Dash8 - Q400 - Pneumatics

12.19.1

Introduction

The Auxiliary Power Unit (APU) replaces the standard composite tailcone with a titanium tailcone
and firewall. The APU is accessed by two clamshell type doors on the bottom of the tail cone. It
consists of a gas turbine engine driving a DC starter-generator. The APU supplies bleed air for
the Environmental Control System (ECS), and 28 VDC to the electrical system. The APU cannot
be operated in flight.
12.19.2

General

The start control, normal operation, and malfunction monitoring of the APU is automatically performed by the APU FADEC. The APU starter-generator can be powered from either the
aeroplane main battery or external power. Intake air is drawn through a screened inlet duct on
the right rear of the fuselage. Exhaust gases flow through an exhaust ejector and are discharged
through an upwards pointing outlet at the aft end of the titanium tailcone. The APU is protected
by its own automatic fire detection and extinguishing system that continuously monitors the APU
and its compartment whenever electrical power is supplied to the system. The APU control panel
is mounted on the overhead console in the flight deck.
APU Louvered Inlet Cover
A louvered cover is provided for the APU Air Inlet to help prevent snow and sleet from entering
the APU Inlet when the aeroplane is on a long turnaround or remaining overnight.

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Dash8 - Q400 - Pneumatics

12.19.3

Controls and Indications - Auxiliary Power Unit

Page 2

Dash8 - Q400 - Pneumatics

APU CONTROL
PWR

START

GEN

BL AIR

RUN

STARTER

ON

OPEN

FAIL

WARN

Figure 12.19-1 APU Control Panel

Page 3

GEN OHT

Dash8 - Q400 - Pneumatics

APU CONTROL PANEL CALLOUTS

1. PWR SWITCHLIGHT (alternate action)
PUSH - arms APU start circuits and opens the APU fuel valve, APU FUEL VALVE OPEN
light (green) shown on the APU fire protection panel
- only arms if aeroplane on ground, no fire detected, and EXTG switch not selected
RUN segment (green)
- APU is at operating speed after the START switchlight is pushed
PUSH - RUN segment (out)
- closes APU fuel valve, APU FUEL VALVE CLOSED light (white) shown on the APU fire
protection panel, and the APU stops
FAIL - a failure is detected and the APU automatically stops
2. START SWITCHLIGHT (alternate action)
PUSH - STARTER segment (amber)
- starts automatic APU start sequence
STARTER segment (out)
- APU start sequence complete or stopped
3. GEN SWITCHLIGHT (alternate action)
PUSH - ON segment (green)
- APU starter-generator is supplying DC power
PUSH - ON segment (out)
- selects APU generator off line
WARN segment (amber)
- APU starter-generator off line with APU running
4. BL AIR SWITCHLIGHT (alternate action)
PUSH - OPEN segment (green)
- APU bleed air valve open
- APU bleed air supplies air to the ECS if engine bleed air is off
PUSH - OPEN segment (out)
- APU bleed air valve is closed
- selects APU bleed air off
5. GEN OHT ADVISORY LIGHT
GEN OHT segment (amber)
APU starter-generator overheat condition
APU automatically shuts down

Page 4

Figure 12.19-2 APU Fire Protection Panel (1 of 2)

Page 5

FAULT

VALVE

CLOSED

BTL LOW

EXTG

OPEN

FUEL

APU

FIRE
TEST

FIRE

ARM

BTL

FAULT
B

BTL
LOW

FAULT
A

HYD

PULL FUEL/HYD OFF

FAULT
B

FWD BTL

EXTG
AFT BTL

ENGINE 2

CLOSED

VALVES
FUEL

OPEN

TEST
DETECTION

CLOSED

HYD

OPEN

VALVES
FUEL

PULL FUEL/HYD OFF

ENGINE 1

FWD BTL

EXTG
AFT BTL

FAULT
A

EXTG

TEST

FWD

AFT

SMOKE

ARM LOW

ARM LOW

FIRE BOTTLE

EXTG

SMOKE

BAGGAGE
FWD

FWD

AFT

1
TEST

VALVE
OTLT

CLOSED

VENT
INLT

BAGGAGE
AFT

Dash8 - Q400 - Pneumatics

Dash8 - Q400 - Pneumatics

APU FIRE PROTECTION PANEL CALLOUTS

1. FUEL SHUT-OFF VALVE CLOSED ADVISORY LIGHT (white)
- APU fuel shut-off valve closed
- APU shutdown, manually or automatically
2. FUEL SHUT-OFF VALVE OPEN ADVISORY LIGHT (green)
- APU fuel shut-off valve open
- PWR switchlight pushed
- APU circuits armed for starting or fire test
3. FIRE ADVISORY LIGHT (red)
- APU fire detected
- after 7 seconds extiguishing agent automatically releases, and amber BTL ARM light
goes out
4. EXTG SWITCHLIGHT (guarded, alternate action)
EXTG segment (white)
- APU fire extinguisher bottle can be activated manually if BTL ARM light is on
PUSH - EXTG segment (out)
- APU fire bottle cannot be activated, or has been set off
5. BOTTLE LOW LIGHT (amber)
- fire extinguisher bottle is low or empty
6. FAULT ADVISORY LIGHT (AMBER)
- fault in the APU fire extinguisher system
- fault in the APU FPP
7. BTL ARM ADVISORY LIGHT
(amber)
- APU fire extinguisher bottle ready to be activated
(OUT)
- APU fire extinguishing bottle cannot be activated
- no power to the system

Page 6

Figure 12.19-3 APU Fire Protection Panel (2 of 2)

Page 7

FAULT

VALVE

CLOSED

BTL LOW

EXTG

OPEN

FUEL

APU

FIRE
TEST

FIRE

ARM

BTL

ENGINE 1

FAULT
B

BTL
LOW

FAULT
A

HYD

PULL FUEL/HYD OFF

FAULT
B

FWD BTL

EXTG
AFT BTL

ENGINE 2

CLOSED

VALVES
FUEL

OPEN

TEST
DETECTION

CLOSED

HYD

OPEN

VALVES
FUEL

PULL FUEL/HYD OFF

FAULT
A

FWD BTL

EXTG
AFT BTL

EXTG

TEST

FWD

SMOKE

ARM LOW

FWD

BAGGAGE
FWD

ARM LOW

AFT

AFT

EXTG

SMOKE

FIRE BOTTLE

TEST

VALVE
OTLT

CLOSED

VENT
INLT

BAGGAGE
AFT

Dash8 - Q400 - Pneumatics

Dash8 - Q400 - Pneumatics

APU FIRE PROTECTION PANEL CALLOUTS (contd)

8. FIRE TEST PUSHBUTTON (momentary action)
PUSH AND HOLD - check the following lights:

MASTER WARNING light (red) flashes

CHECK FIRE DET warning light (red) flashes

FIRE segment (red)

BTL ARM light (amber)

FUEL VALVE OPEN (out)

FUEL VALVE CLOSED (white)

FAULT light (amber)

EXTG segment (white)

RELEASE - check the FUEL VALVE OPEN (green) comes on

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Dash8 - Q400 - Pneumatics

2
3

ELECTRICAL

DC EXT
POWER

MAIN
AUX
STBY

+1. 00 +22
- . 34 +22
- . 34 +22

LOAD

+1. 00
DC GEN
1

ESS
MAIN
SEC

VOLT

28.1
28.1
28.1

VOLT

LOAD

A 115
B 115
C 115

. 06
. 06
. 06

LOAD

. 00

+0. 00

DC BUS
L

AC GEN 1

APU GEN

BATT
LOAD

AC EXT
POWER

AC GEN 1

28.1
28.1
28.1

TRU
1

LOAD

+1. 00 +1. 00

VOLT

LOAD

A 115
B 115
C 115

. 06
. 06
. 06

SPOILERS

0 5
10
FLAP
DEG

LO

RUD

L1

R1

35

ELEV
HYD PRESS PSI
PK BRK

3000

STBY

3000

3000

ELEV
HYD QTY %
3

0 100

100

100

Figure 12.19-4 MFD Electrical Page with APU Generator Operating (1 of 2)

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Dash8 - Q400 - Pneumatics

MFD ELECTRICAL PAGE CALLOUTS

1. DIGITAL DISPLAY OF APU GENERATOR LOAD (white)
APU GEN (white)
LOAD (cyan)
- is activated only if the APU is installed depending on the aeroplane configuration
- the load demand on the APU generator is displayed in the following format:
+ sign is displayed on the left of the lead digit to indicate an overload of the DC
power source

nothing is displayed when the load is in the expected range

leading zero is suppressed in the lead digit position

digital number gives the rate of the load, with 1.00 equal to 100% of load

examples:
.60 indicates that the DC generator or TRU is loaded at 60% of the maximum output.

+1.30 indicates an overload of the DC generator or TRU

2. BATTERY LOAD AND TEMPERATURE ANNUNCIATION AREA

BATT (white)
LOAD, C (cyan)
MAIN, AUX, STBY (white)
3. DIGITAL DISPLAY OF BATTERY LOAD (white)
- displays the load of the battery
- + or - sign on the left of the lead digit, whether the battery is in overcharge (+ displayed)
or in discharge (- displayed)
- nothing is displayed when the battery is in charge within the expected range
- leading zero is suppressed in the lead digit position
- digital number gives the rate of the load, with 1.00 equal to 100% of load
- examples:

.60 indicates that the battery is in charge at 60% of its maximum rate of charge

-1.30 indicates that the battery is discharging at 30% over the maximum rate of
discharge

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Dash8 - Q400 - Pneumatics

ELECTRICAL

. 06 +22
. 04 +19
. 03 +20

MAIN
AUX
STBY

LOAD

. 64

ESS
MAIN
SEC

VOLT

27.2
28.0
28.0

. 00

TRU
1

LOAD

. 00

LOAD

0
0
0

. 00
. 00
. 00

AC GEN 2

27.2
28.0
28.0

VOLT

LOAD

.00

DC BUS
L

A
B
C

DC GEN
1

4
5

AC GEN 1

APU GEN

BATT
LOAD

. 00

A
B
C

VOLT

LOAD

0
0
0

. 00
. 00
. 00

SPOILERS

0 5
10
FLAP
DEG

LO

RUD

LI

RI

35

ELEV
HYD PRESS PSI
PK BRK

3000

STBY

3000

3000

RO

ELEV
HYD QTY %
3

0 100

100

100

Figure 12.19-5 MFD Electrical Page with APU Generator Operating (2 of 2)

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Dash8 - Q400 - Pneumatics

MFD ELECTRICAL PAGE CALLOUTS (contd)

4. DIGITAL DISPLAY OF BATTERY TEMPERATURE
- battery temperature digits are displayed in white with a + or - sign on the left of the lead
digit, when the temperature is within normal limits
- when the temperature is in the range (+50C to +65C) the digits turn yellow
- when the temperature exceeds 65C the digits are displayed in red
5. DC bus voltage annunciation area
DC BUS (white)
L, R (white)
VOLT (cyan)
6. DIGITAL DISPLAY OF DC BUS VOLTAGE (white)
- indicates the voltage on the associated bus

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Dash8 - Q400 - Pneumatics

ENGINE

CL
AIRCRAFT

DRY
BAY

INLET
STRAINER

COLLECTOR
BAY

ENGINE FEED LINE

Figure 12.19-6 APU Fuel Schematic

Page 13

APU SHUT-OFF
VALVE

APU FEED
LNE

Dash8 - Q400 - Pneumatics

12.19.4

Auxiliary Power Unit - Description

12.19.4.1

APU Fuel System

APU fuel is supplied from the left wing collector bay through an APU shutoff valve. A rigid fuel
line is routed from the shutoff valve to the APU/tailcone firewall (Figure 12.19-6), outside the
pressurized part of the fuselage. A gravity-fed, APU-driven fuel pump keeps positive fuel pressure to the APU engine.
The APU shutoff valve opens when the APU PWR switchlight is pushed and closes when the
APU is shut down. The position of the shutoff valve is shown on the APU Fire Protection Panel
(FPP). Fuel is automatically scheduled for starting, acceleration, and speed regulation.
The APU shutoff valve will close if:

PWR switchlight is pushed off

Fire is detected in the tailcone
EXTG switchlight is pushed
Aeroplane is in flight

Page 14

Figure 12.19-7 Starter-Generator Schematic

Page 15

#1 AC
GEN

OFF

GCU

L VARIABLE
AC BUS

L TRU

#1 DC
GEN

OFF

EXT
PWR

#1 AC
GEN

L SEC
FEEDER

L MAIN
FEEDER

AUX
BATT

AUX
GCU

BATTERY
BUS

#1 DC
GEN

GALLEY
BUS

STBY

STBY
BATT

LEFT ESSENTIAL

OFF

EPCU

MAIN BUS
TIE

OFF

#2 AC
GEN

R SEC
FEEDER

R MAIN
FEEDER

GCU

OFF

ON

GEN
WARN

OFF

APU
DC GEN

GALLEY
BUS

GCU

#2 DC
GEN

#2 AC
GEN

R VARIABLE
AC BUS

R TRU

#2 DC
GEN

OFF

GCU

OFF

BATTERY
MASTER

MAIN

MAIN
BATT

RIGHT ESSENTIAL

Dash8 - Q400 - Pneumatics

Dash8 - Q400 - Pneumatics

12.19.4.2

Starter-Generator

The APU has a gearbox mounted 28 VDC starter-generator (Figure 12.19-7). An APU start
requires either aeroplane batteries or external power. When the APU starts, the starter stays
engaged until the APU reaches half its operating speed. When the APU is operating, the RUN
segment comes on to show that, the generator mode is available to supply 28 VDC. The WARN
segment of the GEN switchlight comes on when the generator is off-line.
The APU electrical load and voltage can be monitored on the ELECTRICAL page of the MFD.
After the aeroplane main engine DC starter-generators are on line, the APU generator will continue to supply power in parallel to the DC buses. The APU system will automatically prevent
APU generator output if external AC or DC power is applied to the aeroplane.
The starter-generator is cooled by air which enters through a metal mask screen at the bottom
forward section of the tailcone and exits through the tailcone right side door (Figure 12.19-8). If
the starter-generator overheats, the APU will automatically shut down, shown by the GEN HOT
advisory light.
If a starter-generator fault is detected, the:

Starter-generator is removed from the right main feeder bus

ON segment of the GEN switchlight goes out
WARN segment of the GEN switchlight goes amber

The APU utilizes 2x40 Ahr NiCad batteries for starting when external DC power is not available.
The characteristics of the Starter are such that the initial current draw is not highly dependent on
the load being driven, but more a function of the Starter itself, and the state of charge of the batteries.Due to the high inrush currents during starting, there is a drop in voltage at the battery, and
hence the Main Buses. Technical data shows that for a battery only start, with a battery at
100% charge, bus voltage drops to about 20 VDC (value varies with temperature). For a battery
at 50% charge, the bus voltage drops to about 18 VDC (value varies with temperature). It is clear
that starting the APU using batteries with insufficient charge can lead to system problems which
jeopardize successful start.

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Dash8 - Q400 - Pneumatics

2
3

4
5

5
9

10

LEGEND
1. Drain Mast.
7. Starter/Generator Exhaust
2. Drain Outlet Opening.
Air Duct Opening.
3. Left APU Door.
8. Starter/Generator Exhaust
4. Strut Push-Release.
Air Duct.
5. Telescopic Struts.
9. APU Ventilation Inlet.
6. Right APU Door.
10. APU Starter/Generator
Air Inlet.

Figure 12.19-8 APU Starter-Generator Air Inlet and Exhaust

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Dash8 - Q400 - Pneumatics

12.19.4.3

APU Compartment Ventilation

APU compartment ventilation is established by the APU exhaust air ejector system.
Ventilation air enters the compartment through a screened inlet near the bottom forward end of
the tailcone. Air is directed by a composite duct to the area in front of the oil sump to assist oil
cooling. Air then flows around the APU before being mixed with the exhaust ejector flow.

Page 18

APU BLEED
MANIFOLD
APU BLEED
VALVE

FIREWALL

FROM
ENGINE #1

APU CHECK
VALVE

WING DUCT
CHECK VALVE

FROM
ENGINE #2

Figure 12.19-9 APU Bleed Air System Schematic

Page 19

TO AFT SAFETY VALVE

(CABIN PRESSURE
CONTROL SYTEM)

TO ENVIRONMENTAL
CONTROL SYSTEM

WING DUCT
CHECK VALVE

Dash8 - Q400 - Pneumatics

Dash8 - Q400 - Pneumatics

12.19.5

APU Bleed-Air System

When the APU is operating, the APU bleed air valve can be opened by pushing the BL AIR
switchlight on the APU control panel. APU bleed air supplies bleed air for the ECS, and holds the
Cabin Pressure Control System (CPCS) aft safety valve open (Figure 12.19-9). When the valve
is not closed, the OPEN segment (green) of the BL AIR switchlight will come on. The bleed air
supply is reduced if the APU exhaust temperature reaches an established temperature limit. This
gives APU generator load priority over bleed air.
An APU check valve and wing duct check valves prevent APU bleed air from entering the engine
bleed air supply, including airframe de-icing. If either main engine BLEED air toggle switch is set
to 1 or 2, the APU BL AIR switchlight is automatically de-energized. This ensures that bleed air is
not supplied from the engines and APU at the same time.

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Dash8 - Q400 - Pneumatics

APU
APD
SENSOR

CONTROL
AMPLIFIER

SQUIB
PRESSURE
SWITCH

APU
FIRE
BTL

FIRE
PROTECTION
PANEL

Figure 12.19-10 APU Fire Detection Schematic

Page 21

MASTER
WARNING/
CAUTION PANEL

Dash8 - Q400 - Pneumatics

12.19.6

APU Fire Protection

The APU has a fully automatic fire detection and extinguishing system (Figure 12.19-10). The
system monitors the APU hot section and exhaust whenever the right essential 28 VDC bus is
energized. The system is operated from the Fire Protection Panel (FPP) on the overhead console.
The system includes a:

stainless steel fire extinguisher bottle and distribution tubing

loop sensor routed along the tailcone above the APU
control circuit

When a fire or overheat condition is sensed by the detection loop, the:

FIRE light (red), on the FPP illuminates

MASTER WARNING light flashes (red)
CHECK FIRE DET warning light flashes (red)
BTL ARM advisory light (amber); then (out) if the bottle has discharged
APU FUEL VALVE CLOSED advisory light (white) illuminates
APU FUEL VALVE OPEN advisory light (out) extinguishes
EXTG segment of EXTG switchlight (white) illuminates
MASTER CAUTION light flashes (amber)
APU caution light (amber) illuminates

12.19.7

APU Fire Extinguishing

If a fire is detected, the APU automatically shuts down and the fire extinguishing agent is
released after 7 seconds. If automatic fire extinguisher discharge fails, the BTL ARM light stays
on. The guarded EXTG switchlight can be pushed to discharge the fire extinguishing agent, if the
BTL ARM is on.
NOTE:

Once the bottle has been discharged, restarting the APU is prevented until the bottle
has been replaced.

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Dash8 - Q400 - Pneumatics

12.19.8

APU Faults

The APU system can detect a fault, isolate the APU and supply protection. The system will protect and shut down the APU.
The APU system monitors:

Overspeed
Underspeed
Start Failure
Accelerate Failure
EGT overtemperature
Low Oil pressure
High Oil temperature
Failed Sensors
Failed Valves, relays, circuits
Internal failure

If an APU fault is detected, the:

APU automatically shuts down

FAIL segment of the APU PWR switchlight (amber)
APU FUEL VALVE CLOSED advisory light (white)
APU FUEL VALVE OPEN advisory light (out)
APU caution light (amber)

The APU PWR switchlight must be reselected after an automatic shutdown or failure to start.
12.19.9

APU Shutdown

Before shutting down the APU, close the bleed air valve and select the GEN off. Normal APU
shut down is then accomplished by pushing the PWR switchlight.
12.19.10

Limitations

Item 1 (Ref. Para 12.19.4.3)

With composite cooling duct removed, ambient temperature limitation applies: 30C or ISA
+25C, whichever is lower.
Item 2
For APU operation with the Air Inlet Louvre installed, ambient temperature Limitation of 21C
applies.

Page 23