Aviation Maintenance Technician Certification Series

MODULE 15

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GAS TURBINE ENGINE

The publishers of this Aviation Maintenance Technician Certification Series welcome you to the world of aviation maintenance. As you move towards EASA certification, you are required to gain suitable knowledge and experience in your chosen area. Qualification on basic subjects for each aircraft maintenance license category or subcategory is accomplished in accordance with the following matrix. Where applicable, subjects are indicated by an "X" in the column below the license heading.

For other educational tools created to prepare candidates for licensure, contact Aircraft Technical Book Company.

We wish you good luck and success in your studies and in your aviation career!

VERSION EFFECTIVE DATE DESCRIPTION OF CHANGE

001 2019 01 Module Creation and Release

iiiModule 15 A - Gas Turbine Engine

WELCOME

REVISION LOG

CONTENTS

ixModule 15 A - Gas Turbine Engine

GAS TURBINE ENGINEWelcome ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ iiiRevision Log ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ iiiForward ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ ivEASA License Category Chart ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ vGeneral Knowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ vContents ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ xiv

SUB-MODULE 01FUNDAMENTALSKnowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.115.1 - Fundamentals ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.2

Turbine Engine Fundamentals ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.2Energy ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.2

Potential Energy ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.2Kinetic Energy ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.2

Newton's Laws of Motion ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.3Newton's First Law ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.3Newton's Second Law ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.3Newton's Third Law ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.3

The Brayton Cycle ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.3Force, Work, Power, Motion ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.4

Force ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.4Work ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.5Power ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.5Energy ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.6Motion ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.6

Speed and Velocity ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.6Acceleration ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.6

Fundamentals of Operation ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.7Fuel Economy ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.8Durability and Reliability ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.8Operating Flexibility ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.9Compactness ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.9

Constructional Arrangement ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.9Turbo-jet ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.9Turbo-fan Engines ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.9Turboshaft ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.11Turboprop ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 1.11

SUB-MODULE 02ENGINE PERFORMANCEKnowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 2.1

SUB-MODULE 03INLETKnowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 3.115.3 - Inlet ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 3.2

Compressor Inlet Ducts ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 3.2Turbine Engine Inlet Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 3.2

Divided Entrance Duct ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 3.3Variable Geometry Duct ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 3.4Turboprop and Turboshaft Compressor Inlets ‥‥ 3.5Turbofan Engine Inlet Sections ‥‥‥‥‥‥‥‥‥‥‥‥‥ 3.5Bellmouth Compressor Inlets ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 3.6Compressor Inlet Screens ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 3.7

Ice Protection ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 3.7System Operation ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 3.8Regulator Valve ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 3.8Anti-icing with Electronic Engine Control and FADEC ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 3.9Electrothermal Anti-icing Systems ‥‥‥‥‥‥‥‥‥‥ 3.9

SUB-MODULE 04COMPRESSORSKnowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 4.115.4 - Compressors ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 4.2

Compressor Section ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 4.2Bleed Air ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 4.2Compressor Types and Applications ‥‥‥‥‥‥‥‥‥ 4.2

Construction Features, Operating Principles, and Applications ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 4.2

Centrifugal Flow Compressor ‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 4.2Axial Flow Compressor ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 4.3Advantages and Disadvantages ‥‥‥‥‥‥‥‥‥‥‥‥‥ 4.5

Fan Balance ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 4.5Operation ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 4.5Cause and Effect of Compressor Stall ‥‥‥‥‥‥‥‥‥‥‥ 4.6Airflow Control ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 4.6

Bleed Valves ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 4.6Variable Inlet Guide Vanes ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 4.7Variable Angle Stator Vanes ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 4.7Rotating Stator Blades ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 4.7

Compressor Ratio ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 4.7

SUB-MODULE 05COMBUSTION SECTIONKnowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 5.115.5 - Combustion Section ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 5.2

Constructional Features ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 5.2Can Type Combustion Chambers ‥‥‥‥‥‥‥‥‥‥‥ 5.2

CONTENTS

x Module 15 A - Gas Turbine Engine

Can Annular Combustion Chambers ‥‥‥‥‥‥‥‥ 5.3Annular Type Combustion Chambers ‥‥‥‥‥‥‥‥ 5.3

Operation of Combustion Chambers ‥‥‥‥‥‥‥‥‥‥‥ 5.4Combustion Chamber Performance ‥‥‥‥‥‥‥‥‥‥ 5.4Combustion Stability ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 5.4

SUB-MODULE 06TURBINE SECTIONKnowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 6.115.6 - Turbine Section ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 6.2

The Turbine Section ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 6.2Axial Flow Turbines ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 6.2Radial Inflow Turbine ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 6.2Stator and Turbine Blade Sections ‥‥‥‥‥‥‥‥‥‥‥ 6.2

Stator Nozzle Guide Vanes ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 6.3Turbine Blades Operation and Characteristics ‥‥‥‥ 6.3

Turbine Blade Types ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 6.4Impulse Configuration ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 6.4Reaction Configuration ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 6.4Reaction-Impulse Configuration ‥‥‥‥‥‥‥‥‥‥ 6.4

Turbine Blades Construction ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 6.4Blade to Disk Attachment ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 6.4

Turbine Blade Stress and Creap ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 6.6Turbine Section Configuration ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 6.7

Turbine Stages ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 6.7Turbine Housing ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 6.8

SUB-MODULE 07EXHAUSTKnowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 7.115.7 - Exhaust Section ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 7.2

Construction and Principles of Operation ‥‥‥‥‥‥‥ 7.2Exhaust Nozzle Shapes ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 7.4

Convergent Exhaust Nozzle ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 7.5Convergent-Divergent Exhaust Nozzle ‥‥‥‥‥‥‥ 7.5

Engine Noise Suppression ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 7.5Thrust Reverser's ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 7.6

SUB-MODULE 08BEARINGS AND SEALSKnowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 8.1

SUB-MODULE 09LUBRICANTS AND FUELSKnowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 9.115.9 - Lubricants and Fuels ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 9.2

Properties and Specifications of Lubricants ‥‥‥‥‥‥ 9.2

Spectrometric Oil Analysis Programs ‥‥‥‥‥‥‥‥ 9.2Properties and Specifications of Turbine Engine Fuels 9.2

Turbine Fuel Types ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 9.3Fuel Identification ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 9.4Fuel Purity ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 9.4

Fuel Additives ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 9.4Anti-ice Additives ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 9.4Anti-microbial Additives ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 9.4

Safety Precautions ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 9.5

SUB-MODULE 10LUBRICATION SYSTEMSKnowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.115.10 - Lubrication Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.2

Turbine Lubrication Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.2Turbine Lubrication System Components ‥‥‥‥‥‥‥ 10.2

Oil Tank ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.2Oil Pump ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.2Oil Filters ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.4Oil Pressure Regulating Valve ‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.5Oil Pressure Relief Valve ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.5Oil Jets ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.6Lubrication System Breather Systems (Vents) ‥‥ 10.6Lubrication System Check Valve ‥‥‥‥‥‥‥‥‥‥‥‥ 10.6Thermostatic Bypass Valves ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.7Air Oil Coolers ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.7Fuel Oil Coolers ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.7Deoiler ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.7Magnetic Chip Detectors ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.7Lubrication System Instrumentation ‥‥‥‥‥‥‥‥‥ 10.8

Scavenge System ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.8Breather Pressurizing System ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.9

Typical Dry Pump Lubrication Systems ‥‥‥‥‥‥‥‥‥ 10.9Pressure Regulated Lubricating ‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.9

Pressure System ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.10Variable Dry Pump Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.10

Pressure Subsystem ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.10Scavenger Subsystem ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.11Breather Subsystems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 10.11

Typical Wet sump Lubrication Systems ‥‥‥‥‥‥‥‥‥ 10.11

SUB-MODULE 11FUEL SYSTEMSKnowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 11.115.11 - Fuel Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 11.2

General Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 11.2Turbine Engine Fuel Metering Systems ‥‥‥‥‥‥‥‥‥ 11.2

CONTENTS

xiModule 15 A - Gas Turbine Engine

Hydromechanical Fuel Controls ‥‥‥‥‥‥‥‥‥‥‥‥ 11.3Hydromechanical/Electronic Fuel Controls ‥‥‥ 11.3FADEC Fuel Control Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 11.4

FADEC for an Auxiliary Power Unit ‥‥‥‥‥‥ 11.4FADEC for Propulsive Engines ‥‥‥‥‥‥‥‥‥‥ 11.5

Fuel System Operation ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 11.7Fuel System Layout and Components‥‥‥‥‥‥‥‥‥‥‥ 11.8

Engine Driven Fuel Pumps ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 11.8Fuel Heater ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 11.8Fuel Filters ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 11.9Fuel Spray Nozzles and Manifolds ‥‥‥‥‥‥‥‥‥‥‥ 11.10

Simplex Fuel Nozzle ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 11.11Duplex Fuel Nozzle ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 11.11

Fuel Pressurizing and Dump Valves‥‥‥‥‥‥‥‥‥‥ 11.11Fuel Quantity Indicating Units ‥‥‥‥‥‥‥‥‥‥‥‥‥ 11.12

SUB-MODULE 12AIR SYSTEMSKnowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 12.115.12 - Air Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 12.2

Turbine Engine Cooling ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 12.2Zone Cooling ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 12.2Compressor Cooling ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 12.3Combustion Chamber Cooling ‥‥‥‥‥‥‥‥‥‥‥‥‥ 12.3Turbine Section Cooling ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 12.4Accessory Cooling ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 12.4

Engine Bleed Air ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 12.4Engine Anti-Icing Control ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 12.4

Bearing Chamber Seal ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 12.4External Air ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 12.4

SUB-MODULE 13STARTING AND IGNITION SYSTEMSKnowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 13.115.13 - Starting and Ignition Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥ 13.2

Turbine Engine Starters ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 13.2Electric Starting Systems and Starter Generator Starting Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 13.2Air Turbine Starters ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 13.3

Turbine Engine Ignition Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 13.7Capacitor Discharge Exciter Unit ‥‥‥‥‥‥‥‥‥‥‥ 13.7Igniter Plugs ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 13.8Ignition System Inspection and Maintenance ‥‥ 13.9

Inspection ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 13.9Check System Operation ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 13.10

Maintenance Safety Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥ 13.10Starting System Safety Precautions ‥‥‥‥‥‥‥‥‥‥ 13.10

Ignition System Safety ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 13.10

SUB-MODULE 14ENGINE INDICATION SYSTEMSKnowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 14.115.14 - Engine Indicating Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 14.2

Gas Temperature Indicator ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 14.2Exhaust Gas Temperature Indicator ‥‥‥‥‥‥‥‥‥ 14.2Turbine Inlet Temperature Indicator ‥‥‥‥‥‥‥‥‥ 14.2

Engine Pressure Ratio (EPR) ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 14.3Oil Pressure and Temperature ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 14.3

Oil Pressure Indicator ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 14.3Oil Temperature Indicator ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 14.3

Fuel Pressure and Flow ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 14.4Fuel Pressure Indication ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 14.4Fuel Flow Indicator ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 14.4

Synchronous Mass Flow Meter ‥‥‥‥‥‥‥‥‥‥‥ 14.4Motorless Mass Flow Meter ‥‥‥‥‥‥‥‥‥‥‥‥‥ 14.4

Engine Speed ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 14.4Vibration Monitoring ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 14.5Torque (Turboprop Engines) ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 14.6

SUB-MODULE 15POWER AUGMENTATION SYSTEMSKnowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 15.1

SUB-MODULE 16TURBOPROP ENGINESKnowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 16.115.16 - Turboprop Engines ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 16.2

Introduction ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 16.2Gas (Free Turbine) and Gear Coupled Engines ‥‥‥ 16.2

Gas Coupled ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 16.2Gear Coupled Turbines ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 16.3

Reduction Gears ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 16.3Parallel Spur Gears ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 16.4Epicyclic Reduction Gears ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 16.4

Integrated Engine and Propeller Controls ‥‥‥‥‥‥‥ 16.4Control levers ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 16.6

Single Lever Control ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 16.6Dual Lever Controls ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 16.6

Overspeed Safety Devices ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 16.7Automatic Drag Limiting System (Double Acting Propeller) ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 16.7Pitch Locking ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 16.7Propeller Overspeed Governor ‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 16.8Turbine Shaft Failure ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 16.8

CONTENTS

xii Module 15 A - Gas Turbine Engine

SUB-MODULE 17TURBOSHAFT ENGINESKnowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 17.115.7 - Turboshaft Engines ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 17.2

Arrangements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 17.2Drive System ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 17.2Reduction Gearing ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 17.3Couplings and Drive Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 17.3Control Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 17.3

SUB-MODULE 18AUXILIARY POWER UNITS (APUS)Knowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 18.115.18 - Auxiliary Power Units (APU's) ‥‥‥‥‥‥‥‥‥‥‥‥ 18.2

Purpose ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 18.2Construction ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 18.2Engine Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 18.2

Operation ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 18.3APU Control ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 18.3Crew Control and Monitoring ‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 18.4Inspection and Servicing ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 18.4

Protective Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 18.5

SUB-MODULE 19POWERPLANT INSTALLATIONKnowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 19.115.19 - Powerplant Installation ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 19.2

Powerplant Installations ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 19.2Firewalls ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 19.2Cowlings ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 19.3

C Duct Cowls ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 19.3Acoustic Panels ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 19.4

Engine and Anti-Vibration Mounts ‥‥‥‥‥‥‥‥‥‥‥‥ 19.4Engine Mountings ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 19.4Turbine Vibration Isolation Engine Mounts ‥‥‥ 19.4

Hoses and Pipes ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 19.4Fuel Lines ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 19.5Hydraulic Lines ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 19.5O-Ring Seals ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 19.6

Feeders, Connectors and Wiring Looms ‥‥‥‥‥‥‥‥ 19.6Control Cables and Rods ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 19.8Lifting Points ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 19.8Drains‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 19.8

SUB-MODULE 20FIRE PROTECTION SYSTEMSKnowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.115.20 - Fire Protection Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.2

Fire Protection Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.2Components ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.2

Engine Fire Detection Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.2Thermal Switch Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.2Thermocouple Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.3Optical Fire Detection Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.3

Infrared Optical Fire Protection ‥‥‥‥‥‥‥‥‥‥ 20.3Pneumatic Thermal Fire Detection ‥‥‥‥‥‥‥‥‥‥ 20.3Continuous Loop Detector Systems ‥‥‥‥‥‥‥‥‥‥ 20.4

Fenwal Continuous loop System ‥‥‥‥‥‥‥‥‥‥ 20.4Kidde Continuous loop System ‥‥‥‥‥‥‥‥‥‥‥ 20.4Sensing Element ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.4Combination Fire and Overheat Warning ‥‥ 20.5System Test ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.5

Dual Loop Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.6Support Tube Mounted Sensing Elements ‥‥‥‥ 20.6

Fire Zones ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.6Engine Fire Extinguishing System ‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.7

Fire Extinguishing Agents ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.7Turbine Engine Ground Fire Protection ‥‥‥‥‥‥ 20.7Containers ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.7

Discharge Valves ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.8Pressure Indication ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.8Two Way Check Valve ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.8

Discharge Indicators ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.8Red Disk Thermal Indicator ‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.8Yellow Disk Discharge Indicator ‥‥‥‥‥‥‥‥‥‥ 20.9

Fire Switches ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.9Warning Systems ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 20.9

SUB-MODULE 21ENGINE MONITORING AND GROUND OPERATIONKnowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.115.21 - Engine Monitoring and Ground Operation ‥‥ 21.2

Procedures for Starting and Ground Run-up ‥‥‥‥‥ 21.2Turbine Engine Operation ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.2Ground Operation Engine Fire ‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.2Engine Checks ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.2

Checking Takeoff Thrust ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.2Engine Shutdown ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.3Turboprop Operation ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.4

Interpretation of Power Output and Parameters ‥‥ 21.4

CONTENTS

xiiiModule 15 A - Gas Turbine Engine

Interpretation for Turboprop Engines ‥‥‥‥‥‥‥‥ 21.4Health and Trend Monitoring ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.4

In-flight Recorders ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.4Ground Indicators ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.4

Magnetic Plugs or Chip Detectors ‥‥‥‥‥‥‥‥ 21.4Oil Debris Monitoring ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.6Oil Filters ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.6

Spectrometric Oil Analysis Program ‥‥‥‥‥‥‥‥‥ 21.6Typical Wear Metals and Additives ‥‥‥‥‥‥‥ 21.8

Vibration Monitoring ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.8Borescope ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.8

Inspection of Engines and Components ‥‥‥‥‥‥‥‥‥ 21.8Compressor Section ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.9

Inspection and Cleaning ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.9Causes of Blade Damage ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.11Blending and Replacement ‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.12

Combustion Section Inspection ‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.12Marking Materials for Combustion Section Parts‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.13Inspection and Repair of Combustion Chambers ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.13Fuel Nozzle and Support Assemblies ‥‥‥‥‥‥ 21.14

Turbine Section ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.14Turbine Disk Inspection ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.14Turbine Blade Inspection ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.14Turbine Blade Replacement ‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.15Turbine Nozzle Inlet Guide Vane Inspection 21.15Clearances ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.15

Exhaust Section ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.16Engine Accessories ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.17

Compressor Washing ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.17Foreign Object Damage (FOD) ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 21.18

SUB-MODULE 22ENGINE STORAGE AND PRESERVATIONKnowledge Requirements ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ 22.1

Acronym Index ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ A.1

3.2 Module 15 A - Gas Turbine Engine

15.3 - INLET

COMPRESSOR INLET DUCTSThe air entrance is designed to conduct incoming air to the compressor with a minimum energy loss resulting from drag or ram pressure loss; that is, the flow of air into the compressor should be free of turbulence to achieve maximum operating efficiency. Proper inlet design contributes materially to aircraft performance by increasing the ratio of compressor discharge pressure to duct inlet pressure. (Figure 3-1)

This is also referred to as the compressor pressure ratio. This ratio is the outlet pressure divided by the inlet pressure. The amount of air passing through the engine is dependent upon three factors:1. The compressor speed (rpm).2. The forward speed of the aircraft.3. The density of the ambient (surrounding) air.

Turbine inlet type is dictated by the type of gas turbine engine. A high bypass turbofan engine inlet is completely different from a Turboprop or turboshaft inlet. Large gas turbine powered aircraft almost always have a turbofan engine. The inlet on this type of engine is bolted to the front (A f lange) of the engine. These engines are mounted on the wings, or nacelles, on the aft fuselage, and a few are in the vertical fin. A typical turbofan inlet can be seen in Figure 3-2.

Since on most modern turbofan engines the huge fan is the first part of the aircraft the incoming air comes into contact with, icing protection must be provided.

This prevents chucks of ice from forming on the leading edge of the inlet, breaking loose, and damaging the fan. Warm air is bled from the engine's compressor and is ducted through the inlet to prevent ice from forming. If inlet guide vanes are used to straighten the air flow, then they also have anti-icing air flowing through them. The inlet also contains some sound reducing materials that absorb the fan noise and make the engine quieter.

Turboprops and turboshafts can use an inlet screen to help filter out ice or debris from entering the engine. A deflector vane and a heated inlet lip are used to prevent ice or large chunks from entering the engine.

On military aircraft, the divided entrance permits the use of very short ducts with a resultant small pressure drop through skin friction. Military aircraft can f ly at speeds above Mach 1, but the airf low through the engine must always stay below Mach 1. Supersonic air flow in the engine would destroy the engine. By using convergent and divergent shaped ducts, the air f low is controlled and dropped to subsonic speeds before entering the engine. Supersonic inlets are used to slow the incoming engine air to less than Mach 1 before it enters the engine.

TURBINE ENGINE INLET SYSTEMSThe engine inlet of a turbine engine is designed to provide a relatively distortion free f low of air, in the required quantity, to the inlet of the compressor. (Figure 3-3) Many engines use inlet guide vanes (IGV)

Figure 3-1. Military style short intake duct. Figure 3-2. Typical turbofan inlet.

3.3Module 15 A - Gas Turbine Engine

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to help straighten the airflow and direct it into the first stages of the compressor. A uniform and steady airflow is necessary to avoid compressor stall (airflow tends to stop or reverse direction of flow) and excessive internal engine temperatures in the turbine section. Normally, the air inlet duct is considered an airframe part and not a part of the engine. However, the duct is very important to the engine's overall performance and the engine's ability to produce an optimum amount of thrust.

A gas turbine engine consumes considerable more airf low than a reciprocating engine. The air entrance passage is correspondingly larger. Furthermore, it is more critical in determining engine and aircraft performance, especially at high airspeeds. Inefficiencies of the inlet duct result in successively magnified losses through other components of the engine. The inlet varies according to the type of turbine engine. Small Turboprop and turboshaft engines have a lower airflow than large turbofan engines which require a completely different type of inlet. Many Turboprop, auxiliary power units, and turboshaft engines use screens that cover the inlet to prevent foreign object damage (FOD).

As aircraft speed increases, thrust tends to decrease somewhat; as the aircraft speed reaches a certain point, ram recovery compensates for the losses caused by the increases in speed. The inlet must be able to recover as much of the total pressure of the free airstream as possible. As air molecules are trapped and begin to be compressed in the inlet, much of the pressure loss is recovered. This added pressure at the inlet of the engine increases the pressure and airflow to the engine. This is known as "ram recovery" or "total pressure recovery." The

inlet duct must uniformly deliver air to the compressor inlet with as little turbulence and pressure variation as possible. The engine inlet duct must also hold the drag effect on the aircraft to a minimum.

Air pressure drop in the engine inlet is caused by the friction of the air along both sides of the duct and by the bends in the duct system. Smooth f low depends upon keeping the amount of turbulence to a minimum as the air enters the duct. On engines with low f low rates, turning the airflow allows the engine nacelle to be smaller and have less drag. On turbofan engines, the duct must have a sufficiently straight section to ensure smooth, even airflow because of the high airflows. The choice of configuration of the entrance to the duct is dictated by the location of the engine within the aircraft and the airspeed, altitude, and attitude at which the aircraft is designed to operate.

DIVIDED ENTRANCE DUCTThe requirements of high speed, single or twin engine military aircraft, in which the pilot sits low in the fuselage and close to the nose, render it diff icult to employ the older type single entrance duct, which is not used on modern aircraft. Some form of a divided duct, which takes air from either side of the fuselage, has become fairly widely used. This divided duct can be either a wing root inlet or a scoop at each side of the fuselage. (Figure 3-4)

Either type of duct presents more problems to the aircraft designer than a single entrance duct because of the difficulty of obtaining sufficient air scoop area without imposing prohibitive amounts of drag. Internally, the

Figure 3-3. Inlet fan blades and guide vanes

(inner circle) on a GEnx engine.

Figure 3-4. An example of a divided entrance duct.

3.4 Module 15 A - Gas Turbine Engine

problem is the same as that encountered with the single entrance duct: to construct a duct of reasonable length with as few bends as possible. Scoops at the sides of the fuselage are often used. These side scoops are placed as far forward as possible to permit a gradual bend toward the compressor inlet, making the airflow characteristics approach those of a single entrance duct. A series of turning vanes is sometimes placed in the side scoop inlet to assist in straightening the incoming airflow and to prevent turbulence.

VARIABLE GEOMETRY DUCTThe main function of an inlet duct is to furnish the proper amount of air to the engine inlet. In a typical military aircraft using a turbojet or low bypass turbofan engine, the maximum airf low requirements are such that the Mach number of the airf low directly ahead of the face of the engine is less than Mach 1. Airflow through the engine must be less than Mach 1 at all times. Therefore, under all flight conditions, the velocity of the airf low as it enters the air inlet duct must be reduced through the duct before the airflow is ready to enter the compressor. To accomplish this, inlet ducts are designed to function as diffusers, decreasing the velocity and increasing the static pressure of the air passing through them. (Figure 3-5)

As with mil itary supersonic a ircraft, a diffuser progressively decreases in area in the downstream direction. Therefore, a supersonic inlet duct follows this general configuration until the velocity of the incoming air is reduced to Mach 1. The aft section of the duct then increases in area, since this part must act as a subsonic diffuser. (Figure 3-6)

In practice, inlet ducts for supersonic aircraft follows this general design only as much as practical, depending upon the design features of the aircraft. For very high speed aircraft, the inside area of configuration of the duct is changed by a mechanical device as the speed of the aircraft increases or decreases. A duct of this type is usually known as a variable geometry inlet duct.

Military aircraft use the three methods described above to diffuse the inlet air and slow the inlet airflow at supersonic flight speeds. One is to vary the area, or geometry, of the inlet duct either by using a movable restriction, such as a ramp or wedge, inside the duct.

Another system is some sort of a variable airflow bypass arrangement, which extracts part of the inlet airflow from the duct ahead of the engine. In some cases, a combination of both systems is used.

The third method is the use of a shock wave in the airstream. A shock wave is a thin region of discontinuity in a flow of air or gas, during which the speed, pressure, density, and temperature of the air or gas undergo a sudden change. Stronger shock waves produce larger changes in the properties of the air or gas. A shock wave is willfully set up in the supersonic flow of the air entering the duct, by means of some restriction or small obstruction which automatically protrudes into the duct at high flight Mach numbers. The shock wave results in diffusion of the airflow, which, in turn, decreases the velocity of the airflow. In at least one aircraft installation, both the shock method and the variable geometry method of causing diffusion are used in combination. The same device that changes the area of the duct also sets up a shock wave that further reduces the speed of

Figure 3-5. An inlet duct acts as a diffuser to decrease the airflow velocity and to increase the static pressure of air.

ConvergentSupersonic

Section

DivergentSubsonicSection

Figure 3-6. The aft section of an inlet duct acting as a subsonic diffuser.

3.5Module 15 A - Gas Turbine Engine

the incoming air within the duct. The amount of change in duct area and the magnitude of the shock are varied automatically with the airspeed of the aircraft.

TURBOPROP AND TURBOSHAFT COMPRESSOR INLETSThe air inlet on a Turboprop is more of a problem than some other gas turbine engines because the propeller drive shaft, the hub, and the spinner must be considered in addition to other inlet design factors. The ducted arrangement is generally considered the best inlet design of the Turboprop engine as far as airf low and aerodynamic characteristics are concerned. (Figure 3-7) The inlet for many types of Turboprops are anti-iced by using electrical elements in the lip opening of the intake. Ducting either part of the engine or nacelle directs the airflow to the intake of the engine. Deflector doors are sometimes used to def lect ice or dirt away from the intake. (Figure 3-8) The air then passes through a screen and into the engine on some models. A conical spinner, which does not allow ice to build up on the surface, is sometimes used with Turboprop and turbofan engines. In either event, the arrangement of the spinner and the inlet duct plays an important function in the operation and performance of the engine.

TURBOFAN ENGINE INLET SECTIONSHigh bypass turbofan engines are usually constructed with the fan at the forward end of the compressor. A typical turbofan intake section is shown in Figure 3-9 Sometimes, the inlet cowl is bolted to the front of the engine and provides the airflow path into the engine. In dual compressor (dual spool) engines, the fan is integral with the relatively slow turning, low pressure compressor, which allows the fan blades to rotate at low tip speed for best fan efficiency. The fan permits the use of a conventional air inlet duct, resulting in low inlet duct loss. The fan reduces engine damage from ingested foreign material because much of any material that may be ingested is thrown radially outward and passes through the fan discharge rather than through the core of the engine. Warm bleed air is drawn from the engine and circulated on the inside of the inlet lip for anti-icing. The fan hub or spinner is either heated by warm air or is conical as mentioned earlier. Inside the inlet by the fan blade tips is an abraidable rub strip that allows the fan blades to rub for short times due to flightpath changes. (Figure 3-10) Also, inside the inlet are sound reducing materials to lower the noise generated by the fan.

Figure 3-7. An example of a ducted arrangement on a Turboprop engine.

Figure 3-8. Deflector doors used to deflect ice or dirt away from the intake.

Figure 3-9. A typical turbofan intake section.

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