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:
16.Compression – Ambient air is drawn in and compressed to high pressure
17.Combustion – Fuel is injected and burned at approximately constant pressure
18.Expansion – Hot gases expand through the turbine, extracting energy
19.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:
73.Rotor blades: Add kinetic energy to the air (increase velocity)
74.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:
124.Primary zone: Fuel-air mixture is ignited and burned at near-stoichiometric ratios
125.Intermediate zone: Combustion completes with additional air
126.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:
154.Drive the compressor (via the connecting shaft)
155.Drive the fan (in turbofan engines)
156.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
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.
