PRATT & WHITNEY CANADA

MAINTENANCE MANUAL
MANUAL PART NO. 3015442

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MANUAL PART NO. 3015442

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MANUAL PART NO. 3015442

TABLE OF CONTENTS
SUBJECT PAGE

PART 1 - DESCRIPTION OF ENGINE

SECTION 1 - GENERAL INFORMATION AND DESCRIPTION

INTRODUCTION

1. GENERAL 1-1-1
2. MAINTENANCE CONCEPT 1-1-2
3. CUSTOMER SUPPORT 1-1-2
4. ACCESSORIES SUPPORT 1-1-2

5. SUPPLEMENTARY PUBLICATION 1-1-2

6. SERVICE BULLETINS 1-1-2
7. DIRECTIONAL REFERENCES 1-1-3
8. FITS AND CLEARANCES 1-1-3
9. TOOL PROCUREMENT 1-1-3
10. SUPPLIERS AND SUPPLIER SERVICES 1-1-3
11. CONSUMABLE MATERIALS 1-1-3
SECTION 2 - LEADING PARTICULARS AND SPECIFICATIONS

1. GENERAL 1-2-1
2. FRONT AND REAR ACCESSORY DRIVES 1-2-2
3. FUEL AND LUBRICATION SYSTEMS 1-2-2
4. METAL SHIPPING CONTAINER 1-2-4
5. FIBERBOARD SHIPPING CONTAINER 1-2-4
SECTION 3 - GENERAL INFORMATION

1. GENERAL 1-3-1
SECTION 4 - DETAILED INFORMATION

1. GENERAL 1-4-1
2. COMPRESSOR INLET CASE 1-4-1

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TABLE OF CONTENTS
SUBJECT PAGE

SECTION 4 - DETAILED INFORMATION (Cont’d)

3. COMPRESSOR ROTOR AND STATOR ASSEMBLY 1-4-1

4. GAS GENERATOR CASE 1-4-7
5. COMBUSTION CHAMBER LINER 1-4-11
6. TURBINE SECTION 1-4-11
7. COMPRESSOR TURBINE VANES 1-4-11
8. COMPRESSOR TURBINE 1-4-15
9. INTERSTAGE BAFFLE 1-4-15
10. POWER TURBINE VANES 1-4-15
11. POWER TURBINE 1-4-15
12. POWER TURBINE SUPPORT HOUSING 1-4-16
13. NO. 3 AND 4 BEARINGS 1-4-16
14. REDUCTION GEARBOX (PT6A-SERIES ENGINES) - TWO
STAGE 1-4-16
15. TORQUEMETER 1-4-20
16. EXHAUST DUCT 1-4-21
17. ACCESSORY GEARBOX 1-4-21
18. ENGINE LIFTING POINTS 1-4-23

SECTION 5 - AIR SYSTEMS

1. GENERAL 1-5-1

2. COMPRESSOR BLEED VALVE 1-5-1

3. BEARING COMPARTMENT SEALS, TURBINE COOLING AND
AIR BLEED SYSTEMS 1-5-1
SECTION 6 - LUBRICATION SYSTEM

1. GENERAL 1-6-1
2. OIL TANK 1-6-1
3. OIL PUMP 1-6-1

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MANUAL PART NO. 3015442

TABLE OF CONTENTS
SUBJECT PAGE

SECTION 6 - LUBRICATION SYSTEM (Cont’d)

4. OIL FILTER ASSEMBLY 1-6-1

5. OIL PRESSURE SYSTEM 1-6-3
6. PITCH REVERSING VALVE SYSTEM (PT6A-6A ENGINES) 1-6-7
7. SCAVENGE OIL SYSTEM 1-6-7
8. BREATHER SYSTEM 1-6-8
9. CENTRIFUGAL BREATHER 1-6-8
10. OIL-TO-FUEL HEATER 1-6-9
11. OIL SYSTEM INSTRUMENTATION 1-6-9
SECTION 7 - IGNITION SYSTEM

GLOW PLUG TYPE (PRE-SB1429)

1. GENERAL 1-7-1
2. IGNITION CURRENT REGULATOR 1-7-1
3. GLOW PLUGS 1-7-1
SPARK IGNITER TYPE (POST-SB1429)

4. GENERAL 1-7-4
5. IGNITION EXCITER 1-7-4
6. SPARK IGNITERS 1-7-4

SECTION 8 - FUEL SYSTEM

INTRODUCTION

1. GENERAL 1-8-1
FUEL PUMP (VANE TYPE)

2. GENERAL 1-8-1
FUEL PUMP (GEAR TYPE)

3. GENERAL 1-8-5

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MANUAL PART NO. 3015442

TABLE OF CONTENTS
SUBJECT PAGE

SECTION 8 - FUEL SYSTEM (Cont’d)

FUEL CONTROL SYSTEM

4. GENERAL 1-8-5
FUEL CONTROL UNIT

5. GENERAL 1-8-9
6. FUEL SYSTEM 1-8-9
7. POWER INPUT, SPEED GOVERNOR, AND ENRICHMENT
SECTION 1-8-11
8. BELLOWS SECTION 1-8-13
POWER TURBINE GOVERNOR (PT6A-6 SERIES AND PT6A-20 ENGINES)

9. GENERAL 1-8-16
PROPELLER GOVERNOR (PT6A-20A AND -20B ENGINES)

10. GENERAL 1-8-17

TEMPERATURE COMPENSATOR

11. GENERAL 1-8-17

FUEL CONTROL SYSTEM - COMPLETE OPERATION (TYPICAL)

12. ENGINE STARTING 1-8-17

13. ACCELERATION 1-8-20
14. GOVERNING 1-8-20
15. ALTITUDE COMPENSATION 1-8-20
16. DECELERATION 1-8-21
17. REVERSE THRUST OPERATION 1-8-21
18. STOPPING THE ENGINE 1-8-22
FUEL DUMP VALVE

19. GENERAL 1-8-22

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MANUAL PART NO. 3015442

TABLE OF CONTENTS
SUBJECT PAGE

SECTION 8 - FUEL SYSTEM (Cont’d)

FUEL MANIFOLD AND SPRAY NOZZLES

20. GENERAL 1-8-22

21. FUEL MANIFOLD ADAPTER ASSEMBLY 1-8-24
SECTION 9 - ENGINE CONTROLS AND INSTRUMENTATION

1. GENERAL 1-9-1
2. POWER CONTROL LEVER 1-9-1
3. FUEL SHUT-OFF LEVER (PT6A-6A, -6B, -6/C20, -20, -20A AND
-20B ENGINES) 1-9-1
4. PROPELLER CONTROL LEVER (PT6A-6, -6A, -6B, -6/C20, -20,
-20A AND -20B ENGINES) 1-9-1

5. INTERTURBINE TEMPERATURE SENSING SYSTEM (T5)

(PT6A-6/C20, -20, -20A AND -20B ENGINES) 1-9-2
6. TURBINE TEMPERATURE SENSING SYSTEM (T4) (PT6A-6, -6A
AND -6B ENGINES) 1-9-2
7. OIL PRESSURE INDICATOR 1-9-2
8. OIL TEMPERATURE INDICATOR 1-9-5
9. TACHOMETER-GENERATOR - GAS GENERATOR (Ng) 1-9-5
10. TACHOMETER-GENERATOR - POWER TURBINE (Nf) 1-9-5
11. TORQUEMETER 1-9-5
SECTION 10 - PROPELLER NON-REVERSING INSTALLATION

(PT6A-6 ENGINES)

1. GENERAL 1-10-1
2. PROPELLER 1-10-1
3. PROPELLER GOVERNOR 1-10-1
4. PROPELLER GOVERNOR ON-SPEED CYCLE 1-10-3
5. PROPELLER GOVERNOR OVERSPEED CYCLE 1-10-3

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TABLE OF CONTENTS
SUBJECT PAGE

SECTION 10 - PROPELLER NON-REVERSING INSTALLATION

(Cont’d)

6. PROPELLER GOVERNOR UNDERSPEED CYCLE 1-10-3

7. PROPELLER OVERSPEED GOVERNOR 1-10-5
8. POWER TURBINE GOVERNOR 1-10-5
SECTION 11 - PROPELLER REVERSING INSTALLATION

(PT6A-6A ENGINES)

1. GENERAL 1-11-1
2. PROPELLER 1-11-1
3. PROPELLER GOVERNOR 1-11-3

4. PROPELLER GOVERNOR ON-SPEED CYCLE 1-11-3

5. PROPELLER GOVERNOR OVERSPEED CYCLE 1-11-5

6. PROPELLER GOVERNOR UNDERSPEED CYCLE 1-11-5
7. PROPELLER OVERSPEED GOVERNOR 1-11-6
8. POWER TURBINE GOVERNOR 1-11-6
9. PITCH REVERSING VALVE AND LINKAGE 1-11-6
10. PROPELLER REVERSING INTERCONNECTING LINKAGE 1-11-7
11. LOW PITCH CONTROL CYCLE 1-11-7

12. HIGH PITCH CONTROL CYCLE 1-11-9

13. REVERSE PITCH CONTROL CYCLE 1-11-9
14. REVERSE TO LOW PITCH CONTROL CYCLE 1-11-10
SECTION 12 - PROPELLER REVERSING INSTALLATION

(PT6A-6B & PT6A-20)

1. GENERAL 1-12-1
2. PROPELLER 1-12-1
3. PROPELLER GOVERNOR 1-12-3

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MANUAL PART NO. 3015442

TABLE OF CONTENTS
SUBJECT PAGE

SECTION 12 - PROPELLER REVERSING INSTALLATION (Cont’d)

4. PROPELLER GOVERNOR ON-SPEED CYCLE 1-12-5

5. PROPELLER GOVERNOR OVERSPEED CYCLE 1-12-5
6. PROPELLER GOVERNOR UNDERSPEED CYCLE 1-12-5
7. PROPELLER OVERSPEED GOVERNOR 1-12-6
8. POWER TURBINE GOVERNOR 1-12-6
9. FUEL CUT-OFF LEVER 1-12-6
10. POWER CONTROL LEVER 1-12-9
11. NON-STOL APPLICATIONS 1-12-10
SECTION 13 - PROPELLER REVERSING INSTALLATION

(PT6A-20A AND PT6A-20B ENGINES)

1. GENERAL 1-13-1
2. PROPELLER 1-13-1
3. PROPELLER GOVERNOR 1-13-5
4. PROPELLER GOVERNOR ON-SPEED CYCLE 1-13-10
5. PROPELLER GOVERNOR OVERSPEED CYCLE 1-13-10
6. PROPELLER GOVERNOR UNDERSPEED CYCLE 1-13-11
7. PROPELLER OVERSPEED GOVERNOR 1-13-11
8. NON-STOL APPLICATIONS 1-13-11

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MANUAL PART NO. 3015442

LIST OF FIGURES
FIGURE PAGE

Front and Rear Accessory Drives - Leading Particulars Figure 1-2-1 1-2-3
PT6 Engine -Cross Section (Typical) Figure 1-3-1 1-3-3
PT6 Engine - Airflow Figure 1-3-2 1-3-5
Typical PT6 Engine Figure 1-3-3 1-3-7
PT6A-6 Series Engines - Right Front View Figure 1-3-4 1-3-9
PT6A-6 Series Engines - Right Rear View Figure 1-3-5 1-3-10

PT6A-6 Series Engines - Left Front View Figure 1-3-6 1-3-11

PT6A-6 Series Engines - Left Rear View Figure 1-3-7 1-3-12
PT6A-20 Engine - Right Front View Figure 1-3-8 1-3-13
PT6A-20 Engine - Right Rear View Figure 1-3-9 1-3-14
PT6A-20 Engine - Left Front View Figure 1-3-10 1-3-15
PT6A-20 Engine - Left Rear View Figure 1-3-11 1-3-16
PT6A-20A Engine - Right Front View Figure 1-3-12 1-3-17
PT6A-20A Engine - Right Rear View Figure 1-3-13 1-3-18
PT6A-20A Engine - Left Front View Figure 1-3-14 1-3-19
PT6A-20A Engine - Left Rear View Figure 1-3-15 1-3-20
PT6 Engine - Exploded View (Typical) Figure 1-4-1 1-4-3
Compressor Inlet Case Figure 1-4-2 1-4-5
Compressor Rotor and Stator Assembly and No.1 Bearing Area Figure 1-4-3 1-4-6
Main Bearing Installation (Typical) Figure 1-4-4 1-4-9
Combustion Chamber Liner Figure 1-4-5 1-4-12
Compressor and Power Turbine Area Figure 1-4-6 1-4-13
Power Turbine No. 3 and No. 4 Bearing Areas Figure 1-4-7 1-4-17
Two Stage Reduction Gearbox Cross Section Figure 1-4-8 1-4-18
Two Stage Reduction Gearbox Cross Section Figure 1-4-9 1-4-19
Torquemeter - Schematic Figure 1-4-10 1-4-22

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MAINTENANCE MANUAL
MANUAL PART NO. 3015442

LIST OF FIGURES
FIGURE PAGE

Twin Outlet Port Exhaust Duct (Typical) Figure 1-4-11 1-4-24

Accessory Gearbox - Cross Section Figure 1-4-12 1-4-25
Compressor Bleed Valve - Schematic Figure 1-5-1 1-5-2
Bearing Compartment Seals, Turbine Cooling and Air Bleed Systems (Typical)
Figure 1-5-2 1-5-3
Accessory Gearbox - Lubrication Schematic Figure 1-6-1 1-6-2
Engine Lubrication - Schematic PT6A-6/20 Figure 1-6-2 1-6-5
Oil-to-Fuel Heater Flow - Schematic Figure 1-6-3 1-6-10
Ignition Current Regulator - Circuit Diagram Figure 1-7-1 1-7-2
Glow Plug - Installation Figure 1-7-2 1-7-3
Engine Fuel System - Schematic Figure 1-8-1 1-8-3
Fuel Control System - Schematic Figure 1-8-2 1-8-7
Fuel Control Unit Figure 1-8-3 1-8-10
Drive Body Assembly - Operation Figure 1-8-4 1-8-12

FCU Bellows Section - Functional Diagram Figure 1-8-5 1-8-14

Manual Override System Figure 1-8-6 1-8-15
Temperature Compensator - Cross Section Figure 1-8-7 1-8-18
Fuel Ratio Curve for Starting and Acceleration Schedules (Typical) Figure 1-8-8 1-8-19

Fuel Manifold Adapter Assembly - Front Cross Section Figure 1-8-9 1-8-23
Fuel Manifold Adapter Assembly - Front Cross Section Figure 1-8-10 1-8-25
Interturbine Temperature (T5) Sensing System Figure 1-9-1 1-9-3
Turbine Inlet Temperature (T4) Sensing System Figure 1-9-2 1-9-4
Typical Single-acting Propeller - Schematic Figure 1-10-1 1-10-2
Propeller Governor -Schematic Figure 1-10-2 1-10-4
Propeller Reversing Installation - Schematic Figure 1-11-1 1-11-2
Reduction Gearbox Propeller Reversing Installation Figure 1-11-2 1-11-4

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PART 1 LIST OF FIGURES Aug 27/2004
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MAINTENANCE MANUAL
MANUAL PART NO. 3015442

LIST OF FIGURES
FIGURE PAGE

Propeller Control Lever - Cam Slot Terminology Figure 1-11-3 1-11-8

Propeller Reversing Installation - Schematic Figure 1-12-1 1-12-2
Propeller Reversing Installation - Reduction Gearbox Figure 1-12-2 1-12-4
Engine Controls and Reversing Installation Figure 1-12-3 1-12-7
Propeller Reversing Installation - Schematic Figure 1-13-1 1-13-3
Engine Controls and Reversing Installation Figure 1-13-2 1-13-7

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PART 1 LIST OF FIGURES Aug 27/2004
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MAINTENANCE MANUAL
MANUAL PART NO. 3015442

LIST OF TABLES
TITLE PAGE
TABLE 1-2-1, Engine Specifications 1-2-1
TABLE 1-2-2, Engine Leading Particulars 1-2-1
TABLE 1-2-3, Accessory Drives - Leading Particulars 1-2-2
TABLE 1-2-4, Fuel and Lubrication System Specifications 1-2-4
TABLE 1-2-5, Metal Shipping Container Data 1-2-4
TABLE 1-2-6, Fiberboard Shipping Container Data (PT6A-6, -6A, -6B Engines)
-Type PK562 1-2-4
TABLE 1-2-7, Fiberboard Shipping Container Data (PT6A-20) -Type PK1325 1-2-5

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MAINTENANCE MANUAL
MANUAL PART NO. 3015442

PART 1 - DESCRIPTION OF ENGINE

SECTION 1 - GENERAL INFORMATION AND DESCRIPTION

INTRODUCTION

1. GENERAL

A. This publication is compiled and issued by the Customer Support Department of Pratt &
Whitney Canada, Longueuil, Quebec, Canada. It includes approved and recommended
maintenance procedures for turboprop engine models PT6A-6, -6A, -6B, -6/C20,
-20, -20A and -20B. The PT6A-6/C20 is a -6 reworked by the manufacturer to -20
configuration. The PT6A-20A is similar to the -20 except for reversing linkage and exhaust
duct. The PT6A-20B is modified from the -20 ( SB1206) to have reversing linkage
similar to the -20A. All turboprop models except PT6A-6 have reversing pitch propeller
installations.

B. Because the PT6A-Series engines are under a continuing program of improvements in

design and manufacture, it is anticipated that the appearance of certain parts or details
may change as refinements are introduced. However, this publication will be revised
as necessary to incorporate the latest approved data.

C. Requests for pertinent information not covered in this publication and suggestions for
modification or amplification of these instructions so as to increase their usefulness,
will be welcomed by the Customer Support Department of Pratt & Whitney Canada.

D. Any discrepancies, problems or suggestions regarding this publication should be

forwarded in writing, using the Pratt & Whitney Canada Customer Feedback Sheet
(RSVP), to:

Pratt & Whitney Canada Corp.

1000 Marie-Victorin Blvd
Longueuil, Quebec
Canada J4G 1A1
Attention: Manager Publications Dept (01PB4)
Website: www.pwc.ca
E-mail: publications@pwc.ca

E. Customer Feedback Sheets are enclosed with new manuals and each subsequent
revision. Additional forms may be obtained by contacting: The Supervisor, Publications
Customer Services, at the above address.

F. The on-line Customer Feedback Sheet (RSVP form) is also available on the Pratt &
Whitney Canada website (www.pwc.ca).

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MANUAL PART NO. 3015442

2. MAINTENANCE CONCEPT

A. The maintenance functions for the PT6 engines described in this manual are divided
into two basic levels; Line Maintenance and Heavy Maintenance. The scope of Line
Maintenance consists of the removal and installation of external components, engine
accessories and Hot Section Inspection. Heavy Maintenance details repairs normally
considered beyond the capabilities of the average line maintenance shop. The scope
consists of the removal and installation of engine components and limited repairs to the
hot section area, reduction gearbox and accessory gearbox. Repairs beyond this
level are not recommended and should be accomplished by an approved overhaul facility.
For Engine Condition Trend Monitoring System refer to Part 2, Section 6.

3. CUSTOMER SUPPORT

A. Customer Support representatives maintain contact with operators and service activities
and are available for the investigation of any specific difficulty or problem. Requests for
assistance and/or AOG support should be directed to:

Pratt & Whitney Canada Corp.

1000 Marie-Victorin Blvd.
Longueuil, Quebec
Canada J4G 1A1
Attention: Customer Support

B. Customer Help Desk (24-hour service):

US and Canada: 1-800-268-8000
International: 450-647-8000
Fax: 450-647-2888
Telex: DELETED
Email: customerhelpdesk@pwc.ca

4. ACCESSORIES SUPPORT

Accessories may be sent for repair or complete overhaul to:

P&WC Accessories Services

333 rue d’Auvergne (Area 2K)
Longueuil, Quebec
Canada J4H 3Y3

5. SUPPLEMENTARY PUBLICATION

A. Personnel concerned with the maintenance of these engines should familiarize

themselves with the contents of the Parts Catalog (P/N 3015444), which lists and
describes the saleable parts of the engines and illustrates their interrelationship.

6. SERVICE BULLETINS

A. Service Bulletins will be issued as required to provide information or instructions for

modifying earlier production engines or parts to the latest configuration.

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MANUAL PART NO. 3015442

7. DIRECTIONAL REFERENCES

A. The terms right and left, clockwise and counterclockwise, upper and lower, and similar
directional references will apply to the engine as viewed from the rear and the engine in
a horizontal position.

8. FITS AND CLEARANCES

A. During all operations which involve measurements, frequent reference should be made
to the Table of Fits and Clearances, Part 4.

9. TOOL PROCUREMENT

A. Depending on the geographical location of the operator, requests for the purchase of
special tools, should be sent to:
(1) United States:

Kell-Strom Tool Co.

214 Church Street
Wethersfield, CT 06109
USA
TEL: 860-529-6851 or 1-800-851-6851 (USA & Canada), or
860-721-0658 (24 hour service number)
FAX: 860-257-9694
(2) Canada and Elsewhere:

Pratt & Whitney Canada Inc.

1000 Marie-Victorin
Longueuil, Quebec
Canada J4G 1A1
Attention: Sales Department

10. SUPPLIERS AND SUPPLIER SERVICES

A. The names of any companies provided in this publication as a possible source for
required services or supplies are furnished for information purposes only. Pratt & Whitney
Canada does not endorse the work performed or supplies procured from these
companies. Further, Pratt & Whitney Canada does not accept responsibility, to any
degree, for the selection of such companies for such work performed or supplies procured.

11. CONSUMABLE MATERIALS

A. Part 2, Section 8, Table 2-8-1, lists the consumable materials used in the maintenance
of the engine.

B. Material Safety Data Sheets (MSDS) containing information about Trade Name, Safety
Hazards, Health Hazards, Reactivity, Spill or Leak Procedures, Special Protection
Information, Special Precautions, and Transportation and Labelling are available from
manufacturer. Read prior to using consumable materials.

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PART 1 - DESCRIPTION OF ENGINE

SECTION 2 - LEADING PARTICULARS AND SPECIFICATIONS

1. GENERAL

A. Leading particulars of the PT6A-6, PT6A-6A, PT6A-6B, PT6A-20, PT6A-20A and

PT6A-20B engines are as follows: (Refer to Tables 1-2-1 and 1-2-2)

TABLE 1-2-1, Engine Specifications

JP-4 Fuel
Consumption
LB/ESHP/HR
Shaft RPM Jet Thrust AT 15°C
ESHP SHP (3) (lb.) (59°F)
A-6 A-20 A-6 A-20 A-6 A-20 A-6 A-20 A-6 A-20
A-6A A-20A A-6A A-20A A-6A A-20A A-6A A-20A A-6A A-20A
Rating A-6B A-20B A-6B A-20B A-6B A-20B A-6B A-20B A-6B A-20B
Takeoff 578 579 550 550 2200 2200 70 72 0.650 0.649
(2) (2)
Max. 525 579 500 550 2200 2200 62 72 0.670 0.649
Cont (1) (2)
Max. 525 566 500 538 2200 2200 62 70 0.670 0.653
Climb (1)
Max. 495 522 471 495 2200 2200 60 68 0.680 0.670
Cruise
(1) Available to 18.3 °C (65°F) Ambient
(2) Available to 21.1 °C (70 °F) ambient
(3) Rotor Speed - Power Turbine 33,000 rpm
NOTE: Maximum Continuous is an emergency rating only.

TABLE 1-2-2, Engine Leading Particulars

LEADING PARTICULARS
Engine Type Free Turbine
Type of combustion chamber Annular
Compression ratio 6.3:1
Propeller shaft rotation Clockwise
(looking FWD)
Propeller shaft configuration Flanged
(PT6A-6 and -20 Series)

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TABLE 1-2-2, Engine Leading Particulars (Cont’d)

LEADING PARTICULARS
Propeller shaft gear ratio 0.0668:1
(PT6A-6 and -20 Series)
Engine diameter 19.0 in.
Basic at Room Temperature (482.6 mm)
Engine length 62.0 in.
Basic at Room Temperature (1575 mm)
(PT6A-6 and -20 Series)
Oil Consumption 0.2 lb/hr
(maximum) (0.091 kg/hr)
(10 hour period)
Dry weight, (Approximate) 275 lb.
(Standard equipment) (125 kg)
(PT6A-6 and -20 Series)

2. FRONT AND REAR ACCESSORY DRIVES

A. For leading particulars of the front and rear accessory drives, refer to Table 1-2-3.

TABLE 1-2-3, Accessory Drives - Leading Particulars

Max. Torque
(in.lb.)
Drive Pad Rotation Ratio Continuous Static
1 Starter-generator (1) CW 0.293:1 170 1600
2 Fuel Pump/FCU (1) CCW 0.167:1 - -
3 Ng Tach Generator (1) CCW 0.112:1 7 100
4 Vacuum Air Pump (Optional) (1) CCW 0.103:1 60 800
5 Optional Accessory Drive (1) CW 0.321:1 135 800
6 Optional Accessory Drive (1) CCW 0.203:1 150 800
7 Propeller Governor (2) CW 0.1273:1 50 850
8 Nf Tach Generator (2) CW 0.1273:1 7 100
9 Propeller OverspeedGovernor (2) CW 0.1273:1 70 850
NOTE: 1. Rear Accessory Drives (100 % Ng 37500 rpm).
NOTE: 2. Front Accessory Drives (100 % Nf 33000 rpm).

3. FUEL AND LUBRICATION SYSTEMS

A. For specifications of the fuel and lubrication systems, refer to Table 1-2-4.

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6 2

4 3

AS VIEWED FROM REAR

8
9

AS VIEWED FROM FRONT

C7819B
Front and Rear Accessory Drives - Leading Particulars
Figure 1-2-1

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TABLE 1-2-4, Fuel and Lubrication System Specifications

Nomenclature Specification
Fuel Specification Refer to SB No. 1244
Oil Specification Refer to SB No. 1001
Oil Tank - Total Capacity 2.3 US gallons (1.92 Imp. gallons, 8.70
liters)
Oil Tank - Expansion Space 0.7 US gallons (0.58 Imp. gallons, 2.64
liters)
Oil Tank - Usable Quantity 1.5 US gallons (1.25 Imp. gallons, 5.68
liters)

4. METAL SHIPPING CONTAINER

A. For leading particulars of the reusable engine metal shipping container, refer to Table
1-2-5.

TABLE 1-2-5, Metal Shipping Container Data

Description Data
Length 79 inches (2001 mm)
Width 34 inches (864 mm)
Height 38 inches (965 mm)
Weight 400 lb. (181.4 kg)
Pressure Tested at 10 psig
Material Specification SAE 1010-1020
Paint Primer Coat TT-P-664
Paint Finish Coat - MIL-E-7729
Semi-gloss

5. FIBERBOARD SHIPPING CONTAINER

A. For leading particulars of the non-reusable engine fiberboard shipping container, refer to
Table 1-2-6 or 1-2-7.

TABLE 1-2-6, Fiberboard Shipping Container Data (PT6A-6, -6A, -6B Engines) -Type PK562
Description Data
Length 73.5 inches (1867 mm)
Width 31.5 inches (800 mm)
Height 37 inches (940 mm)
Weight (approximate) 160 lbs (72.6 kg)

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TABLE 1-2-7, Fiberboard Shipping Container Data (PT6A-20) -Type PK1325

Description Data
Length 68 inches (1727 mm)
Width 26 inches (660 mm)
Height 26 inches (660 mm)
Weight (approximate) 210 lbs (95.3 kg)

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PART 1 - DESCRIPTION OF ENGINE

SECTION 3 - GENERAL INFORMATION

1. GENERAL

A. The engines described throughout this publication are variants of the basic PT6
powerplant which is a lightweight free-turbine engine designed for use in fixed or
rotary-wing aircraft. The PT6A series engines for use in fixed-wing aircraft are available in
two configurations: the PT6A-6 which utilizes a non-reversing propeller and the
PT6A-6A, -6B, -6/C20, -20, -20A and -20B which, though essentially similar, are equipped
with additional features to provide control for reversing propeller applications.

B. All engines utilize two independent turbines; one driving a compressor in the gas
generator section, the second driving reduction gearing for propeller, or rotary-wing
operation. (See Figure 1-3-1)

C. Inlet air enters the engine through and annular plenum chamber formed by the
compressor inlet case. From there it is directed to the compressor. The compressor
consists of three axial stages and one centrifugal stage assembled as an integral unit,
which provides a compression ratio of 6.3:1.

D. A row of stator vanes, located between each stage of compression, diffuses the air,
raises its static pressure and directs it to the next stage of compression. The
compressed air is passed through diffuser pipes, turned ninety degrees in direction, then
led through straightening vanes to the combustion chamber. (See Figure 1-3-2)

E. The combustion chamber liner located in the gas generator case consists of an annular
reverse flow weldment with varying size perforations which provide an entry for the
compressed air. The flow of air changes direction to enter the combustion chamber liner
where it reverses direction and mixes with fuel. The location of the combustion
chamber liner eliminates the need for a long shaft between the compressor and the
turbine, thus reducing the overall length and weight of the engine.

F. Fuel is injected into the combustion chamber liner by 14 simplex nozzles supplied by a
common manifold. The fuel/air mixture is ignited by two glow plugs (Pre-SB1429) or
spark igniters (Post-SB1429) which protrude into the combustion chamber liner. The
resultant gases expand from the combustion chamber liner, reverse direction and pass
through the turbine guide vanes to the compressor turbine. The turbine guide vanes
ensure that the expanding gases impinge on the turbine blades at the correct angle, with
minimum loss of energy. The still expanding gases pass forward through a second set
of stationary guide vanes to drive the power turbine.

G. The compressor and power turbines are located in the approximate center of the engine
with their shafts extending in opposite directions. This provides for simplified installation
and inspection procedures. The exhaust gas from the power turbine is directed
through an exhaust plenum to atmosphere via dual exhaust ports provided in the duct.

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H. An accessory gearcase is located at the rear of the engine with the relevant accessory
drives and mounting pads. The accessories are driven from the compressor by means
of a coupling shaft which extends the drive through a conical tube in the oil tank
center-section. An integral oil tank with a capacity of 2.3 U.S. gallons (1.92 Imperial
gallons: 8.70 liters) is provided in the compressor inlet case.

I. A planetary gearbox located in the front of the engine, provides the speed reduction
between the power turbine and the propeller shaft (PT6A series engines). The PT6A
series engines have a two stage planetary gearbox. Engine output is accurately indicated
by means of an integral torquemeter device located in the gearbox.

J. Stations and flanges are shown on Figure 1-3-3.

K. For specifications of the engine, refer to Table 1-2-1.

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C5B
PT6 Engine -Cross Section (Typical)
Figure 1-3-1

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C552A
PT6 Engine - Airflow
Figure 1-3-2

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COMPRESSOR TURBINE INLET COMBUSTION CHAMBER INLET

INTERTURBINE COMPRESSOR INLET

EXHAUST OUTLET EXHAUST DUCT ENGINE INLET

STATIONS 7 6 5 4 3 2 1

FLANGES A B C D E F G
REDUCTION GEARBOX TO OIL TANK TO ACCESSORY
EXHAUST CASE GEARBOX DIAPHRAGM
INNER EXIT DUCT TO COMPRESSOR GAS GENERATOR CASE TO
REDUCTION GEARBOX REAR CASE TURBINE SHROUD HOUSING COMPRESSOR INLET CASE
TO POWER TURBINE HOUSING SUPPORT
EXHAUST DUCT TO POWER
EXHAUST DUCT TO GAS TURBINE VANE HOUSING
GENERATOR CASE

C553B
Typical PT6 Engine
Figure 1-3-3

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C77465
PT6A-6 Series Engines - Right Front View
Figure 1-3-4

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C77464
PT6A-6 Series Engines - Right Rear View
Figure 1-3-5

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C77463
PT6A-6 Series Engines - Left Front View
Figure 1-3-6

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C77466
PT6A-6 Series Engines - Left Rear View
Figure 1-3-7

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C77459
PT6A-20 Engine - Right Front View
Figure 1-3-8

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C77460
PT6A-20 Engine - Right Rear View
Figure 1-3-9

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C77461
PT6A-20 Engine - Left Front View
Figure 1-3-10

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C77462
PT6A-20 Engine - Left Rear View
Figure 1-3-11

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C77470
PT6A-20A Engine - Right Front View
Figure 1-3-12

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C77468
PT6A-20A Engine - Right Rear View
Figure 1-3-13

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C77467
PT6A-20A Engine - Left Front View
Figure 1-3-14

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C77471
PT6A-20A Engine - Left Rear View
Figure 1-3-15

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PART 1 - DESCRIPTION OF ENGINE

SECTION 4 - DETAILED INFORMATION

1. GENERAL

A. This section describes in detail all major items of the engine. (See Figure 1-4-1)

2. COMPRESSOR INLET CASE

A. The compressor inlet case (see Figure 1-4-2) consists of a circular casting, the front of
which forms a plenum chamber for the passage of compressor inlet air. The rear portion,
which consists of a hollow compartment, is used to form an integral oil tank. A large
area circular wire mesh screen is bolted around the air intake and the rear of the gas
generator case to preclude foreign object ingestion by the compressor.

B. The No. 1 bearing, bearing support and labyrinth seal are contained within the
compressor inlet case centerbore. The bearing support is secured to the inlet case
centerbore flange by four bolts. A nut and shroud washer retains the No. 1 bearing outer
race in its support housing. An oil nozzle, fitted at the end of a cored passage,
provides lubrication to the rear of No. 1 bearing at approximately the 1 o’clock position.
Other cored passages are provided for pressure and scavenge oil (see Figure
1-4-3).

C. The engine main oil filter, incorporating check and bypass valve assemblies, and a
pressure relief valve are respectively located on the right hand side of the inlet case
in 3 and 1 o’clock positions. A fabricated conical tube, complete with preformed packings,
is located between the wall of the inlet case centerbore and centerbore of the accessory
diaphragm, to form a passage for the coupling shaft driving the accessories. The oil
pressure pump, connected to the oil filter housing, is driven by an accessory drive gear.
The pump located in the bottom of the oil tank, is secured to the accessory diaphragm
by four bolts.

3. COMPRESSOR ROTOR AND STATOR ASSEMBLY

A. The compressor rotor and stator assembly (see Figure 1-4-3) consists of three axial
rotor stages, three interstage spacers, three stator assemblies and a single-stage
centrifugal impeller and housing. The blades fit into dove-tail grooves machined in the
rotor disk rims, and limited clearance between blade root and groove produces the
characteristic metallic clicking heard during compressor rundown. Blade axial
movement is limited by interstage spacers between the disks. The airfoil cross-section of
the first-stage blades differ from those of the second and third stages which are
identical. The length of the blades differ in each stage, decreasing from first to third
stage. The wide chord first stage blades provide increased impact resistance.

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12

13
11
14

10
8
7
6
5

4 15

16 LEGEND
3 17 1. PROPELLER REDUCTION GEARBOX
2. POWER TURBINE SUPPORT HOUSING
3. EXHAUST DUCT
2 4. POWER TURBINE
5. COMPRESSOR TURBINE
9 6. COMBUSTION CHAMBER LINER
1 7. FUEL MANIFOLD
8. GAS GENERATOR CASE
9. COMPRESSOR BLEED VALVE
10. COMPRESSOR ASSEMBLY
18 11. COMPRESSOR INLET CASE
12. OIL−TO−FUEL HEATER
19 13. DIP−STICK AND FILLER CAP
14. ACCESSORY GEARBOX
20 15. IGNITION CURRENT REGULATOR
16. FUEL CONTROL UNIT AND PUMP
17. AIR INLET SCREEN
18. IGNITION GLOW PLUG
19. COMPRESSOR TURBINE GUIDE VANES
20. POWER TURBINE GUIDE VANES

C555A
PT6 Engine - Exploded View (Typical)
Figure 1-4-1

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OIL INLET FROM FUEL

HEATER AND OIL COOLER
PRESSURE RELIEF
VALVE HOUSING

INLET CASE

FWD

PRESSURE OIL

DRAIN PLUG
ORIFICE OIL FILTER
HOUSING PORT

PRESSURE OIL BOSS

C562B
Compressor Inlet Case
Figure 1-4-2

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SHORT SLEEVE AND COMPRESSOR

SPACER ASSEMBLY (2) COMPRESSOR BLADE INLET CASE

COMPRESSOR INTERSTAGE
AIR BLEED NO. 1 BEARING
OIL NOZZLE

IMPELLER
HOUSING
OIL TANK
CENTRIFUGAL CENTER TUBE
IMPELLER

TIE ROD (6)

COMPRESSOR
FRONT STUB
SHAFT
ACCESSORIES GEARBOX
LONG SPACER AND COUPLING SHAFT
SLEEVE ASSEMBLY
NO. 1 BEARING
VANE AND
SHROUD
ASSEMBLY
GAS GENERATOR CASE (3)

COMPRESSOR NO. 1 BEARING

REAR HUB OIL TRANSFER TUBE

C556A
Compressor Rotor and Stator Assembly and No.1 Bearing Area
Figure 1-4-3

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B. The first and second stage stator assemblies each contain 44 vanes and the third stage
contains 40 vanes. Each set of compressor stator vanes is held in position by a circular
ring through which the vane ends protrude and to which they are brazed. The vane
rings provide shrouds for the first, second and third stage rotor blades. Stator assemblies
and impeller housing are retained to prevent rotation of the assembly, by lugs on vane
rings engaging grooves on vane rings and impeller housing. Interstage pressure air (P2.5)
is vented to the compressor bleed valve chamber in the gas generator case through
slots in the third stage vane ring.

C. The compressor stubshaft, centrifugal impeller, and impeller housing are positioned in
that order, followed sequentially by an interstage spacer, a stator assembly and a rotor
assembly. The rotating assembly is stacked and secured together by six tie-rods which are
numbered during initial assembly. The impeller housing is secured in the gas generator
case by eight eccentric bolts. The compressor stubshaft is a hollow forging, machined
to accommodate the No. 2 bearing double labyrinth air seal rotor and the No. 2 bearing
assembly. The No. 2 bearing, a roller type, supports the compressor stubshaft and
the compressor rotor assembly in the gas generator case. (See Figure 1-4-4).

D. The compressor rear hub is an integral part of the first stage compressor rotor disk. The
hub, a hollow forging, is machined to accommodate the No. 1 bearing labyrinth air seal
rotor, a spacer, and the No. 1 bearing. The No. 1 bearing is a ball type, and supports
the compressor rear hub and the compressor rotor assembly in the compressor inlet case.
A short, hollow coupling, externally splined at its protruding end, is secured within the
compressor rear hub by a transverse pin which passes through the hub and coupling.

E. The complete compressor rotor and stator assembly is fitted into the center rear section
of the gas generator case, which forms the compressor housing, and secured by the
impeller housing at the front and the compressor inlet case at the rear.

F. A short, hollow, coupling shaft, internally splined, locates on the splined hub coupling, to
extend the compressor drive to the accessory gearbox input gearshaft. The coupling
embodies a ball-lock at the front end to prevent end thrust on the input gearshaft roller
bearings.

4. GAS GENERATOR CASE

A. The gas generator case is attached to the front flange of the compressor inlet case. It
consists of two sections fabricated into a single structure. The rear inlet section
provides a housing support for the compressor assembly. The No. 2 bearing with two air
seals is located in the centerbore of the gas generator case. The bearing has a
flanged outer race which is secured in the support housing by four bolts. The front and
rear stator air seals with their spiral wound gaskets are each secured in the centerbore
of the case by eight bolts. An oil nozzle with two jets, one in the front and the other in the
rear of the bearing, provides lubrication. The 14 radial vanes brazed inside the double
skin center section of the gas generator case, provide a pressure increase to the
compressed air as it leaves the centrifugal impeller. The compressed air is then
directed through seventy straightening vanes welded inside the gas generator case
diffuser and out to the combustion chamber area through a slotted diffuser outlet baffle.

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SERIAL NO. THRUST SERIAL NO. SECOND SPLIT INNER RACE− INNER RACE− INNER RACE SERIAL SPLIT INNER RACE−
TO FRONT BEARING TO FRONT STAGE PULLER GROOVE TO SERIAL NO. TO FRONT NO. TO FRONT AND PULLER GROOVE TO REAR.
CARRIER FRONT ’X’ MARKS AND ALIGNED WITH ALIGNED WITH STUB− ’X’ MARKS ALIGNED WITH
BEARING ALIGNED WITH MASTER SPLINE SHAFT MASTER SPLINE OFFSET HOLES ON REAR HUB
MASTER SPLINE

OUTER RACE− OUTER RACE− OUTER RACE− OUTER RACE−

SERIAL NO. SERIAL NO. FLANGE AND SERIAL SERIAL NO. TO REAR
TO FRONT TO FRONT NO. TO FRONT
CAGE AND BALLS− CAGE AND BALLS−
SERIAL NO. TO SERIAL NO. TO REAR
FRONT

ACCESSORY INPUT DRIVE BEARINGS

MASTER OFFSET
SPLINE HOLE REAR HUB

FWD

STUBSHAFT
NO. 4 NO. 3 MASTER NO. 2 NO. 1
SPLINE

CAUTION NOTE
ALL ACCESSORY DRIVE BEARINGS MUST BE
1. INNER RACES CAGE AND BALLS OF REINSTALLED IN THEIR ORIGINAL LOCATIONS.
NO. 1 AND NO. 4 BEARING ARE ID IDENTICAL.
2. NO. 2 AND NO. 3 BEARINGS ARE IDENTICAL.
3. INTERMIXING OF BEARING COMPONENTS
IS STRICTLY UNACCEPTABLE.

C1769A
Main Bearing Installation (Typical)
Figure 1-4-4

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B. The front section of the gas generator case forms an outer housing for the combustion
chamber liner. It consists of a circular structure provided with mounting bosses for the
14 fuel nozzle assemblies and common manifold. Mounting bosses are also provided at
the six o’clock position for the fuel dump valve and combustion chamber front and
rear drain valves. Two igniter plugs at the 4 and 8 o’clock positions, protrude into the
combustion chamber liner to ignite the fuel/air mixture. Three equi-spaced pads on outer
circumference of gas generator case, provide accommodation for flexible type engine
mounts. The compressor bleed valve outlet port is located at the 8 o’clock position.

5. COMBUSTION CHAMBER LINER

A. The combustion chamber liner (see Figure 1-4-5) is of the reverse-flow type and
consists primarily of an annular heat-resistant liner open at one end. A series of straight,
plunged and shielded perforations allow air to enter the liner in a manner designed to
provide the best fuel/air ratios for starting and sustained combustion. Direction of air flow
is controlled by cooling rings located opposite the perforations. The perforations
ensure an even temperature distribution at the compressor turbine inlet. The domed
front end of the combustion chamber liner is supported inside the gas generator case by
seven of the fourteen fuel nozzles sheaths. The rear of the liner is supported by
sliding joints which fit into the inner and outer exit duct assemblies. The duct assemblies
form an envelope which effectively changes the direction of the gas flow by providing
an outlet in close proximity to the compressor turbine vanes. The outlet duct and heat
shield assembly is attached to the gas generator case by seven equi-spaced support
brackets in the case. The heat shield forms a passage through which compressor
discharge air is routed for cooling purposes. A scalloped flange on the rear of the
outer duct locks in the support brackets and secures the assembly. The center section
of the assembly is bolted with the compressor turbine guide vane support to the centerbore
of the gas generator case. (See Figure 1-4-6).

6. TURBINE SECTION

A. The turbine rotor section consists of two separate single-stage turbine slocated in the
center of the gas generator case and completely enveloped by the annular combustion
chamber liner. (See Figure 1-4-6).

7. COMPRESSOR TURBINE VANES

A. The compressor turbine vane assembly consists of 29 vanes located between the
combustion chamber exit ducts and compressor turbine. The vanes, incorporating
individual dowel pins on the inner platforms, are positioned in holes on the outer
circumference of the inner support. Sealing is ensured by ceramic fiber cord packing on
the outside diameter of the support which is secured to the centerbore of the gas
generator by eight bolts. The outer platforms of the vanes, sealed with a chevron packing,
fits into the shroud housing and small exit duct which are secured together with twelve
bolts. The shroud housing extends forward into two classified interstage sealing rings on
the power turbine stator housing, to form a seal between both housings. Fourteen
compressor turbine shroud segments positioned in the shroud housing act as a seal and
provide a running clearance for the compressor turbine assembly.

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FUEL NOZZLE PORT

WITH SUPPORT BRACKET (7)

FUEL NOZZLE
PORT PLAIN (7)
GLOW PLUG SLEEVE (2)

C11
Combustion Chamber Liner
Figure 1-4-5

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FUEL MANIFOLD COMBUSTION CHAMBER

TERMINAL BLOCK ADAPTER ASSEMBLY LINER ASSEMBLY GAS GENERATOR
CASE ASSEMBLY

Tt4 THERMOCOUPLE
PROBE T5 THERMOCOUPLE PROBE

COMBUSTION CHAMBER
COMPRESSOR TURBINE SMALL (INNER) EXIT
GUIDE VANES DUCT ASSEMBLY

COMPRESSOR TURBINE
EXHAUST DUCT ASSEMBLY VANE SUPPORT

COMBUSTION CHAMBER
LARGE (OUTER) EXIT
DUCT ASSEMBLY

SINGLE BOLT ATTACHMENT

MULTI−BOLT
ATTACHMENT SINGLE PLANE BALANCING
DISK ASSEMBLY

INTERSTAGE SINGLE BOLT

BAFFLE ASSEMBLY ATTACHMENT

TWO PLANE BALANCING

POWER TURBINE GUIDE VANES DISK ASSEMBLY

COMPRESSOR TURBINE
POWER TURBINE

POWER TURBINE INTEGRAL

VANE RING

C557C
Compressor and Power Turbine Area
Figure 1-4-6

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MAINTENANCE MANUAL
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8. COMPRESSOR TURBINE

A. The compressor turbine which consists of a two-plane balanced turbine disk, blades and
classified weights, drives the compressor in a counterclockwise direction. The two-plane
balanced assembly is secured to the front stubshaft by a simplified center lock bolt
(see Figure 1-4-6). A master spline is provided to ensure the disk assembly is always
installed in a predetermined position to retain proper balance. The disk embodies a
reference circumferential groove to enable checking of disk growth when required.
The 58 blades in the compressor turbine disk are secured in fir-tree serrations machined
in the outer circumference of the disk and held in position by individual rivets. The
blades embody squealer tips that ensure a minimum amount of pick-up should the rotating
blades come into contact with the shroud segments. The required number of classified
weights is determined during balancing procedures and riveted to the relevant flanges
machined on the turbine disk. A small machined area on the rear face of the disk
provides a sealing surface to control the flow of turbine disk cooling air.

9. INTERSTAGE BAFFLE

A. The compressor turbine is separated from the power turbine by an interstage baffle
which prevents dissipation of turbine gases and consequent transmission of heat to
the turbine disk faces. (See Figure 1-4-6). The baffle is secured to and supported by the
power turbine vane ring. The center section of the baffle includes small circular lipped
flanges on the front and rear faces. The flanges fit over mating rotor seals machined on the
respective turbine disk faces to provide control of cooling air flow through the perforated
center of the baffle.

10. POWER TURBINE VANES

A. On PT6A-6 series, PT6A-6/C20, early PT6A-20 and PT6A-20B engines, the power
turbine vane assembly consists of 19 cast vanes. The vanes are cast with dowel pin
platforms which fit into the interstage baffle rim. The stator housing with the enclosed vane
assembly is bolted to the exhaust duct and supports the two classified sealing rings at
flange D. The rings are self-centering and held in position by retaining plates bolted to the
rear face of the stator housing. On most PT6A-20, PT6A-20B and all PT6A-20A
engines, the individual vanes are replaced by a power turbine vane ring with 19 integrally
cast vanes.

11. POWER TURBINE

A. The power turbine assembly consisting of a turbine disk, blades, and classified weights,
drives the reduction gearing through the power turbine shaft in a clockwise direction.
The disk is held to close tolerances and embodies a reference circumferential groove to
permit disk growth check measurements when required. The turbine disk is splined to
the turbine shaft and secured by a single center lockbolt and keywasher (See Figure
1-4-6). A master spline ensures the power turbine disk is always installed in a
predetermined position to retain the original balance. The required number of classified
weights is determined during balancing procedures and riveted to a flange on the
rear face of the turbine disk. The power turbine blades differ from those of the compressor
turbine in that they are cast complete with notched and shrouded tips. The 41 blades
are secured by fir-tree serrations in the turbine disk and held in position by individual rivets.
The blade tips rotate inside a double knife-edge shroud and form a continuous seal
when the engine is running. This reduces tip leakage and increases turbine efficiency.

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MANUAL PART NO. 3015442

12. POWER TURBINE SUPPORT HOUSING

A. The power turbine support housing consists of a cylindrical housing attached to the
reduction gearbox rear case by 12 studs. The housing provides support for the
power turbine shaft assembly and two bearings (see Figure 1-4-7). A labyrinth type
seal, secured at the rear of the housing, prevents oil leakage into the power turbine
section. An internal oil transfer tube with four oil nozzles, provides front and rear lubrication
to number three and four bearings. A scavenge tube inside the housing transfers
bearing scavenge oil to the front of the engine.

13. NO. 3 AND 4 BEARINGS

A. The power turbine disk and shaft assembly is supported and secured in the power
turbine shaft housing by two bearings, the No. 3 bearing which is a roller type and
the No. 4 bearing which is a ball bearing (see Figures 1-4-7 and 1-4-4). The No. 3 bearing
includes an inner race and a flanged outer race. The inner race is secured on the rear
of the power turbine shaft together with the power turbine air seal and power turbine disk
by the center lock bolt. The outer race is secured inside the power turbine shaft
housing by four bolts and tabwashers.

B. The No. 4 bearing includes a split inner race and a flanged outer race. The split inner
race is secured on the front of the power turbine shaft, together with the first stage
reduction gear coupling and a coupling positioning ring, by a keywasher and spanner
nut. A puller groove is incorporated on the front half of the split inner race to facilitate
removal. The flanged outer race is secured inside the power turbine shaft housing by
four bolts and tabwashers.

14. REDUCTION GEARBOX (PT6A-SERIES ENGINES) - TWO STAGE

A. The reduction gearbox (see Figure 1-4-8 for PT6A-6 series engines, and Figure 1-4-9
for PT6A-20 series engines) is located at the front of the engine and consists of two
castings bolted to the front flange of the exhaust duct. The first stage of reduction is
contained in the rear case. Torque from the power turbine is transmitted through the power
turbine shaft to the first stage sun gear.

B. The first stage reduction sun gear consists of a short hollow shaft which has an integral
spur gear at the front end and externally splined at the rear. The external splines fit over
the end of the power turbine shaft and engage the internal splines of a retainer
coupling. The coupling is in turn splined to the turbine shaft and secured there by a
retaining nut and keywasher. Two double coil retaining rings hold the sun gear in the
retainer coupling. The spur gear end of the sun gear drives the three planetary gears in the
first stage planet carrier.

C. The first stage ring gear is located in helical splines provided in the first stage reduction
gearbox rear case. The torque developed by the power turbine is transmitted through
the sun gear and planet gears to the ring gear which is opposed by the helical splines. This
results in rotation of the planet carrier. The ring gear, though secured by the helical
splines, is allowed to move axially between the case and three retaining plates. This
movement is used in the torquemeter application (refer to Paragraph 15., following).

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MANUAL PART NO. 3015442

THERMAL INSULATION
BLANKET
EXHAUST DUCT
OIL NOZZLE ASSEMBLY
OIL STRAINER
TURBINE BLADES
(SHROUDED TIP)
FROM
TORQUEMETER

FIRST STAGE
CARRIER TURBINE DISK

CARRIER
SLEEVE
TURBINE
LOCKING BOLT

PLAIN BEARING
POWER TURBINE
REDUCTION AIR SEAL
GEARBOX
REAR CASE
SCAVENGE
RETAINER OIL TUBE
RING
TRANSFER TUBE
NO. 3 BEARING
NO. 4 BEARING POWER TURBINE
SHAFT HOUSING
POWER TURBINE SHAFT

C558B
Power Turbine No. 3 and No. 4 Bearing Areas
Figure 1-4-7

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MAINTENANCE MANUAL
MANUAL PART NO. 3015442

GOVERNOR DRIVE
SHAFT GEAR FLANGE A
FRONT CASE ASSEMBLY 2ND STAGE SUN GEAR
OIL TRANSFER
GALLERY FLEXIBLE COUPLING
REAR CASE
BEVEL GEAR ASSEMBLY
OIL SEAL

1ST STAGE
PROPELLER PLANET GEAR
SHAFT
SPLINE
ADAPTER

FLANGE B
THRUST
BEARING

THRUST BEARING TORQUEMETER

COVER CYLINDER AND PISTON
1ST STAGE RING GEAR
PROPELLER OIL
TRANSFER HOUSING
2ND STAGE RING GEAR
ROLLER BEARING
2ND STAGE
PLANET GEAR

(PT6A-6 Series Engines) C419A

Two Stage Reduction Gearbox Cross Section
Figure 1-4-8

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MANUAL PART NO. 3015442

GOVERNOR DRIVE
FLANGE A
FRONT CASE ASSEMBLY SHAFT GEAR
SECOND STAGE SUN GEAR
OIL TRANSFER FLEXIBLE COUPLING
GALLERY
REAR CASE
BEVEL GEAR ASSEMBLY

OIL SEAL

FIRST STAGE
PROPELLER PLANET GEAR
SHAFT
SPLINE
ADAPTER
FLANGE B

THRUST BEARING TORQUEMETER

CYLINDER AND
THRUST BEARING PISTON
COVER FIRST STAGE
RING GEAR
PROPELLER OIL
TRANSFER HOUSING SECOND STAGE
RING GEAR
ROLLER BEARING
SECOND STAGE
CHIP DETECTOR PLANET GEAR

(PT6A-20 Series Engines) C1163A

Two Stage Reduction Gearbox Cross Section
Figure 1-4-9

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MANUAL PART NO. 3015442

D. The second stage reduction gearing is contained at the front of the reduction gearbox.
The first stage planetary gears drive the second stage sun gear via a flexible coupling
which also dampens any vibrations between the two rotating gear assemblies. The second
stage sun gear drives five planet gears in the second stage carrier. A second stage
ring gear is fixed by splines to the reduction gearbox front case and secured by three
bolted retaining plates. The second stage carrier is in turn splined to the propeller shaft and
secured by a retaining nut and shroud washer. A flanged roller bearing assembly,
secured by four bolts to the case provides support for the second stage carrier and
propeller shaft. An oil transfer tube and nozzle assembly fitted with preformed packings
is secured within the propeller shaft by a retaining ring to provide lubrication to No. 4
bearing.

E. The accessories located on the reduction gearbox front case are driven by a bevel gear
mounted on the propeller shaft behind the thrust bearing assembly (see Figures 1-4-8
and 1-4-9). Propeller thrust loads are absorbed by a flanged ball bearing located in the
front face of the reduction gearbox centerbore. The bevel drive gear, adjusting spacer,
thrust bearing and seal runner are stacked and secured to the propeller shaft by a single
spanner nut and keywasher. The thrust bearing cover is secured to the front of the
reduction gearbox and incorporates a removable oil seal retaining ring to facilitate
replacement of the oil seal.

15. TORQUEMETER

A. The torquemeter is a hydro-mechanical torque measuring device located inside the first
stage reduction gear housing to provide an accurate indication of engine power output.
The mechanism consists of a torquemeter cylinder, torquemeter piston, valve plunger
and spring. (See Figure 1-4-10).

B. Rotation of the ring gear is resisted by the helical splines which impart an axial
movement to the ring gear and therefore to the torquemeter piston. This in turn moves a
valve plunger against a spring, opening a metering orifice and allowing an increased
flow of pressure oil to enter the torquemeter chamber. This movement will continue until
the oil pressure in the torque chamber is proportional to the torque being absorbed
by the ring gear. Any change in power control lever setting will re-cycle the sequence
until a state of equilibrium is again reached.

C. Hydrostatic lock is prevented by allowing oil to bleed continuously from the pressure
chamber into the reduction gear casing through a small bleed hole provided in the top
of the torquemeter cylinder.

D. Because the external pressure within the reduction gearbox may vary and affect the
total pressure on the exterior face of the torquemeter piston, the internal pressure is
measured. The difference between the torquemeter pressure and the reduction gearbox
internal pressure accurately indicates the torque being produced. The two pressures
are internally routed to bosses located on the top of the reduction gearbox front case
where connections can be made to suit individual cockpit instrumentation requirements.

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16. EXHAUST DUCT

A. The exhaust duct (see Figure 1-4-11), a divergent heat resistant duct has two outlet
ports, one on each side of the case. The duct is attached to the front flange of the gas
generator case and consists of inner and outer sections. A reinforcing ring is provided at
the rear of the exhaust duct at flange D. This consists of a scalloped ring machined in
two halves and coupled to form a circle by two equi-spaced clevis pin joints. The power
turbine stator housing is secured to flange D by twelve bolts which screw into and
also secure the reinforcing ring. The outer conical section which has two flanged exhaust
outlet ports forms the outer gas path and also functions as a structural member to
support the reduction gearbox. The inner section forms the inner gas path and provides
a compartment for the reduction gearbox rear case and the power turbine support
housing. A removable sandwich-type heat shield insulates the power turbine support
housing from the hot exhaust gases. A short No. 3 bearing cover and spacer are secured
at the rear of the power turbine support housing by a retaining ring. A drain passage,
located at the 6 o’clock position in the exhaust duct, enables residual fuel accumulating
in the exhaust duct during engine shutdown to drain into the gas generator case
where it is discharged overboard through the front drain plug.

17. ACCESSORY GEARBOX

A. The accessory gearbox housing, located at the rear of the engine, consists of two
castings both of which are attached to the rear flange of the compressor inlet case by 16
studs. The front casting, which incorporates front and rear preformed packings, forms
an oil-tight diaphragm between the oil tank compartment of the inlet case and the
accessory drives. The diaphragm provides support for the accessory drive gear
bearings, seals, and main pressure oil pump which is secured to the diaphragm by four
bolts. The diaphragm is attached to the accessory gearbox rear casting by four
countersunk screws and nuts located at the 4th, 8th, 14th and 18th positions in clockwise
rotation, assuming the 1st to be at the 12 o’clock position. (See Figure 1-4-12).

B. The rear casting forms an accessory gearbox cover and provides support bosses for the
accessory drive bearings and seals. The internal scavenge oil pump is secured inside
the housing and a second scavenge pump is externally mounted. Mounting pads and
studs are provided on the rear face for the combined starter-generator, the fuel
control unit with the sandwich mounted fuel pump, and the Ng tachometer-generator. A
large access plug located below the starter-generator mounting pad provides passage
for a puller tool which must be used to disengage and hold the ball-locked coupling shaft
and input gearshaft during disassembly. Three additional pads are available for
optional requirements. Accessory drives are supported on similar roller bearings fitted
with garter-type oil seals. An oil tank filler cap and integral dipstick is located at the
11 o’clock position on the rear housing to facilitate servicing of the oil system. A
centrifugal oil separator mounted on the starter-generator drive gearshaft separates the
oil from the engine breather air in the accessory gearbox housing. A cored passage
in the diaphragm connects the oil separator to an external mounting pad located at the
12 o’clock position on the rear housing. A carbon face seal located on the front of
the gearshaft in the diaphragm precludes pressure leakage through the bearing assembly.

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11
13
2
10

12 4

6
7

C14B
Torquemeter - Schematic
Figure 1-4-10

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MAINTENANCE MANUAL
MANUAL PART NO. 3015442

Key to Figure 1-4-10

1. Gearbox Pressure
2. Torquemeter Pressure
3. Oil Control Piston
4. Control Spring
5. Metering Orifice
6. Piston
7. Torquemeter Chamber
8. First Stage Planet Gear
9. First Stage Ring Gear
10. Helical Splines
11. Casting
12. Cylinder
13. Bleed Hole
18. ENGINE LIFTING POINTS

A. Engine installation or removal is facilitated by front and rear lifting sling suspension
points. The front suspension point consists of a single metal bracket secured to the
top of the exhaust duct at flange A for all engine models. The bracket embodies a special
eye to accommodate the sling attachment and is identified for this use; a bolted
service plate is incorporated to accommodate the front linkage swivel joint. On PT6A-6
and PT6A-6A engines there are two rear suspension points. These consist of two metal
brackets located at the 3 and 9 o’clock positions on flange G. On PT6A-6B and
PT6A-20 Series engines, the rear suspension point consists of a single metal bracket
secured to the top of the accessory gearbox at flange G.

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FLANGE A FLANGE C FLANGE D

C454B
Twin Outlet Port Exhaust Duct (Typical)
Figure 1-4-11

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MANUAL PART NO. 3015442

ACCESSORY GEARBOX
HOUSING

FLANGE G

CENTRIFUGAL
BREATHER IMPELLER
ACCESSORY
DIAPHRAGM

STARTER−GENERATOR
DRIVE GEARSHAFT
CARBON FACE SEAL

BALL LOCK
ROLLER BEARING
(TYPICAL)

INPUT GEARSHAFT

OIL FILTER AND OIL PUMP DRIVE

NON−RETURN GEARSHAFT
VALVE
OIL SEAL
(TYPICAL)
MAIN PRESSURE
PUMP BODY
ASSEMBLY

INLET OIL SCREEN

INTERNAL SCAVENGE
PUMP GEARS

OIL TRAY

C554A
Accessory Gearbox - Cross Section
Figure 1-4-12

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MAINTENANCE MANUAL
MANUAL PART NO. 3015442

PART 1 - DESCRIPTION OF ENGINE

SECTION 5 - AIR SYSTEMS

1. GENERAL

A. The engine has three separate air bleed systems: a compressor air bleed control, a
bearing compartment air seal and bleed system and a turbine disk cooling system. A
fourth system is available as an optional source of high-pressure air for use in operating
auxiliary airframe equipment. A blanked mounting flange located on the gas generator
case is provided for external connections.

2. COMPRESSOR BLEED VALVE

A. The compressor bleed valve automatically opens a port in the gas generator case to
spill interstage compressor air (P2.5), thereby providing antistall characteristics at low
engine speeds (less than 80% Ng). The port closes gradually as high engine speeds are
attained.

B. The compressor bleed valve consisting of a piston type valve in a ported housing, is
installed on the gas generator case at the 7 o’clock position, and is secured by two
bolts. The piston assembly is supported in the bore of the housing and is guided by a
seal support plate, guidepin and a guidepin bolt, which holds the piston assembly together.
A rolling diaphragm permits the piston full travel in either direction, to open or close
the port, while at the same time effectively sealing the compartment at the top of the piston.
A port in the gas generator case provides a direct passage for the flow of compressor
interstage air (P2.5) to the bottom of the bleed valve piston. (See Figure 1-5-1).

C. Compressor delivery air (P3) is tapped off and applied to the bleed valve through a
nozzle (fixed orifice) in the bleed valve cover, then passed through an intermediate
passage and out to atmosphere through a metering plug (convergent-divergent orifice).
The control pressure (Px) between the two orifices acts upon the upper side of the
bleed valve piston, so that when Px is greater than P2.5, the bleed valve closes. In the
closed position, the interstage air port is sealed by the seal support plate which is
forced against its seat by the action of Px. Conversely, when Px is less than P2.5, the
bleed valve opens and allows interstage air (P2.5) to be discharged to atmosphere. The
piston is prevented from closing off the Px feed line to the upper section of the valve
chamber and from damaging the piston assembly by the stop formed by the hexagon of
the guidepin, which is screwed into the valve cover, and the hexagon of the piston
guidepin bolt.

3. BEARING COMPARTMENT SEALS, TURBINE COOLING AND AIR BLEED SYSTEMS

A. Engine air pressure is utilized to seal the 1st, 2nd and 3rd bearing compartments and
also to cool both the compressor and the power turbines (see Figure 1-5-2). The air
pressure is used in conjunction with air seals which establish and control the required
pressure gradients. The air seals used on the engine consist of a series of stationary
expansion chambers (labyrinths) formed by deep annular grooves machined in the bore
of a circular seal. The clearance between the rotating and stationary parts is kept as
small as possible consistent with mechanical safety.

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P3 − COMPRESSOR
DISCHARGE AIR

FIXED ORIFICE

Px − POTENTIOMETRIC
PRESSURE TO TOP
OF PISTON

AMBIENT
PRESSURE

FINAL ORIFICE

CENTRAL GUIDE PIN

ROLLING
DIAPHRAGM BODY

PISTON
DISCHARGE TO
ATMOSPHERE
OUTER GUIDE

INTERSTAGE AIR
GAS GENERATOR CASE (P2.5)

C829
Compressor Bleed Valve - Schematic
Figure 1-5-1

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MANUAL PART NO. 3015442

A SEALING AND COOLING B AIR BLEED SYSTEM C TURBINE COOLING

C563A
Bearing Compartment Seals, Turbine Cooling and Air Bleed Systems (Typical)
Figure 1-5-2

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B. Compressor interstage air is utilized to provide a pressure drop across the air seal
located in front of No. 1 bearing. The air is led through perforations in the rim of the
compressor long spacer and sleeve assembly into the center of the rotor. It then flows
rearward through passages in the three compressor disks and out to an annulus machined
in the center of the air seal via passages in the compressor rear hub. The pressure air
is allowed to leak through the labyrinth and so provides the required pressure seal.

C. The No. 2 bearing is protected by an air seal at the front and at the rear of the bearing.
Pressure air for this system is bled either from the centrifugal impeller tip or, depending
on engine speed, from the labyrinth seal connecting it to the turbine cooling air
system. The air flows through passages in the No. 2 bearing support, equalizing the air
pressure at the front and rear of the bearing compartment and ensures a pressure
seal in the front and rear labyrinths.

D. The compressor and power turbine disks are both cooled by compressor discharge air
bled from the slotted diffuser baffle area down the rear face of the outer exit duct
assembly. It is then metered through holes in the compressor turbine vane support into
the turbine baffle hub, where it divides into three paths. Some of the air is metered
to cool the rear face of the compressor turbine disk, and some to pressurize the bearing
seals. The balance is led forward through passages in the compressor turbine hub to
cool the front face of the compressor turbine. A portion of this cooling air is also led through
a passage in the center of the interstage baffle where the flow divides. One path flows
up the rear face of the power turbine disk while the other is led through the center of the
disk hub and out through drilled passages in the hub of the No. 3 bearing air seals
and front face of the power turbine disk.

E. The cooling air from both turbine disks is dissipated into the main gas stream flow to
atmosphere. The bearing cavity leakage air is scavenged with the scavenge oil into
the accessory gearbox and vented to atmosphere through the centrifugal breather.

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MANUAL PART NO. 3015442

PART 1 - DESCRIPTION OF ENGINE

SECTION 6 - LUBRICATION SYSTEM

1. GENERAL

A. The lubrication system is designed to provide a constant supply of clean lubricating oil
to the engine bearings, reduction gears, torquemeter, propeller and all accessory drive
gears. The oil lubricates and cools the bearings, and conducts any foreign matter to the
main oil filter where it is precluded from further circulation. Calibrated oil nozzles are
used on the main engine bearings to ensure that an optimum oil flow is maintained for all
operating conditions. A main pressure pump located in the oil tank supplies oil to the
accessory section and through an external transfer tube to the gas generator section and
reduction gearbox.

2. OIL TANK

A. The oil tank is an integral part of the compressor inlet case and is located in front of the
accessory gearbox. The tank capacity is sufficient to meet all normal operating
requirements, since the average oil consumption is less than 0.2 lbs/hr. For tank capacities
and other data, refer to Section 2, preceding.

B. The oil tank is provided with an oil filler neck and a quantity dipstick and cap which
protrudes through the accessory gearbox at the 11 o’clock position. The markings on
the dipstick correspond to U.S. quarts, and indicate the quantity of oil required to service
the tank to the full mark. A drain plug provided in the bottom of the tank facilitates oil
drainage. SB1506 introduces a check valve at the lower end of the Oil Filter Tube,
restricting oil loss should the oil-filler-cap be improperly installed.

3. OIL PUMP

A. Oil supplied to a gear type pump, in the lowest portion of the oil tank, is delivered under
pressure to the engine lubricating system. The oil pump consists of two gears within a
pump body and cover which are bolted together to the front face of the accessory
diaphragm. Drive is imparted to the pump by an accessory gearshaft which also
drives the internal double element scavenge pump. A removable screen is provided at
the oil pump inlet. A circular mounting boss on the pump body accommodates a spring
loaded check valve, and through which oil delivery pressure is pumped into the filter
housing.

4. OIL FILTER ASSEMBLY

A. The oil filter assembly consists of either a disposable cartridge type filter element or an
all-metal cleanable element, with a perforated flanged end, a spring-loaded bypass valve
and check valve, (see Figure 1-6-1). These are secured in a alloy housing located in
the compressor inlet case at the 3 o’clock position. A filter cover, secured to the inlet case
by four nuts, holds the filter element and housing in position. Oil pressure flows
through the check valve into the housing, through the filter element and out to the
engine system via a cored passage, leaving any foreign matter deposited on the exterior
of the element. The check valve, positioned in the end of the housing, prevents
gravity flow into the engine after shutdown and permits the filter element to be changed
or removed for cleaning without having to drain the oil tank.

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MANUAL PART NO. 3015442

OVERBOARD
BREATHER VENT

RELIEF VALVE BODY

OIL TANK FILLER CAP
AND DIPSTICK

ADJUSTING SPACER
ACCESSORY GEARBOX OIL PRESSURE
HOUSING RELIEF VALVE

FILTER ELEMENT

OIL FILTER COVER

TEFLON SPACER

STARTER GENERATOR
CAVITY DRAIN FILTER HOUSING

BYPASS VALVE

CHECK VALVE

REDUCTION GEARBOX AND INTERNAL DOUBLE ELEMENT

POWER SECTION SCAVENGE SCAVENGE OIL PUMP
OIL RETURN
EXTERNAL DOUBLE ELEMENT
SCAVENGE OIL PUMP PRESSURE OIL TO REDUCTION
GEARBOX AND POWER SECTION

OIL TANK DRAIN PLUG

C308A
Accessory Gearbox - Lubrication Schematic
Figure 1-6-1

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B. A filter bypass is provided by a spring-loaded bypass valve located in the downstream

side of the check valve. The bypass valve is normally closed and sealed against
leakage by a molded seal located on the front end of the disposable filter element, or by
a preformed packing located on the front of the all-metal element. In the event of a
blocked filter, the increased pressure overcomes the bypass valve spring, opens the
bypass valve and provides an alternate passage for unfiltered oil flow through the engine.

C. Engine oil pressure is regulated by a plunger type pressure relief valve located on the inlet
case just above the oil filter assembly, (see Figure 1-6-1). Oil in excess of the regulated
pressure is bypassed back to the tank through a cored passage inside the case. Oil
pressure is adjusted at the factory by the addition or deletion of adjusting spacers and
does not normally require any further adjustment.

5. OIL PRESSURE SYSTEM

A. Oil lubrication for the accessory drives and bearings is supplied from the oil filter through
cored passages and transfer tubes in the accessory diaphragm and rear housing, (see
Figure 1-6-1). The No. 1 bearing is lubricated by oil routed via an internal passage in the
compressor inlet case, through a filter and nozzle assembly secured in the center
section of the case at the rear of the bearing. The oil is sprayed by the nozzle into a
collector ring mounted on the rear of the compressor hub and directed through passages
in the split inner race to the bearing assembly by centrifugal force.

B. A direct supply of oil is provided to lubricate the No. 2 bearing, reduction gearbox, front
accessories, and power turbine shaft bearings, (see Figure 1-6-2). From the filter, the
oil is directed through a cored passage to an outlet boss located at the bottom of the
compressor inlet case. An external transfer tube, connected to the boss, leads oil
pressure forward to the reduction gearbox. Oil is tapped from the tube to lubricate No. 2
bearing via an internal transfer tube secured in the gas generator case. The tube
passes through a diffuser vane to an oil gallery with two nozzles. A strainer protects the
two nozzles which are positioned one in front and the other at the rear of the No. 2
bearing.

C. The main oil pressure is delivered at 70 psig to the reduction gearbox where it is divided
into two branches. One branch leads to the first stage reduction gears, splines,
torquemeter, Nos. 3 and 4 bearings. The second branch delivers oil to the second stage
reduction gears and spray lubrication for the No. 4 bearing.

D. Oil pressure to the torquemeter is directed through a metering valve which controls the
flow into the torquemeter chamber. The position of the metering valve is dependent on
the torquemeter piston which reacts in direct proportion to engine torque. (For torquemeter
details, refer to Section 4, preceding.) The oil from the torquemeter supply is directed
via an internal oil transfer tube and strainer in the power turbine support housing to four
nozzles; two of the nozzles are positioned one in front and one in the rear of No. 3
bearing. The other two nozzles are located one in front and the other in the rear of No. 4
bearing to provide additional lubrication to the first stage sun gear coupling and the
bearing.

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MAINTENANCE MANUAL
MANUAL PART NO. 3015442

FROM OIL COOLER OIL−TO−FUEL HEATER

DEAERATOR

PROPELLER INTEGRAL OIL TANK OIL TANK BREATHER

GOVERNOR CAPACITY 2.3 GALLONS

OIL FILLER AND

DIP STICK
TORQUEMETER OIL
SPLINES, BEARINGS AND CONTROL VALVE CENTRIFUGAL
REDUCTION GEARS BREATHER
NO. 4 BEARING NO. 3 BEARING NO. 2 BEARING NO. 1 BEARING
TO OIL COOLER
(30 PSI MAX.)

POWER OIL FILTER AND

OIL SUPPLY CHECK VALVE
TURBINE POWER TURBINE
BEARINGS COMPRESSOR ASSEMBLY
BEARINGS
BEARINGS NO. 2 BEARING SCAVENGE PUMP
SCAVENGE PUMP
FILTER BYPASS VALVE
THRUST PRESSURE RELIEF VALVE REDUCTION CASE
BEARING TORQUEMETER WITH RETURN TO OIL TANK SCAVENGE PUMP

ACCESSORY CASE
SCAVENGE PUMP

OIL PRESSURE PUMP GRAVITY DRAIN TO

ACCESSORY
TO TORQUEMETER PRESSURE OIL TANK
TRANSMITTER GEARBOX
PRESSURE INDICATOR DRAIN
BYPASS VALVE

SCAVENGE OIL FROM PROPELLER AND REDUCTION GEARBOX TO COCKPIT TEMPERATURE

INSTRUMENTS BULB

OIL SUPPLY TO PROPELLER AND REDUCTION GEARBOX 70 PSI

TO REDUCTION GEARBOX
OIL SUMP TO
PROPELLER CONTROL REDUCTION
OIL GEARBOX
SUPPLY
REVERSING VALVE PRESSURE OIL SCAVENGE OIL

FLOW DIVIDER
DRAIN OIL BREATHER LINE

PROPELLER PRESSURE OIL

PT6A−6A ENGINES ONLY

C13815
Engine Lubrication - Schematic PT6A-6/20
Figure 1-6-2

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E. The second branch oil pressure is directed to No. 4 bearing through an oil nozzle
secured in the rear of the propeller oil transfer tube. Passages in the split inner races of the
bearing line up with a drilled annulus on the power turbine shaft to provide access for
centrifugal oil flow into the bearing assembly. The second branch also provides oil from the
internal annulus through cored passages to the propeller constant speed control unit,
the accessory drive gears and the propeller thrust bearing. Oil mist provides lubrication to
the second stage carrier bearing.

6. PITCH REVERSING VALVE SYSTEM (PT6A-6A ENGINES)

A. The pitch reversing valve works in connection with a flow divider to provide a hydraulic
control medium for use on reversing propeller installations. The reversing valve serves
as a variable pivot for the reversing valve lever; its position is therefore a function of
reversing valve lever travel. It consists of a ported sleeve valve and housing secured
in the propeller thrust bearing cover.

B. The flow divider consists of a ported sleeved valve which is spring-loaded to the closed
position. It is contained in a separate housing, secured to the reduction gearbox and
interconnects the reversing valve to the gearbox oil sump. It also provides an alternate
oil passage through a check valve to the reduction gearbox main oil pressure inlet. During
normal propeller operation, a high-pressure oil bleed opens the flow divider valve
ports, allowing the oil dumped by the reversing valve to return directly to the reduction
gearbox oil sump. However, during windmilling operations, with consequent low gas
generator rpm, the flow divider valve spring tension overcomes the oil bleed pressure,
closing the valve ports and directing the propeller control oil flow back to the main oil
pressure inlet where it is recirculated through various passages in the reduction
gearbox.

7. SCAVENGE OIL SYSTEM

A. The scavenge oil system includes two double-element scavenge pumps connected by
internal passages and lines to two main external transfer tubes. One pump is secured
inside the accessory gearbox and the other is externally mounted. They are contained in
separate housings and driven off accessory gearshafts.

B. The oil from the No. 1 bearing compartment is returned by gravity through an internal
cored passage to the bottom of the compressor inlet case and then through oil transfer
tube in the oil tank and accessory diaphragm into the accessory gearbox. No. 2
bearing oil drains down from its compartment into an external transfer tube leading
rearward to the bottom of the oil tank. It is then scavenged through a cored passage in
the accessory diaphragm to the front element of the internal scavenge pump which
forces oil into the accessory gearbox. The oil from the centrifugal breather and input
gearshaft and bearings drains to the bottom of the accessory gearbox. It is scavenged
from the gearbox together with the oil from Nos. 1 and 2 bearings and the scavenged
oil from Nos. 3 and 4 bearings (see Paragraph 17, following). The double-element
scavenge pump is connected by a quillshaft to the main oil pump in the oil tank and
drives an externally mounted tachometer-generator.

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C. The Nos. 3 and 4 bearing compartment oil is scavenged through a transfer tube into the
reduction gearbox, and drains rearward through an external transfer tube to the front
element of the external scavenge pump, where it is returned into the accessory gearbox.
Oil from the propeller control, front thrust bearing, reduction gears and torquemeter
bleed orifice drains into the reduction gearbox sump, and is scavenged by the rear element
of the external scavenge pump via a separate external transfer tube. The oil from the
pump is delivered through a tee connection into a tube to the airframe cooler, from where
it is returned to the oil tank. Oil returned to the accessory gearbox from Nos. 1, 2, 3
and 4 bearings is scavenged by the rear element of the internal scavenge pump circulated
through the airframe oil cooler and returned to the oil tank.

8. BREATHER SYSTEM

A. Breather air from the engine bearing compartments and the accessory gearbox is
vented overboard through a centrifugal breather located in the accessory gearbox. The
various engine compartments are connected to the accessory gearbox by cored
passages or existing scavenge oil transfer tubes. The No. 1 bearing vents rearward
through the bearing rear housings and the oil tank center tube into the accessory gearbox
via the input gearshaft bearing. The No. 2 bearing is vented to the accessory gearbox
by an interconnecting vent tube in the compressor assembly and the hollow passages in
the extension drive coupling and gearshaft. A bypass valve, immediately upstream of
the front element of the internal scavenge pump, allows oil and air to be vented into the
accessory gearbox under certain transient operating conditions to prevent
overpressurizing the number 2 bearing area. Under normal operating conditions, the
valve is closed to prevent oil flooding back into the tube assembly. The Nos. 3 and 4
bearings and the reduction gearbox are vented by utilizing the present large diameter
internal and external scavenge tubes which lead back from the reduction gearbox to
the external scavenge pump mounted on the accessory gearbox and dumped into the oil
tank after passing the airframe cooler. The oil tank, in turn, vents into the accessory
gearbox through a breather tube.

9. CENTRIFUGAL BREATHER

A. The centrifugal breather consists of a shrouded aluminum alloy impeller secured to the
rear face of the starter-generator gearshaft by a retaining ring. Rotational torque is
transmitted from the gearshaft to the impeller by three equi-spaced alloy pins. Breather
air flows radially inward through the rotating impeller housing where the oil particles
are separated from the air mist by centrifugal force. The oil particles are thrown outward
and drain freely to the bottom of the accessory gearbox. The relatively oil-free
breather air passes forward through the hollow center section of the gearshaft to a
cored passage in the accessory diaphragm. It is then routed through a transfer tube to a
breather boss on the rear face of the accessory housing where a connection for an
overboard vent line is provided.

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MANUAL PART NO. 3015442

10. OIL-TO-FUEL HEATER

A. The oil-to-fuel heater is a self-contained engine option secured to flange G on the

accessory gearbox. It functions as a heat exchanger when the engine is running, utilizing
heat from the engine lubricating system to pre-heat fuel in the engine fuel system. The
system incorporates a two-pass engine oil circuit and a two-pass fuel circuit, at right angles
to one another (see Figure 1-6-3). A temperature control valve regulates the fuel
temperature by either permitting oil flow through the heater core, or bypassing it through
the valve back to the engine oil tank through an external line.

B. Hot engine oil enters the oil circuit through the oil inlet port of the heater under engine
pressure. If the outlet fuel temperature is less than the ball valve setting, the vernatherm
plunger will be retracted and the ball will seal the bypass port, forcing the engine oil
to circulate through the core of the heat exchanger. The heat from the engine oil is
transferred through the walls of the heat exchanger to warm the engine fuel.

C. If the fuel outlet temperature is above the ball valve setting, the vernatherm plunger will
be in an extended position and the ball will be lifted off its seat by the plunger. This
permits the hot engine oil to flow through the bypass port and oil outlet without circulating
through the heat exchanger core. The fuel outlet temperature is accordingly reduced.

11. OIL SYSTEM INSTRUMENTATION

A. Main oil pressure is measured from the transmitter located at the 3 o’clock position on
the accessory housing. Oil temperature is measured by a temperature bulb located at
the 4 o’clock position on the accessory gearbox.

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MAINTENANCE MANUAL
MANUAL PART NO. 3015442

FUEL IN

FUEL OUT

OIL OIL
IN OUT

OIL OIL
IN OUT

BY−PASS
CONDITION

C23
Oil-to-Fuel Heater Flow - Schematic
Figure 1-6-3

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MAINTENANCE MANUAL
MANUAL PART NO. 3015442

PART 1 - DESCRIPTION OF ENGINE

SECTION 7 - IGNITION SYSTEM

GLOW PLUG TYPE (PRE-SB1429)

1. GENERAL

A. The glow plug type ignition system has been developed and adapted to provide the
engine with an ignition system capable of quick light-up at extremely low ambient
temperatures. The basic system consists of an ignition current regulator with a selectable
circuit to the two sets of ballast tubes, two shielded ignition cable and clamp assemblies
and two glow plugs.

2. IGNITION CURRENT REGULATOR

A. The ignition current regulator is secured by three bolts to the accessory gearbox casting
at flange G. The regulator may be airframe mounted if required. The regulator box has a
removable cover and contains four ballast tubes (See Figure 1-7-1) Each tube
consists of a pure iron filament surrounded by helium and hydrogen gases and enclosed
in a glass envelope sealed to an octal base. The iron filament, having a positive
coefficient of resistance (resistance increased with temperature increase caused by
current flow), provides a stabilizing effect on the current passing through it. At low
temperatures ballast tube resistance is decreased to counteract power loss and as
temperatures rise ballast tube resistance increases to maintain nearly constant
glow-plug current. Each glow plug is wired in series with two parallel-connected ballast
tubes and either or both glow plugs may be selected for light-up. Ballast tubes provide an
initial current surge when switched on, and a lower stabilized current after approximately
30 seconds. This characteristic provides rapidly heated glow plugs for fast light-up.

3. GLOW PLUGS

A. The glow plug (see Figure 1-7-2) consists of a helical heating element fitted into a short
plug body. The plugs are secured to the gas generator case in threaded bosses
provided at the 4 and 8 o’clock positions. The heating element consists of a helically
wound coil which lies slightly below the end of the plug body. Four holes, equi-spaced on
the periphery of the plug body lead into an annulus provided below the coil. During
starting procedure, the fuel sprayed by the fuel nozzles traverses the lower wall of the
combustion chamber liner into the annulus, The fuel is vaporized and ignited by the hot coil
element which heats up (yellow hot) to approximately 1260°C (2300°F). The four air
holes bleed compressor discharge air from the gas generator case into the body then past
the hot coil into the combustion chamber to produce a hot streak or torching effect
which ignites the remainder of the fuel. The air also serves to cool the coils when the
engine is running with the ignition system switched off.

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MANUAL PART NO. 3015442

GLOW PLUGS
LEFT RIGHT
(9 O’CLOCK POSITION) (4 0’CLOCK POSITION)

UPPER INPUT CONNECTOR

LOWER CENTER
OUTPUT OUTPUT
CONNECTOR CONNECTOR
A B

3 4 4 5

2 5 3 6

1 6 2 7

8 7 1 8
#1 #3

BALLAST TUBE BASE

3 4 3 4

2 5 2 5

1 6 1 6

8 7 8 7
#2 #4

2 & 3 COMMON
7 & 8 COMMON

C448D
Ignition Current Regulator - Circuit Diagram
Figure 1-7-1

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MAINTENANCE MANUAL
MANUAL PART NO. 3015442

EXHAUST CASE
INNER SECTION

COMBUSTION CHAMBER
LINER

GLOW PLUG

COOLING AIR PASSAGE

LEAD CONNECTOR FROM

GAS GENERATOR IGNITION REGULATOR BOX
CASE

C3785A
Glow Plug - Installation
Figure 1-7-2

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MAINTENANCE MANUAL
MANUAL PART NO. 3015442

SPARK IGNITER TYPE (POST-SB1429)

4. GENERAL

A. The spark ignition system has been developed to provide the engine with an ignition
system capable of quick light-ups over a wide temperature range. The system
consists of an engine-mounted ignition exciter, two individual high tension cable
assemblies and two spark igniters. The system is energized from the aircraft nominal
28-volt d.c. supply and will operate in the 9- to 30-volt range.

5. IGNITION EXCITER

A. The ignition exciter is a sealed unit containing electronic components encased in an

epoxy resin. The unit is energized only during the engine starting sequence to initiate
combustion in the combustion chamber. The exciter transforms the DC input to a pulsed
high voltage output through solid-stage circuitry, a transformer and diodes.

B. When the unit is energized, a capacitor on the high voltage side of the output transformer
is progressively charged, until the energy stored, approximately four joules, is sufficient
to ionize a spark gap in the unit and discharge the capacitor across the two spark
igniters through a dividing and step-up transformer network. The network is designed so
that if one igniter is open or shorted, the remaining igniter will continue to function.
The network also enables the capacitor to discharge automatically in the event of either
or both igniters becoming inoperative, or input voltage is switched off.

6. SPARK IGNITERS

A. The spark igniters, located at the 4 and 8 o’clock positions on the gas generator case
and adjacent to the fuel manifold, are in the form of a double-ended, threaded plug with
a central positive electrode, enclosed in an annular semi-conducting material. The
electrical potential developed by the ignition exciter is applied across the gap between
the central conductor and the igniter shell (ground). As this potential increases, a small
current passes across the semi-conducting material. This current increases until the
air between the central conductor and the shell ionizes. When ionization occurs, high
energy discharges between the electrodes. The spark always occurs somewhere in the
annular space between the central conductor and shell.

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MAINTENANCE MANUAL
MANUAL PART NO. 3015442

PART 1 - DESCRIPTION OF ENGINE

SECTION 8 - FUEL SYSTEM

INTRODUCTION

1. GENERAL

A. The basic fuel system (see Figure 1-8-1) consists of a single engine driven pump, a
fuel control unit, a fuel manifold and 14 simplex fuel nozzles. A fuel dump valve and
two drain valves are provided on the gas generator case to ensure drainage of residual
fuel after engine shutdown.

FUEL PUMP (VANE TYPE)

2. GENERAL

A. The fuel pump is mounted on the accessory gearbox and is driven through a splined
coupling. The coupling splines are lubricated by oil mist from the accessory gearbox
through a hole in the gearshaft. Another splined coupling shaft extends the drive to the
fuel control unit which is bolted to the rear face of the fuel pump. Fuel from the
aircraft fuel boost pump enters the pump through a 74 micron (200 mesh) inlet screen.
The filter screen is spring-loaded to lift and allow unfiltered fuel to flow in the event
of the screen becoming blocked. The fuel is pumped by rotation of the slotted rotor which
contains 10 vanes. The vanes move within their slots, following the inside contour of a
cam ring. The vanes are held against the contour of the ring by centrifugal force and
system pressure. The moving vanes in the cam ring form opposing chambers connected
to pump inlet and outlet ports. As each chamber, formed between the vanes, moves
around in the cam ring, it changes in volume, drawing in fuel and then ejecting it.

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MAINTENANCE MANUAL
MANUAL PART NO. 3015442

LEGEND
BYPASS FUEL PRESSURE Po
TRANSFER TUBE FUEL MANIFOLD
UNMETERED FUEL PRESSURE P1 ADAPTER ASSEMBLY
METERED FUEL PRESSURE P2 NOZZLE
AMBIENT PRESSURE Pa SHEATH
DRAIN
GOVERNOR SERVO PRESSURE Py COMBUSTION CHAMBER LINER

ACCELERATION & SPEED ENRICHMENT PRESSURE Px GAS GENERATOR

DRAIN VALVE TO CASE
COMPRESSOR DISCHARGE PRESSURE Pc ATMOSPHERE

MINIMUM CUT−OFF VALVE FUEL CONTROL

PRESSURIZING
VALVE
MIN. FLOW STOP
UNIT AUTOMATIC FUEL
NOTE PROPELLER SPEED SPECIFIC GRAVITY DUMP VALVE
METERING
PROPELLER GOVERNOR ON MAX. STOP VALVE ADJUSTMENT OUTPUT FILTER (10 MICRON)
PT6A−6 SERIES AND PT6A−20
ENGINES IS AIRFRAME SUPPLY DISK GROUP GEAR TYPE
(BIMETALLIC)
BELLOWS STOP
FUEL PUMP FILTER BYPASS VALVE
METERING HEAD
REGULATOR
INPUT COUPLING
PROP. GOVERNOR DIFFERENTIAL PRESSURE
RESET ARM BELLOWS REGULATING
VALVE
BLEED RETURN
MAXIMUM OR
TO TANK
STOP
RATIO ARM RELIEF
VALVE
UNDERSPEED FUEL ACCELERATION PUMP GEAR AND SHAFTGEAR
GOVERNING SPEED BELLOWS
ADJUSTMENT (EVACUATED) BYPASS RETURN TO
MAX. FLOW TORQUE TUBE PUMP (INTERNAL)
STOP

PROPELLER GOVERNOR Nf SECTION INLET SCREEN (74 MICRON)

PT6A−20A OR 20B ENGINES ONLY BYPASS PRESSURE
REGULATING VALVE
CUT−OFF STOP
FUEL INLET
(FROM BOOST PUMP)
PRIMARY AND Pc NEEDLE VALVE

IDLE RESET STOP

HOT
VIEW OF CUT−OFF & IDLE
RESET LINKAGE
DISK GROUP (BIMETALLIC)
POWER TURBINE
MAX. STOP MIN. STOP
GOVERNROR Nf GOVERNOR
SCHEDULING CAM COMPRESSOR DISCHARGE PRESSURE

PT6A−6 SERIES AND GAS SPEED

PT6A−20 ENGINES ONLY SCHEDULING Ng GOVERNOR
CAM
TEMPERATURE COMPENSATOR
IDLE SPEED
VIEW OF POWER TURBINE STOP
GOVERNOR LEVER

C1854A
Engine Fuel System - Schematic
Figure 1-8-1

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FUEL PUMP (GEAR TYPE)

3. GENERAL

A. This is a positive displacement gear type pump which incorporates spring and pressure
loaded bushings and is driven off the accessory gearbox. A coupling is incorporated to
transmit the drive to the pump gears. The coupling splines are lubricated by oil mist from
the accessory gearbox, through a hole in the engine gearshaft. Another coupling is
used to transmit a speed signal to the fuel control unit from the pump gears. Fuel from
the aircraft boost pump enters the fuel pump through a 74 micron (200 mesh) inlet screen
and flows into the pump gear chamber, from where the fuel is delivered at high
pressure to the fuel control unit through a 10 micron pump outlet filter. The inlet screen
is spring-loaded to lift and allow unfiltered fuel to flow into the pump in the event of
the screen becoming blocked. A bypass valve, in parallel with the outlet filter and
connected to the fuel control unit by cored passages in the pump casing, enables
unfiltered high pressure fuel to pass from the pump to the fuel control unit when the
outlet filter becomes blocked. An internal cored passage originating at the mating face
with the fuel control unit mounting flange returns bypass fuel from the fuel control unit to
the pump inlet downstream of the inlet screen. A pressure regulating valve in this line
serves to pressurize the gear bearings.

FUEL CONTROL SYSTEM

4. GENERAL

A. The fuel control system consists of three separate units with interdependent functions:
The fuel control unit (FCU), the power turbine governor, or propeller governor package
(PT6A-20A and PT6A-20B engines) and temperature compensator, (see Figure 1-8-2).
The FCU determines the correct fuel schedule for the engine, to provide the power
required as established by the power control lever. This is accomplished by controlling
the speed of the compressor turbine (Ng). The power turbine governor, or power turbine
governor section (Nf) of the propeller governor package (PT6A-20A and PT6A-20B
engines), provides power turbine overspeed protection during normal operation. On
propeller reversing installations, the propeller governor is inoperative during reverse thrust
operation, and control of the power turbine speed is accomplished by the power
turbine governor, or power turbine governor section (Nf) of the propeller governor package
(PT6A-20A and -20B engines). The temperature compensator alters the acceleration
fuel schedule of the fuel control unit, to compensate for variations in compressor inlet air
temperature. Engine characteristics vary with changes in inlet air temperature and the
acceleration fuel schedule must, in turn, be altered to prevent compressor stall and/or
excessive turbine temperatures.

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1 FUEL METERING VALVE

2 MINIMUM PRESSURIZING VALVE
3 HIGH−PRESSURE RELIEF VALVE
4 TORQUE TUBE ASSEMBLY
5 BYPASS VALVE FUEL CONTROL 1
METERING SECTION 2
6 NEEDLE VALVE ORIFICE TEMPERATURE UNIT
COMPENSATOR PNEUMATIC (COMPUTING)
7 ROD SECTION
8 COMPRESSOR TURBINE GOVERNOR SPRING
T2
8’ COMPRESSOR TURBINE ENRICHMENT SPRING P1 P2
9 COMPRESSOR TURBINE SPEED SET LEVER 5
Py
10 LINK
11 IDLE SPEED ADJUSTMENT 7
FUEL BLEED TO TANK
12 POWER TURBINE SPEED SET LEVER 4
13 POWER TURBINE GOVERNOR SPRING Px FUEL OUTLET TO NOZZLES
14 POWER TURBINE GOVERNOR PLATFORM 3 P0
17
15 NEEDLE VALVE ROD 15
16 MINIMUM Ng CUT−BACK Pc
17 CUT−OFF VALVE
18 COMPRESSOR TURBINE GOVERNOR PLATFORM 6 FUEL
19 MINIMUM Nf GOVERNING PUMP
20 SPEED SET LEVER FUEL FROM
21 SPEEDER SPRING Pa
BOOSTER PUMP
22 PLATFORM 11 9
23
23 OVERSPEED ADJUSTMENT
24 UNDERSPEED ADJUSTMENT Pa 19 12
8 O6
25 YOKED BELLCRANK
25 20
Pa O5 8’ 21 Pa O8 13
Pa AMBIENT AIR 10
Pc COMPRESSOR DICHARGE AIR
P1 UMMETERED PUMP DELIVERY FUEL 18 16 14
P2 METERED FUEL 22
Po BYPASS FUEL 24
Px ENRICHMENT PRESSURE
Py GOVERNING PRESSURE Nf GOVERNOR SECTION
O5 ENRICHMENT ORIFICE Nf GOVERNOR
O6 GOVERNING ORIFICE PROPELLER GOVERNOR
Nf SECTION PT6A−6 SERIES AND
O8 GOVERNING ORIFICE PT6A−20 ENGINES ONLY
T2 AIR INLET TEMPERATURE PT6A−20A AND −20B
ENGINES ONLY

C1852A
Fuel Control System - Schematic
Figure 1-8-2

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FUEL CONTROL UNIT

5. GENERAL

A. The gas turbine fuel control unit (FCU) (see Figure 1-8-3) is mounted on the engine
driven fuel pump and driven at a speed proportional to compressor turbine speed
(Ng). The FCU determines the correct fuel schedule for the engine, to provide the power
required by the power control lever setting. This is accomplished by controlling the
speed of the compressor turbine (Ng). Engine power output is directly dependent upon
compressor turbine speed. The FCU governs compressor turbine speed (Ng), thereby
governing the actual power output of the engine. Control of Ng is accomplished by
regulating the amount of fuel supplied to the combustion chamber.

6. FUEL SYSTEM

A. The FCU is supplied with fuel at pump pressure (P1), (see Figures 1-8-1 and 1-8-2).
Fuel flow is established by a metering valve and bypass valve system. Fuel at P1
pressure is applied to the metering valve inlet. The fuel pressure at the metering valve
outlet is referred to as metered fuel pressure (P2). The bypass valve maintains an
essentially constant fuel pressure differential (P1 - P2) across the metering valve. The
orifice area of the metering valve changes to meet specific engine requirements. Fuel
pump output in excess of these requirements is returned to pump inlet, and is referred
to as Po. The bypass valve consists of a sliding valve operating in a ported sleeve. The
valve is actuated by means of a diaphragm and spring. In operation, the spring force
is balanced by the pressure differential (P1 - P2) operating on the diaphragm. The bypass
valve is always positioned to maintain the P1 - P2 differential and to bypass fuel in
excess of engine requirements.

B. A relief valve is incorporated parallel to the bypass valve to prevent a buildup of

excessive P1 pressure in the control body. The valve is spring loaded closed and remains
closed until the inlet fuel pressure (P1) overcomes the spring force and opens the
valve. As soon as the inlet pressure is reduced, the valve again closes.

C. The metering valve consists of a contoured needle working in a sleeve. The metering
valve regulates the flow of fuel by changing orifice area. Fuel flow is a function of
metering valve position only, as the bypass valve maintains an essentially constant
differential fuel pressure across the orifice regardless of variations in inlet or discharge
fuel pressures. The pressurizing valve is located between the metering valve and the
cut-off valve. Its function is to maintain adequate pressure within the control to assure
correct fuel metering. The cut-off valve provides a positive means of stopping fuel flow to
the engine. During normal operation, this valve is fully open and offers no restriction to
the flow of fuel to the nozzles. On installations not incorporating a reversing propeller, the
cut-off valve is actuated by the power control lever. On installations incorporating a
reversing propeller, an additional linkage system is employed to operate the cut-off valve.

D. An external adjustment is provided on the bypass valve spring cover to match

accelerations between engines on multi-engine installations. Compensation for variations
in specific gravity resulting from changes in fuel temperature is accomplished by the
bimetallic disks under the bypass valve spring.

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HI−IDLE STOP CUT−OFF STOP

ACCELERATION
ADJUSTMENT

FCU CONTROL MAX Ng SPEED

ARM ADJUSTMENT

PART POWER
TRIM STOP

FUEL
OUTLET

FUEL INLET FUEL CUT−OFF

VALVE LEVER

C27B
Fuel Control Unit
Figure 1-8-3

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7. POWER INPUT, SPEED GOVERNOR, AND ENRICHMENT SECTION

A. For details of the governor and enrichment levers, see Figure 1-8-4. Views A and B
identify the individual levers and their relationship to each other. Views C and D show
these levers in operation. The power input shaft incorporates a cam which depresses an
internal lever when the power is increased. A spring connects this cam follower lever
to the governor lever. The governor lever is pivoted and one end operates against an
orifice to form the governor valve. The enrichment lever pivots at the same point as
the governor lever. It has two extensions which straddle a portion of the governor lever
so that after a slight movement a gap will be closed and then both levers must
move together. The enrichment lever actuates a fluted pin which operates against the
enrichment hat valve. Another smaller spring connects the enrichment lever to the
governor lever. A roller on the arm of the enrichment lever contacts the end of the
governor spool.

B. The speed scheduling cam applies tension to the governor spring through the intermediate
lever which applies a force to close the governor valve. The enrichment spring, between
the enrichment and governor levers, provides a force to open the enrichment valve.
As the drive shaft revolves, it rotates a table on which the governor weights are mounted.
Small levers on the inside of the weights contact the governor spool. As compressor
turbine speed (Ng) increases, centrifugal force causes the weights to apply increasing
force against the spool. This tends to move the spool outward on the shaft against the
enrichment lever. As governor weight force overcomes opposing spring force, the
governor valve is opened and the enrichment valve is closed.

C. The enrichment valve will start to close whenever compressor turbine speed (Ng)
increases enough to cause the weight force to overcome the force of the small spring. If
Ng continues to increase, the enrichment lever will continue to move until it contacts
the governor lever as shown in View C, at which time the enrichment valve will be fully
closed. The governor valve will open if compressor turbine speed (Ng) increases
sufficiently to cause the weight force to overcome the force of the larger spring. At this
point the governor valve will be open and the enrichment valve closed, as shown in View
D.

D. The main body incorporates a vent port which vents the inner body cavity to atmospheric
pressure (Pa). Modified compressor discharge pressure, Px and Py, will be bled off to
Pa when the respective enrichment and governor valves are open.

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Pa
Px
SPEED ENRICHMENT VALVE OPEN

Py
GOVERNOR LEVER
ENRICHMENT LEVER

GOVERNOR
VALVE CLOSED

NOTE LOCATION
OF GAP
VIEW B
VIEW A ENRICHMENT LEVER
GOVERNOR LEVER
SPEED ENRICHMENT
VALVE CLOSED
SPEED ENRICHMENT AND GOVERNOR VALVE OPEN
GOVERNOR VALVES CLOSED

NOTE
LOCATION
OF GAP

VIEW C VIEW D
GOVERNOR WEIGHT FORCE GOVERNOR WEIGHT FORCE
OVERCOMES FORCE OF OVERCOMES FORCE OF
SMALL SPRING LARGE SPRING

C451
Drive Body Assembly - Operation
Figure 1-8-4

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8. BELLOWS SECTION

A. The bellows assembly consists of an evacuated (acceleration) bellows and governor

bellows connected by a common rod. The end of the acceleration bellows opposite
the rod is attached to the body casting. The acceleration bellows provides an absolute
pressure reference. The governor bellows is secured in the body cavity and its function is
similar to that of a diaphragm. Movement of the bellows is transmitted to the metering
valve by the cross shaft and associated levers. The cross shaft moves within a torque tube
which is attached to the shaft near the bellows lever. The tube is secured in the body
casting at the opposite end by means of an adjustment bushing. Therefore, any rotational
movement of the cross shaft will result in an increase or decrease in the force of the
torque tube. The torque tube forms the seal between the air and fuel sections of the
control. The torque tube is positioned during assembly to provide a force in a direction
tending to close the metering valve. The bellows act against this force to open the metering
valve. Py pressure is applied to the outside of the governor bellows. Px pressure is
applied to the inside of the governor bellows and to the outside of the acceleration bellows.

B. For explanation purposes the governor bellows assembly is illustrated as a diaphragm

(see Figure 1-8-5). Py pressure is applied to one side of the diaphragm and Px is
applied to the opposite side. Px is also applied to the evacuated bellows attached to the
diaphragm. The force of Px applied against the evacuated bellows is cancelled by
application of the same pressure on an equal area of the diaphragm, as the forces act
in opposite directions. All pressure forces applied to the bellows section can be resolved
into forces acting on the diaphragm only. These forces are: Py pressure acting on the
entire surface of one side; the internal pressure of the evacuated bellows acting on a
portion of the opposite side (within the area of pressure cancellation); and Px acting
on the remainder of that side. Any change in Py will have more effect on the diaphragm
than an equal change in Px, due to the difference in effective area.

C. Px and Py vary with changing engine operating conditions as well as inlet air temperature.
When both pressures increase simultaneously, as during acceleration, the bellows cause
the metering valve to move in an opening direction. When Py decreases as the
desired compressor turbine speed (Ng) is approached (for governing after acceleration),
the bellows will travel to reduce the opening of the metering valve. When both
pressures decrease simultaneously, the bellows will travel to reduce the metering valve
opening because a change in Py is more effective than the same change in Px. This
occurs during deceleration and moves the metering valve to its minimum flow stop.

D. On installations, incorporating a manual override (see Figure 1-8-6), the cover containing
the bellows travel stop (for governor bellows) and the retaining plate is removed and
replaced by a mechanism which can manually move the end of the governor bellows. This,
in turn, will move the metering valve in a direction to increase the fuel flow to the
engine. The mechanism contains an operating pin with a driving pin passing through it
and through the portion of the actuating shaft which is contoured (helix). Both ends of the
driving pin engage in slots in the cover to prevent rotation. A special manual control in
the cockpit connects to the actuating shaft lever and provides rotation of the actuating
shaft. When the lever is in the OFF position, the operating pin acts as the bellows
travel stop. This system is uni-directional in that it can only push on the governor bellows
to increase fuel flow. It has no power to move the bellows in the other direction to
reduce fuel flow. This system provides an emergency control in the event of governor
bellows failure.

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AREA OF PRESSURE
CANCELLATION

PX

PY

PX

DIAPHRAGM REPRESENTS EVACUATED

GOVERNOR BELLOWS (ACCELERATION)
BELLOWS

C434
FCU Bellows Section - Functional Diagram
Figure 1-8-5

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SHIMS DRIVING PIN

COVER

STOP

ENRICH

SHAFT

LEVER

OPERATING PIN

BODY

C30A
Manual Override System
Figure 1-8-6

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POWER TURBINE GOVERNOR (PT6A-6 SERIES AND PT6A-20 ENGINES)

9. GENERAL

A. The power turbine governor, mounted on the reduction gearbox of the engine, is driven
at a speed proportional to power turbine speed (Nf). The function of the power turbine
governor is to limit the maximum speed of the power turbine (Nf) as, during normal
operation, Nf is controlled by the propeller governor. However, if a malfunction occurs
the power turbine governor will prevent power turbine speed (Nf) from exceeding 105%.
This is accomplished by reducing the fuel flow in the FCU.

B. The power turbine governor employs a drive body similar to the drive body of the FCU,
with the main difference being the elimination of the speed enrichment mechanism. The
cover incorporates vent holes which maintain the inner cavity of the governor at
atmospheric pressure (Pa). During normal operation, the governor throttle lever is
positioned against the maximum speed stop. On non-reversing applications, the lever is
locked in this position.

C. Py pressure from the FCU (see Figure 1-8-1) is applied to the power turbine governor
valve. If a power turbine overspeed occurs, the governor weight force overcomes the
spring force which opens the valve to bleed off Py pressure. This, in turn, reduces Py
pressure on the governor bellows on the FCU, and results in a reduction of fuel flow
and consequently, compressor turbine speed (Ng). As the overspeed condition is
corrected, the governor weight force diminishes and the spring force again overcomes
the reduced weight force. This action closes the valve restoring control of Py pressure and
engine fuel flow to the FCU.

D. On installations using a reversing propeller, the propeller governor becomes ineffective

during REVERSE THRUST operation, and power turbine speed (Nf) is then controlled
by the power turbine governor to approximately 95%.

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PROPELLER GOVERNOR (PT6A-20A AND -20B ENGINES)

10. GENERAL

A. The propeller governor, mounted on the reduction gearbox of the engine contains a
normal propeller governor (CSU) section, a reversing valve, and a power turbine
governor section, driven at a speed proportional to power turbine speed (Nf). The Nf
governor section provides power turbine overspeed protection during normal operation.
During reverse thrust operation the propeller governor is inoperative and control of
power turbine speed is accomplished by the Nf governor section. The Nf governor section
of the propeller governor senses Py pressure through an external pneumatic line from
the drive body adapter on the FCU to the governor. In the event of a power turbine
overspeed condition, an airbleed orifice in the Nf governor section is opened by
flyweight action (see Figure 1-8-1), to bleed off Py pressure through the governor.
When this occurs, Py pressure acting on the FCU bellows decreases to move the FCU
metering valve in a closing direction, thus reducing fuel flow. Reduction in fuel flow
decreases Ng speed and consequently Nf speed. The speed at which the airbleed orifice
opens is dependent on the setting of the propeller governor control lever and the
setting of the Nf air bleed link. Normally the airbleed orifice is opened at approximately
6% above propeller governor speed setting with the Nf governor air bleed link at maximum
position, and approximately 4% under propeller governor speed setting at minimum
position. In reverse thrust the propeller reversing interconnect linkage resets the Nf
governor air bleed link to a setting below the propeller governor control lever setting.
Power turbine speed (Nf) and hence propeller speed is then limited by the Nf governor;
power supplied by the gas generator is reduced to allow a propeller speed approximately
5% under the speed set by the propeller governor.

TEMPERATURE COMPENSATOR

11. GENERAL

A. The temperature compensator (see Figure 1-8-7) is mounted on the compressor inlet
case with the bimetallic disks extending into the inlet air stream. Compressor discharge
pressure (Pc) is applied to the compensator. This pressure source is used to provide
a Px pressure signal to the FCU. The compensator changes the Px pressure to provide
an acceleration schedule biased by inlet temperature to prevent compressor stall or
excessive turbine temperature.

FUEL CONTROL SYSTEM - COMPLETE OPERATION (TYPICAL)

12. ENGINE STARTING

A. PT6A-6 engine starting cycle is initiated with the power control lever in the CUT-OFF
position, while PT6A-6A, PT6A-6B, PT6A-6/C20, PT6A-20, PT6A-20A and PT6A-20B
engine starting cycle is initiated with the power control lever in the IDLE position and the
fuel shut-off lever in the CUT-OFF position. The ignition and starter are switched on
and when required compressor turbine speed (Ng) is attained, either the power control
lever on the former engines or the fuel shut-off lever on the latter engines, is moved to the
IDLE position to provide fuel. For the fuel ratio curve (see Figure 1-8-8).

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NEEDLE VALVE
HOT

AIR

BIMETALLIC DISK GROUP

Px
COMPRESSOR
AIR
Pc

C31A
Temperature Compensator - Cross Section
Figure 1-8-7

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STARTING ACCELERATION

105 ° F

59 ° F
Wf
Pc −50° F

0 IGNITION IDLE SPEED 100

ENRICHMENT
Ng SPEED (%)

C32B
Fuel Ratio Curve for Starting and Acceleration Schedules (Typical)
Figure 1-8-8

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B. At the time of ignition, the fuel control metering valve is in a low flow position. As the
engine accelerates, the compressor discharge pressure (Pc) increases, causing an
increase in Px pressure. Px and Py increase simultaneously as Px = Py during engine
acceleration. The increase in pressure sensed by the bellows causes the metering valve to
move in an opening direction. As compressor turbine speed (Ng) approaches idle, the
centrifugal force of the drive body weights begins to overcome the governor spring force
and opens the governor valve. This creates a Px - Py differential which causes the
metering valve to move in a closing direction until the required-to-run idle fuel flow is
obtained.

C. Any variation in engine speed from the selected (idle) speed will be sensed by the
governor weights and will result in increased or decreased weight force. This change
in weight force will cause movement of the governor valve which will then be reflected by
a change in fuel flow necessary to re-establish the proper speed.

13. ACCELERATION

A. As the power control lever is advanced above the IDLE position, the speed scheduling
cam is repositioned, moving the cam follower lever to increase the governor spring
force. The governor spring then overcomes the weight force and moves the lever closing
the governor valve. Px and Py immediately increase and cause the metering valve to
move in an opening direction. Acceleration is then a function of increasing Px (Px = Py).

B. With the increase in fuel flow, the compressor turbine will accelerate. When compressor
turbine speed (Ng) reaches a predetermined point (above 80%), weight force overcomes
the enrichment spring and starts to close the enrichment valve. When the enrichment
valve starts to close, Py and Px pressures increase causing an increase in the movement
rate of the governor bellows and metering valve, thus providing speed enrichment to
the acceleration fuel schedule. Continued movement of the enrichment will then be
discontinued. Meanwhile, as compressor speed (Ng) and hence power turbine speed (Nf)
increase, the propeller governor increases the pitch of the propeller to prevent Nf from
overspeeding and applies the increased power as additional thrust. Acceleration is
completed when the centrifugal force of the weights again overcomes the governor spring
and opens the governor valve.

14. GOVERNING

A. Once the acceleration cycle has been completed, any variation in engine speed from the
selected speed will be sensed by the governor weights and will result in increased or
decreased weight force. This change in weight force will cause the governor valve to either
open or close, which will then be reflected by the change in fuel flow necessary to
re-establish the proper speed. When the FCU is governing, the valve will be maintained
in a regulating, or ‘‘floating’’ position.

15. ALTITUDE COMPENSATION

A. Altitude compensation is automatic with this fuel control system since the acceleration
bellows assembly is evacuated and affords an absolute pressure reference. Compressor
discharge is a measurement of engine speed and air density. Px is proportional to
compressor discharge pressure, so it will decrease with a decrease in air density. This is
sensed by the acceleration bellows which acts to reduce fuel flow.

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16. DECELERATION

A. When the power control lever is retarded, the speed scheduling cam is rotated to a
lower point on the cam rise. This reduces the governor spring force and allows the
governor valve to move in an opening direction. The resulting drop in Py moves the
metering valve in a closing direction until it contacts the minimum flow stop. This stop
assures sufficient fuel to the engine to prevent flameout. The engine will continue to
decelerate until the governor weight force decreases to balance the governor spring force
at the new governing position.

17. REVERSE THRUST OPERATION

A. On PT6A-6A engine installations, the FCU speed scheduling cam has two contoured
lobes. This permits scheduling full power at each end of the power control lever travel.
When the power control lever is moved into the reverse thrust range, the propeller pitch
control and the FCU are integrated. Power control lever movement toward full reverse
will increase gas generator turbine speed (Ng) and propeller reverse pitch. The function of
the propeller governor is eliminated in the reverse thrust range and the limiting of
power turbine speed (Nf) is provided by the power turbine governor.

B. On PT6A-6B, PT6A-6/C20, PT6A-20, PT6A-20A and PT6A-20B engine installations, the

FCU has a single lobe cam. A cam type linkage on the engine provides forward
movement of the fuel control lever at the fuel control when the power control lever is
moved in either the reverse or forward direction. This eliminates the necessity for the two
lobe cam in this control.

C. The idle reset has two settings, low (ground) idle and high (flight) idle. The high idle
setting permits acceleration to maximum rpm to be accomplished in a reduced
amount of time. The idle setting, as well as cut-off, is controlled by the cut-off lever in
the cockpit. The throttle linkage is used only to increase or decrease power.

D. When the power control lever is moved into the reverse thrust position, it integrates the
propeller pitch control and the function of the propeller governor is eliminated. In the
reverse thrust range limiting of power turbine speed (Nf) is provided by the power turbine
governor.

E. Coordinated linkage moves the power turbine governor lever or, air bleed link on
propeller governor (PT6A-20A and PT6A-20B engines) away from the maximum stop, so
limiting will be on the idle adjustment which is set at approximately 95% power turbine
speed (Nf).

F. When power turbine speed (Nf) is reset during reverse thrust operation, the power
turbine governor servo will be partially open, bleeding off some Py pressure. If
power turbine speed drops off, the servo valve will move toward its closed position
increasing Py pressure and fuel flow to increase compressor turbine speed (Ng), thereby
increasing power turbine speed. If power turbine speed exceeds the desired value, the
servo valve will move further open to decrease Py pressure and fuel flow.

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18. STOPPING THE ENGINE

A. The cut-off valve provides a positive means of stopping fuel flow to the engine. During
normal operation, this valve is fully open and offers no restriction to the flow of fuel to
the nozzles. On PT6A-6 engines the cut-off valve is operated by the power control lever.
On PT6A-6A, PT6A-6B, PT6A-6/C20, PT6A-20, -20A and -20B engines an additional
linkage system is employed to operate the cut-off valve. PT6A-6 engines are stopped by
placing the fuel shutoff lever in the CUT-OFF position. This action moves the FCU
cut-off valve to its seated position stopping all fuel flow to the engine.

FUEL DUMP VALVE

19. GENERAL

A. The fuel dump valve consists of a spring-loaded poppet valve which automatically
dumps residual manifold and spray nozzle fuel overboard after engine shutdown. The
valve is connected to the common fuel manifold adapter by a tube coupling at the
6 o’clock position. Pressure fuel is led from the fuel control unit directly to the fuel dump
valve through an external pipe. Fuel pressure in excess of 13 psig opens a port
connecting the valve inlet with the manifold and directs the fuel flow through to the
nozzles. Conversely, closing of the shut-off valve in the fuel control unit will stop the fuel
flow, reduce the fuel pressure at the dump valve which will open and dump residual
fuel overboard, thus preventing the fuel from being coked in the system due to heat
absorption. (See Figure 1-8-9).

FUEL MANIFOLD AND SPRAY NOZZLES

20. GENERAL

A. The fuel manifold delivers a constant supply of high-pressure fuel from the fuel dump
valve to 14 simplex fuel nozzles. The common fuel manifold consists of 14 fuel
manifold adapter assemblies interconnected by 14 short fuel transfer tubes. The transfer
tubes incorporate preformed packings at each end to accomplish sealing. Lockplates
maintain the transfer tube alignment and security in their respective manifold adapters.
Each manifold, with its lockplate, is secured by two bolts to a mounting boss on the
circumference of the gas generator case. Individual transfer tubes and fuel manifold
adapter assemblies can be removed and replaced without removing the remaining
assemblies and tubes.

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LOCKING PLATE FUEL MANIFOLD FUEL MANIFOLD

ADAPTER ASSEMBLY TRANSFER TUBE

GASKET

GAS GENERATOR
CASE ASSEMBLY

COMBUSTION CHAMBER
LINER ASSEMBLY
FUEL NOZZLE
FUEL NOZZLE
SHEATH

(Most PT6A-20 and PT6A-20B, and all PT6A-20A Engines) C1213

Fuel Manifold Adapter Assembly - Front Cross Section
Figure 1-8-9

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21. FUEL MANIFOLD ADAPTER ASSEMBLY

A. The fuel manifold adapter assembly (see Figure 1-8-9 and 1-8-10) consists of 14 fuel
manifold adapters, each fitted with a simplex fuel nozzle assembly. The fuel nozzle
assemblies provide a fine atomized fuel spray in the annular combustion chamber liner.
Each fuel nozzle assembly consists of a sheath, a spray nozzle and tip all protected
by a fine strainer. The sheath, which functions as a heat shield, fits over the fuel nozzle
assembly and is located on the underside of the manifold adapter by a dowel pin.
The sheath is perforated at the flanged end to permit entry of compressor discharge air,
which cools the nozzle tips and assists fuel atomization. The fuel nozzles are positioned
on the manifold adapter extension to produce a continuous tangential spray from one
nozzle to the next in the combustion chamber liner. The combustion chamber liner is
located and supported by alternate fuel nozzle sheaths. The sheaths act as spigots and
pass through the suspension collars and support brackets welded on the outer wall
of the liner. The spigot fit, between the sheaths and suspension collars, securely locates
the front of the liner in the gas generator case.

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FUEL MANIFOLD
ADAPTER ASSEMBLY
LOCKING
PLATE FUEL MANIFOLD
TRANSFER TUBE

GASKET

GAS GENERATOR
CASE ASSEMBLY

COMBUSTION CHAMBER
LINER ASSEMBLY FUEL NOZZLE FUEL NOZZLE
SHEATH

(PT6A-6, -6A, -6B, PT6A-6/C20, early PT6A20 and PT6A-20B Engines) C33A
Fuel Manifold Adapter Assembly - Front Cross Section
Figure 1-8-10

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PART 1 - DESCRIPTION OF ENGINE

SECTION 9 - ENGINE CONTROLS AND INSTRUMENTATION

1. GENERAL

A. The following instrumentation and controls are available and considered necessary for
normal operation of engine in flight, to ascertain its mechanical condition, and to ground
check and adjust the power output when necessary.

2. POWER CONTROL LEVER

A. The power control lever controls the gas generator speed and thus modulates engine
power output from Idle to Takeoff. The position for Idle represents the lowest permissible
level of power.

B. On non-reversing propeller installations, fuel to the engine is automatically shut off at

lever positions below Idle.

C. On reversing propeller installations, the lever is interconnected with the power turbine
governor, or propeller governor on PT6A-20A and PT6A-20B engines only and
modulates engine power from Full Reverse to Takeoff.

3. FUEL SHUT-OFF LEVER (PT6A-6A, -6B, -6/C20, -20, -20A AND -20B ENGINES)

A. The fuel shut-off lever opens and closes the cut-off valve, thus selecting CUT-OFF or
IDLE positions, through external linkage. This lever also selects the HI-IDLE position
where applicable.

4. PROPELLER CONTROL LEVER (PT6A-6, -6A, -6B, -6/C20, -20, -20A AND -20B ENGINES)

A. The propeller lever positions the speed adjusting lever on the propeller governor to
provide a means of selecting the desired propeller rpm. When placed in the maximum
decrease rpm position, the propeller will automatically feather.

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5. INTERTURBINE TEMPERATURE SENSING SYSTEM (T5) (PT6A-6/C20, -20, -20A AND

-20B ENGINES)

A. The interturbine temperature sensing system (Tt5) provides the pilot with an accurate
indication of engine operating temperatures. The system monitors the temperature at a
point between the compressor and power turbines; it consists of 8 individual
thermocouples connected in parallel by a bus-bar assembly. Earlier engines may include
a wiring harness assembly in lieu of the bus-bar. Each thermocouple probe protrudes
through a threaded boss on the power turbine stator housing into the area immediately in
front of the leading edge of the power turbine vanes (see Figure 1-9-1). Each probe
is secured to its boss by means of a floating, threaded fitting which is part of the probe
assembly. The bus-bar assembly consists of two bus-bars with 8 terminal straps on
each bar. Each bus-bar terminal strap is secured to its respective probe assembly terminal
by special screws, the large diameter screws being for the alumel terminals and the
smaller diameter screws for the chromel terminals. The bus-bar assembly is located on
the power turbine stator housing by five mounting clamps. The assembly is secured
together with five sealing ring retaining plates, by five bolts. The shielded leads which
connect each bus-bar to the terminal block are supported by four brackets welded on the
power turbine stator housing and two brackets on the exhaust duct. The terminal
block is mounted at the 2 o’clock position on the gas generator case. The terminal block
and gasket are secured on the inner wall of the gas generator case with the terminals
protruding through the case to provide a connection point for external wiring to cockpit
instrumentation.

6. TURBINE TEMPERATURE SENSING SYSTEM (T4) (PT6A-6, -6A AND -6B ENGINES)

A. The turbine inlet temperature sensing system (T4) provides the pilot with an accurate
indication of engine operating temperatures. The system monitors the temperature of
the gases in the area immediately in front of the compressor turbine guide vanes and
consists of 24 chromel/alumel thermocouple probes, a chromel and an alumel bus-bar
assembly and a wiring harness. The thermocouple probes are mounted in pairs on 12
brackets and connected in parallel to two bus-bars. Each bracket is secured to
flange E by a centrally located bolt. The probes extend through the small exit duct into
the gas stream between the large and small exit ducts, through a locating ring mounted on
the front face of the small exit duct (See Figure 1-9-2). The shielded wiring harness
connected to the bus-bars incorporates an integral terminal block which is mounted at the
2 o’clock position on the gas generator case. The terminal block and gasket are
secured to the inner wall of the gas generator case, with their terminal posts protruding
through a port in the case, to provide the connection point for the trim harness and
for external wiring for cockpit instrumentation. The trim harness is provided on some
overhauled engines to eliminate read out errors.

7. OIL PRESSURE INDICATOR

A. The oil pressure indicator shows the pressure in the lubricating system, taken from the
delivery side of the oil pump. Provision has been made to install an oil pressure
transmitter in a mounting boss located at the 3 o’clock position on the accessory gearbox
housing.

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MAINTENANCE MANUAL
MANUAL PART NO. 3015442

TERMINAL
BLOCK

SHIELDED LEADS
TERMINAL LUG

ALUMEL BUS−BAR

CHROMEL BUS−BAR

SEAL RETAINING
PLATE
TERMINAL STRAPS

BUS−BAR MOUNTING
SCREW
TERMINAL LUG
CONTAINMENT RING

CHROMEL TERMINAL TRIM THERMOCOUPLE

ALUMEL TERMINAL
ALUMEL TERMINAL

CAP SCREW

WASHER DETAIL A
T5 TERMINAL BLOCK
THREADED
COUPLING
PROBE ASSEMBLY CHROMEL TERMINAL
DETAIL OF FLANGE C
THERMOCOUPLE PROBE (REF.)

(PT6A-20 Series Engines) C571D

Interturbine Temperature (T5) Sensing System
Figure 1-9-1

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MANUAL PART NO. 3015442

GAS GENERATOR COMBUSTION CHAMBER

CASE ASSEMBLY LINER ASSEMBLY COMBUSTION CHAMBER
FUEL NOZZLE SMALL EXIT DUCT

WIRING
HARNESS LARGE EXIT
DUCT
TERMINAL
BLOCK
EXHAUST CASE
ASSEMBLY
POWER TURBINE

POWER TURBINE PROBE

GUIDE VANES
FLANGE COMPRSSOR COMPRESSOR TURBINE
TURBINE GUIDE VANES
WIRING
HARNESS
INCONEL
TUBE SLEEVE

PROBE

BRACKET

MOUNTING CHROMEL
BOLT HOLE BUS−BAR

ALUMEL
BUS−BAR

(PT6A-6 Series Engines) C309B

Turbine Inlet Temperature (T4) Sensing System
Figure 1-9-2

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8. OIL TEMPERATURE INDICATOR

A. The oil temperature indicator shows the temperature of the lubricating oil as it leaves the
delivery side of the pressure oil pump. A mounting boss is provided on the accessory
gearbox housing just below the oil pressure transmitter mounting to accommodate an oil
temperature bulb.

9. TACHOMETER-GENERATOR - GAS GENERATOR (Ng)

A. The tachometer-generator provides an electric current which is used in conjunction with

a tachometer to indicate gas generator rpm. The tachometer-generator is located at the
5 o’clock position on the accessory gearbox housing and is driven from the internal
scavenge pump. Rotation is counterclockwise with a drive ratio of 0.112:1.

10. TACHOMETER-GENERATOR - POWER TURBINE (Nf)

A. The power turbine tachometer-generator operates in the same manner as that of the
gas generator. However, the turbine tachometer-generator is installed on the reduction
gearbox case and rotates clockwise with a drive ratio of 0.1273:1.

11. TORQUEMETER

A. The torquemeter indicating system is an integral hydro-mechanical torque measuring

device which provides an accurate indication of the torque being produced by the
power turbine. The torque pressure value is obtained by tapping the two outlets on top
of the reduction gearbox. The pressure differential between the two outlets is then led to an
instrument which indicates the correct torque pressure.

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MAINTENANCE MANUAL
MANUAL PART NO. 3015442

PART 1 - DESCRIPTION OF ENGINE

SECTION 10 - PROPELLER NON-REVERSING INSTALLATION

(PT6A-6 ENGINES)

1. GENERAL

A. The PT6A-6 engines non-reversing installation is designed to utilize a hydraulically

operated propeller controlled by a propeller governor and an overspeed governor. A
power turbine governor prevents turbine overspeed by controlling gas generator output
through the FCU.

2. PROPELLER

A. The engine is normally equipped with a three bladed metal propeller dowelled and
bolted to the front face of the propeller shaft flange. The propeller consists of a
hollow steel spider hub which supports three propeller blades and also houses an
internal oil pilot tube and feather return springs (see Figure 1-10-1). Movement of the
propeller blades is controlled by a hydraulic servo piston which is mounted on the front of
the spider and is connected by individual linkage to the blade trailing roots. Centrifugal
counterweights on each blade, combined with the feathering springs in the servo piston,
tend to drive the piston into the feather or high pitch positions. This movement is
opposed by the oil pressure generated and controlled by the propeller governor. The
pressure oil is routed to the servo piston via an internal oil transfer housing and the oil pilot
tube. An increase in the oil flow moves the servo piston forward, pulling the propeller
blades towards the low pitch position (increased rpm). A decrease in the oil flow allows the
blades to move toward the high pitch position (decreased rpm) under the influence of
the feathering springs and blade counterweights.

3. PROPELLER GOVERNOR

A. The propeller governor consists of a constant speed single-acting centrifugal governor

mounted on the reduction gearbox front case at the 12 o’clock position. The propeller
governor regulates power turbine speed by varying the pitch of the propeller to match the
load torque to the engine torque in response to changing flight conditions. The
propeller governor utilizes pressure oil to decrease blade pitch while the centrifugal force
of the blade counterweights assisted by return springs tend to increase the pitch.

B. The main parts of the propeller governor include a gear type oil pump with a pressure
relief valve, a pair of pivoted flyweights mounted on a rotating flyweight head, a
spring-loaded pilot valve and the necessary cored passages contained in the governor
cover and base. The rotating flyweights determine the position of the spring-loaded pilot
valve while the spring load adjusting control is varied by an externally mounted speed
lever.

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MANUAL PART NO. 3015442

PROPELLER SPEED SETTING SCREW

CONTROL LEVER

SPEEDER SPRING SPEEDER SPRING

THRUST BEARING THRUST BEARING
GOVERNOR FLY−
PUMP WEIGHTS
PROPELLER
GOVERNOR PROPELLER
OVERSPEED
GOVERNOR

RELIEF
VALVE
DRIVE DRIVE
SPLINE SPLINE
OIL DUMP TO OIL DUMP TO
REDUCTION REDUCTION
GEARBOX GEARBOX

COUNTERWEIGHT
ENGINE
OIL SUPPLY FEATHER RETURN
SPRINGS
SERVO PISTON

ROTATION

PISTON SEAL
PROPELLER OIL
TRANSFER HOUSING SLIDING GUIDE
ROD (3)
HIGH PITCH STOP
PROPELLER
COUNTERWEIGHT

FINE PITCH

C548A
Typical Single-acting Propeller - Schematic
Figure 1-10-1

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MANUAL PART NO. 3015442

C. The propeller governor incorporates two safety features. The first is the control lever
spring attached to the speed adjusting lever. In the event of a propeller control linkage
failure, the spring holds the speed adjusting lever in its last selected position or moves it
toward the high speed setting. A high speed stop prevents the lever moving beyond
the 100% position and enables the engine to operate at or near full rated speed and to
develop maximum power. The second safety feather is a positive low speed setting.
Moving the propeller control lever and thence the speed adjusting lever against the low
speed stop, raises the pilot valve to a positive coarse pitch or feathering position
regardless of flyweight force. This enables the pilot to feather the propeller and minimize
propeller drag in the event of engine failure.

D. To provide the propeller governor with a sensing element, the rotating pivoted flyweights
are linked mechanically to the engine by a hollow drive gearshaft (see Figure 1-10-2).
The rotating flyweights, actuated by centrifugal force, position the pilot valve so as to cover
or uncover ports in the drive gearshaft and regulate the oil flow to and from the
propeller servo piston. The centrifugal force exerted by the flyweights is opposed by the
force of the adjustable speeder spring. This determines the engine rpm required to
develop sufficient centrifugal force on the flyweights to center the pilot valve.

4. PROPELLER GOVERNOR ON-SPEED CYCLE

A. During an on-speed condition, (see Figure 1-10-2) the forces acting on the
engine/governor/propeller combination are in a state of balance. With the propeller control
lever set to obtain the desired rpm, and the propeller blades in the correct pitch range
to absorb the power developed by the engine, the centrifugal force of the rotating
flyweights exactly balances the force of the speeder spring with the flyweights in the
vertical position. This positions the pilot valve plungers so that the ports which control the
oil flow from the propeller governor pump to the propeller servo piston are closed. The
oil is therefore diverted through the open relief valve back to the inlet side of the pump.

5. PROPELLER GOVERNOR OVERSPEED CYCLE

A. In an overspeed condition, the rotating flyweights pivot outward, overcome the speeder
spring tension and raise the pilot valve plunger. This uncovers the ports in the drive
gearshaft and dumps the oil from the propeller servo piston to the reduction gearbox sump.
As the propeller blades increase in pitch angle the load on the engine is increased
with a consequent decrease in engine rpm. The centrifugal force exerted on the governor
flyweights is reduced allowing the speeder spring tension to return the flyweights to a
vertical position. The pilot valve plunger once again blocks the flow of control oil to or from
the propeller servo piston.

6. PROPELLER GOVERNOR UNDERSPEED CYCLE

A. With the propeller control lever set to the desired rpm, a condition of underspeed occurs
when the engine rpm drops below the predetermined setting. The speeder spring
tension then overcomes the decreased centrifugal force on the flyweights and pivots
them inward, forcing the pilot valve plunger downward and opening the ports. This directs
a flow of pressure oil to the propeller servo piston which overcomes the combined
forces of the propeller counterweights and return springs and decreases the pitch of the
propeller blades. The load on the engine is reduced with a consequent increase in
engine rpm. This is sensed by the propeller governor flyweights causing the pilot valve
plunger again to be positioned to close the oil ports.

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MANUAL PART NO. 3015442

SPEED ADJUSTING
CONTROL LEVER

HIGH SPEED STOP

CONTROL LEVER
LIFT ROD SPRING

FLYWEIGHT
ADJUSTING WORM

SPEEDER SPRING
DRIVE GEAR SHAFT

PILOT VALVE TOE

PLUNGER
FLYWEIGHT
HEAD
RELIEF
VALVE
DRAIN TO SUMP

OIL PUMP PROPELLER

CONTROL LINE

ENGINE
OIL INLET

PRESSURE

"ON SPEED CONDITION"

C547
Propeller Governor -Schematic
Figure 1-10-2

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MANUAL PART NO. 3015442

7. PROPELLER OVERSPEED GOVERNOR

A. The propeller overspeed governor is installed in parallel with the propeller governor and
is mounted at approximately the 10 o’clock position on the reduction gearbox front case.
It is provided to control power turbine/propeller overspeed condition by immediately
dumping the oil from the propeller servo piston to the reduction gearbox sump (see Figure
1-10-1). The overspeed governor consists of conventional type flyweights mounted on
a rotating hollow splined shaft. The hollow shaft embodies ports which are normally closed
by a pilot valve and held in this position by a speeder spring. The spring tension acts
in opposition to the centrifugal force produced by the rotating flyweights. When a power
turbine/propeller overspeed condition occurs, the increased centrifugal force sensed
by the flyweights overcomes the spring tension, lifts the pilot valve and dumps the
propeller servo piston oil back to the reduction gearbox through the hollow splined shaft.
This permits the combined forces of the counterweights and return springs to move
the blades toward a coarse pitch position, absorbing the engine power and preventing
further power turbine/propeller overspeed.

8. POWER TURBINE GOVERNOR

A. The power turbine governor is mounted on top of a sandwich type tachometer-generator

located at the 2 o’clock position on the reduction gearbox front case. It is driven by the
propeller shaft through bevel gears. This governor provides additional overspeed
protection by utilizing an overall fuel limiting control which becomes effective at
approximately 105% power turbine (Nf) rpm. Further clarification is provided in Section
8, preceding.

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PART 1 - DESCRIPTION OF ENGINE

SECTION 11 - PROPELLER REVERSING INSTALLATION

(PT6A-6A ENGINES)

1. GENERAL

A. The PT6A-6A reversing propeller installation consists of a reversing propeller, controlled

by a propeller governor. An optional overspeed governor may be provided to limit engine
speed in the event of a propeller governor failure. A propeller governor lever determines
the limiting rpm range to which the governor will control the propeller in the normal range.
The power delivered by the engine to the propeller shaft is controlled by a power
control lever through the FCU. However, for reverse propeller operation the FCU works
in conjunction with a power turbine governor to limit power output according to
propeller requirements. An adjustable linkage, control levers and a slotted cam
arrangement interconnect a reversing valve and lever with the power turbine governor
and the FCU, thereby enabling all reverse pitch operations to be controlled by the cockpit
power control lever (see Figure 1-11-1).

2. PROPELLER

A. The engine is normally equipped with a three bladed metal propeller dowelled and
bolted to the front face of the propeller shaft flange. The propeller consists of a
hollow steel spider hub which supports three propeller blades and houses an internal oil
pilot tube and feather return springs (see Figure 1-11-1). Movement of the propeller
blades is controlled by a hydraulic servo piston which is mounted on the front of the
propeller spider hub and connected by individual linkage to the blade trailing roots.
Centrifugal counterweights on each blade combined with the feathering springs in the
servo piston, tend to drive the piston into the feather or high pitch position. This movement
is opposed by the propeller governor. The pressure oil is routed to the servo piston via
an internal oil transfer housing the oil pilot tube. An increase in the oil pressure moves the
servo piston forward pulling the propeller blades towards the low pitch position
(increased rpm). A decrease in the oil pressure allows the blades to move toward the
high pitch position (decreased rpm) under the influence of the feathering springs and blade
counterweights.

B. The servo piston is also connected by three spring loaded sliding rods to a feedback
ring mounted behind the propeller. The three rods are actuated by the servo piston,
automatically repositioning the feedback ring. Movement of the feedback ring is
transmitted by a carbon block through linkage to the reversing valve which opens
progressively and diverts oil flow from the propeller to the reduction gearbox, precluding
any further change in pitch. Conversely, closing of the valve will increase the oil flow
and so decrease the pitch (increased rpm).

C. The reversing valve function is to schedule the blade angle between the low and
maximum reverse pitch positions and also limit the low pitch. An internal sleeve located
on the pilot valve provides a mechanical stop to limit blade pitch at the maximum
reverse angle.

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MANUAL PART NO. 3015442

PROPELLER PROPELLER PROPELLER OVERSPEED

GOVERNOR GOVERNOR SPEED SETTING
CONTROL SCREW
LEVER
SPEEDER SPRING SPEEDER SPRING
THRUST BEARING THRUST BEARING

GOVERNOR PUMP FLY−

WEIGHTS

RELIEF
VALVE
DRIVE DRIVE
SPLINE SPLINE
OIL DUMP TO OIL DUMP TO
REDUCTION REDUCTION
GEARBOX GEARBOX

COUNTERWEIGHT
FEEDBACK − RING
MAIN OIL REDUCTION
FEATHER RETURN
SUPPLY GEARBOX SPRINGS
SUPPLY SERVO
PISTON
PROPELLER OIL
TRANSFER ROTATION
FULL HOUSING
REVERSE CHECK PISTON
IDLE VALVE SEAL
TO REDUCTION REVERSE RETURN SPRINGS
TAKEOFF GEARBOX SUMP
POWER FLOW CARBON BLOCK
DIVIDER REVERSING VALVE LEVER
POWER POWER TURBINE
REAR REVERSING
CONTROL LEVER
FOLLOWER VALVE GOVERNOR
LEVER

MAX. STOP−SET
PROPELLER AT U.A.C.L.
CONTROL LEVER
ADJUSTABLE
CENTER TURNBUCKLE
FIRESEAL GOVERNOR ACTUATING
FUEL CONTROL CONTROL LEVER REAR
MOUNTING BRACKET LEVER ASSEMBLY
UNIT LEVER FIRESEAL

(PT6A-6A Engines) C549A

Propeller Reversing Installation - Schematic
Figure 1-11-1

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MANUAL PART NO. 3015442

3. PROPELLER GOVERNOR

A. The propeller governor consists of a constant speed single-acting centrifugal governor

mounted at the 12 o’clock position on the reduction gearbox front case. The governor
regulates power turbine speed by varying propeller blade pitch to match the load torque
to the engine torque, in response to changing flight conditions. The governor utilizes
pressure oil to decrease blade pitch while the centrifugal force of the blade
counter-weights assisted by return springs tend to increase the pitch. (See Figure
1-11-1).

B. The main parts of the propeller governor include a gear-type oil pump with a pressure
relief valve; a pair of pivoted flyweights mounted on a rotating flyweight head; a
spring-loaded pilot valve and the necessary cored oil passages contained in the propeller
governor housing and base. The rotating flyweights determine the position of the pilot
valve while the spring load can be varied by an externally mounted propeller control lever.

C. The propeller governor incorporates two safety features. The first is a control lever spring
which is attached to the speed adjusting lever. In the event of a propeller control linkage
failure, the spring holds the speed adjusting lever in its last selected position, or tends
to move it toward the high speed setting. The high speed stop prevents the lever moving
beyond the 100% position and enables the power turbine to operate at or near
full-rated speed and develop maximum power. The second safety feature is a positive
low speed setting. Moving the speed adjusting control lever against the low speed stop
raises the pilot valve to a positive coarse pitch or feathering position regardless of
flyweight force. This enables the pilot to feather the propeller and minimize propeller
drag in the event of engine failure.

D. To provide the propeller governor with a sensing element, the rotating pivoted flyweights
are linked mechanically to the engine by a hollow drive gearshaft (see Figure 1-11-2).
The rotating flyweights actuated by centrifugal force, position the pilot valve so as to cover
or uncover ports in the drive gearshaft and regulate the oil flow to and from the
propeller servo piston. The centrifugal force exerted by the flyweights is opposed by the
force of the adjustable speeder spring. This determines the engine rpm required to
develop sufficient centrifugal force on the flyweights to center the pilot valve.

4. PROPELLER GOVERNOR ON-SPEED CYCLE

A. During an on-speed condition (see Figure 1-11-2) in forward thrust the forces acting on
the engine/governor/propeller combination are in a state of balance with the propeller
control lever set to obtain the desired rpm and the propeller blades in the correct pitch
range to absorb the power developed by the engine, the centrifugal force of the
rotating flyweights exactly balances the force of the speeder spring with the flyweights in
the vertical position. This positions the pilot valve plunger so that the ports which
control the oil flow from the governor pump to the propeller servo piston are closed. The
oil is therefore diverted through the open relief valve back to the inlet side of the
governor pump.

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9
10
8
7
11
6
12

13

5 14

2 15

(PT6A-6A Engines) C342B

Reduction Gearbox Propeller Reversing Installation
Figure 1-11-2

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MAINTENANCE MANUAL
MANUAL PART NO. 3015442

Key to Figure 1-11-2

1. Pressure Oil Transfer Tube

2. Flow Divider Valve Housing
3. Cable Tensioner
4. Low Pitch Stop Adjustment
5. Reversing Valve
6. Reversing Valve Lever
7. Power Turbine Governor
8. Governor Actuating Linkage
9. Reversing Lever Guidepin
10. Return Spring
11. Sliding Rod (3)
12. Feedback Ring
13. Carbon Block
14. Propeller Thrust Bearing Cover
15. Scavenge Oil Transfer Tube
5. PROPELLER GOVERNOR OVERSPEED CYCLE

A. In an overspeed condition, the rotating flyweights pivot outward, overcome the speeder
spring tension and raise the pilot valve plunger. This uncovers the ports in the drive
gearshaft and dumps the oil from the propeller servo piston to the reduction gearbox sump.
As the propeller blades increase in pitch angle, the load on the engine is increased
with a consequent decrease in engine rpm. The centrifugal force exerted on the governor
flyweights is reduced allowing the speeder spring tension to return the flyweights to a
vertical position. The pilot valve plunger once again blocks the flow of control oil to or from
the propeller servo piston mechanism.

6. PROPELLER GOVERNOR UNDERSPEED CYCLE

A. With the propeller control lever set to the desired rpm, a condition of underspeed occurs
when the propeller rpm drops below the predetermined setting. The speeder spring
tension then overcomes the decreased centrifugal force on the flyweights and pivots them
inward, forcing the pilot valve plunger downward and opening the ports. This directs a
flow of pressure oil to the propeller servo piston which overcomes the combined forces of
the propeller counterweights and return springs and decreases the pitch of the
propeller blades. The load on the propeller is reduced with a consequent increase in
propeller rpm. This is sensed by the governor flyweights, causing the pilot valve plunger
to again be positioned to close the oil ports.

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7. PROPELLER OVERSPEED GOVERNOR

A. The propeller overspeed governor is installed in parallel with the propeller governor and
is mounted at approximately the 10 o’clock position on the reduction gearbox front case.
It is provided to control a power turbine/propeller overspeed condition by immediately
dumping the oil from the propeller servo piston to the reduction gearbox sump (see Figure
1-11-1). The governor consists of conventional type flyweights mounted on a rotating
hollow splined shaft. The hollow shaft embodies ports which are normally closed by a pilot
valve and held in this position by a speeder spring. The spring tension acts in opposition
to the centrifugal force of the rotating flyweights. When a power turbine/propeller
overspeed condition occurs, the increased centrifugal force sensed by the flyweights
overcomes the spring tension, lifts the pilot valve and dumps the propeller servo piston
oil back to the reduction gearbox through the hollow splined shaft. This permits the
combined forces of the counterweights and return springs to move the blades toward a
coarse pitch position, absorbing the engine power and preventing further power
turbine/propeller overspeed.

8. POWER TURBINE GOVERNOR

A. The power turbine governor is mounted on the top of a sandwich type

tachometer-generator (Nf). The generator is mounted at approximately the 2 o’clock
position on the reduction gearbox and driven by the propeller shaft through bevel gears.
The power turbine governor is set to approximately 106% (Nf) speed for normal pitch
operation. Should the propeller stick in such a way as to prevent the propeller governor
from governing in the normal pitch range, the power turbine governor will act as a
power turbine/propeller overspeed limiter by cutting back fuel flow in the event the
propeller shaft overspeed reaches 106% (Nf).

B. During reverse operation, the power turbine governor is progressively reset to

approximately 95% (Nf) before the propeller reaches a negative pitch angle. This ensures
that the engine power is limited to maintain a propeller rpm somewhat less than that
of the propeller governor setting. The propeller governor will, therefore, always sense an
underspeed condition and direct pressure oil to the propeller servo piston. The power
turbine governor lever is connected by adjustable linkage to the reversing valve lever
system.

9. PITCH REVERSING VALVE AND LINKAGE

A. The pitch reversing valve consists of a plug type sleeve valve located in the propeller
thrust bearing cover at the 6 o’clock position (see Figure 1-11-2). The reversing valve
oil inlet passage is connected by an external oil transfer tube to an internal transfer tube
on the reduction gearbox. The dump side of the reversing valve is connected to the
reduction gearbox through an externally mounted oil flow divider and pressure check
valve. These ensure an adequate supply of oil to the power section bearings during engine
windmilling. The propeller reversing valve accommodates a pivoted reversing lever at
its center. One end of the reversing lever is attached to a push-pull control wire rope which
is interconnected through propeller control and cam follower levers to the FCU, then
to the cockpit power control lever. The opposite end of the reversing lever is attached to
and positioned by a carbon block located in the propeller feedback ring.

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10. PROPELLER REVERSING INTERCONNECTING LINKAGE

A. The propeller reversing interconnecting linkage provides the correct relationship between
engine power and propeller blade pitch during reversing operations. The system (see
Figure 1-11-1) comprises three major parts: a control lever mounting bracket bolted to the
accessory gearbox at flange G; a rear control lever, with a circumferential and radial
cam slot (see Figure 1-11-3), mounted on a pivot in the mounting bracket, and a rear
follower lever. The rear follower and rear control levers are interconnected by a follower pin
which rides in the contoured cam slot, transmitting movement of either lever. The
cockpit power control lever is connected directly to the attaching lug on the rear follower
lever, or through the lever pivot pin which is splined and has a special extension to
accommodate an airframe supplied type lever. Two lugs are provided on the control lever
mounting bracket for use by the airframe manufacturer. These enable attachment of
flexible cable supports and provide a terminal point for the static cable casing. A ball
bearing ended interconnecting rod completes the linkage from the rear follower lever to the
FCU control arm.

B. The rear control lever is connected forward to the reversing valve lever on the reduction
gearbox by a push-pull wire rope. Swivel joint telescopic units are used at each end of
the cable to allow for angular variations between the rear control lever and the reverse
lever relative to the wire rope outer casing. The telescopic unit at the rear control
lever end includes an adjustment to enable varying the effective wire rope length. The
clevis end of the wire rope is attached to the rear control lever by using one of a number
of holes provided to effect the desired pitch change rate. The telescopic unit at the
reversing valve lever end includes a wire rope tensioning spring which eliminates
backlash, and an adjustable stop to limit the forward travel of the reversing valve lever. The
stop is set to ensure that the reversing valve is in a ‘‘null’’ position for normal propeller
operation.

C. During normal propeller operation (from IDLE to TAKEOFF), the follower pin moves in
the circumferential portion of the cam slot, but imparts no motion through the rear
control lever to the reversing lever. However, on selection of reverse pitch, the follower
pin moves up the radial portion of the slot, rotating the rear control lever and transmitting
this motion forward through the cable to the reversing lever.

11. LOW PITCH CONTROL CYCLE

A. The cockpit power control lever is normally placed in the IDLE position prior to engine
lightup. At this point, the follower pin is at the intersection between the radial and
circumferential ports on the cam slot. The feedback ring and therefore the propeller end
of the reversing lever is fixed by stops and return springs to its maximum rearmost
position. The wire rope clevis end of the reversing lever is positioned by the wire rope
spring stop so that the reversing valve is shut off. Both the propeller and power turbine
governors are set for maximum rpm (100% propeller rpm and 106% Nf) respectively.

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PROPELLER CONTROL CAM

CAM SLOT

RADIAL SECTION

CIRCUMFERENTIAL SECTION

C1212
Propeller Control Lever - Cam Slot Terminology
Figure 1-11-3

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B. As the engine lights up and the rpm increases, the propeller governor senses an
underspeed and directs pressure oil to the propeller servo piston to effect a pitch change.
At an angle somewhat above the low pitch setting, the propeller servo piston will pick
up the low pitch adjustment stops on the guide rods. The feedback ring moves axially
forward transmitting this motion through the carbon block to the reversing lever. The
reversing lever pivots on the wire rope clevis end which is presently static and gradually
opens the reversing valve. Oil from the propeller is dumped through the flow divider
to the reduction gearbox. The process continues until the amount of oil dumped by the
valve equals the amount being supplied by the propeller governor, at which point there will
be no further change in pitch and the propeller will be in low pitch.

12. HIGH PITCH CONTROL CYCLE

A. With the engine operating and the propeller in low pitch (high rpm), the power control
lever is advanced from IDLE to the TAKEOFF position. The movement transmitted
through the wire rope moves the follower pin in the circumferential position of the cam
slot without altering the position of the rear control lever. Movement of the rear follower
lever simultaneously advances the FCU control lever to full power position (100%
rpm). As the propeller governing speed is reached, the propeller governor will dump
pressure oil from the propeller to increase pitch and maintain speed. The feedback ring
returns to a rearward position shutting off the reversing valve and the propeller will
operate in the conventional manner.

13. REVERSE PITCH CONTROL CYCLE

A. To go into reverse pitch from the low pitch position, the power control ever is pulled
back through a gate into the REVERSE position. This moves the follower pin up the
radial portion of the cam slot, rotating the rear control lever and transmitting a pulling force
to the wire rope against the tension spring. This moves the end of the reversing lever
which pivots on the feedback ring end, partially closing the reversing valve and reducing
the amount of oil being dumped to an amount less than that supplied by the propeller
governor. Since this condition is one of underspeed, the propeller will move through low
pitch into the negative pitch range as long as the reversing lever continues to move.
As the propeller moves into low pitch, the feedback ring changes position in an attempt
to open the reversing valve and prevent a further change in pitch. This cannot occur
while the reverse lever continues to be moved.

B. As the propeller reaches the full reverse pitch position, movement at the wire rope clevis
end of the reversing lever will cease. The reversing lever will have travelled only far
enough to move the reversing valve to a ‘‘null’’ position at which point the oil flow into the
propeller is equal to the flow out. The power turbine governor is simultaneously reset
from 106% to 95% (Nf). This final setting leads the propeller and will be reached before
the propeller attains full reverse pitch. Selection of reverse pitch also repositions the
FCU control lever increasing fuel flow and consequently power output. The chosen
relationship of engine power to propeller pitch depends largely on operating requirements.
Should the relationship selected be such as to require 95% (Nf), the power turbine
governor will limit the engine power according to the ability of the propeller to absorb the
power at that speed.

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MANUAL PART NO. 3015442

14. REVERSE TO LOW PITCH CONTROL CYCLE

A. The process of returning to low pitch from full reverse is exactly opposite to going into
reverse pitch. As the power control lever is moved towards the low pitch gate, the
reversing valve is opened past its ‘‘null’’ position, dumping more oil into the reduction
gearbox than is being supplied by the propeller governor. As the propeller pitch moves out
of the reverse pitch range, the fuel flow is progressively reduced and the power
turbine governor reset to 106% (Nf). The full stabilized low pitch position is obtained
when the amount of oil dumped by the reversing valve equals the quantity supplied by
the propeller governor.

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MANUAL PART NO. 3015442

PART 1 - DESCRIPTION OF ENGINE

SECTION 12 - PROPELLER REVERSING INSTALLATION

(PT6A-6B & PT6A-20)

1. GENERAL

A. The PT6A-6B and PT6A-20 reversing propeller installation utilizes a single-acting

hydraulic reversing propeller controlled by a propeller governor. An optional propeller
overspeed governor may be provided to limit power turbine speed in the event of
failure of the propeller governor. A power turbine governor controls propeller speed in
the Beta range and will also limit propeller speed in the remote event of the propeller
overspeeding.

B. The integrated engine and propeller combination has three cockpit controls: a fuel cut-off
lever, a power control lever and a propeller control lever. The fuel cut-off lever has two
positions: CUT-OFF and IDLE. An additional HI-IDLE position is provided on certain
installations. This lever controls the cut-off function of the fuel control unit (FCU) and
resets the power control lever Idle Stop to provide 50% minimum Ng in the IDLE position
and up to a minimum of 80% Ng in the HI-IDLE position, where applicable. The Ng
selected by the power control lever is a maximum value which is reached only if the power
turbine governor does not act to reduce Ng below the selected value. The power
control lever also selects low blade angle (Beta) and resets the power turbine governor
from over propeller selected speed at full forward power to under propeller selected
speed in the Beta range. The propeller control lever is connected to the propeller governor
through linkage. Movement of the linkage determines and limits the rpm range of the
propeller governor and so governs the propeller. When the control lever is moved to the
minimum rpm position, the propeller will automatically feather.

2. PROPELLER

A. The engine is normally equipped with a three bladed metal propeller dowelled and
bolted to the front face of the propeller shaft flange. The propeller consists of a
hollow steel spider hub which supports three propeller blades and also houses an
internal oil pilot tube and the feather return springs (see Figure 1-12-1). Movement of
the propeller blades is controlled by a hydraulic servo piston mounted on the front of the
propeller spider hub. The servo piston is connected by a link to each of the blade
trailing edge roots. Centrifugal counterweights on each blade and feathering springs in
the servo piston tend to drive the servo piston into the feather, or high pitch position. This
movement is opposed by the oil pressure generated and controlled by the propeller
governor. The pressure oil is transferred to the servo piston via an oil transfer housing
and the hollow center of the propeller shaft. An increase in the oil pressure moves the
propeller blades toward the low pitch position (increased rpm). A decrease in the oil
pressure allows the blades to move toward the high pitch position (decreased rpm) under
the influence of the feathering springs and blade counterweights.

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MANUAL PART NO. 3015442

SPEED ADJUSTING PROPELLER

BETA CONTROL OVERSPEED
LEVER
GOVERNOR

PROPELLER GOVERNOR
GOVERNOR PUMP

ENGINE
OIL SUPPLY

STOP SPRING
STOP
RELIEF VALVE
OIL DUMP TO OIL DUMP TO
REDUCTION REDUCTION
GEARBOX GEARBOX

COUNTERWEIGHT

REVERSE RETURN
LOW STOP SPRINGS
COLLAR
FEEDBACK SERVO PISTON
RING

ROTATION

PISTON SEAL
PROPELLER OIL
TRANSFER HOUSING LOW STOP ROD (3 OFF)
FEATHER RETURN
SPRINGS
CARBON BLOCK

(PT6A-6B and -20Engines) C37B

Propeller Reversing Installation - Schematic
Figure 1-12-1

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MANUAL PART NO. 3015442

B. The servo piston is also connected by three spring-loaded sliding rods to a feedback
ring mounted behind the propeller. Movement of the feedback ring is transmitted by
carbon blocks through a propeller reversing lever and linkages to the propeller governor
override rod. The lever functions as a link between the push-pull wire rope and the
override rod in the propeller governor and also functions as a feedback link in the reverse
pitch and low pitch selections.

3. PROPELLER GOVERNOR

A. The propeller governor performs two functions. Under normal flight conditions it acts as
a constant speed governing unit (CSU) to regulate power turbine speed (Nf) by varying
the propeller blade pitch to match the load torque to the engine torque in response
to changing flight conditions. During low airspeed operations, the propeller governor can
be used to select the required blade angle (Beta control); while in the Beta control
range, engine power is adjusted by the FCU and the power turbine governor to maintain
power turbine speed (Nf) at a speed slightly less than the selected rpm.

B. The propeller governor consists of a single-acting centrifugal governor mounted at the

12 o’clock position on the reduction gearbox front case (see Figure 1-12-2). The
propeller governor function in forward thrust is to boost engine oil pressure to decrease
propeller blade pitch, while the centrifugal force of the blade counterweights, assisted
by feathering springs, tend to increase the pitch.

C. The propeller governor includes an integral gear type oil pump with a pressure relief
valve, a pair of pivoted flyweights mounted on a rotating flyweight head, a spring-loaded
pilot valve and the necessary cored oil passages contained in the governor housing and
base. The rotating flyweights determine the position of the pilot valve while the spring
load can be varied by an externally mounted propeller control lever.

D. In the event of a propeller control linkage failure, a spring attached to the propeller
speed adjusting lever holds the lever in its last selected position or moves it toward
the high speed setting. The high speed stop prevents the lever moving beyond the 100%
position and enables the engine to be operated at or near full rated speed and
develop maximum power. Moving the propeller speed adjusting lever against the low
speed stop raises the pilot valve to a positive coarse pitch or feathering position regardless
of flyweight force. This enables the pilot to feather the propeller and minimize propeller
drag in the event of engine failure.

E. To provide the propeller governor with a sensing element, the rotating pivoted flyweights
are linked mechanically to the engine by a hollow drive gearshaft. The rotating
flyweights, actuated by centrifugal force, position the pilot valve so as to cover or
uncover oil ports in the drive gearshaft and regulate the oil flow to and from the propeller
servo piston. The centrifugal force exerted by the flyweights is opposed by the force of
the adjustable speeder spring. This determines the engine rpm required to develop
sufficient centrifugal force on the flyweights to center the pilot valve, thereby preventing
oil flow.

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MAINTENANCE MANUAL
MANUAL PART NO. 3015442

PROPELLER GOVERNOR
HIGH SPEED CABLE TERMINAL
ADJUSTMENT
STOP

LOW PITCH
STOP ADJUSTER
PROPELLER REVERSING
LEVER
POWER TURBINE
GOVERNOR
INTERCONNECTING
ROD

PROPELLER OVERSPEED PROPELLER

GOVERNOR TEST GOVERNOR
SOLENOID
PROPELLER OVERSPEED
GOVERNOR

FEEDBACK
RING

C38A
Propeller Reversing Installation - Reduction Gearbox
Figure 1-12-2

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F. An override rod is located inside the hollow center of the propeller governor pilot valve
and is connected through a linkage to the propeller reversing lever. The rod is ineffective
when the engine is in the constant speed range as the rotating flyweights hold the
pilot valve clear of the rod. However, when the propeller is running at a speed lower than
that selected on the propeller control lever, the flyweights allow the pilot valve to drop,
causing the blade angle to decrease. As the propeller blades approach the angle selected
by the power control lever, the feedback ring repositions the propeller reversing lever
to move the pilot valve, through the override rod, to an on-speed or null position and so
prevent further blade movement.

4. PROPELLER GOVERNOR ON-SPEED CYCLE

A. During an on-speed condition in forward thrust the forces acting on the engine/propeller
governor/propeller combination are in a state of balance. With the propeller control lever
set to obtain the desired rpm and the propeller blades in the correct pitch range to
absorb the power developed by the engine, the centrifugal force of the rotating flyweights
balances the force of the speeder spring with the flyweights in the vertical position.
This positions the pilot valve plunger so that the ports which control the oil flow from the
governor pump to the propeller servo piston are closed. The oil pressure is therefore
diverted through the relief valve, back to the inlet side of the governor pump.

5. PROPELLER GOVERNOR OVERSPEED CYCLE

A. In an overspeed condition, the rotating flyweights pivot outward, overcome the speeder
spring tension and raise the pilot valve. This uncovers the ports in the drive gearshaft
and dumps the oil from the propeller servo piston to the reduction gearbox sump. As the
propeller blades increase in pitch angle, the load on the engine is increased with a
consequent decrease in engine rpm. The centrifugal force exerted on the propeller
governor flyweights is reduced allowing the speeder spring tension to return the flyweights
to a vertical position. The pilot valve once again blocks the flow of control oil to or
from the propeller servo piston.

6. PROPELLER GOVERNOR UNDERSPEED CYCLE

A. With the propeller control lever set to the desired rpm, a condition of underspeed occurs
when the engine rpm drops below the predetermined setting. The speeder spring
tension then overcomes the decreased centrifugal force on the flyweights and pivots
them inward, forcing the pilot valve downward and opening the ports. This directs a flow
of pressure oil to the propeller servo piston which overcomes the combined forces of
the propeller counterweights and return springs and decreases the pitch of the propeller
blades. The load on the engine is reduced with a consequent increase in engine rpm.
This is sensed by the propeller governor flyweights, which cause the pilot valve plunger to
again be positioned to close the oil ports.

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MANUAL PART NO. 3015442

7. PROPELLER OVERSPEED GOVERNOR

A. The propeller overspeed governor is installed in parallel with the propeller governor and
is mounted at approximately the 10 o’clock position on the reduction gearbox front case.
It is provided to control any power turbine/propeller overspeed condition by immediately
bypassing the oil from the propeller servo piston to the reduction gearbox (see Figures
1-12-1 and 1-12-2). The propeller overspeed governor consists of conventional type
flyweights mounted on a rotating hollow-splined shaft. The hollow shaft embodies ports
which are normally closed by a pilot valve and held in this position by a speeder
spring. The spring tension acts in opposition to the centrifugal force of the rotating
flyweights. When an engine overspeed condition occurs, the increased centrifugal force
sensed by the flyweights overcomes the spring tension, lifts the pilot valve and
bypasses the propeller servo piston oil back to the reduction gearbox through the hollow
splined shaft. This permits the combined forces of the counterweights and return
springs to move the blades toward a coarse pitch position, absorbing the engine power
and preventing further engine overspeed. A ground test solenoid valve in the governor
is provided to permit testing the unit. The valve resets the governor to approximately 10%
below its normal overspeed setting.

8. POWER TURBINE GOVERNOR

A. The power turbine governor is mounted on the top of a sandwich type

tachometer-generator (Nf). The generator is mounted at approximately the 2 o’clock
position on the reduction gearbox and is driven by the propeller shaft through bevel gears
(see Figure 1-12-3). The power turbine governor is set to approximately 106% (Nf)
speed for normal pitch operation. Should the propeller stick in such a way as to prevent
the propeller governor from governing in the normal pitch range, the power turbine
governor will act as an overspeed limiter by bleeding FCU control air, thereby reducing
the fuel flow to limit the power and so govern the propeller shaft speed.

B. The power turbine governor is progressively reset to below 100% Nf as the propeller
control cam is pulled back into the Beta range, i.e. below the 90% position of the
power control lever. If Nf exceeds the setting of the power turbine governor, the governor
will reduce the fuel flow, through the FCU, and so reduce engine power by reduction
of Ng. The power turbine governor can reduce Ng to a minimum of 50%. If with Ng at 50%,
Nf still exceeds the governor setting, the propeller reverts to a constant-speed
condition controlled by the propeller governor. This will occur under high airspeed
conditions and under these conditions Beta control and reverse pitch selection are not
possible.

9. FUEL CUT-OFF LEVER

A. The cockpit fuel cut-off lever is connected through a combined lever and stop on the top
of the FCU to the cut-off valve lever. The lever and stop also function as an idle stop for
the FCU control rod. With the cockpit fuel cut-off lever in the IDLE position, the FCU
control rod stop is set to the 50% Ng position while with the cockpit fuel cut-off lever in
the HI-IDLE position, (where applicable) the FCU control rod stop is reset to the
80% Ng position.

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MANUAL PART NO. 3015442

PROPELLER CONTROL CAM 1 NON−STOL CAM SLOT

2 REAR FORK END CONFIGURATION
3 LOCKNUT
REAR WIRE ROPE
TERMINAL

4 CLAMPING BOLT CLAMPING BOLT 38 COUNTERWEIGHT

37 ACTUATING LEVER SUPPORT ARM POWER TURBINE GOVERNOR

39 INTERCONNECT ROD (UPPER)
PROPELLER GOVERNOR STOP HARTZELL PROPELLER HC−B3TN−3
(IN FEATHERED POSITION)
90°± 1° 5 23 PROPELLER SPEED SELECT LEVER 24 FRONT FORK END
TAKE−OFF POSITION REVERSE POSITION RING, ROD END
IDLE POSITION IDLE DEADBAND TO COCKPIT PROPELLER
ADJUSTING BOLT CONTROL LEVER
36 PROPELLER GOVERNOR ARM SERVO PISTON
(NON−STOL
APPLICATION) 22 PROPELLER GOVERNOR FEATHER RETURN SPRINGS
20 FRONT LIFTING BRACKET
DIM. Z 35 SPRING STOP DIM. X
PROPELLER CONSTANT 19 LOCKNUT 30 LOCKNUT
21 SPEED CONTROL UNIT STOP SPRING RETAINER CUP
31
18 REAR SWIVEL JOINT FRONT SWIVEL
6 FCU ACTUATING LEVER JOINT 25
LOW PITCH LOW PITCH AND REVERSE
STOP ADJUSTER
1 PROPELLER CONTROL CAM HIGH PITCH AND FEATHER
PROPELLER GOVERNOR
26 ACTUATING LEVER LOW−STOP ROD
9 CAM FOLLOWER LEVER WIRE ROPE
CASING SPRING, REVERSE RETURN
DETAIL A POWER TURBINE
GOVERNOR LINE (Py)
33 PROPELLER REVERSING LEVER
(CARBON BLOCKS (25) NOT SHOWN)
27 PROPELLER 40 POWER TURBINE GOVERNOR MAXIMUM STOP
PUSH−PULL CONTROL FEEDBACK RING
8 ADJUSTING SPACER WIRE ROPE 41 POWER TURBINE GOVERNOR ARM
42 POWER UTRBINE GOVERNOR INTERCONNECT ROD (LOWER)
7 HI−IDLE STOP TO COCKPIT POWER
CONTROL LEVER 43 POWER TURBINE GOVERNOR LEVER
POWER TURBINE
15 17 GOVERNOR LINE (Py)
STOL CAM SLOT CAM FOLLOWER PIN 32 LOCK−BOLT
FCU REVERSING
CONFIGURATION LEVER CENTER FIRESEAL FRONT LIFTING
16 20 BRACKET
FCU
14 INTERCONNECT POWER CONTROL LOW−PITCH
ROD LEVER 29 ADJUSTER STOP
REAR FIRESEAL 28 LOCKNUT
13 Ng MAX. STOP RETAINING
44 PLATE 34 SPRING
FUEL CONTROL
12 FCU ARM EXTENSION DETAIL B
UNIT (FCU) 11 FCU CONTROL ARM DETAIL C 25 LOW−PITCH STOP
ADJUSTER
10 CUT−OFF VALVE LEVER POST−SB298

(PT6A-6B and -20 Engines) C1849A

Engine Controls and Reversing Installation
Figure 1-12-3

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10. POWER CONTROL LEVER

A. The cockpit power control lever controls engine power through the full range from
maximum Take-off power through to Full Reverse. It also selects the propeller pitch (Beta
control) from the reverse selection up to the constant speed rpm as selected by the
cockpit propeller control lever (See Figure 1-12-3)

B. A cam assembly, secured to the accessory gearbox is the connecting point for the
cockpit power control lever and provides scheduled control movements to the FCU, power
turbine governor and the low blade angle selection system. The cam assembly
consists of an input lever, which is connected through a shaft to a cam follower assembly,
and three cam type levers. The FCU reversing lever is free-mounted on the input
lever shaft while the propeller control cam and FCU actuating lever are free-mounted on
a second shaft. The FCU actuating lever is spring-loaded toward idle and is connected
through an adjustable control rod to the FCU control arm. In operation, movement of the
cockpit power control lever rotates the cam follower. The cam follower pin contacts
one face of the FCU actuating lever and slides in the slotted propeller control cam.

C. The propeller control cam is coupled through a push-pull wire rope to the upper end of
the propeller reversing lever. The mid-point of the reversing lever is connected through a
linkage to the propeller governor interconnect rod. The upper end of the lever is also
coupled through two adjustable control rods and a lever to the power turbine governor.

D. When the upper end of the reversing lever is moved by the push-pull wire rope, both the
power turbine governor and the propeller low blade angle selection mechanism are
reset. If the propeller rpm is below constant speed rpm, movement of the reversing lever
causes the propeller governor to change the propeller pitch. The resulting movement
of the propeller feedback ring moves the lower end of the lever to return the propeller
governor to a null position and prevent further blade movement when the selected
blade angle is reached.

E. In the Beta control range, movement of the power control lever from IDLE up to
approximately the 90% position will select increasing propeller blade pitch angles.
Movement of the lever from IDLE to REVERSE will select decreasing propeller pitch
angles until the full reverse pitch is obtained.

F. The FCU control arm is positioned by the cockpit power control lever through the cam
follower and in the reverse selection through the FCU reversing lever. In the IDLE
position (see Figure 1-12-3), the cam follower pin is contacted by the adjuster on the
FCU reversing lever and by the FCU actuating lever. When the power control lever is
pushed toward the TAKEOFF position, the cam follower pin pushes the FCU actuating
lever forward against the action of the spring and moves the FCU control arm until the
required power is selected. In the TAKEOFF position, the controls will be in the
positions shown (see Figure 1-12-3).

G. Moving the power control lever back from the IDLE to the REVERSE position, the cam
follower pin contacts the adjuster on the FCU reversing lever and rotates the lever. The
lug at the bottom of the FCU reversing lever contacts the underside of the FCU
actuating lever, lifting it clear of the cam follower pin and rotating it toward the TAKEOFF
position. When the power control lever is in the full REVERSE selection, the FCU
actuating lever will be in the Takeoff position. (See Figure 1-12-3).

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MANUAL PART NO. 3015442

11. NON-STOL APPLICATIONS

A. The installation of Non-Stol applications is identical to the Stol reversing propeller

installation except that a special propeller cam is used. This cam has a different curved slot
configuration for the Lo-idle position up to the Take-Off position of the power control
lever travel (refer to Figure 1-12-3 for a detail of the Non-Stol cam). The reverse position
of the propeller control cam is the same as that for Stol applications except that the
beta range of the Stol application is limited to that part of the control range from Lo-Idle
to Reverse. Movement of the cockpit power control lever from Idle to Take-Off position,
therefore, has no direct effect upon propeller pitch.

B. Reduced idle power may be obtained by selecting below idle at which point a 12° dead
band allows selection of a flat, fine low without increasing the FCU power lever travel.

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MANUAL PART NO. 3015442

PART 1 - DESCRIPTION OF ENGINE

SECTION 13 - PROPELLER REVERSING INSTALLATION

(PT6A-20A AND PT6A-20B ENGINES)

1. GENERAL

A. The engine reversing propeller installation consists of a single-acting hydraulic propeller

controlled by a propeller governor, combining the functions of a normal propeller
governor (CSU), a reversing valve and a power turbine governor (Nf).

B. The integrated engine and propeller combination has three cockpit controls: a fuel cut-off
lever, a power control lever and a propeller control lever. The fuel cut-off lever has two
positions: CUT-OFF and IDLE. An additional HI-IDLE position is provided on certain
installations. This lever controls the cut-off function of the fuel control unit (FCU) and
resets the power control lever Idle Stop to provide 50% minimum Ng in the IDLE position
and up to a minimum of 80% Ng in the HI-IDLE position, where applicable. The Ng
selected by the power control lever is a maximum value which is reached only if the power
turbine governor section of the propeller governor does not act to reduce Ng speed
below the selected value. The power control lever also selects low blade angle (Beta)
and resets the power turbine governor section of the propeller governor from over propeller
selected speed at full forward power to under propeller selected speed in the Beta
range. The propeller control lever is connected to the propeller governor through linkage.
Movement of the linkage determines and limits the rpm range of the propeller governor
and so governs the propeller. When the control lever is moved to the minimum rpm
position, the propeller will automatically feather.

2. PROPELLER

A. The engine is normally equipped with a three bladed metal propeller dowelled and
bolted to the front face of the propeller shaft flange. The propeller consists of a
hollow steel spider hub which supports three propeller blades and also houses an
internal oil pilot tube and the feather return springs (see Figure 1-13-1). Movement of
the propeller blades is controlled by a hydraulic servo piston mounted on the front of the
propeller spider hub. The servo piston is connected by a link to each of the blade
trailing edge roots. Centrifugal counterweights on each blade and feathering springs in
the servo piston tend to drive the servo piston into the feather, or high pitch position. This
movement is opposed by the oil pressure generated and controlled by the propeller
governor. The pressure oil is transferred to the servo piston via an oil transfer housing
and the hollow center of the propeller shaft. An increase in the oil pressure moves the
propeller blades toward the low pitch position (increased rpm). A decrease in the oil
pressure allows the blades to move toward the high pitch position (decreased rpm) under
the influence of the feathering springs and blade counterweights.

B. The servo piston is also connected by three spring-loaded sliding rods to a feedback
ring mounted behind the propeller. Movement of the feedback ring is transmitted by a
carbon block through the propeller reversing lever and thence to the Beta valve. This
motion is used to control blade angle from normal forward fine pitch stop to full
reverse blade angle. The motion is available only from approximately 20 degrees positive
to 20 degrees reverse blade angle.

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STOL CAM PUSH−PULL CONTROL

SPEED ADJUSTING
LEVER
SPEEDER
SPRING
FCU ARM PILOT VALVE
Py

RESET POST
BETA CONTROL VALVE
AIR BLEED LINK

MINIMUM
GOVERNING
ADJUSTMENT

RETURN TO PUMP
TO SUMP FROM PUMP
PITCH LOCK
SOLENOID VALVE

TEST SOLENOID
PROPELLER OVERSPEED
GOVERNOR

BETA ROD

HYDRAULIC LOW PITCH ADJUSTMENT

PITCH LOCK SOLENOID
ACTUATING SWITCH
COUNTERWEIGHT

TO SUMP

(PT6A-20 and -20B Engines) C1853

Propeller Reversing Installation - Schematic
Figure 1-13-1

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3. PROPELLER GOVERNOR

A. The propeller governor performs three functions. Under normal flight conditions it acts
as a constant speed governing (CSU), maintaining the propeller speed selected by the
pilot by varying the propeller blade pitch to match the load to the engine torque in response
to changing flight conditions. During low airspeed operations, the propeller governor
can be used to select the required blade angle (Beta control); while in the Beta control
range, engine power is adjusted by the FCU and the power turbine section of the propeller
governor to maintain power turbine speed (Nf) at slightly less than the selected rpm.

B. The propeller governor is a single-acting centrifugal type mounted at the12 o’clock

position on the reduction gearbox front case (see Figure 1-13-2). In forward thrust
operation, the function of the propeller governor is to boost engine pressure oil to
decrease propeller blade pitch, while the centrifugal force of the blade counterweights,
assisted by feathering spring, tends to increase the pitch.

C. The propeller governor consists of an integral gear type oil pump with a pressure relief
valve, two pivoted flyweights mounted on a rotating flyweight head, a spring loaded pilot
valve and necessary cored passages contained in an aluminum housing. The flyweight
head is attached to a hollow drive shaft which protrudes below the housing flange. The
shaft is externally splined to mate with the corresponding coupling shaft in the
reduction gearbox. The spring loaded pilot valve is installed in the drive shaft centerbore.
Ports in the drive shaft and the position of the pilot valve control the direction of oil
flow within the housing. The rotating shaft speed and hence rotating flyweight determine
the position of the pilot valve, while the opposing spring load on the valve is varied by
the speed adjusting lever at top of the governor. The speed adjusting lever is connected
through airframe linkage to the propeller control lever in the cockpit.

D. A maximum stop prevents the speed adjusting lever moving beyond 100% position and
enables the propeller to be operating at or near, full rated speed, and the engine to
develop maximum power. Movement of the adjusting lever towards the minimum preset
(feathering) stop, raises the pilot valve and decreases oil pressure to propeller servo
piston. This decrease in pressure allows the propeller piston to move under the influence
of the feathering return springs and blade counterweights, to rotate the propeller
blades to a positive coarse pitch or feathering position, regardless of governor flyweight
force acting on the pilot valve. This enables the pilot to feather the propeller and
minimize drag in the event of engine failure.

E. In the event of a malfunction in the linkage of the propeller control ever, a spring
attached to the speed adjusting lever holds the lever in its last selected position or moves
it against the maximum stop.

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MANUAL PART NO. 3015442

PROPELLER
CONTROL CAM 1 2 REAR FORK END
NON−STOL CAM SLOT CONFIGURATION
3 LOCKNUT
REAR WIRE
ROPE RERMINAL TO COCKPIT PROPELLER 23 PROPELLER SPEED
CONTROL LEVER ADJUSTING LEVER COUNTERWEIGHT PROPELLER CUTAWAY (REF. ONLY)

24 FRONT FORK END PROPELLER − TYPICAL

(IN FEATHERED POSITION)
4 CLAMPING BOLT EXTENSION
SPRING PROPELLER GOVERNOR 22 RING, ROD END
SERVO PISTON
5 SETTING MAX. STOP 21
IDLE DEADBAND CAM FOLLOWER
90 ° ± 1° ADJUSTING BOLT LEVER REVERSE POSITION FRONT LIFTING BRACKET 20 FEATHER RETURN SPRINGS
(NON−STOL
IDLE POSITION APPLICATION) TAKE−OFF POSITION COUPLING NUT 40
WIRE ROPE CASING 43
SPRING RETAINER CUP
WIRE ROPE 46
A
FCU ACTUATING LEVER 6 25 LOW PITCH AND REVERSE
LOW PITCH
STOP ADJUSTER HIGH PITCH AND FEATHER
20 FRONT LIFTING BRACKET
REAR FORK END 2 REAR SWIVEL
LOW STOP ROD
JOINT
PROPELLER GOVERNOR AIR 18 SPRING, REVERSE RETURN
PRESSURE TUBE (Py) WIRE ROPE
CASING CASTELLATED
PROPELLER CONTROL CAM 1 26 27 NUT
PROPELLER GOVERNOR
INTERCONNECT ROD PROPELLER FEEDBACK LOW PITCH 38 PROPELLER GOVERNOR
INTERCONNECT ROD
CAM FOLLOWER LEVER 9 PUSH−PULL CONTROL RING LOCKNUT 28 29 ADJUSTER
WIRE ROPE 44 BRACKET STOP
26
37 RETAINING PLATE ADJUSTING SPACER 8 40 COUPLING NUT LOW PITCH STOP ADJUSTER 25
DETAIL A 19 LOCKNUT
CLAMP
POST−SB298 HI−IDLE STOP 7 TO COCKPIT POWER SPRING 34 FRONT WIRE FRONT
CONTROL LEVER ROPE TERMINAL 24 FORK
17 CAM FOLLOWER PIN
42 SELF−LOCKING NED
NUT
16 POWER CONTROL LEVER LOCKNUT 30
15 45
TELESCOPIC TERMINAL SELF−LOCKING NUT 39
HOUSING COMPRESSOR SPRING FCU REVERSING CENTER FIRESEAL 36 CLAMPING
LEVER INSULATION BOLT
GASKET MAX. STOP
25 34 28 29 47 36 38
26 REAR FIRESEAL 31
PROPELLER
FRONT SWIVEL JOINT REVERSING
24 14 FCU INTERCONNECT ROD 33 LEVER

13 Ng MAX. STOP
LOCK BOLT 32 BETA
FUEL CONTROL 41 VALVE
30 MAX. STOP UNIT (FCU) 12 FCU ARM EXTENSION GOVERNOR AIR BLEED LINK 35 CLEVIS

48 39 CARBON BLOCK
11 FCU CONTROL ARM (REF.)
31 33 10 CUT−OFF VALVE PROPELLER GOVERNOR 26
INTERCONNECT ROD
35 PT6A−20A PUSH−PULL CONTROL
32 41 ROD ARRANGEMENT
26
PT6A−20B PUSH−PULL CONTROL
ROD ARRANGEMENT (TELESCOPIC UNIT)

PT6A-20A and -20B Engines) C1850B

Engine Controls and Reversing Installation
Figure 1-13-2

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MAINTENANCE MANUAL
MANUAL PART NO. 3015442

F. The Beta valve, incorporated in the propeller governor pump output line to the pilot
valve, and mechanically connected to the propeller reversing lever, is designed so
that forward movement of the valve will initially block off high pressure oil to the propeller
servo piston; as forward movement continues, pressure oil in the servo will be
dumped back to the reduction gearbox. Axial movement in reverse direction has no
effect on normal propeller control. When the propeller is rotating at a speed lower than
selected by the speed adjusting lever, the governor oil pump provides pressure oil to the
servo piston thereby producing finer propeller pitch, until the feedback ring pulls back
the Beta valve to block off pressure oil to the servo piston, thus preventing further pitch
change. The pitch angle will coarsen automatically to maintain selected propeller
speed as higher engine power is selected. The Beta control does not forcibly select
reverse blade angles and very fine pitch, reverse can only occur when conditions are such
that the propeller is underspeeding relative to that selected.

G. On some installations, a lock pitch solenoid is mounted at the front of the propeller
governor and connected by cored passages to the pump oil pressure line to the
servo piston. The solenoid is energized automatically by a microswitch through movement
of the feedback ring, at a predetermined propeller blade angle. When the solenoid is
energized the valve will close to block oil flow to the servo piston; this action will lock oil
in the piston chamber and prevent any further movement of blades. However, if the
engine power lever is moved into the reverse position, a secondary microswitch installed
in the power lever quadrant, will deenergize the solenoid valve actuating switch,
opening the valve. This enables the propeller governor, power turbine section, to return
to its governing role. A test switch is incorporated in the system to functionally test
valve operation.

H. To provide the governor with a sensing element, the rotating pivoted flyweights are
mechanically coupled to the engine by a hollow drive shaft and accessory drive
shaft in the reduction gearbox. The flyweights, actuated by centrifugal force, position the
pilot valve so as to cover or uncover ports in the drive gearshaft and regulate oil flow
to and from the servo piston. The centrifugal force exerted by the flyweights is opposed
by the force of the speeder spring, this determines the engine rpm required to
develop sufficient centrifugal force on the flyweights to center the pilot valve, thereby
preventing oil flow to the servo piston.

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MAINTENANCE MANUAL
MANUAL PART NO. 3015442

I. The function of the propeller governor Nf section, during normal forward thrust
operation, is to protect the engine against a possible power turbine overspeed in the event
of a propeller governor failure. During reverse thrust operation, the propeller governor
power turbine section is set below the propeller governor selected speed. This acts to
control propeller speed via the FCU servo system (Py), and thus reduce the power
supplied by the gas generator to below that required to maintain approximately 5% less
than the selected propeller speed. A yoked bellcrank, operating off the flyweight
head, opens a pneumatic orifice as speed is increased. The orifice is connected to the
pneumatic servo system of the FCU and opening of the orifice reduces fuel metered to the
engine. The speed at which the propeller governor power turbine section operates is
dependent on the speed selected on the governor and the position of the air bleed link.
This link is normally set so that the propeller governor power turbine section will
control Nf at approximately 6% higher than the selected speed in its maximum position
and approximately 4% lower in its minimum position. The propeller governor power
turbine section ‘‘droop’’ is approximately 4%, thus in maximum position, the governor will
commence governing at 102% and fully govern at 106%. It should be noted that
repositioning the air bleed link to its minimum position brings the yoke into contact with
the pilot valve and brings in a spring load in addition to the speeder spring which
the flyweights must overcome to control propeller speed. This effect causes propeller
governed speed to increase to about 1% higher than nominally selected.

4. PROPELLER GOVERNOR ON-SPEED CYCLE

A. During an on-speed condition in forward thrust, the forces acting on the engine,
propeller governor and propeller combination are in a state of balance. With the propeller
speed adjusting lever set to obtain the desired rpm and the propeller blades in the
correct pitch range to absorb the power developed by the engine, the centrifugal force of
the rotating flyweights balances the force of the speeder spring with the flyweights in
the vertical position. This positions the pilot valve plunger so that the ports which control
the oil flow from the governor pump to the propeller servo piston are closed. The oil
pressure is therefore diverted through the relief valve, back to the inlet side of the governor
pump.

5. PROPELLER GOVERNOR OVERSPEED CYCLE

A. In an overspeed condition, the rotating flyweights pivot outward, overcome the speeder
spring tension and raise the pilot valve. This uncovers the ports in the drive gearshaft
and dumps the oil from the propeller servo piston to the reduction gearbox sump. As the
propeller blades increase in pitch angle, the load on the engine is increased with a
consequent decrease in engine rpm. The centrifugal force exerted on the governor
flyweights is reduced allowing the speeder spring tension to return the flyweights to a
vertical position. The pilot valve once again blocks the flow of control oil to or from
the propeller servo piston.

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MAINTENANCE MANUAL
MANUAL PART NO. 3015442

6. PROPELLER GOVERNOR UNDERSPEED CYCLE

A. With the propeller speed adjusting lever set to the desired rpm, a condition of underspeed
occurs when the engine rpm drops below the predetermined setting. The speeder spring
tension then overcomes the decreased centrifugal force on the flyweights and pivots
them inward, forcing the pilot valve downward and opening the ports. This directs a flow
of pressure oil to the propeller servo piston which overcomes the combined forces of
the propeller counterweights and return springs and decreases the pitch of the propeller
blades. The load on the engine is reduced with a consequent increase in engine rpm.
This is sensed by the governor flyweights, which cause the pilot valve plunger to again be
positioned to close the oil port.

7. PROPELLER OVERSPEED GOVERNOR

A. The propeller overspeed governor is installed in parallel with the propeller governor and
is mounted at approximately the 10 o’clock position on the reduction gearbox front case.
It is provided to control any propeller overspeed condition by immediately bypassing
the oil from the propeller servo piston to the reduction gearbox (see Figure 1-13-1). The
propeller overspeed governor consists of conventional type flyweights mounted on a
rotating hollow splined shaft. The hollow shaft embodies ports which are normally closed
by a pilot valve and held in this position by a speeder spring. The spring tension acts
in opposition to the centrifugal force of the rotating flyweights. When an engine overspeed
condition occurs, the increased centrifugal force sensed by the flyweights overcomes
the spring tension, lifts the pilot valve and bypasses the propeller servo piston oil back to
the reduction gearbox through the hollow splined shaft. This permits the combined
forces of the counterweights and return springs to move the blades toward a coarse pitch
position, absorbing the engine power and preventing further engine overspeed. A
solenoid valve, which resets the governor to a value below its normal overspeed setting,
is provided to permit ground testing of the unit.

8. NON-STOL APPLICATIONS

A. The installation for Non-Stol applications is identical to the Stol reversing propeller
installation except that a special propeller cam is used. This cam has a different curved slot
configuration for the Low-idle position up to the Take-Off position of the power control
lever travel (refer to Figure 1-13-2 for a detail of the Non-Stol cam). The reversing position
of the propeller control cam is the same as that for Stol applications except that the
Beta range of the Stol application is limited to that part of the control range from Lo-Idle
to Reverse. Movement of the cockpit power control lever from Lo-Idle to Take-Off
position, therefore, has no direct effect upon propeller pitch.

B. Reduced idle power may be obtained by selecting below idle at which point a 12° dead
band allows selection of a flat, fine low without increasing the FCU power lever travel.

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