and course information from
provides a pictorial display of the aircraft’s
VOR/LOC, GPS, Loran, or RNAV is combined
position with respect to the selected course.
on the KI-525A.

Compass Card Deviation Scale

When tuned to a VOR frequency, each white
Responding from inputs from the slaved
dot represents 2° of deviation either left or right
directional gyro, this card rotates within the
of course. When tuned to a Localizer frequency
display so that the aircraft heading is always
the deviation is 1/2° per dot. In RNAV “APPR”
at the top, under the lubber line.
mode the scale is 1/4 nm per dot. In RNAV
“ENROUTE” mode the scale is 1 nm per dot.
Lubber Line
The lubber line is a fixed white marker at the
top of the display that indicates the aircraft
magnetic heading on the compass card.

FOR TRAINING PURPOSES ONLY 34-21

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Heading Select Bug NOTES

This is a moveable marker on the outer
perimeter of the display, used primarily to
select the desired heading you wish to fly. This
desired heading can be coupled to the autopilot
or flight director with the appropriate autopilot
adapter to provide a “Heading Select” function.

Heading Select Knob

34 NAVIGATION

The knob is turned to rotate the heading select

bug to a desired point on the compass card.

To/From Indicator
This is a white triangle near the center of the
display that indicates, with reference to the
OBS setting, whether the aircraft is flying to or
from the selected VOR/RNAV station.

Dual Glideslope Pointers

The chartreuse triangular pointers drop into
view when a usable glideslope signal is received
and go out of view when the signal is marginal.
During an ILS approach the pointer represent the
vertical orientation of the aircraft with respect
to the center line of the glideslope beam. When
on the center line the pointers will align with the
center markers on the glideslope scale.

Glideslope Deviation Scale

White dots on each side of the display which,
in conjunction with the glideslope pointers,
indicate “too high”, “too low” or on the glide
path when doing an ILS approach.

Compass Warning Flag

A red flag labeled “HDG” becomes visible
in the upper right quadrant of the display
whenever the electrical power is inadequate or
the directional gyro is not up to speed.

NAV Warning Flag

A red flag labeled “NAV” becomes visible in the
upper left quadrant of the display whenever an
unusable VOR or Localizer signal is being received.

34-22 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

DIRECTIONAL GYRO (KG-102A)

Refer to Figure 34-13. Directional Gyro
(KG-102A).

The remote mounted directional gyro provides

gyro-stabilization for the system and contains the
slaving circuits necessary for system operation.

MAGNETIC AZIMUTH

34 NAVIGATION
TRANSMITTER (FLUX VALVE)
KMT-112
Refer to Figure 34-14. M
 agnetic Azimuth
Transmitter (Flux
Valve) KMT-112.

This remote mounted sensor is usually installed

in a wingtip to eliminate the possibility of
magnetic interference. The transmitter senses
the direction of the earth’s magnetic field and
Figure 34-13. Directional Gyro (KG-102A)
continuously transmits this information through
the slaving circuitry to the directional gyro
which is then corrected for precession.

Figure 34-14. M
 agnetic Azimuth Transmitter
(Flux Valve) KMT-112

FOR TRAINING PURPOSES ONLY 34-23

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

AUTOPILOT ADAPTERS KA-52

OR KA-57
Refer to Figure 34-15. A
 utopilot
Adapters KA-52
or KA-57.

The remote mounted autopilot adapters are

optional units that convert direct current
34 NAVIGATION

heading select and course select signals

from the pictorial navigation indicator into
alternating current signals that are compatible
with most autopilot and flight director systems. Figure 34-15. A
 utopilot Adapters KA-52
or KA-57
SLAVING CONTROL AND
COMPENSATING UNIT KA-51B
Refer to Figure 34-16. S
 laving Control and
Compensating Unit
KA-51B.
Slaving Meter
This panel mounted unit provides a means of
selecting either the “slaved gyro” or “free gyro”
modes. The free gyro mode allows manual Free and Slave
slaving when fully automatic operation is not Gyro Switch
desired. The unit also contains a meter that
deflects to indicate when there is a difference Clockwise/
between the pictorial navigation indicator’s Counter-clockwise
compass card and the aircraft’s magnetic Adjustment Switch
heading.

Figure 34-16. S
 laving Control and
Compensating Unit KA-51B

34-24 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Slaving Meter •• The gyro spin motor is at least 50% of

normal speed.
This meter indicates the difference between the
displayed heading and the magnetic heading. •• The fast slave mode has been executed.
Right deflection indicates a clockwise error of
•• Normal system power is present.
the compass card and left deflection indicates
a counter-clockwise error of the compass card. If the system is in the slaved gyro mode the
Whenever the aircraft is in a turn and the card compass card will fast slave at the rate of 180°
rotates the meter will show a full deflection to per minute towards the magnetic heading. The
one side or the other. system will remain in this fast slave mode for

34 NAVIGATION
120 seconds after which it will revert to the
normal slaving mode and slave at a constant
NOTE rate of 3° per minute to keep the system aligned
During level flight it is with the earth’s magnetic field.
normal for the meter needle to
continuously move from side Until a usable navigation signal is being
to side. If the needle stays fully received the red NAV flag will be visible.
deflected during level flight the
Free Gyro mode can be used to
center the needle.
Instructions For Continued
Airworthiness
Slave and Free Gyro Locking The instructions for continued airworthiness
given in the TC or STC approvals for this
Switch product supplements or supersedes the
When in the “slave” position, the system is instructions for continued airworthiness in
using inputs from the flux valve to counter this manual.
the effects of precession. When in the “free”
position the system is not engaged and a Most Honeywell products are designed
manual input from the Adjustment switches is and manufactured to allow “on condition
required to correct precession. maintenance”.

On condition maintenance is described as

Clockwise Adjustment follows:
When the system is in the free gyro mode a
clockwise manual heading drive switch will •• T h e r e a r e n o p e r i o d i c s e r v i c e
rotate the compass card to the right to eliminate requirements necessary to maintain
left compass card error. continued airworthiness.
•• No maintenance is required until the
Counter-clockwise Adjustment equipment does not properly perform
its intended function. When service is
When the system is in the free gyro mode
required, a complete performance test
a counter-clockwise manual heading drive
should be accomplished following any
switch will rotate the compass card to the left
repair action.
to eliminate right compass card error.
Consult the appropriate unit Maintenance/
Overhaul Manual for complete performance
Operation test information.
When power is applied to the system and the
DG is up to speed the HDG flag will disappear
from view. The HDG flag will remain in view
until the following conditions are satisfied:

FOR TRAINING PURPOSES ONLY 34-25

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

ROLL ATTITUDE
GO AROUND LIGHT INDEX POINTER
FLIGHT ATTITUDE FLAG
DIRECTOR
FLIGHT DIRECTOR ATTITUDE FLAG
COMMAND SPHERE DECISION HEIGHT LIGH
POINTER
FIXED ROLL
GA DH
ATTITUDE SCALE
34 NAVIGATION

SPEED CONTROL FD ATT

INDICATOR

FAST GLIDE SLOPE

POINTER

AIRPLANE
SYMBOL
GLIDE SLOPE
SCALE

SLOW

RADIO ALTITUDE
DISPLAY
DECISION HEIGHT DH RAD ALT
DISPLAY ATT DIM
DH
SEL/
TEST

DIM CONTROL
ATTITUDE TEST SWITCH
EXPANDED LOCALIZER DECISION HEIGHT
INCLINOMETER DEVIATION POINTER SELECTOR/TEST KNOB

Figure 34-17. Attitude Director Indicator

34-26 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-22-00 ATTITUDE NOTES

DIRECTOR INDICATOR
INTRODUCTION
The attitude indicator provides the pilot with
the attitude of the aircraft relative to the
ground. It indicates pitch and roll information.

34 NAVIGATION
GENERAL
Refer to Figure 34-17. Attitude Director
Indicator.

The attitude director indicator (ADI) combines

a true sphere type attitude display with a
single-cue flight guidance command pointer
to show the commands required to intercept
and maintain a desired flight path. The ADI
also shows the displays and controls for the
following functions:

•• Pitch and roll attitude data.

•• Flight director commands.
•• Glideslope deviation.
•• Attitude test button.
•• Warning flags.

A stationary triangular-shaped airplane

symbol is located in the center of the display,
representing the aircraft in relationship to the
pitch, roll and command bar information. The
ADI requires 26VAC primary power for its
operation, supplied from the 26VAC bus.

Vertical Gyro
The vertical gyro senses the pitch and roll
attitude of the airplane and provides this data
to the flight director indicator, the computer/
control, and the turn and slip indicator.

FOR TRAINING PURPOSES ONLY 34-27

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

Figure 34-18. C-14A Compass System

34-28 FOR TRAINING PURPOSES ONLY FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-00-00 C-14A NOTES

COMPASS SYSTEM
GENERAL
Refer to Figure 34-18. C-14A Compass System.

The C-14A gyro magnetic compass system

34 NAVIGATION
provides visual indication of the magnetic
heading of the airplane. The magnetic heading
display is provided from a gyro which is
corrected to the heading reference by the output
signal from a magnetic azimuth detector (flux
valve). This signal is applied via an internal
slaving amplifier. System power is 115VAC,
400-Hz input to the C-14A directional gyro
(DG) and 26VAC output to the flux valve and
compensator. The basic system consists of:

•• Flux valve and compensator.

•• Directional gyro.
•• Synchronizer annunciator.
The system is normally operated in slaved (to
the magnetic heading provided by the flux
valve) mode. The system may be unslaved
in higher latitudes or areas of magnetic
disturbance.

NOTE
When running unslaved, the
gyro is subject to inherent, real
and transport errors.

FOR TRAINING PURPOSES ONLY 34-29

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

Figure 34-19. Magnetic Standby Compass

34-30 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-25-00 STANDBY NOTES

MAGNETIC COMPASS
GENERAL
Refer to Figure 34-19. Magnetic Standby
Compass.

34 NAVIGATION
The standby magnetic compass, installed on a
bracket near the center windshield post gives
indication of the aircraft’s magnetic heading.
The compass is fluid damped with a built-in
compensator and is integrally lighted. The
intensity of the lighting is controlled by the
PLT ENG INS & EMER PNL LTS dimmer
control. A compass correction card is installed
in a holder mounted above the instrument.

FOR TRAINING PURPOSES ONLY 34-31

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

Figure 34-20. Compass Swing (Sheet 1 of 2)

Figure 34-21. Compass Swing (Sheet 2 of 2)

34-32 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of compass, right wing). Rotate the flux
the maintenance practices and is intended for valve to cancel out the index error.
training purposes only.
For a more detailed description of the practice,
NOTE
refer to the task in the Viking AMM PSM 1-63-2. Rotate the flux valve in a clockwise
direction, as seen from above,
34-25-00 MAINTENANCE to cancel out plus (+) error, or
counter-clockwise to cancel out
PRACTICES minus (−) error. The amount of

34 NAVIGATION
rotation must equal the index error.
ADJUSTMENT/TEST e. Secure the flux valve mounting screws.
Compass Swing f. C o n f i r m i n d e x e r r o r t o e n s u r e
that it is zero. Readjust (step 4) if necessary.
Refer to:
3. Compensating for Magnetic Error
•• Figure 34-20. Compass Swing (Sheet 1 of 2).
a. Remove the cover from the compensator
•• Figure 34-21. Compass Swing (Sheet 2 of 2). (two screws).
The compass swing procedure is done in three b. Position the aircraft north to within +5°
stages and must be accomplished in order. Complete (using compass rose or sighting compass).
the procedures for the No.1 compass system first,
c. Check the RDI heading. Remove all
and then repeat the procedures for the No.2 compass
error by adjusting the N-S compensator
system. The three stages are listed as below.
until the indicator reading agrees with
1. Preparing for swing the actual magnetic heading.
a. Position the aircraft on a compass rose, d. Position the aircraft east to within +5°.
when available. If a rose is not available,
e. Check the RDI heading. Remove all
a magnetic compass sighting can be used.
error by adjusting the E-W compensator
b. Start and run the engines. Increase until the indicator reading agrees with
power to cruise rpm for each reading. the actual magnetic heading.
c. Check that the battery is fully charged. f. Position the aircraft south to within +5°.
d. Switch on all communications and g. Check the RDI heading. Remove half
navigation systems. the error with the N-S compensator.
2. Removing compass index error h. Position the aircraft west to within +5°.
a. Position the aircraft on each of the four i. Check the RDI heading. Remove half
cardinal headings in turn and record the error with the E-W compensator.
each reading. Allow enough time for the
j. Accomplish the swing check at 45°
compass reading to settle down before
increments, and record the indicator
taking each reading.
readings. All readings must be within
b. Calculate the deviation between the ± 3° of actual magnetic headings. If
remote indicator (RDI) reading and the any reading is not within tolerance,
actual magnetic heading. repeat the index error and magnetic
compensation adjustment procedures.
c. Add the deviations algebraically, and
divide the sum by four. The result is the k. Record the deviation readings and the
index error. date on the indicator correction card.
d. L o o s e n t h e f l u x v a l v e m o u n t i n g l. Replace the compensator cover.
screws (No.l compass, left wing; No.2

FOR TRAINING PURPOSES ONLY 34-33

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

Figure 34-22. Attitude Indicator

34-34 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-22-00 ATTITUDE NOTES

INDICATOR
GENERAL
Refer to Figure 34-22. Attitude Indicator.

The standard attitude indicator (artificial

34 NAVIGATION
horizon) is a 115VAC-powered gyro
instrument. It provides the pilot with a constant
visual indication of the airplane lateral and
longitudinal attitude relative to the horizon. A
symbolic airplane (reference bar) in the center
of the instrument represents the airplane and
can be adjusted in accordance with the pitch
attitude of the airplane. The gyro has a caging
knob on the lower right side marked PULL
FOR QUICK ERECT. The instrument has a
power-off flag and receives power through the
PILOT ART HORIZ fuse on the fuse panel.

FOR TRAINING PURPOSES ONLY 34-35

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Indicators
34 NAVIGATION

“Push” Transfer
Buttons

Figure 34-23. Radio Magnetic Indicator

Figure 34-24. Outside Air Temperature Gage and Probe

34-36 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-24-11 RADIO NOTES

MAGNETIC INDICATOR
GENERAL
Refer to Figure 34-23. Radio Magnetic
Indicator.

34 NAVIGATION
The radio magnetic indicator (RMI) is a dual
pointer, rotating, servo-driven azimuth dial
instrument, compatible with either sine/cosine
or conventional ARINC receiver output. A
fixed-index mask is marked with a lubber
line and triangular markers at 45° increments.
A single-bar pointer and a wide, double-
bar pointer provide bearing indication for
either ADF or VOR, as read on the face of
the instrument. The pointers can be switched
to either ADF or VOR modes by means of
rotary switches or “push” transfer buttons,
depending on the particular instrument. Servo
error, compass valid, and instrument power are
monitored by a single power-off flag.

Outside Air Temperature Gage

Refer to Figure 34-24. Outside Air Temperature
Gage and Probe.

An outside air temperature gage is installed

in the left side of the flight compartment roof
with the dial facing down and the stem of the
instrument protruding through the roof into
the slipstream. The gage reads in Celsius and
Fahrenheit.

FOR TRAINING PURPOSES ONLY 34-37

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

Figure 34-25. Navigation General

34-38 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-00-00 NAVIGATION The VHF navigation system supplies VOR

information, localizer and glide-slope
deviation, flag signals, to-from signals,
GENERAL marker beacon signals, VOR localizer, and
marker beacon audio outputs. The VHF NAV
Refer to Figure 34-25. Navigation General. receiver provides sine/cosine VOR outputs to
a radio magnetic indicator (RMI). Magnetic
Navigation systems are covered in Chapter compass information is combined with the
34 of the Maintenance Manual, “Navigation”. VOR data within the RMI to display VOR
Before starting work on either a VHF navigation position. The RMI also displays ADF bearing.

34 NAVIGATION
or VHF communications system, check both King and Collins are commonly used VHF
chapters (34 and 23) of the Maintenance Manual navigation systems in the Twin Otter. The
because there may be additional information navigation systems are listed in Chapter 34 of
given which is pertinent to all VHF radio the Maintenance Manual and are available for
systems (e.g., the location of all VHF antennas, Twin Otter installation.
given in Chapter 23 only). Also, because most
navigation and communications equipment is
optional, the aircraft maintenance engineer is
likely to find major differences in components
installed when comparing two different Twin
Otters. Always check the Maintenance Manual
furnished with the particular airplane, when
possible.

The aircraft maintenance engineer should

thoroughly understand, and be able to operate,
the Twin Otter navigation and communications
equipment. This knowledge allows more
comprehensive functional testing of, and better
evaluation of, equipment when any system
is malfunctioning. In addition, it may save
the unnecessary replacement cost of a high-
value item which does not need replacing. This
section of the “Avionics” chapter gives a broad
description and operation of the major avionics
systems available in the Twin Otter.

Removal and installation of navigation and

communications equipment is self-evident.
To gain access to the electrical cable runs,
however, it i s necessary t o remove the
upholstery by peeling back the roof upholstery,
removing the screws holding the panel to the
fuselage formers, and allowing the panel to
fall free of the velcro tape, thus exposing the
electrical wiring. The work area is buttoned
up in reverse. Figure 34-25 shows the typical
locations of navigation and communications
equipment and antennas.

FOR TRAINING PURPOSES ONLY 34-39

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

10 NM

20 NM ROUND TRIP IN
123.6 MICROSECONDS

Figure 34-26. Radar Transmitting/Receiving Speed

34-40 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-00-00 WEATHER When a pulse intercepts a target, the energy is

reflected as an echo, or return signal, back to the
RADAR SYSTEM (ART antenna. From the antenna, the returned signal
2000) - OPTIONAL is transferred to the receiver and processing
circuits located in the receiver transmitter unit.
The echoes, or returned signals, are displayed
INTRODUCTION on an indicator.

The primary use of this radar is to aid the Radio waves travel at the speed of 300 million
pilot in avoiding thunderstorms and associated meters per second and thus yield nearly

34 NAVIGATION
turbulence or major terrain features such as instantaneous information when echoing
rivers, coastlines, towns and cities for position back. Radar ranging is a two-way process that
fixes. The proficient operator manages antenna requires 12.36 micro-seconds for the radio
tilt control to achieve best knowledge of storm wave to travel out and back for each nautical
height, size, and relative direction of movement. mile of target range.

As shown in Figure 34-26, it takes 123.6

GENERAL micro-seconds for a transmitted pulse of radar
energy to travel out and back from an area of
The weather radar system detects dangerous areas
precipitation 10 nautical miles away.
of weather in a 240 NM range ahead of the aircraft
and in an arc of approximately ± 15°, providing
the flight crew with a visual indication of their SYSTEM DESCRIPTION
intensity, based on the color of the display.
The weather radar system includes the
Although the system’s primary function is components that follow:
weather detection, the system can also be used
for ground mapping. This secondary function •• One weather radar antenna receiver/
can supply useful additional guidance on areas transmitter.
with limited ground based navigational aids.
•• One weather radar indicator/controller.

NOTE
Radar is fundamentally a
distance measuring system using
the principle of radio echoing.
The term RADAR is an acronym
for radio detecting and ranging.
It is a method for locating targets
by using radio waves.

The transmitter generates microwave energy

in the form of pulses. These pulses are then
transferred to the antenna where they are
focused into a beam by the antenna. The radar
beam is much like the beam of a flashlight. The
energy is focused and radiated by the antenna
in such a way that it is most intense in the
center of the beam with decreasing intensity
near the edge. The same antenna is used for
both transmitting and receiving.

FOR TRAINING PURPOSES ONLY 34-41

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

Figure 34-27. Antenna Receiver/Transmitter

BRT BENDIX LOG

ON
TST
SBY
U 2.0
18 OFF
80
Wx
WxA RNG

RNG
VP

60
MAP TRK

40
TRK
NAV WX
20
UP

TILT
GAIN
DN

Figure 34-28. Weather Radar Indicator/Controller

34-42 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Antenna Receiver/Transmitter NOTES

Refer to Figure 34-27. Antenna Receiver/
Transmitter.

The ART, mounted on the forward bulkhead

and protected by the radome is an integrated
assembly that incorporates an antenna, receiver
and transmitter.

34 NAVIGATION
The antenna is a flat plate phased array,
combining receive and transmit signals of the
radar system.

The ART provides antenna movement in the

vertical and horizontal planes. Signals from the
receiver-transmitter operate the azimuth motor
to sweep the antenna horizontally for a total
sweep of 60° or 120°. The ART also contains
self-test circuits.

The bottom of the ART contains maintenance

switches and an electrical connector. The
maintenance switches are used to enable/
disable the scan and to enable/disable the
transmitter.

Weather Radar Indicator/

Controller
Figure 34-28. Weather Radar Indicator/
Controller.

The indicator/controller is normally mounted

on the instrument panel where it is easily
accessed by both pilots.

FOR TRAINING PURPOSES ONLY 34-43

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

STAB OFF U 7.0

80

60
TEST
40
20

Figure 34-29. Radar Indicator/Controller Test Pattern

34-44 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

OPERATION GND MAP - Places the radar system in ground

mapping mode. Gain control capability is
Operation of the weather radar system and configurable at installation to be enabled or
the display is controlled by several switches disabled in ground map mode. Ground map
and potentiometer controls located on the colors are: green for weak returns, yellow for
front of the indicator. The display and various moderate returns and magenta for intense
indications are directly related to these controls returns. “MAP” will appear on the lower left
and are described in the paragraphs that follow. of the display.

GAIN - The gain knob adjusts the radar gain

34 NAVIGATION
BRT BRT - Controls brightness of the indicator from 0 to -20 dB (CCW rotation reduces
display (CW rotation for max brightness). GAIN gain). The gain knob will only function when
in the MAP mode.

Wx/WxA - Alternately selects between LOG - Used when Bendix/King radar

the Wx (weather) and WxA (weather-alert) graphics units are installed. A listing of
modes of operation. “Wx” or “WxA” will the latitudes and longitudes of selected
appear in the lower left of the display. Wx LOG waypoints are displayed. When a
ON
Wx or WxA colors are: Black for no returns, TST
compatible navigation source is installed,
WxA Green for weak returns, Yellow for SBY the selected VOR frequencies along with
moderate returns, Red for heavy returns OFF bearings and distances are also displayed.
and Magenta for intense returns. When The radar transmits in the LOG mode,
the WxA mode is selected, magenta unless a Bendix/King radar graphics unit
areas of storms flash between magenta (IU-2023, GC-360A or GC-381A) is installed.
and black at a 1 Hz rate.
ON - Selects the normal condition of
VP - Selects and deselects the Vertical operation for weather detection and/or
Profile mode of operation. When VP is other modes of operation. The system will
selected on the indicator the radar will transmit after a 60 second warm-up time is
provide a vertical scan of ±30° at the location completed. The radar system initializes to
of the horizontal track line. Selecting the the Wx mode, 80 nm.
VP VP mode of operation will not change the
selected mode of operation: TST, Wx, WxA NOTE - The 60 second warm up period can
LOG
or GND MAP. Once in VP, these modes may ON be monitored upon power up of the system.
be changed as desired. VP will engage from TST When the knob is switched directly from OFF
SBY
the NAV MAP mode, but NAV data will not OFF
to ON mode (or LOG mode with no Bendix/
be displayed during VP operation. King radar graphics unit installed), the display
will blank. As the radar sweeps the blue/white
MAP - Places indicator in navigation mode
will grow outward. Just before the warm up
so that preprogrammed waypoints may be
period is complete, the screen will turn black
displayed. If other modes are also selected,
for a few seconds, then the radar will begin
the NAV display will be superimposed
transmitting and the screen will display radar
on them. This button is effective only if
returns. No radar transmissions occur until
an optional radar graphics unit and Flight
the warm up period is complete.
Management System are installed. If
activated without these units, NO NAV will TST - The multicolored arc display test pattern
appear at lower left of screen. The radar will (Figure 34-29) is displayed in this mode of
display weather when NAV MAP is selected if operation. The test pattern (typical 4-color
LOG
the radar selector is in the ON position. ON test pattern) is initialized and sized to fit the
TST 80 nm range and can also be scaled with the
SBY
OFF
range select buttons. No radar transmissions
occur while TST is selected. TEST will appear
in the lower left of the display. STAB OFF is
always displayed in top left.

FOR TRAINING PURPOSES ONLY 34-45

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

SBY - Fully energizes the system circuitry NOTES

but no radar transmissions occur in the SBY
mode of operation. The antenna is parked at
LOG 0° azimuth and 30° tilt down with the antenna
ON
TST
drive motors locked. In the standby mode
SBY of operation, NAV MAP, checklist and TCAS
OFF traffic can be activated with a Bendix/King
radar graphics unit (IU-2023B, GC-360A,
GC-362A or GC-381A) installed. SBY will
appear in the lower left of the display.
34 NAVIGATION

OFF - Removes primary power from the

radar indicator, but the radar still has power
applied. The radar will remain active with no
radar transmissions occurring, for up to a
LOG
ON maximum time of 30 seconds. This time delay
TST allows time to park the antenna at 0° azimuth
SBY
OFF
and 30° tilt down.

NOTE - The only way to remove primary

power from the radar is to pull the radar
circuit breaker.

RNG - Clears the display and advances

the indicator to the next range. The upper
RNG button increases range, the lower button
decreases it. The display ranges are: 10,
20, 40, 80, 160, and 240 nm. The selected
RNG
range is displayed in the upper right corner
of the display with the range ring distance
displayed along the right edge.

TRK - Provides a yellow track centerline

for vertical profile. With the radar on and a
track button pushed, the track line position
TRK moves left or right in 1° increments at a rate
of about 15° per second. When Vertical
Profile mode is selected, the antenna scans
TRK the slice at the track line azimuth position.
While in Vertical Profile mode, the TRK
buttons move the slice left and right. The
azimuth position of the antenna is displayed
on the upper left corner of the indicator.
UP TILT - Permits manual adjustment of
TILT antenna tilt 15° up or down for best indicator
DN
presentation. The tilt angle is displayed in
the upper right corner of the display.

34-46 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of

the maintenance practices and is intended for
training purposes only.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2.

34-00-00 MAINTENANCE
PRACTICES

34 NAVIGATION
REMOVAL/INSTALLATION
Antenna Receiver/Transmitter
To remove the weather radar antenna, proceed
as follows:

1. Remove screws holding radome in position.

2. Remove radome.
3. Disconnect electrical connector from ART
assembly.
4. Remove four bolts from mounting bracket.
5. Support antenna assembly and remove
bonding lead from top right bolt.
6. Remove ART assembly.

To install the weather radar antenna, proceed in

reverse the above removal instructions.

System & 1. 2. Number Installed

Sequence 3. Number Required for Dispatch
Numbers 4. Remarks or Exceptions

34 NAVIGATION

8 Weather Radar/ D - 0 As required by regulations.

Thunderstorm Detection
Equipment

Figure 34-30. MMEL - Weather Radar - Thunderstorm Detection

FOR TRAINING PURPOSES ONLY 34-47

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34 NAVIGATION

PAGE INTENTIONALLY LEFT BLANK

34-48 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-00-00 STORMSCOPE NOTES

GENERAL
The Stormscope (WX-500) is a passive sensor
that uses a receive antenna to detect the
bearing and range of electromagnetic signals
(lightning).

34 NAVIGATION
SYSTEM DESCRIPTION
Stormscope (WX-500)
The Stormscope (WX-500) System maps
electromagnetic signals (lightning) 360° around
the aircraft and to a maximum radius of 200
nautical miles.

When selected in the horizontal situation

indicator (HSI) overlay, the partial compass
HSI format displays information from the
lightning sensor system.

There are two components of the lightning

display on the HSI:

•• Mode/fault annunciations/Strike Rate.

•• Lightning cell strike data.
Mode/fault and strike rate annunciations are
placed outside of the HSI Display area while
lightning strike signals are placed inside the
HSI. LX CLR in the HSI overlay clears all
strike signals that have been received.

The Stormscope system consists of a receiver

installed in the rear avionics rack and a
top mounted antenna installed at FS400. A
receiver inhibit line is interfaced to the aircraft
audio control panel ensuring that erroneous
strike signals are not displayed when a radio
transmitter is keyed.

28VDC power is protected by a 3 Amp

STORMSCOPE (M4) (Right Bus) circuit breaker
located on the avionics circuit breaker panel.

FOR TRAINING PURPOSES ONLY 34-49

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

Figure 34-31. Transmitter/Receiver Figure 34-32. Antenna

DH LAMP DH BUG

FAILURE ALTITUDE
FLAG SCALE

SELF TEST DH KNOB

SWITCH

Figure 34-33. KNI 415 Indicator

34-50 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-00-00 RADIO The primary and valid outputs are supplied to

the radio altimeter indicator. The auxiliary and
ALTIMETER valid outputs are supplied to the flight guidance
computers and the enhanced ground proximity
warning system (EGPWS).
GENERAL
The radio altimeter (RA) System calculates the Antennas
low level Above Ground Level (AGL) altitude
for indication and for use by other aircraft Refer to Figure 34-32. Antenna.

34 NAVIGATION
systems.
The RA transmit and receive antennas are
The RA system supplies the reference for located in line and spaced 20 in. (508 mm) apart
decision height (DH) calculations during the on the bottom of the center fuselage section.
approach phase of a flight.
An aluminum foil gasket with an elastomeric
sealant electrically bonds each antenna to the
SYSTEM DESCRIPTION aircraft structure.
The radio altimeter (RA) system has the
The antennas operate in the RA frequency
following units:
spectrum of 4200 through 4400 MHz. The
antennas are linearly polarized and have a
•• Transmitter/receiver (Figure 34-31).
beam width coverage of more than 45° in the
•• T r a n s m i t a n d r e c e i v e a n t e n n a s roll plane and more than 40° in the pitch plane.
Figure 34-32).
The antennas are directional and have markings
•• KNI 415 Indicator (Figure 34-33).
that denote direction of flight and therefore
must be installed correctly.
The RA system transmits a signal to the ground
and it then receives it back to calculate the
above ground level (AGL) altitude. CAUTION
On aircraft with two radio altimeter systems Ensure that the associated radio
the two radio altimeter (RA1, RA2) systems altimeter radio circuit breaker is
function independently. opened before carrying out removal
or installation of the antennas.
The RAs continuously transmit a frequency
modulated carrier wave signal through the RA information is displayed on the pilot and
transmit antenna that reflects off the earth’s co-pilot PFD RA (radio altimeter) altitude is in
surface and is received through the receive the form of a digital readout on the lower part
antenna. The time difference between the of the ADI. Digits are green unless DH has been
transmit frequency and the received frequency reached in which case it will change to amber.
is used to calculate the distance to the ground.
The r a da r /r a dio a ltime te r is c a pa b l e o f
The received signal is mixed with the performing a self-test. Once invoked, the RA
transmitted signal to make an intermediate digital readout of current AGL will change,
frequency (IF) that is proportional to the time (100 feet if a Honeywell system, 50 feet if it is
that it takes the signal to travel to the ground an Allied Signal System). Releasing the button
and back. re-establishes the correct AGL information.

The RA transmitter/receivers give continuous Once over 2500 ft AGL the RA will not display
AGL altitude data from 0 ft to 2500 ft (762 m). an altitude.

FOR TRAINING PURPOSES ONLY 34-51

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

GA DH

FAST

SLOW

DH RAD ALT
ATT DIM
DH
SET/
TEST

Figure 34-34. ADI

34-52 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

OPERATION NOTES
Operation of the radio altimeter system is fully
automatic. In the processor section of the R/T
unit, the detected video pulse from the receiver
unit is compared to the system sync pulse from
the transmitter. The time difference between
these pulses is determined and changed to DC
analog voltages (primary and auxiliary outputs)
in proportion to the aircraft altitude.

34 NAVIGATION
Reflection by the ground sends remnant of
the signal skyward where it is picked up by
the systems receive antenna. The signal is
processed and altitude is determined based on
the time delay between signal transmission and
reception.

RA information is displayed on the KNI 415

indicator or on the ADI in the form of a digital
readout in the lower right side.

The decision height is set by the DH knob

and when reached, the amber DH lamp will
illuminate.

T he radar/radi o al t i met er i s capable of

performing a self-test. Once invoked, the RA
readout of altitude will then indicate 100 feet.
Releasing the button reestablishes correct
altitude information.

Once over 2500 ft AGL the RA will not display

an altitude.

FOR TRAINING PURPOSES ONLY 34-53

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

EXTERNAL
EGPWS FAULT
HONEYWELL INTERNATIONAL INC COMPUTER
REDMOND, WA
OK
COMPUTER
Honeywell FAIL

PUSH
TO
EJECT
PRESS TO
SELF TEST
34 NAVIGATION

IN
PROG

HEADPHONES CARD
CHNG

PC CARD TOP
XFER
COMP

XFER
FAIL

P1

Figure 34-35. Enhanced Ground Proximity Warning System

Figure 34-36. GNS 430

34-54 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-00-00 ENHANCED •• Mode 4: Too close to terrain.

GROUND PROXIMITY •• Mode 5: Excessive deviation below

glideslope.
WARNING SYSTEM •• Mode 6: Advisory callouts.
(EGPWS) - OPTIONAL
SYSTEM DESCRIPTION
INTRODUCTION The major feature of the EGPWS is the global
terrain and obstacle awareness alerting and

34 NAVIGATION
The enhanced ground proximity warning
display functions. This feature locates the
system (Figure 34-35) gives an aural and visual
aircraft geographic position, aircraft altitude and
indication of possible controlled flight into
compares it with the global terrain and obstacle
terrain (CFIT).
database to predict potential conflicts between
the aircraft flight path and the terrain. The feature
GENERAL provides a video display of the conflicting terrain
or obstacle by use of the following:
The enhanced ground proximity warning
system (EGPWS) is designed to monitor the •• A three dimensional topographic map
aircraft relationship to ground and obstacles. database.
The EGPWS is a combination of the ground
•• GPS Location: Lat, Long, Alt, direction
proximity warning system (GPWS) and the
and speed.
terrain awareness warning system (TAWS).
When the aircraft comes within 2500 ft above •• ADC: Aircraft barometric altitude,
ground level (AGL), the radio altimeter system airspeed and vertical speed.
triggers the EGPWS.
•• RA: AGL.
The EGPWS warns by voice, radar like display A multi-color graphic display shows the aircraft
and visual annunciations when it computes an in relation to conflicting terrain or obstacles.
unsafe situation developing. The EGPWS is This display is shown on the GNS 430 (or 530)
built around an enhanced ground proximity (Figure 34-36).
warning computer that serves as the central
processing device. The system is activated
when 28VDC is applied to the aircraft bus. The
Runway Database
EGPWS provides the TAWS functions plus the The EGPWS runway database consists of data
six GPWS modes. records for all airport runways offered for the
coverage provided by the Terrain Database. All
The TAWS enhancements are as follows: hard surface runways in the world 3500 feet or
greater in length are supported. The database
•• A Global Terrain and Obstacle database. provides the means of accessing the records of
runways closest to the current aircraft position.
•• A G l o b a l T e r r a i n a n d O b s t a c l e
awareness alerting and display.
The EGPWS six basic modes of operation are
as follows:

•• Mode 1: Excessive descent rate.

•• Mode 2: Excessive terrain closure rate.
•• Mode 3: Sink after takeoff.

FOR TRAINING PURPOSES ONLY 34-55

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

3000

“SINKRATE SINKRATE” 2500

Radio Altitude (FEET)

34 NAVIGATION

“PULL UP” 2000

“SINKRATE”
“SINKRATE” 1500

1000
“PULLUP” “PULLUP”
500

0 2000 4000 6000 8000 10000

Descent Rate (FEET/MINUTE)

Figure 34-37. Mode 1 - Excessive Descent Rate

3000

2500

“TERRAIN TERRAIN”
Radio Altitude (FEET)

2000
“TERRAIN TERRAIN” Speed Expansion

1500

1000
“PULL UP”
“PULL UP”
500

0 2000 4000 6000 8000 10000

Terrain Closure Rate (FEET/MINUTE)

Figure 34-38. Mode 2A/2B - Terrain Closure Rate

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 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

OPERATION NOTES
Mode 1 - Excessive Descent Rate
Refer to Figure 34-37. Mode 1 - Excessive
Descent Rate.

Mode 1 provides audio and visual alerts

for excessive descent rates into terrain. The
enhanced ground proximity warning.

34 NAVIGATION
Computer calculates the possible flight into
terrain from the radio altitude (RA) and air data
unit (ADC) barometric decent rate data.

When the EGPWS caution alert envelope

is penetrated, the message “SINKRATE”
annunciates and an EGPWS alert light comes
on. Continuing the excessive descent rate
into the warning alert envelope, results in a
“PULL-UP” annunciation.

Mode 2A/2B - Terrain Closure

Rate
Refer to Figure 34-38. Mode 2A/2B - Terrain
Closure Rate.

Mode 2 provides audio and visual alerts for

dangerously high terrain closure rates and
calculates two sub-modes, referred to as Modes
2A and 2B.

2A is active when the flaps are not in the

landing position and the aircraft is not on an
ILS approach within ± 2 dots of glideslope
center.

2B is active when the flaps are in the landing

position or while on an ILS approach within ±
2 dots of glideslope deviation.

When the caution alert envelope is penetrated,

the message “TERRAIN, TERRAIN”
annunciates and alert lights come on.

Continuing into the warning alert envelope

causes a “PULL-UP” annunciation.

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 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

2500

2000

Radio Altitude (FEET)

“DON’T SINK”
1500

1000

“DON’T SINK”
500

0 50 100 150 200 250 300

Altitude Loss (FEET)

Figure 34-39. Mode 3 - Descent After Take-Off

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 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Mode 3 - Descent After Take-Off NOTES

Refer to Figure 34-39. Mode 3 - Descent After
Take-Off.

Mode 3 provides audio and visual alerts

for excessive altitude loss after takeoff, or
after a go-around from below 245 feet AGL,
when flaps and gear are not in the landing
configuration. A barometric altitude decrease of

34 NAVIGATION
approximately 10% of the above ground level
(AGL) altitude starts the Mode 3 indication.

Penetrating the Mode 3 alert envelope causes

the message “DON’T SINK, DON’T SINK”
and illumination of the alert lights.

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 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

“TOO LOW, TERRAIN”

AIRCRAFT SLOWED TO
LESS THAN 190 KTS
34 NAVIGATION

“TOO LOW, TERRAIN”

“TOO LOW, GEAR” RUNWAY

“TOO LOW, TERRAIN”

AIRCRAFT SLOWED TO
LESS THAN 159 KTS

“TOO LOW, TERRAIN”

“TOO LOW, FLAPS” RUNWAY

3000
“TOO LOW, TERRAIN” UNSAFE TERRAIN CLEARANCE
Minimum Terrain Clearance (FEET)

GEAR UP, FLAPS UP

1500 FPM CLIMB RATE
2000 TAKE-OFF OVER FLAP TERRAIN
OR WATER

1000
Speed Expansion
(>250 KTS)
“TOO LOW, TERRAIN” WARNING AREA
(<190 KTS)
0

0 400 800 1200 1600 2000 2400 2800

Radio Altitude (FEET)

Figure 34-40. Mode 4A/4B/4C - Aural Alerts

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 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Mode 4A/4B/4C - Unsafe Terrain NOTES

Clearance
Refer to Figure 34-40. Mode 4A/4B/4C - Aural
Alerts.

Mode 4 provides audio and visual alerts for

unsafe terrain clearance with respect to phase
of flight, height above ground, and speed.

34 NAVIGATION
Three sub-modes, 4A, 4B, and 4C are defined:

4A is active during cruise and approach with

landing gear up. The aural alerts are “TOO
LOW TERRAIN” or “TOO LOW GEAR”
depending on airspeed.

4B is active during cruise and approach with

landing gear down and flaps up. The aural
alerts are “TOO LOW TERRAIN” or “TOO
LOW FLAPS” depending on airspeed.

4C is active during landing when either gear or

flaps are not in the landing configuration. The
aural alert is “TOO LOW TERRAIN”.

EGPWS alert lights come on during all these

alerts.

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 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

1000
MODE 5 BELOW GLIDESLOPE ALERT
GEAR DOWN
Radio Altitude (FEET)
34 NAVIGATION

SOFT ALERT AREA

500

300
HARD ALERT AREA
100

0 1 2 3 4

Glideslope Deviation (DOTS FLY UP)

Figure 34-41. Mode 5 - Descent Below Glideslope

“BANK ANGLE!”
“BANK ANGLE!”

Figure 34-42. Excessive Bank Angle Callout

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 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Mode 5 - Descent Below Excessive Bank Angle Callout

Glideslope Refer to Figure 34-42. Excessive Bank Angle
Callout.
Refer to Figure 34-41. Mode 5 - Descent Below
Glideslope.
The bank angle callout feature provides callout
annunciation for excessive bank angles based
Mode 5 provides audio and visual alerts for
on altitude and bank angle limits defined
excessive glideslope deviation when the aircraft
by aircraft type. It is intended to enhance
descends below the glideslope beam on a front-
situational awareness during intentional or
course ILS approach. Two levels of alerting are

34 NAVIGATION
unintentional maneuvering, and for protection
provided:
against wing or engine strikes when close to
the runway.
If the aircraft is below 1000 feet AGL
and gets to or exceeds 1.3 dots glideslope
When the bank angle limit is reached, the aural
deviation (fly-up), a “soft” (reduced volume)
callout “BANK ANGLE, BANK ANGLE”
“GLIDESLOPE” annunciates.
is given. Follow-on aural messages are only
provided when the aircraft roll angle increases
Exceeding 2 dots below 300 feet AGL provides
an additional 20% from the previous callout.
a “hard” (full volume) “GLIDESLOPE”
annunciation.
Bank angle callouts are enabled by the
installation configuration.
Mode 6 - Advisory Callouts
The EGPWC is programmed to annunciate
Mode 6 advisory callouts based on menu
selectable options. The menu selected
advisory callouts are defined and enabled
in the installation configuration. If altitude
callouts are not enabled, only (DH based)
“MINIMUMS” callouts will be provided.
Only aural callouts are provided for Mode 6.
EGPWS alert lights do NOT come on for Mode
6 callouts. The following table identifies the
Mode 6 callouts that are typically programmed.

Callout Description
At descent below
“MINIMUMS-MINIMUMS”
minimums setting (DH)

“FIVE-HUNDRED” At descent below 500 feet

“FIFTY” At descent below 50 feet

“FORTY” At descent below 40 feet

“THIRTY” At descent below 30 feet

“TWENTY” At descent below 20 feet

“TEN” At descent below 10 feet

FOR TRAINING PURPOSES ONLY 34-63

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

1/2 Runway Length

Envelope Bias Factor
15 NM
12 NM

4 NM
34 NAVIGATION

Figure 34-43. Terrain Clearance Floor

“SINK RATE, SINK RATE”

(Red) “PULL UP”
“TERRAIN, TERRAIN”
Modes 1-4, TA PULL “OBSTACLE, OBSTACLE”
“CAUTION TERRAIN”
All caution and
warning alerts UP “CAUTION OBSTACLE”
“TOO LOW TERRAIN”
“TOO LOW GEAR”
Push to Test “TOO LOW FLAPS”
“DON’T SINK”

(Amber)

Mode 5 Only BELOW

“GLIDESLOPE”
GS
Push to Cancel

Figure 34-44. Pull UP and BELOW G/S Annunciator Switches

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 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Terrain Clearance Floor Operational Test of the EGPWS

Refer to Figure 34-43. Terrain Clearance Floor. The EGPWC supports a manually operated
Self-Test sequence for indicating the
The terrain clearance floor (TCF) alerting operational status of the system. The Self-Test
function adds an additional element of protection aurally annunciates the EGPWS status and
to the standard ground proximity warning activates outputs (lights and displays) for visual
system. It creates an increasing terrain clearance verification. The EGPWC Self-Test includes
envelope around the airport runway to provide detailed configuration and status information
CFIT protection against situations where Mode for the EGPWC and aircraft installation,

34 NAVIGATION
4 provides limited or no protection. TCF alerts maintenance information detailing the cause
are based on current aircraft location, destination of detected faults, and historical information
runway center point position, and AGL. TCF is about faults and alerts that occurred during
active during takeoff, cruise, and final approach. previous flights. The self-test function can only
TCF complements the existing Mode 4 protection be accessed while on the ground.
by providing an alert based on insufficient terrain
clearance even when in landing configuration. There are two Level 1 tests, a long and a short.
The short test is accessed by pressing and
When an aircraft penetrates the TCF alert releasing the PULL UP switch-light. The long
envelope, the aural message “TOO LOW test is accessed by pressing and holding the
TERRAIN” will occur. PULL UP switch-light until the voice alert is
heard.
PULL UP and BELOW G/S To advance to the next level test the PULL
Annunciator Switches UP switch-light must be pressed within 3
seconds of the completion of a test. Pressing
Refer to Figure 34-44. Pull UP and BELOW
the GPWS test switch for more than 2 seconds
G/S Annunciator Switches.
during Level 2 through 5 will bypass the test
information and will immediately jump to the
PULL UP and BELOW G/S annunciator
end of that level (“PRESS TO CONTINUE”
switches are usually attached to the instrument
voice).
panel.
EGPWS Self-Test is divided into six levels:
The switches give the operations that follow:
•• Level 1 - Identifies the status of each of
•• PULL UP indication.
the major functions of the EGPWS. This
•• GPWS TEST selection. is the normal preflight test performed
by the flight crew.
•• BELOW G/S indication.
•• Level 2 - Identifies all failures currently
•• B E L O W G / S a u r a l i n d i c a t i o n
within the system. Level 2 is accessed
cancellation.
by maintenance personnel to help
The EGPWS lamp format provides EGPWS resolve INOP conditions.
warning (red) and caution (amber) alert lamp
•• Level 3 - Identifies the configuration
drive logic such that:
status of the Warning Computer and the
installation.
•• Only a Mode 5 “Glideslope” alert will
activate the caution lamp output (amber). •• Level 4 - Identifies faults that have
occurred during past flights. This
•• All other alerts (except Mode 6 callouts)
information can be used to resolve
will activate the warning lamp output
system problems reported by the flight
(red).
crews.

FOR TRAINING PURPOSES ONLY 34-65

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

•• Level 5 - Identifies alerts that have NOTES

occurred during past flights. This
information can be used to resolve
system problems reported by the flight
crews.
•• Level 6 - Identifies state changes in the
input discretes. This information can
be used to verify the installation and
proper function of all discrete inputs.
34 NAVIGATION

Loading of the EGPWS Terrain

Database
The EGPWS databases are updated as new data
becomes available. The terrain, obstacle, and
runway databases are typically combined and
loaded into the EGPWS as one database.

For each new database, Honeywell will provide

notification via a Service Information Letter
(SIL). The SIL is sent to all registered EGPWS
customers, or they will be available for Internet
download (www.egpws.com).

The installed databases can be determined in

the Level 3 self-test. A database is loaded into
the EGPWC via the PCMCIA card slot in the
front panel.

The database may be loaded either while

the EGPWC is on the aircraft or on a bench
provided the appropriate power is applied.

Each Honeywell database Service Information

Letter includes reference to the procedure for
loading the database in the EGPWC. Refer to
the SIL for this information.

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34 NAVIGATION
PAGE INTENTIONALLY LEFT BLANK

FOR TRAINING PURPOSES ONLY 34-67

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

COLLISION
AREA
WARNING
34 NAVIGATION

AREA

CAUTION AREA RA
20-30
SECONDS

TA
35-45
SECONDS

RA

TA

NOTE: For every aircraft within detection range,

similar Caution and Warning area patterns exist.

Figure 34-45. TCAS Intruder Caution and Warning Areas

34-68 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-00-00 TRAFFIC SYSTEM DESCRIPTION

ALERT AND COLLISION The system transmits interrogations at a
AVOIDANCE SYSTEM frequency of 1030 MHz and receives replies
at a frequency of 1090 MHz. The maximum
(TCAS II) - OPTIONAL surveillance range of the system is 14 nautical
miles with display of detected aircraft of up to
12 nautical miles. TCAS traffic monitoring is
INTRODUCTION continuous and the system can detect and track
up to 30 aircraft simultaneously.

34 NAVIGATION
The traffic collision avoidance system gives an
aural and visual indication when the aircraft
position and flight paths relative to an intruder COMPONENT DETAILS
aircraft may cause a potentially dangerous
condition. It provides the flight crew with 2 The TCAS system includes the components
types of advisories: that follow:

•• TA (traffic advisory): •• A control unit.

Makes flight crew aware of other traffic •• A transmitter/receiver unit.
in the area.
•• Two vertical speed/traffic advisory/
resolution advisory (VSI/TRA)
•• RA (resolution advisory):
indicators.
Provides direction in avoiding a
•• Two antennas.
possible collision.

GENERAL
Refer to Figure 34-45. TCAS Intruder Caution
and Warning Areas.

The traffic alert and collision avoidance system

(TCAS) operates in conjunction with the ATC
Mode “S” transponder system to identify
potential collision threats and to provide visual
and aural advisories for collision avoidance.

FOR TRAINING PURPOSES ONLY 34-69

34-70 34 NAVIGATION

M BUTTON (Mode) A/B BUTTON (Above/Below)

- A VSI with a part time threat only traffic display and a - Push the A/B button to select the altitude volume,
VSI with a full time traffic display. Successive pushes relative to your own aircraft, for which TCAS shows
of the button cycles the display mode. Other Traffic. Successive pushes of the button cycles
- In “the part time threat only, traffic display” mode, the through the available modes. Other Traffic outside of
TVI-920D normally shows a VSI. When TCAS the selected altitude volume does not show on the
detects TA or RA threat traffic, the display automati- vertical speed/TCAS indicator. Proximate, TA and RA
cally changes to a VSI with a TCAS traffic display. traffic are not affected and always show on the display
When TA and RA traffic move out of the selected regardless of the selected altitude volume. (Except RA
TCAS diaplay range and altitude mode or when the traffic does not show on the display in the TA-ONLY
traffic changes to Proximate or Other traffic, the mode). The selected mode is annunciated on the
display changes back to a VSI only display in TCAS traffic display. The available selections are:

TWIN OTTER SERIES

approximately 5 seconds.
- In the full time traffic display mode, the TVI-920D NORMAL - shows Other Traffic from 2700 ft above to
shows the VSI and traffic display at all times. 2700 ft below your own aircraft.
Proximate and Other Traffic within the selected range
and altitude modes also show on the traffic display. ABOVE - shows Other Traffic from 9900 ft above to
FOR TRAINING PURPOSES ONLY

2700 ft below your own aircraft.

TEST BUTTON BELOW - shows Other Traffic from 2700 ft above to

- Push the TEST button to turn on the self-test function MODE A/B RNG 9900 ft below your own aircraft.
of the TCAS-94 system. The TCAS traffic and RA
displays show test pattern traffic symbols, a red and
green resolution advisory, and the TEST annunciator.
FUNCTION KNOB
- The test routine takes approximately 10 seconds to
OFF turns off power to the TCAS receiver/transmitter. For

MAINTENANCE TRAINING MANUAL

complete. After successful completion of the test, the
those units that do not have an OFF position, the aircraft
system returns to the set operating modes and
avionics master switch turns the power on or off to the
aurally annunciates “TCAS SYSTEM TEST OK” on AUTO
the aircraft flight compartment audio system. If a TA TCAS TCAS receiver/transmitter. An annunciation “TCAS OFF”, is
STDBY displayed on the vertical speed/TCAS indicator in this mode.
TCAS systems failure occurs, the annunciator ONLY
“TCAS” or “TCAS FAIL” shows on the appropriate OFF STBY sets the TCAS receiver/transmitter to the standby
TCAS display and the audio system aurally annunci- TEST mode. TCAS does not show traffic on the display; does
ates “TCAS SYSTEM TEST FAIL”.
not give resolution advisories; and does not replay to
other aircraft TCAS interrogations in this mode. The
annunciator “TCAS OFF” shows on the vertical
speed/TCAS indicator in this mode.

RNG BUTTON (RANGE) AUTO is the normal mode of operation. TCAS shows traffic
- Push the R button to set the range of the TCAS on the vertical speed/TCAS indicator and gives traffic alerts
traffic display. An annunciator on the display shows and resolution advisories, as appropriate, in this mode.
the set range. Successive pushes of the button
cycles through the available ranges. The ranges TA ONLY sets the receiver/transmitter to the traffic
available are 6 and 12 nm or 3, 5, 10, 20 and 40 nm advisory (TA) only mode. TCAS does not give resolution
depending on the version installed. advisories (RA) on the indicator or show RA traffic on the
traffic display in this mode. Also, an annunciator (TA
ONLY, ONLY TA, or RA OFF) shows on the TCAS
display(s) to identify this mode.

Figure 34-46. TCAS Control Unit (Collins - 92T)

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Control Unit NOTES

Figure 34-46. TCAS Control Unit (Collins
- 92T).

There are three pushbutton switches on the

top half of the control unit face identified
respectively as MODE, A/B, and RNG.

The MODE pushbutton switch enables the

34 NAVIGATION
selection of either the VSI (pop-up) mode or
the full-time traffic mode on the VSI /TRA
indicators. Each push of the switch toggles the
selection to the next mode.

The A/B pushbutton switch enables the

selection of traffic altitude range relative
to the aircraft to ABV (above), normal (no
annunciation), or BLW (below). Each push
of the switch toggles the selection to the next
altitude range.

The RNG (range) pushbutton switch enables

the selection of the forward range on the VSI/
TRA indicators.

The bottom half of the control unit face features

a TEST pushbutton switch and a mode selector
switch with detented positions labeled OFF,
STBY, AUTO, and TA ONLY.

The TEST pushbutton switch enables the

system to verify proper operation of aural
advisories, traffic and resolution advisory
displays.

The OFF position, the TCAS system power is

off.

The STBY (standby) position, power is

supplied to the TCAS system and the system
is ready for operation.

The AUTO position is the normal operating

mode. TCAS tracks all aircraft in the
surveillance area and provides traffic and
resolution advisories.

The TA ONLY position is the same as the

AUTO mode except that there are no resolution
advisories.

FOR TRAINING PURPOSES ONLY 34-71

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

TTR PASS INDICATOR

LAMP

Collins

PASS
TTR
TTR FAIL INDICATOR FAIL
LAMP X PNDR
UPPER ANT
LOWER ANT
RAD ALT SYSTEM INPUT
HDNG FAIL INDICATOR
R/A LAMPS
T/A
TTR 920
TEST

TEST PUSHBUTTON
CARRY HANDLE
SWITCH

Figure 34-47. TCAS Transmitter/Receiver Unit

34-72 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Transmitter/Receiver Unit NOTES

Refer to Figure 34-47. TCAS Transmitter/
Receiver Unit.

The unit contains circuits to transmit and

receive signals, process received signals,
compute data from received signals and
interface with other components of the TCAS
system. Antenna switching is achieved through

34 NAVIGATION
circuits within the transmitter/receiver unit
to connect transmitter outputs to either the
top or bottom antenna. The front face of the
transmitter/receiver unit consists of a test
pushbutton switch for system self-test and
nine indicator lamps. A system self-test may
be initiated by pressing the TEST pushbutton
switch. If a failure is detected in a circuit(s), the
TTR FAIL and the applicable fail indicator(s)
come on.

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 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

LIGHT SENSOR

TRAFFIC DATA
(SAME COLOUR AS
ASSOCIATED
SYMBOL; TWO DIGIT
PROXIMITY TRAFFIC 6 NM FIGURE REPRESENTS
(PT)SYMBOL (CYAN, ABV RELATIVE ALTITUDE TO
SOLID DIAMOND) OWN AIRCRAFT
IN HUNDREDS OF FEET; +
+10 SIGN INDICATES AIRCRAFT
IS ABOVE AND SIGN
FORWARD RANGE
INDICATES AIRCRAFT
34 NAVIGATION

ARC
IS BELOW;
VERTICAL SPEED DIRECTION
+20 ARROW POINTING UP
OTHER TRAFFIC INDICATES ASCENT AND
(OT) SYMBOL VERTICAL SPEED DIRECTION
(CYAN; OPEN ARROW POINTING
DIAMOND) DOWN INDICATES
DESCENT > 500 FPM;
VERTICAL SPEED DIRECTION
ARROW NOT PRESENT
INDICATES ASCENT OR
DESCENT < 500 FPM OR
LEVEL AIRCRAFT

AIRCRAFT RANGE RING (WHITE;

SYMBOL (WHITE) 12 CLOCK POSITION
MARKERS INDICATE
2 NM RADIUS AROUND AIR
CRAFT; DISPLAY
OF RING IS SMALLER
WHEN FORWARD RANGE IS
SELECTED TO 12 NM)

Figure 34-48. VSI/TRA Indicator Display Mode (1 of 3)

‘ONLY TA’ ANNUN

CIATION (WHITE
LETTERS IN
WHITE BORDER)
TRAFFIC ADVISORY
APPEARS WHEN
(TA) SYMBOL
MODE SELECTOR
(YELLOW; SOLID
SWITCH ON
CIRCLE)
TCAS CONTROL
UNIT IS SET
TO TA ONLY OR
APPEARS
AUTOMATICALLY
WHEN AIRCRAFT IS
< 500 FEET AGL.

Figure 34-49. VSI/TRA Indicator Display Mode (2 of 3)

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 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Vertical Speed/Traffic Advisory/ Each indicator consists of a color liquid crystal

display panel and a bezel-mounted light sensor.
Resolution Advisory (VSI/TRA) Each indicator display is continuously updated
Indicator by microprocessors within the transmitter/
receiver unit. The light sensor is a photo
Refer to:
detector which senses ambient light and
automatically adjusts the display brightness.
•• Figure 34-48. VSI/TRA Indicator
The indicators display traffic information from
Display Mode (1 of 3).
the receiver/transmitter, as well as vertical
•• Figure 34-49. VSI/TRA Indicator speed information from the air data system.

34 NAVIGATION
Display Mode (2 of 3).
•• Figure 34-50. VSI/TRA Indicator
Display Mode (3 of 3).

Two VSI/TRA indicators, one each for the pilot

and co-pilot, replace the conventional IVSI
instruments on the flight instrument panel.

RECOMMENDED
VERTICAL SPEED
(GREEN ARC)

VERTICAL
SPEED
POINTER

RESTRICTED RESOLUTION
VERTICAL ADVISORY (RA)
SPEED RANGE TRAFFIC SYMBOL
(RED ARC) (RED; SOLID SQUARE

Figure 34-50. VSI/TRA Indicator Display Mode (3 of 3)

FOR TRAINING PURPOSES ONLY 34-75

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

BLUE(J3)
RED(J4)
34 NAVIGATION

YELLOW(J1)

BLACK(J2)
BOTTOM VIEW

SIDE VIEW

SCREW
(TYPICAL FOUR
PLACES)
DO NOT PAINT

FWD A/C CL

TOP VIEW

NOTE
Antenna coaxial connectors
are identified with colour
band as shown.

Figure 34-51. TCAS Antennas

34-76 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Antennas A Mode S interrogation is sent from the selected

Mode S transponder through an ARINC 429
Refer to Figure 34-51. TCAS Antennas.
data bus to the TCAS transmitter/receiver unit.
The transmitter/receiver unit processes the
There are two separate but identical antennas in
interrogation data and transmits it through the
the TCAS system, one mounted on the fuselage
selected directional antenna to the addressed
top and the other mounted on the fuselage
transponder-equipped aircraft. The aircraft reply
bottom. The TCAS antennas are directional,
is received by the TCAS transmitter/receiver
four element, vertically polarized, monopole
unit. Reply information is also sent through the
antennas that can transmit interrogations in
ARINC 429 data bus to the Mode S transponder.

34 NAVIGATION
four directions and receive replies from all
Control data from the TCAS control unit is
directions.
fed along an ARINC 429 data bus to both the
Mode S transponders and the TCAS transmitter/
OPERATION receiver unit. The computer circuits in the
transmitter/receiver unit utilize interrogation-
The traffic alert and collision avoidance system to-reply time to determine range and speed, and
continuously monitors the traffic within the Mode C (altitude) information to determine the
system surveillance range. When the mode vertical speed of the addressed aircraft.
selector switch on the TCAS control unit is set
to an appropriate operating mode and the display The TCAS directional antennas permit the
pushbutton switches set to an appropriate VSI/ system to determine the bearing to the addressed
TRA display mode, the system analyzes traffic aircraft. From this information, the system
information and issues traffic or resolution will compute the speed and flight path of the
advisories to the flight crew should the system addressed aircraft to determine the closest point
detect traffic penetration into the protected of approach (CPA). The CPA is the minimum
airspace around the aircraft. These advisories vertical separation between the TCAS-equipped
include a visual display of traffic proximity aircraft and the addressed aircraft.
as well as synthesized voice commands to the
flight crew of what corrective action to take The system will issue a traffic or resolution
to achieve a safe vertical separation between advisory to the flight crew, depending on the
the aircraft and the intruding traffic. When the CPA of the addressed aircraft.
intruding traffic is no longer a hazard to the
aircraft, a “clear of conflict” voice message is The advisory consists of a synthesized voice
issued to the flight crew. command which is fed through the audio
integrating system to the cockpit speakers and
flight crew headsets, and a visual advisory,
OPERATING MODES which shows on the pilot and co-pilot VSI/
TRA indicators.
STBY
When the mode selector switch on the TCAS The TCAS transmitter/receiver unit provides
control unit is set to STBY (standby), all TCAS traffic and resolution advisory data through the
operation is disabled, including interrogations. ARINC 429 buses to the VSI /TRA indicators.
This mode is annunciated as “TCAS OFF” in The data is then converted to display format in
the top right corner of the VSI/TRA display. the indicators.

AUTO
When the mode selector switch on the TCAS
contr ol unit i s set t o AUTO, al l TC AS
operations are automatic and continuous.

FOR TRAINING PURPOSES ONLY 34-77

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

Systems & Nos de 1. 2. Number installed - Nombre d’articles installés

Sequence système Item - Article
Numbers de série 3. Number required for dispatch - Nombre d’articles à expédier

4. Remarks or Exceptions - Remarques ou exceptions

34 NAVIGATION

3 Vertical Speed Indicators C 2 1 For single pilot operations the pilot flying
side VSI must be operative.

C 2 0 Both may be inoperative for day VFR.

4 Transponder/ Altitude D - - As required by regulations.

Reporting

Figure 34-52. MMEL - VSI and Transponder/Altitude Reporting

34-78 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

SYMBOLOGY ON and no TA or RA in process). The predicted

flightpaths of proximate and other traffic do
Colors not penetrate the collision area of the TCAS
aircraft.
Red - Represents an immediate threat to a
TCAS-equipped aircraft. Prompt action is
Intruders are prioritized and displayed based
required to avoid the intruder. This color will
on which are predicted to pass closest to the
only be used in conjunction with a TCAS
TCAS aircraft.
resolution advisory (RA).

SELF-TEST

34 NAVIGATION
Amber - Represents a moderate threat to a
TCAS-equipped aircraft. A visual search is
recommended to prepare for intruder avoidance. When the TEST pushbutton switch on the
Amber is used only in conjunction with a TCAS TCAS control unit or the TCAS transmitter/
traffic advisory (TA). receiver unit is pressed, a system self-test
is initiated and is annunciated by the word
Cyan - Represents proximate traffic and other “TEST” shown on the bottom of the VSI/TRA
traffic the TCAS surveillance logic has in its indicator displays. All indicator lamps on the
track file. front panel of the TCAS transmitter/receiver
unit will come on for approximately one second
at the start of the self-test. During the self-test,
Traffic Advisory the TCAS processes internal test signals and
Intruder aircraft entering the caution area, compares the results with the preset values
35-45 seconds from the TCAS II collision area to determine if the system is operating within
are represented as a solid amber circle. This specifications. If a failure is detected during the
type of traffic will result in a TCAS traffic test, the appropriate indicator lamp on the front
advisory (TA). panel of the transmitter/receiver unit will come
on and the word “TCAS” will appear on the top
left corner of the VSI/TRA indicator displays.
Resolution Advisory All failure indications remain on until the
Intruder aircraft entering the warning area, failure is corrected. If the system tests okay, the
20-30 seconds from the TCAS II collision “TTR PASS” indicator lamp on the front panel
area are represented as a solid red square. This of the transmitter/receiver unit will come on
type of traffic will result in a TCAS resolution and the system can resume normal operation.
advisory (RA). At the end of the test sequence, if no faults are
detected, the aural message “TCAS SYSTEM
TEST OK” or, if any faults are detected, the
Proximate Traffic aural message “TCAS SYSTEM TEST FAIL”
Aircraft within 6.5 nautical miles and ±1200 will sound over the cockpit speakers and the
feet vertically are represented as a solid cyan flight headsets. The self-test is inhibited when
diamond. Proximate traffic is shown to improve the aircraft is in flight.
situational awareness in the event of a potential
conflict with higher priority RA or TA aircraft.

Other Traffic
Any transponder-replying traffic not classified
as an intruder or proximate traffic, and
within ±2700 feet vertically and the range of
the display, are represented as hollow cyan
diamonds, (only in view with the traffic switch

FOR TRAINING PURPOSES ONLY 34-79

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-00-00 TRAFFIC Intruder numbers assigned by the TCAS

processor are used to associate intruder
ALERT AND COLLISION parameters with the correct intruders. A pattern
AVOIDANCE SYSTEM II of intruders, as transmitted by the TCAS unit, is
displayed on the traffic overlay, when the TCAS
(TCAS II) (MOD S.O.O. is in test mode and traffic is selected for display.
6219) When a TCAS resolution advisory is displayed,
the TCAS pitch command is displayed on the
GENERAL attitude director indicator (ADI). The red goal
34 NAVIGATION

posts indicate the do-not-fly zone (or zones, if

Provision for TCAS II is made and can be both pitch-up and pitch-down avoidance zones
installed as customer order option 6219. A CAS are displayed). The green box indicates the
67A TCAS II provides both traffic advisories fly-to zone. When the aircraft symbol is inside
(TA) and resolution advisories (RA) to the the red do-not-fly zones, it turns red. If the
flight crew for appropriate vertical avoidance aircraft symbol is inside the green fly-to box, it
maneuvers when a possible collision is turns green. Otherwise, it remains white.
threatened with an intruding aircraft.
Traffic has display priority over weather
(WX) radar and flight plan symbols (Terrain
SYSTEM DESCRIPTION has priority over TCAS). Generated TCAS II
processor audio signals are sent to the audio
The CAS 67A TCAS II includes all the isolation amplifier and then to the unmuted audio
functionality of TCAS I, but also includes pitch control panel line. The TCAS voice messages
targets (avoidance and fly-to zones) and aural are suppressed when the EGPWS sends voice
advisories, known as resolution advisories (RAs). messages to the audio control panel. Traffic
Advisories (TA) and resolution advisories
There are two types of RAs: (RA) from the TCAS II processor trigger pulse
light operation when the landing light switch is
•• Corrective - Pilot action is required. selected to the upper TCAS position.
Corrective RAs instruct the pilot to take
avoidance action such as, “DESCEND, The CAS 67A TCAS II processor unit and
DESCEND”, “CLIMB, CLIMB”, or configuration module (CM 2000) are mounted
“ADJUST VERTICAL SPEED”. in the nose avionics rack. A directional antenna
•• Preventative - Current path and speed mounted on the top of the fuselage at station
must be maintained. Preventative RAs FS110 is used to determine intruder bearing.
warn the pilot not to deviate from the An omni-directional antenna is mounted on the
current path and speed by instructing, bottom of the fuselage at station FS67.4.
“MONITOR VERTICAL SPEED” or
“MAINTAIN VERTICAL SPEED”. During TCAS transmission on the directional or
omni-directional antenna, the TCAS generates
TCAS II systems coordinate the resolution a suppression pulse to inhibit other L-band
advisories between aircraft before issuing installed equipment transmission.
commands to the pilots, so that when one aircraft
is instructed to descent, the other is typically 28 VDC power is protected by a 5 amp TCAS
told to climb. Traffic symbols including intruder (H5) (Left Bus) circuit breaker on the avionics
symbol, data tag, and vertical speed arrow are circuit breaker panel
consistent between the multi-function display
(MFD) and horizontal situation indicator (HSI)
overlay. The number of intruders displayed on
the HSI overlay is limited to 32.

34-80 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

The TCAS II interfaces to the following aircraft NOTES

systems:

•• Modular avionics unit (MAU) (refer

to 31-43-00, Modular Avionics Unit
(MAU) (Mod 6/2049) - General Data).
•• Enhanced Ground Proximity Warning
System (EGPWS) (refer to 34-44-00,
Enhance Ground Proximity Warning

34 NAVIGATION
System (TAWS) (Mod 6/2049) -
General Data).
•• Audio Isolation Amplifier (refer to
23-54-00, Audio System Isolation
Amplifier - General data).
•• Lights (refer to 33-00-00, Lights -
General Data).
•• A suppression pulse to/from the Dual
KXP Transponder system (refer to
34-53-00, KXP2290 Transponder
System (Mod 6/2049) - General Data).
•• A s u p p r e s s i o n p u l s e t o / f r o m t h e
Distance Measuring Equipment (DME)
(refer to 34-54-00, Distance Measuring
Equipment (DME) (Mod 6/2049) -
General Data).

FOR TRAINING PURPOSES ONLY 34-81

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

Systems & Nos de 1. 2. Number installed - Nombre d’articles installés

Sequence système Item - Article
Numbers de série 3. Number required for dispatch - Nombre d’articles à expédier

4. Remarks or Exceptions - Remarques ou exceptions

34 NAVIGATION

25 TCAS B 1 0 (M) If TCAS functionality is required by

operating regulations, may be inoperative
provided system is deactivated and
secured.

C 1 0 (M) If TCAS fun c tionality is not required by

operating regulations, may be inoperative
provided system is deactivated and
secured.

Figure 34-53. MMEL - TCAS

34-82 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

*The following is an abbreviated description of 8. Confirm that “TCAS SYSTEM TEST OK”
the maintenance practices and is intended for is audible after the test. Should a failure be
training purposes only. detected during self test, the audio message
says, “TCAS SYSTEM TEST FAIL”.
For a more detailed description of the practice,
refer to the task in the Viking AMM PSM 1-63-2. 9. Select MASTER switch to OFF.
10. Select POWER SOURCE switch to OFF.
34-00-00 MAINTENANCE 11. Disconnect external power source from
PRACTICES aircraft.

34 NAVIGATION
ADJUSTMENT/TEST Scheduled Maintenance
Requirements
Function Test The TCAS II (CAS 67A) contains BITE (Built
1. Apply external power to aircraft. -In Test Equipment) to enable the operational
health of the unit to be constantly monitored.
2. S e l e c t P O W E R S O U R C E s w i t c h t o
EXTERNAL.
Maintenance of the TCAS II (CAS 67A) is “on
3. Select MASTER switch to ON. condition” only. On condition maintenance is
described as follows:
4. Wait several minutes for the aircraft
electrical and avionics systems to initialize.
•• T h e r e a r e n o p e r i o d i c s e r v i c e
5. Ensure that TCAS (H5) circuit breaker is requirements necessary to maintain
closed. continued airworthiness.
6. Initiate a TCAS self test in the XPDR/ •• No maintenance is required until the
TCAS detail page. equipment does not properly perform
its intended function.
7. Verify the following:
a. A white TEST caption is displayed on
the lower left corner of the PFDs.
b. Resolution Advisory (solid red square)
will appear at 3 o’clock, range of 2 nm,
at 200 feet relative altitude, above (+02)
with no VS arrow.
c. Traffic Advisory (solid yellow circle)
will appear at 9 o’clock, range of 2
miles, at 200 feet relative altitude,
below (-02) with ascending arrow.
d. Proximity traffic (solid white diamond,
or Honeywell - Cyan) will appear at
1 o’clock, range 3.6 miles, at 1000
feet relative altitude, below (−10) with
descending arrow.
e. N o n - T h r e a t t r a f f i c ( o p e n w h i t e
diamond, or Honeywell - Cyan) will
appear at 11 o’clock, range of 3.6 miles,
at 1000 feet relative altitude, above
(+10) with no arrow.

FOR TRAINING PURPOSES ONLY 34-83

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

Figure 34-54. KN 63

34-84 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-51-00 DISTANCE the display of false information, the KDI 572 or

KDI 574 will display dashes and the KN 63 will
MEASURING stay in “search” whenever power is turned on
EQUIPMENT (DME) or momentarily interrupted in frequency Hold
mode. Normal operation is re-established by
switching to N1 or N2 channeling.
INTRODUCTION
The KDI 573 has no controls and merely
The distance measuring equipment (DME) provides a duplicate display of the information
system is used to show the slant range distance shown on the KDI 572.

34 NAVIGATION
between the aircraft and a selected ground
station. The DME system also supplies station The KDI 574 requires an external panel
identification to the audio integrating system. mounted switch for ON/OFF, NAV1, Hold,
NAV2 switching.
GENERAL The KDI 572, KDI 573, and KDI 574
simultaneously display DME range, speed, and
The distance measuring equipment (DME) system
time-to-station. In addition a “1” is displayed in
operates in conjunction with ground stations
N1 mode and a “2” is displayed in N2 mode to
on set frequencies. When the aircraft is in the
indicate the selected channeling source on both
range of a ground station, the DME transceiver
indicators. In Hold mode, either a “1H” or H2 is
operates in the normal mode and transmits an
displayed to indicate the channeling source that
interrogation signal to the ground station. The
is being held. “RNV” will be displayed when the
ground station sends a reply signal. The DME
displayed distance, speed, and time-to-station
transceiver monitors the time elapsed between
are derived from an Area Navigation System.
the interrogation signal and the reply signal, and
calculates the slant range distance to the ground
When the KN 63 is locked to a ground station,
station. DME also calculates Time To Station
range is displayed to the nearest 0.1 nautical mile
(TTS) and groundspeed (GSPD) for display.
from 0 to 99.9 nautical miles and to the nearest
1 nautical mile from 100 to 389 nautical miles.
SYSTEM DESCRIPTION Ground speed is displayed to the nearest knot
from 0 to 999 knots. Time-to-station is displayed
Refer to: to the nearest minute from 0 to 99 minutes. The
indicators also show 99 minutes for any computed
•• Figure 34-54. KN 63. time-to-station greater than 99 minutes. When the
KN 63 is in search mode, dashes are displayed
•• Figure 34-55. KN 63 - Description. instead of range, speed, and time-to-station.
It is recommended that power to the KN 63
be turned on only after engine start-up, as this Both indicators have an automatic dimming
procedure increases the reliability of the solid circuit that adjusts the brightness of the display
state circuitry. to compensate for changes in ambient light level.
Dimming is controlled by a photocell mounted
The rotary switch on the front of the KDI 572 behind the front panel below the display.
has four positions: Off, N1, Hold, and N2. In
the Off position, the master and slave indicators The audio output of the KN 63 can be set
and the remote mounted DME are all turned off. as high as 15 mw into 500 ohms using the
In N1 position, the DME is channeled from the audio level adjustment accessible through a
NAV1 control head. In N2 position, the DME hole in one of the inner covers. It is set for
is channeled from the NAV2 control head. In approximately 2 mw output at the factory. It is
Hold position, the DME is channeled to the last desirable to use the audio to identify the DME
selected NAV1 or NAV2 frequency. To prevent ground stations being received.

FOR TRAINING PURPOSES ONLY 34-85

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

Figure 34-55. KN 63 - Description

34-86 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

The effective range of the KN 63 DME depends NOTES

on many factors; most important being the
altitude of the aircraft. When the aircraft is on
the ground, the KN 63 usually will not receive
DME stations due to line-of-sight signal
limitations. Other contributing factors to the
DME’s effective range are the locations and
altitude of the ground transmitter, transmitter
power output, and the degree of maintenance
of the ground facility. The distance measured

34 NAVIGATION
by the KN 63 is slant-range distance (measured
on a slant from aircraft to ground station) and
should not be confused with actual ground
distance. The difference between ground
distance and slant-range distance is smallest at
low altitude and long range. These differences
may differ considerably when in close proximity
to a VOR/DME facility. However, if the range
is three times the altitude or greater, this error
is negligible. In order to obtain accurate ground
speed and time-to-station, the aircraft must be
tracking directly to or from the station.

When operating dual KN63s, the respective

DMEs will interfere with each other when
the NAV frequencies differ by 5.3 MHz (for
example, 108.00 MHz and 113.3 MHz). This
interference results in premature flags or loss
of “lock-on”. Should this occur, one of the KN
63s should be either turned off or turned to a
different NAV frequency so that the 5.3 MHz
difference is eliminated.

The DME system includes the components that

follows:

•• One DME transceiver.

•• One DME antenna.
•• One display indicator.

FOR TRAINING PURPOSES ONLY 34-87

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

Figure 34-56. Transceiver Figure 34-57. Indicator

Figure 34-58. Antenna

34-88 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Transceiver NOTE
Refer to Figure 34-56. Transceiver. Avoid running other cables or
wire near the antenna cable.
The KN 63 is a remote mounted, 200 channels
DME employing the latest state of the art solid-
state transmitter and a single crystal, digital
OPERATION
integrated circuit technology. All tuning is
It is recommended that power to the KN 63
done electronically, using a single crystal,
be turned on only after engine start-up, as this
digital, frequency synthesizer. Range, speed
procedure increases the reliability of the solid

34 NAVIGATION
and time-to-station are measured digitally.
state circuitry.
The KN 63 can be operated with any DC input
The rotary switch on the front of the KDI 572
from 11 to 33 volts. Power consumption is 17
has four positions: OFF, N1, HOLD, and N2.
watts maximum.
In the OFF position, the master and slave indicators
The KN 63 is designed to operate with the
and the remote mounted DME are all turned off.
panel mounted KDI 572/574 master indicator
In the N1 position, the DME is channeled from
(required) and the KDI 573 slave indicator
the NAV1 control head. In N2 position, the DME
(optional).
is channeled from the NAV2 control head. In the
HOLD position, the DME is channeled to the last
Indicator selected NAV1 or NAV2 frequency.
Refer to Figure 34-57. Indicator.
To prevent the display of false information, the
KDI 572 or KDI 574 will display dashes and the
Both indicators have a gas discharge display
KN 63 will stay in “search” whenever power is
that simultaneously indicates range, speed
turned on or momentarily interrupted in frequency
and time-to-station. An automatic dimming
hold mode. Normal operation is re-established by
circuit adjusts the brightness of the display to
switching to N1 or N2 channeling.
compensate for changes in ambient light level.
The KDI 572/574 master indicator accepts
The KDI 572, KDI 573 and KDI 574
channeling data from either of two external
simultaneously display DME range, speed and
NAV control heads.
time-to-station as shown above. In addition
a “1” is displayed in N1 mode and a “2” is
A rotary switch on the KDI 572 selects N1,
displayed in N2 mode to indicate the selected
HOLD, or N2 channeling and also provides a
channeling source on both indicators. In HOLD
system power switch. Both indicators receive
mode, either a “N1” or “N2” is displayed to
DME range, speed, and time-to-station as
indicate the channeling source that is being
digital serial data from the KN 63. The KDI
held. “RNV” will be displayed when the
573 slave indicator has no mode switch and
displayed distance, speed, and time-to-station
merely provides a duplicate display of the
are derived from an Area Navigation System.
information shown on the KDI 572.
When the aircraft is in the range of a ground
Antenna station, the DME transceiver operates in the
normal mode and transmits an interrogation
Refer to Figure 34-58. Antenna.
signal to the ground station. The ground station
sends a reply signal. The DME transceiver
The DME antenna is a single blade type antenna
monitors the time elapsed between the
mounted on the bottom surface of the fuselage
interrogation signal and the reply signal, and
and in a vertical position. It is mounted just aft
calculates the slant range distance to the ground
of the rear baggage door.
station.

FOR TRAINING PURPOSES ONLY 34-89

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

The range is displayed to the nearest 0.1 three times the altitude or greater, this error is
nautical mile from 0 to 99.9 nautical miles negligible. In order to obtain accurate ground
and to the nearest 1 nautical mile from 100 to speed and time-to-station, the aircraft must be
389 nautical miles. Ground speed is displayed tracking directly to or from the station.
to the nearest knot from 0 to 999 knots.

Time-to-station is displayed to the nearest

minute from 0 to 99 minutes. The indicators
also show 99 minutes for any computed time-
to-station greater than 99 minutes. When the
34 NAVIGATION

KN 63 is in search mode, dashes are displayed

instead of range, speed, and time-to-station.

The effective range of the KN 63 DME

depends on many factors, most important
being the altitude of the aircraft. When the
aircraft is on the ground, the KN 63 usually
will not receive DME stations due to line-of-
sight signal limitations. Other contributing
factors to the DME’s effective range are the
location and altitude of the ground transmitter,
transmitter power output, and the degree
of maintenance of the ground facility. The
distance measured by the KN 63 is slant-
range distance (measured on a slant from
aircraft to ground station) and should not
be confused with actual ground distance.
The difference between ground distance and
slant-range distance is smallest at low altitude
and long range. These differences may differ
considerably when in close proximity to a
VOR/DME facility. However, if the range is

Systems & Nos de 1. 2. Number installed - Nombre d’articles installés

Sequence système Item - Article
Numbers de série 3. Number required for dispatch - Nombre d’articles à expédier

4. Remarks or Exceptions - Remarques ou exceptions

34 NAVIGATION

10 DME C - - As required by regulations.

D - 0 May be inoperative for extensive periods

of:
a) day, VFR float operations, or
b) day VFR operations north/south of
55 degrees north/south latitude.

Figure 34-59. MMEL - DME

34-90 FOR TRAINING PURPOSES ONLY

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34 NAVIGATION
PAGE INTENTIONALLY LEFT BLANK

FOR TRAINING PURPOSES ONLY 34-91

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-52-00 AIR TRAFFIC request of the controller. Figure 34-60 shows

a typical ATC transponder control head. To
CONTROL (ATC) avoid interference between the transponder and
the DME, the transponder is inhibited when the
DME transmits and vice versa.
INTRODUCTION
The ATC system utilizes the onboard aircraft SYSTEM DESCRIPTION
Transponder to enable ground facilities to
track aircraft movement through ground The air traffic control (ATC) transponder
34 NAVIGATION

controlled sectors. The ground facilities system is made to operate in an air traffic
monitor the aircraft’s location, direction of control zone. When operational, it becomes
travel, identification and altitude. When the a part of the air traffic control radar system
Transponder onboard the aircraft is interrogated and gives identification (Mode A) and altitude
by the ground station, it sends a coded signal (Mode C) data of the aircraft to the ground
reply depending on the mode of operation. controller’s plan position display screen. The
transponder also supplies Mode S operation,
which gives the capability of responding to
GENERAL unique interrogations directed to specified
aircraft, either from a ground facility or from
Refer to:
another aircraft. This identifier is used in
TCAS (traffic alert and collision avoidance
•• Figure 34-60. Transponder GARMIN
system) operation. The transponder also sends
GTX330.
and receives data for air traffic control and
•• F i g u r e 3 4 - 6 2 . X P D R G T X 3 3 0 aircraft separation assurance functions. The
Schematic. transponder operates continuously, and when
interrogated by radar pulses from a ground
The transponder allows automatic identification station or other aircraft, automatically replies
of the airplane for the air traffic control (ATC) with a series of pulses. These reply pulses are
radar beacon system. It is interrogated by pulses coded to supply identification, altitude and data
received from a ground station and automatically transfer information.
replies with a series of pulses. Reply pulses are
coded to supply identification and altitude. A
special pulse identifier (SPI) is present only
when the IDENT switch is depressed and for
approximately 15 seconds after release. The
identification pulse identifies the airplane at the

GTX 330
ALT
IDENT FUNC CRSR
FLIGHT TIME
ALT
ALT
ST

START
OF

VFR
BY

STOP
CLR

0 1 2 3 4 5 6 7 8 9

Figure 34-60. Transponder GARMIN GTX330

34-92 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Mode Selection Keys Keys for Other GTX 330 Functions

OFF - Powers off the GTX 330. Pressing STBY,
Keys for Other GTX 330 Functions
ON, or ALT key powers on the transponder
displaying the last active identification code. IDENT - Pressing the IDENT key activates
the Special Position Identification (SPI)
STBY - Selects the standby mode. When in Pulse for 18 seconds, identifying your
standby mode, the transponder will not reply transponder return from others on the air
to any interrogations. traffic controller’s screen. The word IDENT
will appear in the upper left corner of the

34 NAVIGATION
ON - Selects Mode A. In this mode, the display during this time.
transponder replies to interrogations, as
VFR - Sets the transponder code to the
indicated by the Reply Symbol (R). Replies do
pre-programmed VFR code selected during
not include altitude information.
installation configuration (this is set to 1200
at the factory). Pressing the VFR key again
ALT - Selects Mode A and Mode C. In ALT
restores the previous identification code.
mode, the transponder replies to identification and
If the VFR Key is pressed when disabled
altitude interrogations as indicated by the Reply
(dependent upon installation configuration)
Symbol (R). Replies to altitude interrogations
a VFR Key Disabled message appears to
include the standard pressure altitude received
indicate that no operation took place.
from an external altitude source, which is not
adjusted for barometric pressure. The ALT mode FUNC - Changes the page shown on the
may be selected in aircraft not equipped with an right side of the display. Display data includes
optional altitude encoder, however, the reply Pressure Altitude, Flight Time, Altitude Monitor,
signal will not include altitude information. Count Up, and Count Down timers. Also displays
Outside Air Temperature, Density Altitude,
Any time the function ON or ALT is selected Contrast, Display, and ADS-B Operation
the transponder becomes an active part of (dependent upon installation configuration).
the air traffic control radar beacon system
START/STOP - Starts and stops the Altitude
(ATCRBS). The transponder also responds to
Monitor, Count Up, Count Down, and Flight
interrogations from TCAS equipped aircraft.
timers.

CRSR - Initiates starting time entry for the

Code Selection Count Down timer and cancels transponder
Code selection is done with eight keys (0 - 7) code entry.
providing 4,096 active identification codes.
CLR - Resets the Count Up, Count Down,
Pushing one of these keys begins the code
and Flight timers. Cancels the previous
selection sequence. Digits that are not yet entered
keypress during code selection and Count
appear as dashes. The new code is activated
Down entry. Returns cursor to the fourth
when the fourth digit is entered. Pressing the
code digit within five seconds after entry.
CLR key moves the cursor back to the previous
digit. Pressing the CLR key when the cursor 8 - Reduces Contrast and Display Brightness
is on the first digit of the code, or pressing the when the respective fields are displayed
CRSR key during code entry, removes the cursor (dependent upon installation configuration)
and cancels data entry, restoring the previous and enters the number eight into the Count
code. You may press the CLR key up to five Down timer.
seconds after code entry is complete to return
9 - Increases Contrast and Display Brightness
the cursor to the fourth digit. The numbers 8 and
when the respective fields are displayed
9 are not used for code entry, only for entering
(dependent upon installation configuration)
a Count Down time, and for adjusting contrast
and enters the number nine into the Count
and display brightness.
Down timer.

FOR TRAINING PURPOSES ONLY 34-93

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

NOTE Automatic ALT/GND Mode

GTX 330 options are normally Switching
set at time of installation.
If the GTX 330 is configured for automated
For changes to the GTX 330
airborne determination, normal operation begins
parameters, contact your Garmin
when take off is sensed. When the aircraft
authorized dealer.
is on the ground the screen automatically
displays GND. The transponder does not
Avoid selecting these codes.
respond to ATCRBS interrogations when
GND is annunciated. When a delay time is set
34 NAVIGATION

0000 - Military use

(dependent upon installation configuration), the
GTX 330 waits a specified length of time after
7500 - Hijacking
landing before changing to GND mode.
7600 - Loss of communication
Failure Annunciation
7700 - Emergency.
If the unit detects an internal failure, the screen
displays FAIL. When FAIL is annunciated no
Altitude Trend Indicator transponder data is transmitted.
When the PRESSURE ALT page is displayed,
an arrow may be displayed to the right of the
altitude, indicating that the altitude is increasing
or decreasing. One of two sizes of arrows may
be displayed depending on the vertical speed
rate. The sensitivity of these arrows is initially
set during transponder installation.

Systems & Nos de 1. 2. Number installed - Nombre d’articles installés

Sequence système Item - Article
Numbers de série 3. Number required for dispatch - Nombre d’articles à expédier

4. Remarks or Exceptions - Remarques ou exceptions

34 NAVIGATION

17 Transponders C 2 1 One transponder may be inoperative

provided second (standby) transponder
operates normally.

B 2 0 Both may be inoperative if not required by

operating regulations.

Figure 34-61. MMEL - Transponders

34-94 FOR TRAINING PURPOSES ONLY

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34 NAVIGATION
Figure 34-62. XPDR GTX330 Schematic

FOR TRAINING PURPOSES ONLY FOR TRAINING PURPOSES ONLY 34-95

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

STANDBY STANDBY Frequency, Flight timer and

ANT/ADF IN USE BFO Frequency FLIGHT TIME or Elapsed timer
Mode Annunciation Frequency Annunciation Annunciation ELAPSED TIME mode annunciation

Frequency
323 2 31 Select
Knobs

Select ANT mode

(out position) Select Frequency Select FLIGHT Set and Reset ON/OFF/VOL
Select ADF mode BFO Transfer TIMER or ELAPSED TIMER Control Switch
(in position) Button Button ELAPSED TIMER

Figure 34-63. Receiver

34-96 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-53-00 ADF window at all times. A standby frequency is

displayed in the right side when “FRQ” is
annunciated. The standby frequency is placed
GENERAL in “blind” memory when either FLT (Flight
Time) or ET (elapsed time) mode is selected.
Refer to:
With “FRQ” annunciated, the standby
•• Figure 34-63. Receiver. frequency is selected using the frequency select
knobs which may be rotated either clockwise
•• Figure 34-64. ADF No.1 KR 87 -
or counterclockwise. Pull the small inner knob

34 NAVIGATION
Interconnect Schematic.
out to tune 1s. Push the smaller inner knob in
to tune 10s. The outer knob tunes the 100s and
The automatic direction finding equipment is
the 1000s up to 1799.
used both for homing and obtaining position
fixes in navigation. The ADF information is
The standby frequency selected may then
displayed on the RMI (radio magnetic indicator).
be put into the active window by pressing
the “FRQ” button. The standby and active
The automatic direction finder (ADF) system is
frequencies will be exchanged (flip-flopped),
a low frequency radio system). The ADF system
the new frequency will become active, and the
is used to show the direction to a selected
former active frequency will go into standby.
ground station in reference to the aircraft center
line. The ADF system also supplies station
identification and voice announcements to the Operating Modes
audio integrating system. The transmitting
Antenna (ANT) mode is selected and
stations can be non-directional beacons (NDB)
annunciated when the “ADF” button is in the
or standard AM broadcasting stations in the
“out” position. ANT provides improved audio
frequency range of 200.0 KHz to 1799.0 KHz.
reception from the station tuned and is usually
used for identification. The bearing pointer in
The ADF system includes the component that
the KI 227 indicator will be deactivated and
follows:
immediately turn to the 90° relative position
and remain there during ANT reception.
•• One receiver
•• One combined antenna. The ADF mode is selected and annunciated
when the “ADF” button is in the depressed
position. ADF activates the bearing pointer in
RECEIVER the RMI indicator, causing it to move without
hesitation to point in the direction of the station
Turn-On relative to the aircraft heading.
Rotate the ON/OFF/VOL knob clockwise from
the detented “OFF” position. The unit will be The compass card on the RMI indicator may be
activated and will be ready to operate. Rotation rotated as desired by using the heading knob.
of this control also adjusts audio volume.
The indication of this compass card should
The KR 87 has “audio muting” which causes be compared with that of the KI 525A master
the audio output to be muted unless the receiver indicator from time to time. Check especially
is locked on a valid station. after steep bank turns and taxi turns. If a
discrepancy between the two readings exists,
the RMI compass card should be synchronized
Frequency Selection to the KI 525A compass card by rotating the
The active frequency (to which the ADF “SYNC” knob on the indicator.
is tuned) is displayed in the left side of the

FOR TRAINING PURPOSES ONLY 34-97

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Outside of the United States some stations are or not. The elapsed timer also
unmodulated and use an interrupted carrier has a “count-down” mode. To
for identification purposes. The BFO mode, enter the countdown mode, the
activated and annunciated when the “BFO” SET/RST button is depressed
button is depressed, permits the carrier wave for about two seconds, or until
and the associated Morse code identifier the “ET” annunciation begins
broadcast on the carrier wave to be heard. to flash. It is now in the ET
set mode, and a time up to 59
minutes, 59 seconds may be
ADF Test (PRE-FLIGHT or preset into the elapsed timer
34 NAVIGATION

IN-FLIGHT) with the concentric knobs.

Select ANT mode. This will cause the bearing
The preset time will be displayed
pointer to move directly to the parked 90°
and remain unchanged until
position. Make sure the unit is tuned to a
SET/RST is pressed again,
usable frequency. Now select ADF mode and
which will start the elapsed timer
the needle should move without hesitation to
counting down from the preset
the station bearing. Excessive sluggishness,
time. When the timer reaches
wavering or reversals indicate a signal that is
:00 it will start to count up as the
too weak or a system malfunction.
display flashes for 15 seconds
and an aural alarm, if installed,
Operating the Timers is activated for about 1 second.
The flight timer will always be automatically
reset to :00 whenever power is interrupted either NOTE
by the avionics master switch or the unit’s ON/
The standby frequency which is
OFF switch. An optional external switch may
in memory while flight time or
be installed which, when activated, will stop or
elapsed time modes are being
start the flight timer. This switch would be of
displayed may be called back
use during a non-refueling stop when resetting
by pressing the FRQ button,
the flight timer is not desired. On some aircraft it
then transferred to active use by
may be desirable to use the aircraft strut switch
pressing the FRQ button again.
instead of a manual switch to stop and start the
flight timer. It should be emphasized that the
While FLT or ET is displayed the “in use”
start/stop function will only operate with power
frequency on the left side of the window may
applied to the unit. Always read flight time prior
be changed, by using the frequency select
to power shutdown.
knobs, without any effect on the stored standby
frequency or the other modes. This feature is
Flight time or elapsed time are displayed and
especially useful when searching for stations
annunciated alternatively by depressing the
with unknown frequencies.
FLT/ET button. The flight timer continues to
count up until the unit is turned off or stopped
with an external switch. The elapsed timer may
be reset back to :00 by pressing the SET/RST
button. It will then start counting up again.

NOTE
Pressing the SET/RST button
will reset the elapsed timer
whether it is being displayed

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 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Erroneous ADF Bearings Due to NOTES

Radio Frequency Phenomena
Station Overlap
In the U.S., the FCC, which assigns AM radio
frequencies, occasionally will assign the same
frequency to more than one station in an area.
Certain conditions, such as night effect, may
cause signals from such stations to overlap.

34 NAVIGATION
This should be taken into consideration when
using AM broadcast stations for navigation.

Sunspots and atmospheric phenomena may

occasionally distort reception so that signals
from two stations on the same frequency will
overlap. For this reason it is always wise to
make positive identification of the station being
tuned, by switching the function selector to
ANT and listening for station call letters.

Electrical Storms
In the vicinity of electrical storms, an ADF
Indicator pointer tends to swing from the station
tuned toward the electrical discharges. Location
of the storm can be useful information, but the
erratic behavior of the pointer should be taken
into account.

Night Effect
This is a disturbance particularly strong just
after sunset and just after dawn. An ADF
indicator pointer may swing erratically at these
times. If possible, tune to the most powerful
station at the lowest frequency. If this is not
possible, take the average of pointer oscillations
to determine relative station bearing.

Mountain Effect
Radio waves reflecting from the surface of
mountains may cause the pointer to fluctuate
or show an erroneous bearing. This should be
taken into account when taking bearings over
mountainous terrain. Coastal Refraction Radio
waves may be refracted when passing from
land to sea or when moving parallel to the
coastline. This should be taken into account
when operating near coastal areas.

FOR TRAINING PURPOSES ONLY 34-99

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

Figure 34-64. ADF No.1 KR 87 - Interconnect Schematic

34-100 FOR TRAINING PURPOSES ONLY FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34 NAVIGATION
Systems & Nos de 1. 2. Number installed - Nombre d’articles installés
Sequence système Item - Article
Numbers de série 3. Number required for dispatch - Nombre d’articles à expédier

4. Remarks or Exceptions - Remarques ou exceptions

34 NAVIGATION

12 ADF C - - As required by regulations.

D - 0 M a y b e i n o pe r a t i v e f o r e x t e n s i v e pe r i od s
of:
a) day, VFR float operations, or
b) day VFR operations north/south of
55 degrees north/south latitude.

Figure 34-65. MMEL -ADF

FOR TRAINING PURPOSES ONLY 34-101

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34 NAVIGATION

PAGE INTENTIONALLY LEFT BLANK

34-102 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-55-00 (NAV) VOR NOTES

OPERATION
In VOR mode the VHF NAV receives course
and bearing information from a selected ground
station. Airplane position with respect to the
station or courses to the station is displayed.

34 NAVIGATION
VOR identifier audio signals are provided to
the airplane audio system.

VOR signals, received and processed by the

navigation system, consist of reference and
variable-phase signals. The two signals are in
phase at magnetic north, and the phase shift
at any other bearing from the VOR station
is equal in degrees to the airplane bearing
from the ground station. This generates
VOR radials that radiate outward from the
ground station. The VHF NAV receives and
processes the composite VOR signal for
display of VOR bearing and course on the
RMI and flight director.

FOR TRAINING PURPOSES ONLY 34-103

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-55-00 (NAV) ILS LOCALIZER

The navigation receiver receives and
OPERATION demodulates two AM signals-90 Hz and 150 Hz
on the localizer signal. The 90-Hz modulation
The VHF NAV receives ILS signals (localizer is predominant when the airplane is to the
and glide slope), provides terminal navigation left of the runway centerline, and 150 Hz is
data for the flight director, and sends localizer predominant when the airplane is to the right.
ground station identifier audio signals to the
airplane audio system. The two demodulated signals are compared to
34 NAVIGATION

determine direction and amount of deviation

Localizer and glide-slope ground station from the runway centerline.
frequencies are paired. When a localizer
frequency is selected on the VHF NAV control
head, the corresponding frequency is also
GLIDE SLOPE
selected within the VHF NAV receiver.
The glide-slope signal is similar to the localizer
signal except that the glide-slope deviation is
If a glide-slope frequency is selected,
perpendicular to the localizer deviation. The
the corresponding localizer frequency is
90-Hz signal is above the glide slope, and the
automatically selected.
150-Hz signal is below.

Glide-slope deviation signals are displayed on

the flight director. The null glide path slopes
up approximately 3°.

PWR/VOL
VLOC VHF NAV
VLOC Vol Flip-Flop Display

1 9
3

8 10

2
4 7 11

13
5 12
6

14 15 16 17 18

Freq Selector CDI Key

Figure 34-66. GNS 430

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 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

The VHF NAV sends localizer and glide-slope 5. The large left knob is used to tune the
deviation information for visual display. When megahertz (MHz) value of the standby
on course and on glide path, the CDI and GS frequency for the COM transceiver or the
pointer are centered. If the ground station VLOC receiver, whichever is currently
signals are lost, the VHF NAV sends GS and selected by the tuning cursor.
NAV flag signals to the flight director.
6. The CDI Key is used to toggle which
navigation source (GPS or VLOC) provides
MARKER BEACON output to an external HSI or CDI.

GPS

34 NAVIGATION
The VHF NAV system receives marker beacon
signals from ground stations on the localizer
flight path. As the airplane passes over each At the heart of the GMS430 unit is a WAAS
marker beacon ground station, the marker upgradable 12-channel GPS receiver. The
beacon indicators illuminate (blue, outer marker; unit has a fault detection and exclusion (FDE)
amber, middle marker; white, inner marker). software for oceanic approval.

The middle marker signal is sent to the flight The GPS receiver performs a consistency
computer, which adjusts command gain rates check on all tracked satellites, so as to allow
during landing. Aural signals are sent to the the receiver to calculate a position within a
airplane audio system as the airplane passes specified protection limit (4.0 NM oceanic/
each marker. remote, 2.0 NM for enroute, 1.0 NM for
terminal and 0.3 NM for non-precision
The RMI bearing pointers are parked approaches).
horizontally when a localizer frequency is
selected, pointing to the right.
ACRONYMS
GNS 430 KEYS AND BUTTONS RAIM - Receiver Autonomous Integrity
Monitoring is a GPS receiver function that
1. The COM Power/Volume Knob controls performs a consistency check on all tracked
unit power and communications radio satellites.
volume. Press momentarily to disable
automatic squelch control. WAAS - Wide Area Augmentation System is
reported by the WAAS satellite system and
2. The VLOC Volume Knob controls audio
only works within the WAAS service.
volume for the selected VOR/Localizer
frequency. Press momentarily to enable/
WAAS approaches require WAAS integrity.
disable the ident tone.
But outside the WAAS service area, such as
3. The VLOC Flip-flop Key is used to swap oceanic flight, RAIM prediction will be used.
the active and standby VLOC frequencies
(i.e., make the selected standby frequency
active).
4. The small left knob is used to tune the
kilohertz (kHz) value of the standby
frequency for the COM transceiver or the
VLOC receiver, whichever is currently
selected by the tuning cursor. Press this
knob momentarily to toggle the tuning
cursor between the COM and VLOC
frequency fields.

FOR TRAINING PURPOSES ONLY 34-105

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34 NAVIGATION

Figure 34-67. GNS 430 No.1 - Interconnect Schematic (Sheet 1 of 2)

34-106 FOR TRAINING PURPOSES ONLY FOR TRAINING PURPOSES ONLY

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34 NAVIGATION
Figure 34-68. GNS 430 No.1 - Interconnect Schematic (Sheet 2 of 2)

FOR TRAINING PURPOSES ONLY FOR TRAINING PURPOSES ONLY 34-107

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
34 NAVIGATION

Figure 34-69. GNS 430 No. 2 - Interconnect Schematic

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 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

34-00-00 SERIES 100

AND 200 DIFFERENCES
There are no major Series 100 or 200
differences in the avionics systems. Refer to
the AFM, Maintenance Manual, and vendor
manuals for information on specific avionics
systems installed in specific airplanes.

34 NAVIGATION
Systems & Nos de 1. 2. Number installed - Nombre d’articles installés
Sequence système Item - Article
Numbers de série 3. Number required for dispatch - Nombre d’articles à expédier

4. Remarks or Exceptions - Remarques ou exceptions

34 NAVIGATION

6 Marker Beacon System D - 0 Provided approach minimums are not

dependent on its use.
9 Navigation Equipment

VOR/ILS C - - Any in excess of those required by

regulations and not powered by an
emergency or standby electrical bus may
be inoperative.

D - 0 May be inoperative for extensive periods

of:
a) day, VFR float operations, or
b) day VFR operations north/south of
55 degrees north/south latitude.

Figure 34-70. MMEL - Marker Beacon System and Navigation Equipment

FOR TRAINING PURPOSES ONLY 34-109

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CHAPTER 22
AUTOFLIGHT
CONTENTS
Page
22-00-00 AUTOFLIGHT............................................................................................. 22-1
Introduction......................................................................................................... 22-1
General................................................................................................................ 22-2
Position-Based Vs. Rate-Based Autopilots.................................................... 22-2
Position or Attitude Based Systems............................................................... 22-2
Advantages of Attitude Based Systems.......................................................... 22-2
Rate-Based Systems...................................................................................... 22-2
Rate Gyro Sensors......................................................................................... 22-2

22 AUTOFLIGHT
Accelerometer Sensors.................................................................................. 22-2
Advantages of Rate Based Systems............................................................... 22-2
22-10-00 AP-106 AUTOMATIC FLIGHT CONTROL SYSTEM................................. 22-5
General................................................................................................................ 22-5
22-10-00 AP-106 AUTOPILOT SYSTEM................................................................... 22-7
General................................................................................................................ 22-7
Operating Modes.................................................................................................. 22-7
General......................................................................................................... 22-7
Attitude (Manual) Mode................................................................................ 22-7
Guidance Mode............................................................................................. 22-7
Component Description........................................................................................ 22-9
General......................................................................................................... 22-9
Autopilot Controls........................................................................................ 22-9
Flight Computer............................................................................................ 22-9
Primary Servos and Servo Mounts................................................................ 22-9

Revision 0.3
FOR TRAINING PURPOSES ONLY 22-i
 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page
Control and Indication.......................................................................................... 22-9
Autopilot Engage Switch............................................................................... 22-9
Turn Knob..................................................................................................... 22-9
UP-DN Pitch Wheel...................................................................................... 22-9
Control Wheel Steering................................................................................. 22-9
GA Pushbutton............................................................................................ 22-11
Autopilot Disengagement............................................................................ 22-11
Autopilot Annunciation............................................................................... 22-11
Servo Actuators........................................................................................... 22-11
22-10-00 S-TEC SYSTEM 65 AUTOPILOT/FLIGHT DIRECTOR SYSTEM........... 22-15
General.............................................................................................................. 22-15
22 AUTOFLIGHT

System Limitations............................................................................................ 22-15

Roll Axis Limits.......................................................................................... 22-15
Pitch Axis Limits........................................................................................ 22-15
System Block Diagram....................................................................................... 22-15
Programmer/Annunciator............................................................................ 22-17
Remote Annunciator.................................................................................... 22-17
Roll Flight Guidance Computer (RFGC)..................................................... 22-17
Pitch Flight Guidance Computer (PFGC) ................................................... 22-17
Turn Coordinator......................................................................................... 22-19
Directional Gyro (DG)................................................................................ 22-19
Horizontal Situation Indicator (HSI)........................................................... 22-19
Absolute Pressure Transducer...................................................................... 22-21
Roll Servo................................................................................................... 22-21
Pitch Servo.................................................................................................. 22-21
Trim Servo.................................................................................................. 22-21
Remote Disconnect Switch.......................................................................... 22-23

Revision 0.3
22-ii FOR TRAINING PURPOSES ONLY
 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Page
Control Wheel Steering Switch................................................................... 22-23
Go Around (GA) Switch.............................................................................. 22-23
General Theory of Operation.............................................................................. 22-25
Heading (HDG)........................................................................................... 22-25
Navigation (NAV)....................................................................................... 22-25
Reverse (REV)............................................................................................ 22-29
GPS Intercept and Tracking......................................................................... 22-29
Pitch Modes of Operation................................................................................... 22-31
Vertical Speed (VS)..................................................................................... 22-31
Altitude (ALT)............................................................................................ 22-31
Intercepting and Coupling the Glideslope.................................................... 22-33

22 AUTOFLIGHT
Elevator Trim Indicator............................................................................... 22-33
Optional Autotrim....................................................................................... 22-33
Autopilot Disengage.................................................................................... 22-35
Flight Director Operation, (Optional).......................................................... 22-35
FD/AP Mode............................................................................................... 22-35
FD Mode..................................................................................................... 22-35
Wide Area Augmentation System (WAAS) Procedures................................ 22-37
Yaw Damper Block Diagram....................................................................... 22-37
Yaw Damper Operation............................................................................... 22-37
Yaw Damper Trim Knob.............................................................................. 22-37
Functional/Pre-Flight Test Procedures................................................................ 22-39
Autotrim..................................................................................................... 22-65
Yaw Damper................................................................................................ 22-67
Glossary............................................................................................................. 22-72

FOR TRAINING PURPOSES ONLY 22-iii

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

ILLUSTRATIONS
Figure Title Page
22-1 AP-106 Automatic Flight Control System..................................................22-4
22-2 Autopilot and Flight Director Controls......................................................22-6
22-3 Autopilot Component Locations................................................................22-8
22-4 Rudder & Elevator Servo Capstan Installation (1 of 3)............................22-10
22-5 Rudder & Elevator Servo Capstan Installation (2 of 3)............................22-12
22-6 Rudder & Elevator Servo Capstan Installation (3 of 3)............................22-13
22-7 System Block Diagram............................................................................22-14
22-8 Programmer/Annunciator.........................................................................22-16
22-9 Remote Annunciator................................................................................22-16

22 AUTOFLIGHT
22-10 
Roll Flight Guidance Computer (RFGC)..................................................22-16
22-11 
Pitch Flight Guidance Computer (PFGC).................................................22-16
22-12 Turn Coordinator.....................................................................................22-18
22-14 Horizontal Situation Indicator..................................................................22-18
22-13 Directional Gyro......................................................................................22-18
22-15 Absolute Pressure Transducer..................................................................22-20
22-16 Servo.......................................................................................................22-20
22-17 Remote Disconnect Switch......................................................................22-22
22-18 Control Wheel Steering Switch................................................................22-22
22-19 Go Around (GA) Switch..........................................................................22-22
22-20 Ready......................................................................................................22-24
22-21 Heading (HDG).......................................................................................22-24
22-22 Navigation (NAV) (1 of 4).......................................................................22-24
22-23 Navigation (NAV) (2 of 4).......................................................................22-26

Revision 0.3
FOR TRAINING PURPOSES ONLY 22-v
 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure Title Page

22-24 Navigation (NAV) (3 of 4).......................................................................22-26
22-25 Navigation (NAV) (4 of 4).......................................................................22-28
22-26 Reverse (REV).........................................................................................22-28
22-27 Vertical Speed (VS).................................................................................22-30
22-28 Altitude (ALT).........................................................................................22-30
22-29 Intercepting and Coupling the Glideslope................................................22-32
22-30 UP/DN Trim............................................................................................22-32
22-31 Flight Director Block Diagram................................................................22-34
22-32 
FD Display, FD/AP Mode Engaged..........................................................22-34
22-33 
FD Display, FD Mode Engaged................................................................22-34
22 AUTOFLIGHT

22-34 Yaw Damper Block Diagram....................................................................22-36

22-35 Yaw Damper Operation............................................................................22-36
22-36 Yaw Damper Trim Knob..........................................................................22-36
22-37 AP Display, RDY For Operation..............................................................22-38
22-38 AP Display, Turn Coordinator Failure, RDY Does Not Appear.................22-38
22-39 Sense Freedom of Movement of Pitch Axis..............................................22-40
22-40 Press/Hold UP.........................................................................................22-40
22-41 Press/Hold DN.........................................................................................22-42
22-42 Press FD/AP Switch................................................................................22-42
22-43 
DG - Position Heading Bug Under Lubber Line.......................................22-44
22-45 Engage Heading Mode, Disengage Yaw Damper Mode............................22-44
22-44 
Sense Freedom of Movement of Roll Axis...............................................22-44
22-46 HSI - Turn Heading Left/Right of Lubber Line........................................22-46
22-47 Sense Freedom of Movement of Pitch Axis..............................................22-46
22-48 Engage Altitude Hold Mode.....................................................................22-48

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Figure Title Page

22-49 Sense Freedom of Movement of Pitch Axis..............................................22-48
22-50 Press/Hold UP, Control Wheel Moves Aft................................................22-50
22-51 Press/Hold DN, Control Wheel Moves Fwd.............................................22-50
22-52 Engage Vertical Speed Mode...................................................................22-52
22-53 Press/Hold UP.........................................................................................22-52
22-54 Press/Hold DN, Control Wheel Moves Fwd.............................................22-54
22-55 Engage Altitude Hold Mode.....................................................................22-56
22-56 NAV and SOFT Annunciations Appear On AP Display, HDG is
Extinguished............................................................................................22-56
22-57 CDI - Turn OBS Knob Right/Left of Center.............................................22-58
22-58 Engage Heading Mode.............................................................................22-58

22 AUTOFLIGHT
22-59 Move Control Wheel Fwd and Aft...........................................................22-60
22-60 Move Control Wheel Aft and Fwd...........................................................22-62
22-61 Autotrim..................................................................................................22-64
22-62 Yaw Damper............................................................................................22-66
22-63 System Interconnect (Sheet 1 of 4)..........................................................22-68
22-64 System Interconnect (Sheet 2 of 4)..........................................................22-69
22-65 System Interconnect (Sheet 3 of 4)..........................................................22-70
22-66 System Interconnect (Sheet 4 of 4)..........................................................22-71

FOR TRAINING PURPOSES ONLY 22-vii

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

CHAPTER 22
AUTOFLIGHT

22 AUTOFLIGHT
22-00-00 AUTOFLIGHT
INTRODUCTION
The DHC-6 Twin Otter avionics covered in this chapter include a typical automatic flight
control systems. It is not inclusive of all the optional autopilot systems available for
installation. The user should consult the Maintenance Manual, applicable AFM supplements
and vendor manuals for additional information on specific manufacturers installations not
included in this chapter.
An autopilot is designed to serve two primary purposes:
1. To enhance the pilot flight control capabilities.
2. To reduce the cockpit workload by putting the airplane into an automatic flight mode.

FOR TRAINING PURPOSES ONLY 22-1

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

GENERAL Advantages of Attitude Based

The Collins AP-106 automatic flight control
Systems
system (AFCS) described in this chapter is Responsiveness is the primary advantage of a
typical of an AFCS which may be installed position-based system. Attitude information
in the Twin Otter. The description does not is better for rapid recovery from in-flight
necessarily reflect any particular airplane or upsets. Most attitude-based autopilots are more
client; it is merely presented as an introduction responsive than rate-based systems because
to a standard integrated AFCS autopilot and they can correct the position-error input faster
flight director. and with less course/heading error during
correction. The roll signal isn’t influenced by
yaw angle or rate. When the yaw angle and
Position-Based Vs. Rate-Based rate are combined, as they are in most turn
Autopilots coordinator based rate systems, they could
diminish the effect of roll.
Autopilots are stabilized by signals from
gyroscopes or sensors that emulate forms
of gyroscopes. Of all the gyro instruments Rate-Based Systems
available, vertical gyros and inclined axis rate
The rate-based autopilot system uses rate gyros
gyros are the most commonly used in the design
and/or accelerometers as primary sensors.
of an autopilot. Thus the terms position-based
and rate-based describe the autopilot system
22 AUTOFLIGHT

relative to the primary sensor type used in the Rate Gyro Sensors
stabilizing function. Here is a summary of the
Aviation rate gyros have detected motions as
differences and advantages of each system.
low as 1/16000th/sec and derive the attitude
synthetically, which drives autopilots and the
Position or Attitude Based attitude instruments.
Systems
Vertical gyros provide a display that
Accelerometer Sensors
represents the attitude of the aircraft in roll Accelerometer sensors give the autopilot
and pitch relative to the earth’s surface. The more authority, which improves performance.
amount of change of this artificial attitude Vertical acceleration detects vertical motions
is determined electronically and produces and rates of pitch attitude change. During
an attitude command for pitch and roll that normal operation vertical acceleration limits
tells the autopilot how rapidly the attitude is pitch maneuvering. During abnormal operation
changing. The autopilot then inputs the amount it limits pitch excursions.
of position change to correct the error.

Position or attitude autopilots typically use

Advantages of Rate Based
the pilot attitude indicator as a gyroscope Systems
signal source. This saves the expense of
A rate gyro won’t tumble in an unusual attitude.
adding a dedicated gyro instrument. Position-
Pilots are instructed to use the turn coordinator
based systems typically bank the aircraft at a
instrument to level the wings during recoveries.
preset maximum bank angle regardless of the
The safety factor is increased by the reliability
aircraft’s speed.
of the system.
The angle would normally be either 20° or 25°,
In a light general aviation (GA) airplane, two
depending on the manufacturer of the system
of the least reliable systems are the vacuum
or the size and type of aircraft.
system and the attitude gyro. The electric turn

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 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

coordinator rate gyro and the autopilot do not NOTES

depend on either. If the vacuum system or
attitude gyro fails or experiences degraded
performance, the turn coordinator and the
autopilot will continue to fly the aircraft.

A consistent turn rate requires a lower bank

angle at lower airspeeds, rate autopilots often
provide more precise aircraft turn control at
low airspeed. Since the turn rate is always the
same, the performance of the system rolling
into and out of turns is consistent. There is
little tendency to overshoot and over control
at low speeds.

22 AUTOFLIGHT

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Figure 22-1. AP-106 Automatic Flight Control System
22 AUTOFLIGHT
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22-10-00 AP-106 NOTES

AUTOMATIC FLIGHT
CONTROL SYSTEM
GENERAL
Maintenance Manual Chapter 22, “Autoflight,’’
covers the automatic flight control systems
(AFCS) available on the Twin Otter. A
representative AFCS commonly installed in
the Twin Otter is the Collins AP-106 autopilot
integrated with the Collins FD-112V flight
director or an optional autopilot S-TEC 65
(Figure 22-1).

When engaged and coupled to the flight director,

the autopilot controls the airplane by using the
commands generated by the flight computer.
When engaged without a flight director mode
selected (not coupled), manual pitch and roll

22 AUTOFLIGHT
commands are inserted using the pitch wheel
and turn knob. The pilot can manually fly the
airplane using command bar guidance commands
if the autopilot is not engaged but desired flight
director modes are selected.

A typical AFCS can perform the following

functions:

•• Maintain a preselected attitude

•• Maintain a barometric altitude
•• Maintain an indicated airspeed
•• Capture and maintain a desired heading
•• Capture and maintain a preselected
radio course
•• Capture and maintain an ILS approach
to published minimums.

Some Twin Otters have only the flight director

portion of the AFCS installed. A flight director-
only installation does not include autopilot
servos, the pitch/turn control, and the yaw
damper. The functions described above can still
be accomplished, but the pilot must manually
fly the airplane to satisfy the flight director
commands as programmed.

Revision 0.3
FOR TRAINING PURPOSES ONLY 22-5
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22 AUTOFLIGHT

Figure 22-2. Autopilot and Flight Director Controls

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22-10-00 AP-106 NOTES

AUTOPILOT SYSTEM
GENERAL
The AP-106 provides autopilot engagement
control and the pitch wheel and turn knob for
manually controlling the autopilot. Figure 2
shows the autopilot controls.

OPERATING MODES
General
The autopilot has two modes of operation-
attitude and guidance.

Attitude (Manual) Mode

When the autopilot is engaged (engage lever in

22 AUTOFLIGHT
the ENG position) and no modes are selected
on the computer/control or with the go-around
button, the autopilot is in the manual mode.
The autopilot accepts pitch and roll rate or
position commands from the pitch wheel and
turn knob.

Guidance Mode
When the autopilot is engaged and a lateral and/
or vertical mode is selected on the computer/
control, the autopilot is in the guidance mode
and accepts steering commands from the
flight computer (the computer section of the
computer/control). Whether the autopilot is
engaged or disengaged, the ADI command bars
are always driven by the flight computer.

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FOR TRAINING PURPOSES ONLY 22-7
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Figure 22-3. Autopilot Component Locations
22 AUTOFLIGHT
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COMPONENT DESCRIPTION Turn Knob

The turn knob is spring-loaded to the center detent
General position. It is used to supply roll commands to the
Figure 22-3 shows the locations of the autopilot autopilot when no lateral modes are selected on
components. Following is a description of the the computer/control. If a lateral mode is selected
system. and the autopilot is engaged, moving the turn
knob from detent deselects that mode. When the
airplane is rolled to a normal roll attitude and the
Autopilot Controls knob is then positioned to the center detent, the
The autopilot controls include the engage/ existing roll attitude is held.
disengage switch, pitch wheel and turn knob,
autopilot disengage switch on the control
wheel, and two trim up/down indicators.
UP-DN Pitch Wheel
The pitch wheel is spring-loaded to the center
detent position. It supplies pitch commands
Flight Computer to the autopilot when no vertical modes are
The flight computer is the heart of the AFCS. selected on the computer/control. Moving this
It is composed of the computer/control, pitch thumb wheel to UP or DN causes an appropriate
computer, roll computer, and trim controller. change in pitch attitude at a rate proportional
The signals received by the flight computer to the amount of pitch wheel displacement. On
from various systems and sensors are converted release, the pitch attitude present at that time

22 AUTOFLIGHT
into proper command signals according to the is held. Movement of the pitch wheel clears
selected mode of operation. The command any selected vertical mode; the autopilot then
signals are sent to the flight director pitch assumes pitch hold mode.
and roll command bars, and with the autopilot
engaged, to the rudder, aileron, and elevator
servos, and the pitch trim system.
Control Wheel Steering
When the AFCS includes control wheel steering
(CWS), pressing the CWS button disengages the
Primary Servos and Servo Mounts autopilot servos from the control surfaces and
The primary servos position the airplane disengages the ALT or IAS hold mode (if selected).
control surfaces in response to commands
from the autopilot flight computer. The use of The effect of CWS on HDG, NAV, or B/C,
servo mounts allows the servos to be removed when selected, depends on the modification
without disturbing the flight controls rigging. status of the 913K-1 computer/control (the
913K-lA has modifications factory-installed).
As supplied from the factory (913K-l only),
CONTROL AND INDICATION HDG, NAV, or B/C disengages when the
CWS button is pressed and movement of the
Autopilot Engage Switch control wheel results in more than 10° of bank.
The autopilot engage switch allows manual When CWS is released, the existing attitude is
engagement or disengagement of the autopilot maintained. Bank angles of less than 10° will
system. The lever returns to the down position not disengage a selected lateral mode.
(DIS) whenever the autopilot is disengaged
by pressing the autopilot disconnect button
on the control wheel, when the autopilot fails
NOTE
to engage, or when automatic disengagement APPR does not disengage when
occurs. The disengaged position, when selected the CWS button is pressed. When
manually, is used as the autopilot master the CWS button is released, the
disconnect. airplane returns to the localizer
course and glide slope.

Revision 0.3
FOR TRAINING PURPOSES ONLY 22-9
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22 AUTOFLIGHT

Figure 22-4. Rudder & Elevator Servo Capstan Installation (1 of 3)

Revision 0.3
22-10 FOR TRAINING PURPOSES ONLY
 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

If the 913K-l computer/control has been field TRIM-IN-MOTION - The legend illuminates
modified by Service Bulletin No. 9 (customer when the autopilot is trimming.
option) or a 913K-lA is being used, HDG,
NAV, APPR, and B/C do not disengage when
the CWS button is pressed and bank angles
Servo Actuators
exceed 10°. When the CWS button is released, The Twin Otter AFCS has three primary
the airplane returns to the selected heading or servos and one trim servo. The primary servos
radio course. are identical units and position the aileron,
elevator, and rudder control surfaces in
response to commands received through the
GA Pushbutton gain programmer from the computer/control
The GA pushbutton on the control wheel and the turn and slip indicator. The trim servo
is used to select go-around mode, a flight- positions the elevator trim surface in response to
director-only mode. When depressed, the GA commands from the trim controller or the pilot.
button commands a wings-level, fixed-pitch-up
command without disengaging the autopilot. Details of the rigging procedures for the
GA may be selected any time after APPR is primary and trim servos are contained in the
selected. Maintenance Manual. Figures 4, 5, 6 shows the
rudder and elevator servo capstan installation.
Before adjusting or testing any of the flight
Autopilot Disengagement control surfaces, refer to the applicable
The autopilot can be disengaged by any of the manufacturer’s Maintenance Manual.

22 AUTOFLIGHT
following:

•• Depressing the AP disengage button on

the control column
•• Moving the ENG-DIS switch to the DIS
position
•• Actuating the electric trim switch.

The autopilot automatically disengages when

either of the following occurs:

•• Autopilot power failure

•• Gyro monitor failure

Autopilot Annunciation
Engage/Disengage indicators - The triangular
green engage light illuminates when the
autopilot is engaged. The triangular amber
disengage light illuminates when the autopilot
is disengaged.

TRIM UP/TRIM DN indicators - The

appropriate flashing amber legend illuminates
when the autopilot is driving the trim servo in
the up or down direction or, if the autopilot is
disengaged, illuminates when manual up or
down trim is required.

Revision 0.3
FOR TRAINING PURPOSES ONLY 22-11
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22 AUTOFLIGHT

Figure 22-5. Rudder & Elevator Servo Capstan Installation (2 of 3)

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22 AUTOFLIGHT

Figure 22-6. Rudder & Elevator Servo Capstan Installation (3 of 3)

Revision 0.3
FOR TRAINING PURPOSES ONLY 22-13
22-14 22 AUTOFLIGHT

NAVIGATION
RECEIVER CDI TURN
(VOR/LOC) (VOR/LOC/GPS) COORDINATOR

BATTERY
G
S MASTER
N SWITCH

S
AV

TWIN OTTER SERIES

DG

NAVIGATION
SOURCE
SELECTOR ROLL SERVO
SWITCH
FOR TRAINING PURPOSES ONLY

ROLL FLIGHT
GUIDANCE
HSI COMPUTER

NAVIGATION
RECEIVER AUTOPILOT TRIM SERVO
(GPS) MASTER
SWITCH

MAINTENANCE TRAINING MANUAL

GLIDESLOPE PITCH SERVO
RECEIVER

PITCH FLIGHT
GUIDANCE
REMOTE PROGRAMMER/ANNINCIATOR
COMPUTER
DISCONNECT FD/AP HDG NAV REV UP PRESSURE
SWITCH TRANSDUCER
MANUAL
A/P DISC ELECTRIC
FD VS ALT YD DN
TRIM
SWITCH

TRIM
MASTER
REMOTE ANNUNCIATOR SWITCH

Figure 22-7. System Block Diagram

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

22-10-00 S-TEC SYSTEM •• ALT - Used to hold altitude.

65 AUTOPILOT/FLIGHT •• VS - Used to hold vertical speed.

DIRECTOR SYSTEM •• GS - Used to intercept and track

glideslope.

GENERAL SYSTEM LIMITATIONS

Maintenance Manual Chapter 22, “Autoflight”,
covers the automatic flight control systems
Roll Axis Limits
(AFCS) available on the Twin Otter. The Turn Rate - 75% Standard Rate Turn (HDG,
autopilot system is an integral part of the NAV, NAV APR, REV, REV APR Modes).
flight control system. A representative AFCS
commonly installed in the Twin Otter is the
S-TEC System 65 Autopilot/Flight Director
Pitch Axis Limits
System. The S-TEC System 65 is a rate based Altitude - 32,000 ft.
autopilot that controls the roll and pitch axis
of the aircraft. The autopilot main function Vertical Force Due to Acceleration - 0.60 g.
is to convert pilot commands to logic inputs
for the roll and pitch computers. As the pilot Vertical Speed - 1600 FPM Climbing or
enters the desired mode by pressing a mode Descending.
selector switch, the computer acknowledges the

22 AUTOFLIGHT
selection by bringing on a related annunciation.
SYSTEM BLOCK DIAGRAM
The roll modes of operation are:
Refer Figure 22-7. System Block Diagram.
•• Heading (HDG)
The System 65 is comprised of the following
•• Navigation (NAV) major components:
•• Approach (APR)
•• Programmer/annunciator
•• Reverse (REV).
•• Remote annunciator
The pitch modes of operation are:
•• Roll flight guidance computer
•• Altitude (ALT) •• Pitch flight guidance computer
•• Vertical Speed (VS) •• Turn coordinator
•• Glideslope (GS). •• Heading system (DG or HSI)
•• Absolute pressure transducer
A brief description of each mode is as follows:
•• Roll servo
•• HDG - Used to turn onto a selected
•• Pitch servo
heading and hold it
•• Trim servo
•• NAV - Used to intercept and track a
VOR/GPS course •• Remote disconnect switch
•• APR - Used to intercept and track a •• Touch control steering switch
localizer front course inbound
•• Yaw damper switch.
•• REV - Used to intercept and track a
localizer back course inbound.

FOR TRAINING PURPOSES ONLY 22-15

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

FD/AP HDG NAV REV UP

FD VS ALT YD DN

Figure 22-8. Programmer/Annunciator

22 AUTOFLIGHT

Figure 22-9. Remote Annunciator

Figure 22-10. Roll Flight Guidance Figure 22-11. Pitch Flight Guidance
Computer (RFGC) Computer (PFGC)

22-16 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Programmer/Annunciator Roll Flight Guidance Computer

Refer to Figure 22-8. Programmer/Annunciator. (RFGC)
Refer to Figure 22-10. R
 oll Flight Guidance
The Programmer/Annunciator, installed on the
Computer (RFGC).
instrument panel within view of both pilots,
provides the means for programming a particular
The RFGC, normally installed on the radio rack,
roll mode and pitch mode of operation, and
will in accordance with the selected roll mode,
acknowledges the resulting engagement of
processes inputs from the turn coordinator,
each mode by annunciating it. Other relevant
directional gyro (DG) or optional horizontal
information is also annunciated as required.
situation indicator (HSI), very high frequency
omnidirectional radio range (VOR)/localizer
Remote Annunciator (LOC), long range navigation (LORAN) and
the global positioning system (GPS) navigation
Refer to Figure 22-9. Remote Annunciator.
receivers, to drive the roll servo until the roll
error has been canceled.
The System 65 Remote Annunciator, installed
on the instrument panel within view of
both pilots, is standard equipment with the Pitch Flight Guidance Computer
System 65 Autopilot. The remote annunciator
displays all the modes selected on the autopilot
(PFGC)
programmer, as well as conditions of those Refer to Figure 22-11. P
 itch Flight Guidance

22 AUTOFLIGHT
modes. Examples of these conditions include: Computer (PFGC).

•• NAV gain The PFGC, normally installed on the radio

rack will, in accordance with the selected
•• NAV failure
pitch mode, processes inputs from the absolute
•• GS disable (DSBL) pressure transducer, internal accelerometer,
glideslope deviation indicator, and off warning
•• Out-Of-Trim Conditions.
flag contained in the glideslope receiver. The
pitch system provides vertical speed control
In addition, both SEL and Glideslope (GS) are
and altitude hold, as well as automatic/manual
annunciated only on the remote annunciator.
glideslope capture to drive the pitch servo until
The “SEL” annunciator indicates that the
the pitch error has been canceled.
optional Altitude Selector / Alerter is in use.
The GS annunciator indicates that the autopilot
Vertical speed reference is provided by the
is in glide-slope mode and the pitch annunciator
barometric pressure transducer, while automatic
in the autopilot programmer will be blank.
and manual pitch trim sensing is provided by
the pitch servo. Drive for the elevator trim
NOTE servo is provided by the pitch computer. All
modes use the transducer signal for a VS or
It is very important that the pilot
ALT reference.
monitor the remote annunciator
as well as the autopilot
programmer during all autopilot
operations and especially during
approach operations.

FOR TRAINING PURPOSES ONLY 22-17

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22 AUTOFLIGHT

Figure 22-12. Turn Coordinator Figure 22-13. Directional Gyro

Figure 22-14. Horizontal Situation Indicator

22-18 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Turn Coordinator NOTES

Refer to Figure 22-12. Turn Coordinator.

The turn coordinator, installed on the

instrument panel, supplies the turn rate signal
to the roll flight guidance computer.

Directional Gyro (DG)

Refer to Figure 22-13. Directional Gyro.

The directional gyro, installed on the instrument

panel, supplies the heading error signal to the
roll flight guidance computer. The heading
error is the error between the heading bug and
the actual aircraft heading. To allow for course
intercept capability, the heading error signal is
also supplied to the course error channel of the
roll flight guidance computer.

Horizontal Situation Indicator (HSI)

22 AUTOFLIGHT
Refer to Figure 22-14. Horizontal Situation
Indicator.

The horizontal situation indicator, installed

on the instrument panel, supplies the heading
error signal and course error signal to the roll
flight guidance computer. The heading error
is the error between the heading bug and the
actual aircraft heading. The course error is the
error between the course pointer and the actual
aircraft heading.

FOR TRAINING PURPOSES ONLY 22-19

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22 AUTOFLIGHT

Figure 22-15. Absolute Pressure Transducer

Figure 22-16. Servo

22-20 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Absolute Pressure Transducer NOTES

Refer to Figure 22-15. Absolute Pressure
Transducer.

The absolute pressure transducer, normally

installed in the forward nose area, supplies
the altitude signal to the pitch flight guidance
computer. The transducer must be located not
more than 20 feet from the static source.

Roll Servo
Refer to Figure 22-16. Servo.

The roll servo normally located on the cabin

sloping bulkhead is splined to its capstan and is
coupled to the ailerons by a mechanical linkage
(typically through a bridle cable).

Pitch Servo

22 AUTOFLIGHT
Refer to Figure 22-16. Servo.

The pitch servo normally located in the rear

fuselage (Sta. 391.0, elevator and rudder), is
splined to its capstan and is coupled to the
elevator by a mechanical linkage (typically
through a bridle cable). Internal trim sensors
(torque) detect an out of trim condition, and
supply this information to the pitch flight
guidance computer.

Trim Servo
Refer to Figure 22-16. Servo.

The trim servo, normally located in the rear

fuselage (Sta. 391.0), is splined to its capstan
and is coupled to the elevator trim tabs by a
mechanical linkage (typically through a bridle
cable).

FOR TRAINING PURPOSES ONLY 22-21

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Figure 22-17. Remote Disconnect Switch

22 AUTOFLIGHT

Figure 22-18. Control Wheel Steering Switch

Figure 22-19. Go Around (GA) Switch

22-22 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Remote Disconnect Switch NOTES

Refer to Figure 22-17. Remote Disconnect
Switch.

The autopilot remote disconnect switch,

normally installed on the control wheel
supplies a signal to disconnect the autopilot.
The functions are the same as disconnecting
using the programmer/annunciator FD/AP
switch.

Control Wheel Steering Switch

Refer to Figure 22-18. Control Wheel Steering
Switch.

The autopilot control wheel steering switch,

installed on the control wheel supplies a signal
to declutch the servos from their capstans.
This switch selection does NOT disconnect the
autopilot. While in the CWS mode new targets

22 AUTOFLIGHT
may be selected (ALT, VS, HDG HOLD) and
when the switch is released the “new” targets
will be commanded.

Go Around (GA) Switch

Refer to Figure 22-19. Go Around (GA) Switch.

The Go around switch installed on the control

wheel supplies a signal to command a wings
level 7° nose up command. This switch
selection does NOT disconnect the autopilot.

FOR TRAINING PURPOSES ONLY 22-23

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Figure 22-20. Ready

FD/AP HDG NAV REV UP

22 AUTOFLIGHT

FD VS ALT YD DN

Figure 22-21. Heading (HDG)

FD/AP HDG NAV REV UP

FD VS ALT YD DN

Figure 22-22. Navigation (NAV) (1 of 4)

22-24 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

GENERAL THEORY OF When operating in the HDG mode, the system

is not coupled to any ground navigation device.
OPERATION It flies a specific heading, only. The pilot must
monitor navigation instruments for course
The autopilot system is configured to be
deviation due to wind drift, and to establish
compatible with a particular heading system,
wind correction angles.
and to operate at a rated voltage (A+) of
14/28VDC. Up to three independent switches,
each associated with a dedicated circuit Navigation (NAV)
breaker, apply power to the autopilot system
Refer to Figure 22-22. Navigation (NAV) (1
when closed. The battery master switch
of 4).
applies power to the turn coordinator, while
the autopilot master switch applies power to
The NAV mode provides roll commands for
the rest of the system except for auto-trim.
automatic intercept and tracking of selected
Such an arrangement preserves the integrity
VOR/LOC/GPS navigational signals.
of the turn coordinator display if the autopilot
master switch is opened or its circuit breaker is
If the VOR needle is at full-scale deviation, the
interrupted.
autopilot will establish a 45º intercept angle to
the desired course.
For an auto-trim, there is a trim master switch.
This switch applies power to the auto-trim
As the aircraft approaches the selected radial, the
section of the system, allowing for use of the
autopilot senses the closure rate, and gradually,

22 AUTOFLIGHT
manual electric trim capability if the autopilot
smoothly, shallows the intercept angle. The
master switch is opened or its circuit breaker
point that this turn begins is variable, depending
is interrupted.
on the aircraft position and closure rate to the
radial. However, the turn will always begin
Upon power-up, the programmer/annunciator
between 100% (full-scale) needle deflection
display remains blank until the rate gyro internal
and 20% of full-scale. During the intercept
to the turn coordinator reaches sufficient speed,
sequence, the system operates in maximum
and then RDY becomes annunciated. However,
gain and sensitivity to needle position and can
should the rate gyro fail to reach sufficient
command 90% of a standard rate turn.
speed, the display will remain blank and no
modes can be engaged (Figure 22-20).

Once RDY is annunciated, each of the

following modes may be engaged by pressing
the respective mode selector switch located on
the programmer/annunciator, although a roll
mode must be engaged prior to a pitch mode.

ROLL MODES OF OPERATION

Heading (HDG)
Refer to Figure 22-21. Heading (HDG).

The HDG mode provides heading preselect and

turns through the use of the heading bug on the
directional gyro (DG) or optional horizontal
situation indicator (HSI).

FOR TRAINING PURPOSES ONLY 22-25

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
22 AUTOFLIGHT

Figure 22-23. Navigation (NAV) (2 of 4)

Figure 22-24. Navigation (NAV) (3 of 4)

22-26 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Refer to Figure 22-23. Navigation (NAV) (2 NOTES

of 4).

When the selected course is intercepted

and the needle is within 15% of centered,
the CAP annunciator illuminates indicating
course capture and initiation of the tracking
gain sequence. This high sensitivity level is
maintained for approximately 15 seconds while
wind correction angle is established. Turn rate
capability is then reduced to 45% standard turn
rate (Capture/Soft Gain) identified by both the
CAP and SOFT annunciations.

Refer to Figure 22-24. Navigation (NAV) (3

of 4).

Approximately 60 seconds later, the maximum

turn rate is reduced to 15% of standard rate
(Soft Gain), and the lowest level of sensitivity
is achieved, identified by the NAV and
SOFT annunciation’s. CAP annunciation

22 AUTOFLIGHT
extinguishes. This condition provides low
activity levels during station passage when
VOR signals are erratic.

The system includes a course deviation

monitor. If the aircraft strays off course or
LOC centerline by 50% needle deflection,
the NAV annunciator flashes as a warning.
It should flash at station passage because of
short-term needle excursion beyond 50%. It
also flashes when the NAV flag is in view.
When that occurs, the FAIL annunciation will
illuminate.

FOR TRAINING PURPOSES ONLY 22-27

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

FD/AP HDG NAV REV UP

FD VS ALT YD DN
22 AUTOFLIGHT

Figure 22-25. Navigation (NAV) (4 of 4)

FD/AP HDG NAV REV UP

FD VS ALT YD DN

Figure 22-26. Reverse (REV)

22-28 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Refer to Figure 22-25. Navigation (NAV) (4 NOTES

of 4).

When a localizer frequency is channeled,

and NAV mode is selected, the system will
automatically execute high sensitivity gain
for the approach and automatically activates
the APR mode. NAV/APR illuminates on the
remote annunciator.

The pilot sets the heading bug to the inbound

localizer course, and engages the NAV mode
to intercept and track the localizer front course
inbound or back course outbound.

Reverse (REV)
Refer to Figure 22-26. Reverse (REV).

REV mode provides roll commands for

automatic intercept and tracking of the back
course localizer inbound or the front course

22 AUTOFLIGHT
localizer outbound.

GPS Intercept and Tracking

The System 65 Autopilot can also be used
to intercept and track valid GPS signals for
cross country or approach operations using the
autopilot NAV mode as follows:

1. Program the desired waypoint or initial

approach fix into the GPS navigator and
verify a valid signal is being received.
2. If using a DG, set the heading bug on the
desired track to the waypoint, select the
NAV mode. The autopilot will intercept
and track the bearing to the waypoint.
3. If using an HSI, set the course arrow to
the desired track to the waypoint before
selecting the NAV mode.

FOR TRAINING PURPOSES ONLY 22-29

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

FD/AP HDG NAV REV UP

FD VS ALT YD DN
22 AUTOFLIGHT

Figure 22-27. Vertical Speed (VS)

FD/AP HDG NAV REV UP

FD VS ALT YD DN

Figure 22-28. Altitude (ALT)

22-30 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

PITCH MODES OF OPERATION engaged, the UP and DN switches may be used

to adjust the altitude.
NOTE The UP and DN switches produce a 20
Before engaging a pitch mode of foot change in altitude for each second of
operation, a roll mode must first depression, up to a maximum of 200 feet.
be engaged. Altitude changes of more than 200 feet require
reactivation of the VS mode.
Vertical Speed (VS)
Refer to Figure 22-27. Vertical Speed (VS).

The VS mode provides pitch synchronization

of the autopilot to the aircraft vertical speed.
To activate, press the VS mode switch. This
activates the UP/DN (Down) pitch modifier
switches for pilot commanded changes of
vertical speed, up to a maximum of ± 1600
feet-per-minute (rate of climb / descent).

UP

22 AUTOFLIGHT
When the VS mode is activated, the UP
modifier switch will increase the rate-of-climb
or decrease the rate-of-descent at 160 FPM for
each second of continuous switch depression.

Down (DN)
When the VS mode is activated, the DN switch
will increase the rate-of descent or decrease
the rate-of-climb 160 FPM for each second of
continuous switch depression.

NOTE
If the VS mode annunciator
flashes, it indicates an excessive
error between the actual VS
compared to the selected VS.
The pilot should adjust the
aircraft power or correct the VS
that has been selected.

Altitude (ALT)
Refer to Figure 22-28. Altitude (ALT).

The ALT mode engages the altitude hold mode,

capturing the altitude attained at the time of
activation. When the altitude hold mode is

FOR TRAINING PURPOSES ONLY 22-31

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

FD/AP HDG NAV REV UP

22 AUTOFLIGHT

FD VS ALT YD DN

Figure 22-29. Intercepting and Coupling the Glideslope

FD/AP HDG NAV REV UP

FD VS ALT YD DN

Figure 22-30. UP/DN Trim

22-32 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Intercepting and Coupling the NOTE

Glideslope If the TRIM annunciation is
illuminated and the autopilot
Refer to Figure 22-29. Intercepting and
is disengaged, there will be a
Coupling the Glideslope.
residual out-of-trim force at
the control wheel. Be alert for
To arm the automatic glide-slope (GS) capture
this condition if the autopilot
function, the following conditions must be met:
is disengaged while the TRIM
lights are on.
•• NAV receiver must be tuned to the
appropriate frequency.
•• The glide-slope signal must be valid;
Optional Autotrim
no flag. If the autopilot is equipped with optional
autotrim, the aircraft elevator trim will be
•• The autopilot must be in NAV/APR/
maintained automatically when the trim master
ALT modes.
switch is ON and a pitch mode is activated.
•• The aircraft must be 60% or more below
the GS centerline during the approach When the trim master switch is ON, the trim
to the intercept point, and within annunciators are disabled.
50% needle deviation of the localizer
centerline at the point of intercept, If the switch is OFF, or a power failure

22 AUTOFLIGHT
usually the outer marker. occurs, the annunciators automatically become
functional.
NOTE The trim system is designed to accept any type of
GS arming will occur when the single failure, mechanical or electrical, without
above conditions have existed uncontrolled operation resulting. To ensure that
for 10 seconds. Illumination of no hidden failures have occurred, conduct a trim
the GS annunciator will occur, preflight check prior to every flight.
indicating arming has been
accomplished.
NOTE
The ALT annunciator remains on. GS capture For aircraft without auto
is indicated by extinguishing of the ALT trim, or where auto trim is
annunciation at GS intercept. This should occur disabled or turned off, the UP/
at 5% below the GS center-line. DN annunciators are used to
annunciate out of trim conditions
when either the VS or ALT
Elevator Trim Indicator modes are engaged. If up trim
The autopilot pitch servo contains a sensor is required, the UP annunciator
for detection of elevator out-of-trim loads. will illuminate. If down trim is
Without optional autotrim, when such forces needed, the DN annunciator
exceed a preset level, the TRIM annunciator will illuminate. In both cases,
will illuminate, and either the UP or DN the TRIM annunciation will
annunciator will light up, indicating the also illuminate. The pilot should
direction of required trim. manually trim the aircraft in the
direction indicated, until the light
Annunciation will be steady for about 5 extinguishes. The aircraft will
seconds, then will flash until proper trim then be trimmed for existing flight
conditions have been met. conditions (Figure 22-30).

FOR TRAINING PURPOSES ONLY 22-33

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

FD/AP HDG NAV REV UP

20 20

10 10

10 10

20 AIR 20

FD VS ALT YD DN

Figure 22-31. Flight Director Block Diagram

22 AUTOFLIGHT

20 20
20
10 10
10 20

10

10 10 10
20
10
AIR 20 AIR 20

Figure 22-32. F
 D Display, FD/AP Mode Figure 22-33. F
 D Display, FD Mode
Engaged Engaged

22-34 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Autopilot Disengage As bank angle and vertical speed approach the

required amounts, bank angle and pitch up attitude
The autopilot can be disconnected by any of the
are shallowed. When the delta reference and the
following means:
steering bars are matched, the commands have
been met. Thereafter, it is necessary to maneuver
1. Press AP DISC / TRIM INTR switch
the aircraft to keep the display elements matched
typically on control wheel.
in order to accurately fly the selected modes.
2. Activate both segments of the manual
electric trim switch on control wheel, Accurate flight-director operation requires
whenever a pitch mode (ALT HOLD, VS, alertness by the pilot and monitoring the
GS) is engaged. movement of the display. Keeping it matched is
quite simple. However, control inputs must be
3. P r e s s F D / A P S w i t c h w h e n e v e r O N
timely and smooth for accurate flight director
annunciation appears directly below it.
responses following the desired command.
4. Press FD Switch whenever ON annunciation
appears directly above it.
FD/AP Mode
5. Pull autopilot circuit breaker.
Refer to Figure 22-32. F
 D Display, FD/AP
Mode Engaged.
Flight Director Operation, (Optional)
Press the FD/AP switch. The ON annunciation
Refer to Figure 22-31. Flight Director Block
will appear directly below the switch to

22 AUTOFLIGHT
Diagram.
acknowledge that the programmer is ON and
the FD/AP mode is engaged. Engage a roll
The S-TEC system 65 integrates both the roll
mode (HDG, NAV, NAV APR, REV, REV
and pitch axis and offers a synchronized display
APR). At this point the Steering Command Bars
of the flight profile. It is automatically activated
remain stowed, but the subsequent engagement
when the autopilot pitch axis is engaged.
of a pitch mode (ALT HOLD, VS, GS) will
cause the Steering Command Bars to move
A flight director (FD) provides a visual
into view. The autopilot will steer the aircraft
indication of how accurately the pilot or
toward the Steering Command Bars, until the
autopilot is tracking the commands of the
ARS is tucked into them. The FD provides a
active mode of operation.
visual indication of how accurately the autopilot
is tracking its own roll and pitch commands.
The FD operates in either the FD/AP mode or
the FD mode.
FD Mode
FD mode disables the autopilot servos, allowing
Refer to Figure 22-33. F
 D Display, FD Mode
the pilot to control the aircraft to flight director
Engaged.
commands.
With a roll mode (HDG, NAV, NAV APR, REV,
For proper flight technique, the system presentation
REV APR) and a pitch mode (ALT HOLD, VS,
requires the pilot to roll and pitch the aircraft
GS) engaged, press the FD switch. The ON
toward the display until the delta shaped reference
annunciation will appear directly above this
is tucked into the steering command bars,
switch, whereas the ON annunciation directly
indicating that commands have been satisfied.
below the FD/AP switch will extinguish, to
acknowledge that the Programmer is ON and
For example, if the display is up and left, the
the FD mode is engaged. This disengages
pilot would be required to establish a left turn,
both the roll servo and pitch servo, although
pitch up attitude.
the previously engaged roll mode and pitch
mode annunciations will still appear on the AP

FOR TRAINING PURPOSES ONLY 22-35

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

FD/AP HDG NAV REV UP

FD VS ALT YD DN

Figure 22-34. Yaw Damper Block Diagram

FD/AP HDG NAV REV UP

22 AUTOFLIGHT

FD VS ALT YD DN

Figure 22-35. Yaw Damper Operation

YAW
DAMPER

TRIM

Figure 22-36. Yaw Damper Trim Knob

22-36 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

display. The pilot must steer the aircraft toward Yaw Damper Block Diagram
the Steering Command Bars, until the ARS is
Refer to Figure 22-34. Yaw Damper Block
tucked into them. The FD provides a visual
Diagram.
indication of how accurately the pilot is tracking
the autopilot’s roll and pitch commands.
The optional yaw damper serves to dampen
excessive adverse yaw.
Wide Area Augmentation System
(WAAS) Procedures Yaw Damper Operation
Refer to Figure 22-35. Yaw Damper Operation.
GPS Approach (With Vertical
Guidance) The yaw damper (YD) mode will become engaged
For those aircraft equipped with both the Garmin upon the initial engagement of any roll mode
400W/500W Series GPS navigation receiver (HDG, NAV, NAV APR, REV, REV APR). The
or equivalent unit, and the S-TEC ST-901 YD annunciation will appear to acknowledge
GPSS converter, with the autopilot heading that the yaw damper mode is engaged. This mode
mode engaged and the GPSS converter’s GPSS can be subsequently disengaged by pressing
mode engaged, when conducting a WAAS the YD mode selector switch, causing the YD
approach the autopilot will execute virtually annunciation to extinguish.
the entire lateral approach sequence (i.e.,
intercept and track front outbound course,
Yaw Damper Trim Knob

22 AUTOFLIGHT
complete procedure turn, intercept and track
front inbound course). Refer to Figure 22-36. Yaw Damper Trim Knob.

In addition, the autopilot will execute the The yaw damper trim knob is turned as
following vertical approaches: required, to center the slip/skid ball whenever
the yaw damper mode is engaged.
1. LPV (precision and LNAV/VNAV).
2. LNAV+V (non-precision).
Once on the front inbound course, the NAV
APR and ALT HOLD modes must be engaged
in order to intercept and track either GPS
glidepath listed above.

The remainder of the approach should be flown

like a straight-in ILS.

CAUTION
The aircraft will not automatically
level off at the decision height
(DH) or minimum descent
altitude (MDA). The pilot
must maintain an awareness of
their altitude at all times, and
disconnect the autopilot at DH
or MDA for either landing or
go-around (GA).

FOR TRAINING PURPOSES ONLY 22-37

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

FD/AP HDG NAV REV UP

FD VS ALT YD DN
22 AUTOFLIGHT

Figure 22-37. AP Display, RDY For Operation

FD/AP HDG NAV REV UP

FD VS ALT YD DN

Figure 22-38. AP Display, Turn Coordinator Failure, RDY Does Not Appear

22-38 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

FUNCTIONAL/PRE-FLIGHT NOTES
TEST PROCEDURES
The following is a typical functional/pre-flight
test and is used for instructional purposes only.
For detailed instructions and certification, the
appropriate approved documentation must be used.

1. Set trim master switch to OFF position (if

installed).
2. Set battery master switch to ON.
3. Set avionics master switch to ON - RDY
annunciation only appears on AP display
within 3 minutes (Figure 22-37).

NOTE
Should a turn coordinator
failure be detected, the RDY
annunciation will not appear as
shown in Figure 22-38, and the

22 AUTOFLIGHT
autopilot will not operate.

FOR TRAINING PURPOSES ONLY 22-39

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 22-39. Sense Freedom of Movement of Pitch Axis

22 AUTOFLIGHT

FD/AP HDG NAV REV UP

FD VS ALT YD DN

Figure 22-40. Press/Hold UP

22-40 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

4. Move the A/C control wheel forward and aft, NOTES

to sense its freedom of movement about the
pitch axis (Figure 22-39).
5. P r e s s / H o l d t h e U P s w i t c h o n t h e
programmer, while maintaining a grasp on
the A/C control wheel. (Verifies the g-force
limit switch operation) The Pitch servo
initially engages, as sensed by a reduced
freedom of A/C control wheel movement
about the pitch axis, and an audible alert
sounds a steady tone. After 2 seconds the
pitch servo disengages, as sensed by the
increased freedom of A/C control wheel
movement about pitch axis (Figure 22-40).
6. Release the UP switch - The audible alert
is silenced.

22 AUTOFLIGHT

FOR TRAINING PURPOSES ONLY 22-41

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

FD/AP HDG NAV REV UP

FD VS ALT YD DN
22 AUTOFLIGHT

Figure 22-41. Press/Hold DN

FD/AP HDG NAV REV UP

FD VS ALT YD DN

Figure 22-42. Press FD/AP Switch

22-42 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

7. P r e s s / H o l d t h e D N s w i t c h o n t h e NOTES
programmer, while maintaining a grasp on
the A/C control wheel. (Verifies the g-force
limit switch operation) The pitch servo
initially engages, as sensed by a reduced
freedom of A/C control wheel movement
about the pitch axis, and an audible alert
sounds a steady tone. After two seconds
the pitch servo disengages, as sensed by
the increased freedom of A/C control wheel
movement about pitch axis (Figure 22-41).
8. Release the DN switch - The audible alert
is silenced.
9. Press FD/AP switch (Figure 22-42).

22 AUTOFLIGHT

FOR TRAINING PURPOSES ONLY 22-43

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure 22-43. D
 G - Position Heading Bug Figure 22-44. S
 ense Freedom of Movement
22 AUTOFLIGHT

Under Lubber Line of Roll Axis

FD/AP HDG NAV REV UP

FD VS ALT YD DN

Figure 22-45. Engage Heading Mode, Disengage Yaw Damper Mode

22-44 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

10. Rotate the heading knob on the directional NOTES

gyro (DG) to position the heading bug under
the lubber line (Figure 22-43).
11. Move the A/C control wheel left and right,
to sense its freedom of movement about roll
axis (Figure 22-44).
12. Press the HDG mode selector switch to
engage the heading mode, and then press
the YD mode selector switch to disengage
the yaw damper mode (if installed). HDG
annunciation appears on the AP display,
and RDY and YD are extinguished (Figure
22-45).
13. Attempt to move the A/C control wheel left
and right. The A/C control wheel’s reduced
freedom of movement indicates that roll servo
is engaged. Verify that the roll servo can be
overridden (Figure 22-44).
14. If not, pull Autopilot circuit breaker and
do not use.

22 AUTOFLIGHT

FOR TRAINING PURPOSES ONLY 22-45

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
22 AUTOFLIGHT

Figure 22-46. HSI - Turn Heading Left/Right of Lubber Line

Figure 22-47. Sense Freedom of Movement of Pitch Axis

22-46 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

15. Turn heading bug to the left side of lubber NOTES

line then turn the heading bug to the right
side of the lubber line.
16. Verify the A/C control wheel turns to the
left then to the right.
17. Set the heading bug under the lubber line
and verify the control wheel centers and
stops (Figure 22-46).
18. Move the A/C control wheel forward and aft,
to sense its freedom of movement about the
pitch axis (Figure 22-47).

22 AUTOFLIGHT

FOR TRAINING PURPOSES ONLY 22-47

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

FD/AP HDG NAV REV UP

FD VS ALT YD DN
22 AUTOFLIGHT

Figure 22-48. Engage Altitude Hold Mode

Figure 22-49. Sense Freedom of Movement of Pitch Axis

22-48 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

19. Press the ALT mode selector switch to engage NOTES

the altitude hold mode (Figure 22-48).
20. Attempt to move the A/C control wheel
forward and aft. The A/C control wheel’s
reduced freedom of movement indicates
that the pitch servo is engaged. Verify that
the pitch servo can be overridden (Figure
22-49).
21. If not, pull Autopilot circuit breaker and
do not use.

22 AUTOFLIGHT

FOR TRAINING PURPOSES ONLY 22-49

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

FD/AP HDG NAV REV UP

FD VS ALT YD DN
22 AUTOFLIGHT

Figure 22-50. Press/Hold UP, Control Wheel Moves Aft

FD/AP HDG NAV REV UP

FD VS ALT YD DN

Figure 22-51. Press/Hold DN, Control Wheel Moves Fwd

22-50 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

22. Move the A/C control wheel until the NOTES

elevator is in the neutral position.
23. Press/Hold the UP switch - A/C control
wheel moves in aft direction, while the
audible alert sounds a steady tone (Figure
22-50).
24. Release the UP switch.
25. The A/C control wheel continues moving
in an aft direction, but the audible alert is
silenced.
26. Press/Hold DN the switch.
27. The A/C control wheel slows to a stop in
aft direction, and then moves in forward
direction while audible alert sounds a steady
tone (Figure 22-51).
28. Release the DN switch.
29. The A/C control wheel continues moving
in a forward direction, but the audible alert

22 AUTOFLIGHT
is silenced.

FOR TRAINING PURPOSES ONLY 22-51

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

FD/AP HDG NAV REV UP

FD VS ALT YD DN
22 AUTOFLIGHT

Figure 22-52. Engage Vertical Speed Mode

FD/AP HDG NAV REV UP

FD VS ALT YD DN

Figure 22-53. Press/Hold UP

22-52 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

30. Press the VS mode selector switch to NOTES

engage vertical speed mode.
31. The A/C control wheel stops.
32. VS annunciation appears on the AP display
and ALT is extinguished (Figure 22-52).
33. Press/Hold the UP switch - A/C control
wheel moves in aft direction, while the
audible alert sounds a steady tone (Figure
22-53).
34. Release the UP switch.

22 AUTOFLIGHT

FOR TRAINING PURPOSES ONLY 22-53

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

FD/AP HDG NAV REV UP

FD VS ALT YD DN
22 AUTOFLIGHT

Figure 22-54. Press/Hold DN, Control Wheel Moves Fwd

22-54 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

35. The A/C control wheel continues moving NOTES

in an aft direction, but the audible alert is
silenced.
36. Press/Hold DN the switch.
37. The A/C control wheel slows to a stop in
aft direction, and then moves in forward
direction while audible alert sounds a steady
tone (Figure 22-54).
38. Release the DN switch.
39. The A/C control wheel continues moving
in a forward direction, but the audible alert
is silenced.

22 AUTOFLIGHT

FOR TRAINING PURPOSES ONLY 22-55

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

FD/AP HDG NAV REV UP

FD VS ALT YD DN

Figure 22-55. Engage Altitude Hold Mode

22 AUTOFLIGHT

FD/AP HDG NAV REV UP

FD VS ALT YD DN

Figure 22-56. NAV and SOFT Annunciations Appear On AP Display, HDG is Extinguished

22-56 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

40. Press the ALT mode selector switch to NOTES

engage the altitude hold mode.
41. The A/C control wheel stops.
42. ALT annunciation appears on the AP
display and VS is extinguished (Figure
22-55).
43. Select a local VOR frequency on the
navigation receiver.
44. Turn the OBS knob until the CDI needle is
centered.
45. Press the NAV mode selector switch to
engage navigation mode.
46. NAV and SOFT annunciations appear
on AP display and HDG is extinguished
(Figure 22-56).

22 AUTOFLIGHT

FOR TRAINING PURPOSES ONLY 22-57

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
22 AUTOFLIGHT

Figure 22-57. CDI - Turn OBS Knob Right/Left of Center

FD/AP HDG NAV REV UP

FD VS ALT YD DN

Figure 22-58. Engage Heading Mode

22-58 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

47. Turn the OBS knob until the CDI needle NOTES
deflection is 2 dots right of center - The
A/C control wheel turns to the right.
48. Turn the OBS knob until the CDI needle
deflection is 2 dots left of center - The A/C
control wheel turns to the left.
49. Turn the OBS knob until the CDI needle
is centered - The A/C control wheel stops
(Figure 22-57).
50. Press the HDG mode selector switch to
engage heading mode.
51. HDG annunciation appears on AP display
and NAV and SOFT are extinguished (Figure
22-58).

22 AUTOFLIGHT

FOR TRAINING PURPOSES ONLY 22-59

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

FD/AP HDG NAV REV UP

22 AUTOFLIGHT

FD VS ALT YD DN

Figure 22-59. Move Control Wheel Fwd and Aft

22-60 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

52. Move the A/C control wheel as far forward NOTES

as possible.
53. After 3 seconds, TRIM and UP annunciations
appear on the AP display and an audible alert
sounds a steady tone.
54. After 7 seconds, TRIM and UP annunciations
flash, and the audible alert becomes periodic.
55. Move the A/C control wheel aft until
the TRIM and UP annunciations are
extinguished and the audible alert is
silenced (Figure 22-59).

22 AUTOFLIGHT

FOR TRAINING PURPOSES ONLY 22-61

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

FD/AP HDG NAV REV UP

22 AUTOFLIGHT

FD VS ALT YD DN

Figure 22-60. Move Control Wheel Aft and Fwd

22-62 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

56. Move the A/C control wheel as far aft as NOTES

possible.
57. After 3 seconds, TRIM and DN annunciations
appear on the AP display and an audible alert
sounds a steady tone.
58. A f t e r 7 s e c o n d s , T R I M a n d D N
annunciations flash, and the audible alert
sounds intermittently.
59. Move the A/C control wheel forward
until the TRIM and DN annunciations
are extinguished and the audible alert is
silenced (Figure 22-60).

22 AUTOFLIGHT

FOR TRAINING PURPOSES ONLY 22-63

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
22 AUTOFLIGHT

FD/AP HDG NAV REV UP

FD VS ALT YD DN

Figure 22-61. Autotrim

22-64 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Autotrim •• Press/Hold aft on both segments of

manual electric trim switch - elevator
Refer to Figure 22-61. Autotrim.
trim wheel runs nose up at full speed,
and TRIM annunciation appears
•• Set the trim master switch to the ON
flashing on AP display.
position.
•• Press/Hold AP DISC / TRIM INTR
•• Move the A/C control wheel as far
switch. elevator trim wheel stops.
forward as possible.
•• Release AP DISC / TRIM INTR switch.
•• After 2-3 seconds the elevator trim
elevator trim wheel resumes running
wheel begins to run nose up with
nose up at full speed.
increasing speed.
•• Release manual electric trim switch.
•• Move the A/C control wheel aft until
elevator trim wheel stops. TRIM
the elevator trim wheel stops.
annunciation is extinguished.
•• Move the A/C control wheel as far aft
•• Press AP DISC / TRIM INTR switch.
as possible.
Autopilot disconnects as follows:
•• After 2-3 seconds the elevator trim
•• RDY annunciation appears flashing
wheel begins to run nose down with
and ON remains, whereas all other
increasing speed.
annunciations are extinguished. After
•• Move the A/C control wheel forward 5 seconds, RDY annunciation stops

22 AUTOFLIGHT
until the elevator trim wheel stops. flashing but remains displayed.
Momentarily press either forward or aft, on
both segments of the manual electric trim
switch.

•• Autopilot disconnects.
•• RDY annunciation appears flashing and
ON remains displayed.
•• All other annunciations are extinguished.
•• After 5 seconds the RDY annunciation
stops flashing but remains visible.
•• Press/Hold forward on both segments of
manual electric trim switch - elevator
trim wheel runs nose down at full
speed, and TRIM annunciation appears
flashing on AP display.
•• Press/Hold the AP DISC / TRIM INTR
switch - elevator trim wheel stops.
•• Release AP DISC / TRIM INTR switch
- elevator trim wheel resumes running
nose down at full speed.
•• Release manual electric trim switch
- elevator trim wheel stops. TRIM
annunciation is extinguished.

FOR TRAINING PURPOSES ONLY 22-65

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
22 AUTOFLIGHT

FD/AP HDG NAV REV UP

FD VS ALT YD DN

Figure 22-62. Yaw Damper

22-66 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Yaw Damper NOTES

Refer to Figure 22-62. Yaw Damper.

•• Actuate the A/C rudder pedals alternately

in succession, to sense their freedom of
movement about the yaw axis.
•• Press the YD mode selector switch to
engage the yaw damper mode - YD
annunciation appears on the AP display.
•• Turn the yaw trim knob until the A/C
Rudder Pedals stop.
•• Attempt to push the A/C Rudder Pedals
alternately in succession.
•• A/C Rudder pedals’ reduced freedom
of movement indicates that yaw damper
servo is engaged.
•• The yaw servo can be overridden.
•• If not, pull the autopilot circuit

22 AUTOFLIGHT
breaker and do NOT use.
•• Turn the yaw trim knob fully CCW
- The Left A/C rudder pedal slowly
moves forward.
•• Turn the yaw trim knob fully CW - The
Right A/C rudder pedal slowly moves
forward.
•• Turn the yaw trim knob CCW until the
A/C rudder pedals stop.
•• Press the YD mode selector switch to
disengage the yaw damper mode - The
YD annunciation is extinguished.
•• A c t u a t e t h e A / C r u d d e r p e d a l s
alternately in succession - Increased
freedom of movement indicates that the
servo is disengaged.

FOR TRAINING PURPOSES ONLY 22-67

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
22 AUTOFLIGHT

Figure 22-63. System Interconnect (Sheet 1 of 4)

22-68 FOR TRAINING PURPOSES ONLY FOR TRAINING PURPOSES ONLY

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

22 AUTOFLIGHT
Figure 22-64. System Interconnect (Sheet 2 of 4)

FOR TRAINING PURPOSES ONLY FOR TRAINING PURPOSES ONLY 22-69

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL
22 AUTOFLIGHT

Figure 22-65. System Interconnect (Sheet 3 of 4)

22-70 FOR TRAINING PURPOSES ONLY FOR TRAINING PURPOSES ONLY

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

22 AUTOFLIGHT
Figure 22-66. System Interconnect (Sheet 4 of 4)

FOR TRAINING PURPOSES ONLY FOR TRAINING PURPOSES ONLY 22-71

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

GLOSSARY LOC..................................................Localizer
LORAN...................... Long Range Navigation
Term Meaning
LT.............................................................Left
A/C..................................................... Aircraft
NAV.............................................. Navigation
AC....................................Alternating Current
N/C..........................................No Connection
ALT................................................... Altitude
PCB...............................Printed Circuit Board
AP.................................................... Autopilot
PFGC............ Pitch Flight Guidance Computer
APR................................................. Approach
Term..................................................Meaning
ARINC...................... Aeronautical Radio, Inc.
PN............................................... Part Number
A+................................Aircraft Rated Voltage
(14VDC or 28VDC) POT...........................................Potentiometer
CAP.......................... Capture Gain Condition, PSS........................ Pitch Stabilization System
Course Captured
REV....................................................Reverse
CAP SOFT.........Capture Soft Gain Condition,
RFGC............. Roll Flight Guidance Computer
Tracking Course or localizer
RT...........................................................Right
CCW.................................. Counter-clockwise
SOFT..............................Soft Gain Condition,
22 AUTOFLIGHT

CDI....................... Course Deviation Indicator

Tracking Course
CMD............................................... Command
SOL.................................................. Solenoid
CRS..................................................... Course
TACH........................................... Tachometer
CW................................................. Clockwise
TSO........................ Technical Standard Order
CWS.......................... Control Wheel Steering
UP.............................................................. Up
DC............................................Direct Current
UUT....................................... Unit Under Test
DG........................................ Directional Gyro
VDC................................ Volts Direct Current
DN......................................................... Down
VHF...............................Very High Frequency
DVM.................................. Digital Volt Meter
VOR...............................Very High Frequency
EFIS........ Electronic Flight Instrument System Omnidirectional
Radio Range
FD............................................ Flight Director
Vpp................................... Volts Peak-to-Peak
FPM....................................... Feet Per Minute
Vrms........................ Volts Root-Mean-Square
GND................................................... Ground
VS............................................ Vertical Speed
GPS........................Global Positioning System
XDCR............................................ Transducer
GPSS...... Global Positioning System Steering
GS.................................................. Glideslope
HDG.................................................. Heading
HI........................................................... High
HSI...................Horizontal Situation Indicator
LO........................................................... Low

22-72 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

APPENDICES
CONTENTS
Page
APPENDIX A..................................................................................................... APP-A-3
Ground Operations....................................................................................... APP-A-3
APPENDIX B..................................................................................................... APP-B-3
APPENDIX C..................................................................................................... APP-C-3
Instrument Markings.................................................................................... APP-C-3
Airspeed Indicator Markings................................................................. APP-C-3
Engine Instrument Markings................................................................. APP-C-3

APPENDICES

FOR TRAINING PURPOSES ONLY APP-i

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

ILLUSTRATIONS
Figure Title Page

APP-B-1 Annunciators..................................................................................... APP-B-2

APP-C-1 Airspeed............................................................................................ APP-C-2
APP-C-2 Torque............................................................................................... APP-C-2
APP-C-3 Propeller RPM................................................................................... APP-C-2
APP-C-4 ITT.................................................................................................... APP-C-4
APP-C-6 Oil Temperature................................................................................. APP-C-4
APP-C-5 Gas Generator RPM........................................................................... APP-C-4
APP-C-7 Oil Pressure....................................................................................... APP-C-4

APPENDICES

FOR TRAINING PURPOSES ONLY APP-iii

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

APPENDIX A

APPENDICES

FOR TRAINING PURPOSES ONLY APP-A-1

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

APPENDIX A NOTES

GROUND OPERATIONS
The ground operations procedures include
normal and abnormal procedures, emergency
procedures, leak check procedures, engine
checks, shutdown and system integration
checks. These are found in the Twin Otter
Ground Run-up Checklist.

APPENDICES

FOR TRAINING PURPOSES ONLY APP-A-3

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

APPENDIX B

APPENDICES

FOR TRAINING PURPOSES ONLY APP-B-1

APP-B-2 APPENDICES

L GENERATOR DOORS PNEUMATIC R GENERATOR

RESET PROPS
OVERHEAT UNLOCKED LOW PRESS OVERHEAT

BOOST PUMP 1 BOOST PUMP 2 BOOST PUMP 2 BOOST PUMP 1

L GENERATOR R GENERATOR
AFT PRESS AFT PRESS FWD PRESS FWD PRESS
FIRE PULL FIRE PULL
L ENGINE AFT FUEL FWD FUEL R ENGINE
DUCT OVERHEAT 400 CYCLE
OIL PRESSURE LOW LEVEL LOW LEVEL OIL PRESSURE

TWIN OTTER SERIES

SEL
ARM
FOR TRAINING PURPOSES ONLY

L R

TRIM IN G/S ARM G/S

MAINTENANCE TRAINING MANUAL

CAPT
MOTION NAV
STALL
STAL
LL DEAD REC NAV ARM CAPT

BACK UP SELECT
DISARMED
L R ARMED
ENG ENG

HDG NAV APPR

AFC AFCS
EMER ALT IAS B/C
DME GA
BCN
INVALID VGR VLF

TRIM UP
TRIM DN

Figure APP-B-1. Annunciators

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

APPENDIX B NOTES

Refer to Figure APP-B-1. Annunciators.

Appendix B presents a color representation of

all the annunciator lights in the airplane.

APPENDICES

FOR TRAINING PURPOSES ONLY APP-B-3

TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

APPENDIX C

APPENDICES

FOR TRAINING PURPOSES ONLY APP-C-1

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure APP-C-1. Airspeed

APPENDICES

Figure APP-C-2. Torque Figure APP-C-3. Propeller RPM

APP-C-2 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

APPENDIX C Engine Instrument Markings

Torque
INSTRUMENT MARKINGS Refer to Figure APP-C-2. Torque.
Airspeed Indicator Markings
Series 100/200
General Maximum (red radial)......................... 42.5 psi
Refer to Figure APP-C-1. Airspeed.
Normal (green arc).......................0 to 42.5 psi
Colored markings are placed on the airspeed Acceleration
indicator dials to assist the pilot in observing (unmarked)............ 48.5 psi (two-second limit)
the airspeed operating limitations. All markings
represent calibrated airspeeds.
Series 300
Maximum (red radial)......................... 50.0 psi
Series 300
Normal (green arc).......................0 to 50.0 psi
Maximum operating
speed (red radial line).................... 170 KCAS Acceleration
(unmarked)............ 68.8 psi (two-second limit)
Normal operating
range (green arc)................... 74 to 170 KCAS
Flap operating
Propeller RPM
range (white arc)..................... 58 to 95 KCAS Refer to Figure APP-C-3. Propeller RPM.
Minimum control
speed (red radial line)...................... 66 KCAS Series 100/200
Speed for the best rate Maximum (red radial)............................ 100%
of climb with one engine
Normal (green arc)........................ 75 to 100%
inoperative, flaps 10°
(blue radial line)............................... 82 KCAS
Series 300
Series 100/200 Maximum (red radial).............................. 96%

APPENDICES
Maximum operating Normal (green arc).......................... 75 to 91%
speed (red radial line).................... 160 KCAS
Normal operating
range (green arc)................... 72 to 160 KCAS
Flap operating
range (white arc):
•• Flaps 0 to 20.............. 62 to 100 KCAS
•• Flaps 20 to 37.5........... 56 to 85 KCAS
Minimum control
speed (red radial line)...................... 65 KCAS
Speed for the best rate
of climb with one engine
inoperative, flaps 10°
(blue radial line)............................... 78 KCAS

FOR TRAINING PURPOSES ONLY APP-C-3

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

Figure APP-C-4. ITT Figure APP-C-5. Gas Generator RPM

APPENDICES

Figure APP-C-6. Oil Temperature Figure APP-C-7. Oil Pressure

APP-C-4 FOR TRAINING PURPOSES ONLY

 TWIN OTTER SERIES MAINTENANCE TRAINING MANUAL

ITT Oil Pressure

Refer to Figure APP-C-4. ITT. Refer to Figure APP-C-7. Oil Pressure.

Series 100/200 Series 100/200

Maximum (red radial).......................... 750° C Maximum (red radial)............................85 psi
Caution (yellow arc)................. 705 to 750° C Normal (green arc)........................65 to 85 psi
Normal (green arc).................... 400 to 705° C Caution (yellow arc).....................40 to 65 psi
Starting Minimum (red radial).............................40 psi
(unmarked)...........1,090° C (two-second limit)
Series 300
Series 300 Maximum (red radial).......................... 100 psi
Red segment............................725 to 1,200° C
Normal (green arc)...................... 80 to 100 psi
Caution (yellow arc)................. 695 to 725° C
Caution (yellow arc).....................40 to 80 psi
Normal (green arc).................... 400 to 695° C
Minimum (red radial).............................40 psi
Starting
(white radial).......1,090° C (two-second limit)

Gas Generator RPM

Refer to Figure APP-C-5. Gas Generator RPM.

All Models
Maximum (red radial)......................... 101.5%
Normal (green arc)..................... 50 to 101.5%

Oil Temperature

APPENDICES
Refer to Figure APP-C-6. Oil Temperature.

All Models
Maximum (red radial)............................ 99° C
Normal (green arc)........................ 10 to 99° C
Caution (yellow arc)................... −40 to 10° C

FOR TRAINING PURPOSES ONLY APP-C-5