Module 15: Gas Turbine Engine – Comprehensive Study Material

Overview

Module 15 of the EASA Part-66 syllabus covers the complete gas turbine engine, from fundamental principles to detailed component construction, operation, and maintenance. This module is essential for certifying staff working on turbine-powered aircraft, as it provides the theoretical foundation for understanding engine performance, systems, and troubleshooting.

The module encompasses the following key areas:

Fundamentals of gas turbine operation and thermodynamics
Engine construction and component design (inlet, compressor, combustion, turbine, exhaust)
Engine systems (fuel, ignition, oil, bleed air, anti-icing, starting)
Engine control systems (hydromechanical and FADEC)
Engine monitoring and indicating systems
Maintenance practices, inspection techniques, and troubleshooting

1. Engine Fundamentals and Construction

1.1 Basic Principles of Gas Turbine Operation

A gas turbine engine operates on the Brayton cycle, consisting of four continuous processes:

Compression – Ambient air is drawn in and compressed to high pressure
Combustion – Fuel is injected and burned at approximately constant pressure
Expansion – Hot gases expand through the turbine, extracting energy
Exhaust – Remaining energy is expelled as jet velocity to produce thrust

The engine converts chemical energy in fuel into kinetic energy of the exhaust jet. The fundamental thrust equation is:

F = ṁ × (Vj - Va)

Where:

F = thrust (N)
ṁ = mass flow rate (kg/s)
Vj = jet velocity (m/s)
Va = aircraft velocity (m/s)

1.2 Engine Configurations

Twin-spool engines feature two mechanically independent rotors:

N1 (Low Pressure rotor): LP compressor driven by LP turbine
N2 (High Pressure rotor): HP compressor driven by HP turbine

Each rotor operates at its optimal rotational speed, improving overall efficiency. The N1 and N2 rotors are coaxial, with the LP shaft passing through the hollow HP shaft.

Bypass ratio is defined as:

BPR = Mass flow through bypass duct / Mass flow through engine core

High-bypass turbofan engines (BPR > 5:1) move a large mass of air at lower velocity, resulting in:

Higher propulsive efficiency
Reduced specific fuel consumption
Lower noise levels
Greater thrust at low speeds

1.3 Engine Sections

The engine is divided into two main sections:

Cold Section:

Air inlet
Compressor (fan, LP compressor, HP compressor)
Diffuser

Hot Section:

Combustion chamber
Turbine (HP, LP)
Exhaust system

2. Air Inlet and Compressor

2.1 Air Inlet

The inlet duct must deliver air to the compressor face with minimum pressure loss and distortion. Key design considerations:

Subsonic inlets: Divergent shape to decelerate airflow, converting kinetic energy to static pressure
Supersonic inlets: Complex geometry with variable geometry to manage shock waves

The inlet must also:

Protect the engine from foreign object damage (FOD)
Provide anti-icing protection
Minimise noise propagation

2.2 Compressor Types

Axial Flow Compressor:

Air flows parallel to the axis of rotation
Multiple stages, each consisting of a rotor blade row followed by a stator vane row
A stage is defined as one rotor disc plus one stator row
Each stage provides a pressure ratio of approximately 1.15–1.35:1

Centrifugal Compressor:

Air enters at the centre and is flung outward by centrifugal force
Higher pressure ratio per stage (up to 4:1)
Used in smaller engines or as the final HP stage in some designs

Mixed Flow Compressor:

Combines features of both axial and centrifugal designs

2.3 Compressor Aerodynamics

The compressor increases air pressure by:

Rotor blades: Add kinetic energy to the air (increase velocity)
Stator vanes: Convert kinetic energy to static pressure (decelerate airflow)

The diffuser at the compressor exit further decelerates airflow, converting remaining velocity energy into static pressure before the air enters the combustion chamber.

2.4 Compressor Instability

Compressor surge occurs when airflow separates from the blades, causing a momentary flow reversal. This is characterised by:

Loud bang or rumble
Fluctuating airflow and pressure
Possible flameout
Potential mechanical damage

Compressor stall is a disruption of airflow through the compressor, often caused by:

Rapid throttle advancement
Engine deterioration
Foreign object damage
Icing
Incorrect variable geometry scheduling

2.5 Compressor Airflow Control

Variable Inlet Guide Vanes (VIGVs) and Variable Stator Vanes (VSVs):

Adjust airflow angle into the compressor at low speeds
Prevent stall and surge
Improve compressor operating range
Scheduled by the engine control system based on N2 speed and other parameters

Compressor Bleed Valves (VBVs):

Open at low engine speeds to release air from the compressor
Prevent airflow instability and surge
Close progressively as engine speed increases
Also used during deceleration to prevent surge

2.6 Compressor Blade Inspection

Compressor blades are subject to:

FOD: Nicks, dents, bending, or tearing
Erosion: Sand and dust particles erode leading edges
Fatigue cracking: Especially at blade roots
Tip rubbing: Contact with the casing due to thermal expansion or rotor imbalance

Inspection limits are specified in the AMM. Key considerations:

Nicks and dents within limits may be blended (if permitted by AMM)
Cracks at blade roots are critical and generally require replacement
Titanium blades require special attention due to susceptibility to stress corrosion cracking
Any damage exceeding AMM limits requires blade replacement

3. Combustion Section

3.1 Combustion Chamber Design

The combustion chamber must:

Burn fuel efficiently with minimal pressure loss
Produce uniform exit temperature profile
Maintain stable combustion across the entire operating range
Cool the liner to survive high temperatures

Types of combustion chambers:

Can type (individual chambers)
Annular type (single continuous chamber)
Can-annular type (individual cans within an annular casing)

3.2 Combustion Process

The combustion process involves:

Primary zone: Fuel-air mixture is ignited and burned at near-stoichiometric ratios
Intermediate zone: Combustion completes with additional air
Dilution zone: Cooling air is introduced to reduce gas temperature to acceptable turbine inlet levels

Fuel atomisation is critical for efficient combustion. Fuel nozzles must produce a fine spray to ensure:

Complete combustion
Uniform temperature distribution
Minimal carbon formation
Reliable ignition

3.3 Combustion Chamber Inspection

Common defects found during borescope inspection:

Cracks: In the liner or casing (limits specified in AMM)
Missing cooling holes: Can cause local overheating and cracking – requires liner replacement
Distortion: Buckling or bowing of the liner
Carbon deposits: Indicate poor atomisation or combustion
Hot spots: Discolouration indicating local overheating

Important: Missing cooling holes must not be drilled or plugged unless specifically approved by the AMM. A missing hole compromises the cooling pattern and can lead to premature failure.

3.4 Hot Starts

A hot start occurs when EGT exceeds the allowable limit during the start sequence. This indicates:

Over-temperature condition during start
Potential thermal damage to turbine components
Often caused by:
Excessive fuel flow during start
Delayed ignition
Starter system malfunction
Incorrect fuel scheduling

After a hot start, the engine must be inspected per AMM procedures before further operation.

4. Turbine Section

4.1 Turbine Function

The turbine extracts energy from the hot gases to:

Drive the compressor (via the connecting shaft)
Drive the fan (in turbofan engines)
Drive accessory gearbox components

4.2 Turbine Components

Nozzle Guide Vanes (NGVs):

Stationary vanes located ahead of each turbine rotor stage
Convert part of the gas pressure energy into kinetic energy (velocity)
Direct the flow onto the rotating blades at the optimal angle
Experience the highest temperatures in the engine
Often cooled with compressor bleed air

Turbine Rotor Blades:

Extract energy from the gas stream
Subject to extreme temperatures, centrifugal stress, and thermal cycling
Often coated with Thermal Barrier Coating (TBC) for protection
May be shrouded (with tip shroud) or unshrouded

4.3 Turbine Blade Cooling

Turbine blades operate at temperatures exceeding the melting point of the base material. Cooling methods include:

Internal cooling: Compressor bleed air passes through internal passages
Film cooling: Coolant exits through small holes, creating a protective layer
Thermal barrier coatings: Ceramic coatings reduce heat transfer to the base metal

4.4 Turbine Case Cooling (TCC)

Turbine case cooling uses fan air to cool the turbine casing, causing it to contract and maintain optimal blade tip clearance. This:

Reduces tip leakage losses
Improves turbine efficiency
Reduces specific fuel consumption
Is most effective during cruise conditions

4.5 Turbine Deterioration

Turbine components degrade over time due to:

Creep: Time-dependent deformation under stress and high temperature, causing blade elongation
Erosion: Particle impact on blade surfaces
Corrosion/Sulphidation: Chemical attack, especially in marine environments
Thermal fatigue: Cracking due to repeated thermal cycling
TBC spallation: Loss of thermal barrier coating, exposing the substrate to high temperatures

EGT margin is the difference between the actual EGT and the maximum allowable EGT at a given thrust setting. A decreasing EGT margin indicates:

Turbine deterioration (blade erosion, TBC loss)
Compressor efficiency loss
Increased tip clearances

4.6 Turbine Inspection

Critical inspection points:

Blade cracks: Chordwise cracks are reject conditions – blending is not permitted
TBC spallation: Must be within allowable limits (typically percentage of blade surface)
Tip damage: Missing tips indicate FOD or blade failure
NGV cracks: Trailing edge cracks have specific limits
Tip rubbing: May be acceptable within limits

5. Exhaust Section

5.1 Exhaust Components

Exhaust Cone (Tail Cone/Bullet):

Smoothly collects and straightens the exhaust flow
Reduces turbulence and losses
Houses the rear bearing support structure

Exhaust Duct:

Directs the exhaust gases rearward
May include a mixer in mixed-flow turbofans

Exhaust Mixer:

Combines hot core flow with cooler bypass flow
Reduces exhaust noise
Can improve propulsive efficiency

5.2 Thrust Reversers

Thrust reversers redirect the exhaust airflow forward to create a deceleration force during landing.

Types:

Cascade type: Blocker doors redirect fan airflow through cascade vanes
Clamshell type: Two half-shell doors rotate to reverse the flow
Target type: Bucket doors deploy to reverse the flow

Operational considerations:

Deployment and stow times are specified in the AMM
Incomplete stow indicates misrigging – rigging must be adjusted per AMM
Slow stow time is typically caused by low hydraulic pressure or flow restriction
Faulty sensors cause indication issues, not slow movement

6. Engine Systems

6.1 Fuel System

Fuel Control Unit (FCU):

Meters the correct amount of fuel to the combustion chamber
Based on throttle position, engine speed, and other parameters
May be hydromechanical or electronic (FADEC)

Fuel Components:

Fuel pumps: Provide pressurised fuel to the FCU
Fuel filters: Protect sensitive components from contamination
Fuel heaters: Prevent ice crystal formation that could block filters
Fuel nozzles: Atomise fuel for efficient combustion
Fuel manifolds: Distribute fuel to the nozzles

Fuel System Maintenance:

Leak checks with specified limits (e.g., drops per minute)
Filter replacement at specified intervals
Nozzle inspection and testing
FCU calibration checks

6.2 Ignition System

The ignition system provides high-energy sparks for:

Engine start: Initial ignition of the fuel-air mixture
In-flight relight: Re-ignition after a flameout

Components:

Ignition exciter: Generates high-voltage pulses
Igniter plugs: Produce the spark in the combustion chamber
Ignition leads: Transmit high-voltage energy

Operational characteristics:

Ignition is typically turned off once the engine is self-sustaining
Spark rate is specified (e.g., 1 spark per second)
Low spark rate indicates a faulty exciter
No spark may indicate faulty plugs or leads

6.3 Lubrication System

Functions:

Lubricate bearings, gears, and moving parts
Reduce friction and wear
Remove heat from components
Clean and protect against corrosion

Components:

Oil tank: Stores the oil supply
Oil pumps: Pressure pump and scavenge pumps
Oil cooler: Removes heat (fuel-cooled or air-cooled)
Oil filters: Remove contaminants
Magnetic chip detectors (MCD): Capture metallic particles
Breather system: Vents the oil system

Oil System Maintenance:

Oil level checks must be performed correctly (overfilling causes foaming and oil loss)
High oil consumption without external leaks indicates internal consumption (e.g., through bearing seals)
Metallic particles on the MCD indicate internal damage – investigate before further operation
Low oil pressure at high thrust indicates pump wear or internal leakage

6.4 Bleed Air System

Bleed air is extracted from the compressor and used for:

Air conditioning and pressurisation
Wing and engine anti-icing
Engine starting (air turbine starters)
Hydraulic system pressurisation (in some aircraft)
Pneumatic systems

6.5 Anti-Icing System

Engine anti-icing prevents ice accumulation on:

Inlet surfaces
Compressor components
Sensors and probes

Ice accumulation disrupts airflow, reduces performance, and can cause compressor surge or flameout.

6.6 Engine Control Systems

Hydromechanical Control:

Uses mechanical linkages, cams, and hydraulic components
Schedules fuel flow based on throttle position and engine parameters
Limited flexibility and accuracy

Full Authority Digital Engine Control (FADEC):

Digital computer controls all aspects of engine operation
Manages fuel flow, variable geometry, bleed valves, and ignition
Provides protection against exceedances
Optimises performance across the operating envelope
Includes self-test and fault diagnostics

6.7 Engine Monitoring Systems

Vibration Monitoring:

Uses accelerometers to detect excessive vibration
Indicates bearing wear, rotor imbalance, or blade damage
Provides alerts to flight crew and maintenance personnel
Vibration at a specific N1 speed often indicates fan imbalance

Exhaust Gas Temperature (EGT):

Measured by thermocouples in the exhaust section
Critical parameter for turbine health monitoring
EGT margin trending indicates engine deterioration

Engine Pressure Ratio (EPR):

Ratio of turbine outlet pressure to compressor inlet pressure
Common thrust-setting parameter on turbofan engines

6.8 Accessory Gearbox (AGB)

The accessory gearbox:

Is driven by the high-pressure rotor
Provides mechanical power to engine accessories:
Fuel pumps
Oil pumps
Hydraulic pumps
Electrical generators
Tachometer generators

6.9 Engine Mounting System

Engine mounts:

Secure the engine to the airframe
Transmit thrust, weight, and aerodynamic loads
Provide vibration isolation
Allow for thermal expansion

Important: Over-torqued mount bolts must be replaced, even if not visually damaged, due to potential permanent deformation.

7. Engine Performance and Operation

7.1 Performance Parameters

Thrust is affected by:

Air density (colder air = denser = more thrust)
Aircraft speed
Engine speed (N1 or N2)
Compressor and turbine efficiency

Specific Fuel Consumption (SFC) :

Fuel flow per unit of thrust
High-bypass engines have lower SFC than low-bypass engines

7.2 Engine Operating Limits

Critical operating parameters:

EGT limit: Maximum allowable exhaust gas temperature
N1/N2 limits: Maximum rotor speeds
Oil pressure and temperature limits
Vibration limits

EGT exceedance requires:

Immediate shutdown to prevent further damage
Troubleshooting per AMM
Borescope inspection to assess damage

7.3 Engine Starting

The start sequence involves:

Starter motor rotates the engine (N2 speed)
Fuel is introduced at the appropriate speed
Ignition is activated
Light-up occurs (EGT rises)
Engine accelerates to idle
Starter and ignition disengage

Start malfunctions:

Hot start: EGT exceeds limit – abort start and inspect
No light-up: Ignition present but no fuel reaching combustion chamber
Hung start: Engine fails to accelerate to idle

7.4 Engine Surge During Operation

If a surge occurs during ground run:

The engine may return to idle and operate normally
However, a surge can cause mechanical damage
Mandatory action: Inspect for damage per AMM before further operation

8. Maintenance Practices and Inspection

8.1 Borescope Inspection

Borescope inspection is a critical non-destructive testing method for:

Compressor blades and vanes
Combustion chamber liner
Turbine blades and NGVs
Hot section components

Key principles:

Compare findings against AMM limits
Document all findings
Damage within limits requires no corrective action
Damage exceeding limits requires repair or replacement

8.2 Damage Assessment

Damage within AMM limits:

No repair required
Record findings in maintenance records
Return engine to service

Damage exceeding AMM limits:

Component must be repaired or replaced per AMM
Blending is only permitted if specified in AMM
Cracks in turbine blades are generally reject conditions
Missing cooling holes require liner replacement

8.3 Torque Procedures

Torque values are only valid under specified conditions:

Thread condition (lubricated or dry as specified)
Fastener condition
Washer and nut condition

If a nut does not seat properly at specified torque:

Remove and inspect
Lubricate if specified
Re-torque per AMM

8.4 Engine Storage and Preservation

Engines in storage require:

Preservation procedures to prevent corrosion
Periodic inspections
After prolonged storage (e.g., 12 months without preservation) :
Borescope inspection is mandatory
Filters must be replaced
Full inspection per AMM before installation

8.5 Engine Installation

Engine installation requires:

Following AMM procedures exactly
Correct routing and connection of all harnesses
Proper torque of all fasteners
Verification of all connections
Functional tests after installation

9. Troubleshooting and Fault Diagnosis

9.1 Symptom-Based Diagnosis

High N2 at given EGT with high fuel flow:

Indicates reduced compressor efficiency
Often due to VSV misadjustment
Engine uses more fuel to achieve the same thrust

High EGT with normal fuel flow:

Indicates reduced turbine efficiency
Due to blade damage or increased tip clearances

Decreased EGT margin:

Classic sign of turbine deterioration
First action: borescope inspection

High fuel flow at idle with normal EGT and N1:

Idle speed set too high
Adjust idle stop per AMM

Low oil pressure at high thrust:

Pump wear or internal leakage
Pump cannot maintain pressure at high RPM

Vibration at specific N1 speed:

Fan imbalance or blade damage
Sensor on fan frame picks up fan vibrations

Slow thrust reverser stow:

Low hydraulic pressure or flow restriction
Not a sensor issue

Incomplete thrust reverser stow:

Misrigging
Adjust rigging per AMM

9.2 Critical Actions

Low oil pressure:

Immediate shutdown required
Prevents bearing failure and severe damage

EGT exceedance:

Immediate shutdown
Troubleshoot per AMM

Surge event:

Inspect for damage before further operation
Surge can cause mechanical damage even if engine appears normal

10. Regulatory Framework

10.1 EASA Part-66 Requirements

Module 15 is assessed at three knowledge levels:

Level 1: Overview of the subject
Level 2: General knowledge with some detail
Level 3: Detailed theory with comprehensive understanding

10.2 Maintenance Documentation

Key documents referenced:

AMM (Aircraft Maintenance Manual): Contains all maintenance procedures and limits
IPC (Illustrated Parts Catalogue): Parts identification
AMC (Acceptable Means of Compliance): Guidance for compliance
Part-145: Maintenance organisation requirements

10.3 Recording Requirements

Per Part-145 requirements:

All defects must be recorded
Damage within limits must be documented
Maintenance actions must be certified
Records must be retained per regulations

11. Common Relationships Between Concepts

11.1 Performance Relationships

Thrust ∝ Mass flow × Jet velocity
Higher bypass ratio → Lower SFC → Lower noise
Colder air → Denser air → Higher mass flow → Higher thrust
EGT margin ↓ → Turbine deterioration ↑

11.2 Component Relationships

Compressor efficiency ↓ → Fuel flow ↑ at constant thrust
Turbine efficiency ↓ → EGT ↑ at constant thrust
Tip clearance ↑ → Turbine efficiency ↓ → EGT ↑
VSV misadjustment → Compressor efficiency ↓ → N2 ↑ at constant EGT

11.3 System Relationships

N2 drives the accessory gearbox
N1 drives the fan (in turbofan engines)
Bleed air extraction → Compressor stability ↓ → Bleed valves close
Oil temperature ↑ → Oil viscosity ↓ → Bearing protection ↓

12. Typical Exam Focus Points

12.1 Component Functions

Candidates must be able to identify the function of:

Fan, compressor stages, diffuser
Combustion chamber and fuel nozzles
NGVs and turbine blades
Exhaust cone and mixer
Accessory gearbox
Thrust reversers

12.2 System Operations

Key operational knowledge:

Ignition system operation (start and relight only)
Fuel control system functions
Oil system components and functions
Bleed air system applications
Anti-icing system requirements

12.3 Inspection and Limits

Critical inspection knowledge:

Damage assessment against AMM limits
Borescope inspection procedures
Reject conditions (cracks, missing tips, TBC spallation beyond limits)
Blending limitations
Torque procedures and conditions

12.4 Troubleshooting Scenarios

Common troubleshooting scenarios:

High EGT with normal fuel flow → Turbine deterioration
High fuel flow with normal EGT → Fuel control issue
Low oil pressure at high thrust → Pump wear
Vibration at specific N1 → Fan imbalance
Slow thrust reverser → Hydraulic issue
Incomplete stow → Misrigging

12.5 Safety Considerations

Critical safety actions:

Immediate shutdown for low oil pressure
Immediate shutdown for EGT exceedance
Inspection after surge events
Investigation of metallic particles in oil system

Summary

Module 15 provides the comprehensive knowledge required for certifying staff to maintain gas turbine engines safely and effectively. The key to success is understanding the interrelationships between engine components, systems, and performance parameters. Maintenance decisions must always be based on AMM limits and procedures, with proper documentation of all findings.

Remember: When in doubt, consult the AMM. When damage exceeds limits, replace. When symptoms indicate potential damage, inspect before further operation.