Module 14: Propulsion (B2) — Study Material

1. Overview of Module 14

This module covers the fundamental principles, construction, operation, and maintenance of aircraft propulsion systems, with a specific focus on the systems and components relevant to the B2 avionics technician. The syllabus is divided into key areas that build upon each other:

14.1 Fundamentals: Engine construction, basic gas turbine theory, engine performance parameters, and the layout of major sections.
14.2 Engine Control Systems: From hydromechanical units to Full Authority Digital Engine Control (FADEC), including sensors, actuators, and control laws.
14.3 Engine Instrumentation & Indicating Systems: The measurement, transmission, and display of critical engine parameters like N1, N2, EGT, fuel flow, and oil pressure.
14.4 Engine Systems & Protection: Air, lubrication, fuel, ignition, and fire protection systems.
14.5/14.6 Engine Systems & Operation: Thrust reversers, bleed air systems, and the Auxiliary Power Unit (APU).
14.7 Propellers: Fundamentals of propeller operation, including control systems like constant speed units and synchrophasing.

The B2 technician's role is heavily focused on the electrical, electronic, and digital aspects of these systems. This material synthesises the core knowledge required for the Part-66 B2 examination, focusing on system interfaces, signal processing, fault diagnosis, and the interaction between mechanical components and their electronic controls.

2. Key Concepts Explained in Detail

2.1 Gas Turbine Engine Fundamentals

The gas turbine engine operates on the Brayton cycle. Air is drawn in, compressed, heated by combustion, and expanded through a turbine to produce thrust. The main sections, in order, are:

Air Inlet (Intake): Directs air into the compressor with minimal pressure loss and distortion.
Compressor: Increases air pressure. It can be a single spool or split into multiple spools (e.g., LP and HP) for higher efficiency. Variable Inlet Guide Vanes (VIGV) and Variable Stator Vanes (VSV) are used to prevent stall and surge at low RPM by matching airflow angles to rotor blades.
Combustion Chamber: Fuel is injected and burned, adding heat energy to the air.
Turbine: Extracts energy from the hot gas to drive the compressor and fan. The turbine is also where Exhaust Gas Temperature (EGT) is measured.
Exhaust: The remaining gas is expelled to produce thrust.

Turbofan Engines: In a turbofan, a large fan (part of the LP spool) accelerates a large mass of air through the bypass duct. This bypass air produces most of the thrust. The core flow passes through the compressor, combustor, and turbine. The N1 speed is the rotational speed of the LP spool (fan), and N2 is the speed of the HP spool. N1 is a primary thrust indication.

2.2 Engine Instrumentation and Indicating Systems

The B2 technician must understand how engine parameters are measured and converted into usable data.

Speed Sensing (N1, N2): Typically measured by magnetic pick-ups. A toothed wheel (phonic wheel) rotating near a coil generates an AC signal with a frequency proportional to engine speed. The signal conditioning unit converts this frequency to a digital value for display and for use by the EEC. If the pick-up coil short-circuits, the voltage output drops to zero, resulting in a low or zero speed indication. If the sensor fails and produces no signal, the frequency is zero, and the display will show zero.
Exhaust Gas Temperature (EGT): Measured by thermocouples. A thermocouple generates a small voltage proportional to temperature (Seebeck effect). Multiple probes are often connected in parallel to provide an average reading. A high EGT reading is a critical warning of thermal distress, often caused by an actual engine condition (e.g., overtemperature event) or a faulty indicator. An open circuit in a thermocouple typically causes a low reading, while a short circuit can cause erratic readings.
Oil Pressure: Measured by a pressure transmitter. A common type is a variable resistance transmitter. The resistance changes with pressure, affecting the current in the indicating circuit. A short to ground in this type of transmitter would cause the resistance to drop to zero, resulting in a zero reading on the indicator. An open circuit would typically cause a full-scale or high reading, depending on circuit design.
Signal Conditioning: Modern systems use transmitters with standardised outputs, such as 4-20 mA. This is a linear system where 4 mA represents zero pressure and 20 mA represents full scale. For example, a reading of 12 mA is 8 mA above the zero point, which is 50% of the 16 mA range, indicating 50% of full-scale pressure.
Engine Vibration Monitoring Unit (EVMU): This unit processes analog signals from accelerometers mounted on the engine. Its primary function is to filter, process, and convert these signals into digital data for the engine indication system and health monitoring. It does not physically dampen vibrations. A cracked sensor mounting bracket can cause inaccurate readings and must be rectified.

2.3 Full Authority Digital Engine Control (FADEC)

The FADEC is the electronic brain of the modern engine. Its core component is the Electronic Engine Controller (EEC).

Function: The EEC automatically manages the engine, including fuel metering and variable geometry, to achieve the desired thrust and efficiency. It receives inputs from various sensors and commands actuators.
Key Inputs:
Thrust Lever Angle (TLA): The pilot's demand.
T2 (Inlet Total Temperature): Used to calculate thrust settings and correct for air density variations.
P2 (Inlet Pressure): Used for engine pressure ratio (EPR) calculations.
N1, N2: Rotor speeds.
Turbine Temperature (TGT/ITT): For limiting and monitoring.
Key Outputs & Actuators:
Fuel Metering Valve (FMV): The EEC commands the FMV via an electrical signal. The valve position is sensed by an LVDT (Linear Variable Differential Transformer) and fed back to the EEC, forming a closed-loop control loop. If the torque motor controlling the FMV fails, the valve is typically spring-loaded to a safe position (e.g., minimum fuel flow) to allow safe engine operation.
Variable Stator Vane (VSV) Actuator: Position is fed back to the EEC via an LVDT. If the LVDT fails, the FADEC uses a default schedule or last valid position to maintain safe operation.
Control Laws & Modes:
Normal Mode: The EEC uses all available sensors for full-authority control (e.g., EPR mode).
Alternate Mode: If a sensor fails (e.g., EPR), the EEC degrades to a simpler control law, typically using N1 as the primary controlled parameter. The engine remains automatic but with reduced performance and operational limitations.
Backup Mode: In some systems, if the electronic control is lost, the hydromechanical unit (HMU) can revert to a mechanical backup mode providing a predetermined fuel schedule for safe operation.
Dual-Channel Architecture: FADEC systems are typically dual-channel (A and B). One channel is active, and the other is in standby. If a sensor failure is detected in the active channel, the system automatically switches control to the other channel to maintain full functionality. The failure is stored in non-volatile memory (NVM) for maintenance action. If a channel fails a self-test, the first step is to reset the EEC and re-run the test to determine if the failure is intermittent. If the fault is specific to one channel's sensor supply, a wiring or connector issue between the sensor and that channel is likely.
Fail-Safe Philosophy: FADEC systems are designed to be fail-safe. If a TLA resolver fails out-of-range, the EEC typically uses the last valid signal or a predefined default value to maintain a safe thrust level.

2.4 Engine Systems

Fuel Systems: The fuel pump provides pressurised fuel to the Fuel Control Unit (FCU), which meters fuel to the combustion chamber. Fuel heaters prevent ice formation that can block filters. A turbine flow meter is used to measure fuel flow; air bubbles in the fuel can cause fluctuating readings. Fuel leaks are detected using UV dye and a UV lamp.
Ignition Systems: Igniter plugs produce a high-energy spark to ignite the fuel-air mixture during start and in-flight relight. The system uses exciters to generate the high voltage. Igniter plug leads have a specific low resistance; an infinite resistance indicates an open circuit, requiring replacement. 'Cross-ignition' or 'alternate ignition' selection allows either exciter to be connected to either igniter plug, providing flexibility and redundancy. The correct torque for igniter plugs is specified in the engine maintenance manual (EMM).
Lubrication Systems: The oil system lubricates and cools engine bearings and gears. Oil coolers maintain oil temperature within limits, using fuel or air as the cooling medium.
Bleed Air Systems: Bleed air is extracted from the compressor and used for various aircraft systems, including air conditioning and anti-icing. The High-Pressure Bleed Valve (HPV) opens to supply bleed air from a later, higher-pressure stage when the normal (low-pressure) bleed source is insufficient (e.g., at low power or high demand).
Thrust Reversers: These redirect exhaust (or fan) airflow forward to provide reverse thrust, aiding deceleration on landing. They are actuated by hydraulic actuators controlled by solenoid valves. The system uses proximity sensors to indicate the locked/unlocked position. If the 'REVERSER UNLOCKED' light remains illuminated after deployment, it usually indicates a sensor feedback problem, such as a misadjusted proximity sensor or a wiring fault. If the reverser deploys but will not stow, a faulty feedback sensor may be preventing the system from completing the stow cycle.
Fire Protection: Fire detection systems use sensors (e.g., thermal, rate-of-rise, continuous loop) to detect fire/overheat and provide warnings. A continuous loop pneumatic detector contains a gas-filled tube; when heated, the gas expands, closing an electrical switch. Thermocouple detectors generate a voltage when heated. Fire extinguishing systems use fire bottles with squibs; if the squib circuit is open, the bottle will not discharge, and the discharge indicator will not illuminate. Built-in test functions simulate a fire condition without applying actual heat.
Auxiliary Power Unit (APU): A small gas turbine that provides electrical and pneumatic power on the ground and in flight. An over-temperature condition during acceleration is commonly caused by an excessively rich fuel/air mixture, which could be due to a faulty fuel control unit.

2.5 Propellers

Constant Speed Unit (CSU): Automatically adjusts propeller blade pitch to keep RPM constant despite changes in engine power and airspeed.
Feathering: Sets the propeller blades edge-on to the airflow, reducing drag in the event of an engine failure.
Synchrophasing: Adjusts the relative phase of propellers to minimise noise and vibration in the cabin.

3. Important Formulas, Regulations, and Procedures

3.1 Key Formulas

4-20 mA Transmitter Output: % of Full Scale = (Measured Current - 4 mA) / (20 mA - 4 mA) × 100
Frequency-to-Speed: For magnetic pick-ups, speed is directly proportional to the frequency of the generated AC signal: Speed (RPM) = (Frequency (Hz) × 60) / Number of Teeth on Phonic Wheel

3.2 Regulations and References

Regulation (EU) No 1321/2014, Annex III (Part-66): Defines the requirements for certifying staff. Module 14 is the basis for the B2 propulsion syllabus.
Part-145.A.40: Mandates that maintenance must be performed in accordance with approved data, including the Aircraft Maintenance Manual (AMM) and Engine Maintenance Manual (EMM).
Continuing Airworthiness Instructions: These are the manufacturer's instructions for continued airworthiness, including maintenance, repair, and overhaul procedures. They are the primary source for allowable damage limits and troubleshooting procedures.

3.3 Key Procedures

Borescope Inspection: A visual inspection of internal engine components (e.g., compressor, turbine, combustion chamber) using a specialised scope. Findings must be compared against the EMM's allowable damage limits. Damage not explicitly covered requires engineering evaluation or consultation with the manufacturer.
Engine Ground Run: A functional test of the engine after maintenance. Certifying staff must hold an appropriate type rating for the engine to perform certification tasks, including engine ground runs.
EEC Installation: After installing a new EEC, it must be configured with the correct software and engine-specific data (e.g., rating, trim) as per the AMM before engine start.
Engine Trim: The process of adjusting the fuel control unit to achieve correct maximum and idle RPM. The procedure typically follows a sequence: maximum RPM, then idle RPM, then acceleration check.
Fault Isolation: The process of identifying the root cause of a fault. For FADEC systems, this involves following the approved troubleshooting procedure in the AMM, which may include checking wiring, sensor resistance, or signal output. Intermittent faults must be investigated via approved procedures; replacing components without confirming the fault is contrary to Part-66 and Part-145 philosophy.

4. Common Relationships Between Concepts

N1 and Thrust: In a turbofan, N1 (fan speed) is the primary thrust indication. The EEC uses N1 as a control parameter in alternate mode.
EGT and Turbine Health: EGT is a direct indicator of turbine thermal stress. High EGT or EGT exceedances are recorded by the engine monitoring system and are a primary indicator of overtemperature conditions.
FADEC and Actuators: The EEC forms closed-loop control systems with its actuators. The FMV and VSV actuators have position feedback sensors (LVDTs) that are essential for accurate control. A failure in the feedback loop will cause the EEC to revert to a default schedule.
Sensor Failure and FADEC Response: The FADEC's response to a sensor failure is designed to be fail-safe. It may switch channels, use a default value, or degrade to a simpler control law, but it will not shut down the engine unless the failure is critical.
Thrust Reverser and Sensors: The thrust reverser system relies on proximity sensors to confirm the locked and unlocked positions. A discrepancy between the physical position and the sensor indication points to a fault in the sensor, wiring, or the EEC input.
Bleed Air and Engine Power: The HPV opens to supplement bleed air from the LP stage when the engine is at low power or when demand is high, ensuring adequate pressure for aircraft systems.

5. Typical Exam Focus Points

System Fundamentals: The correct order of engine sections, the primary function of the fan, the purpose of a thrust reverser, and the function of a CSU.
Instrumentation: Understanding what N1, N2, and EGT indicate. The effect of open and short circuits on thermocouple and magnetic pick-up systems. The linear relationship of 4-20 mA transmitters.
FADEC Architecture: The role of the EEC, the function of the FMV and its feedback sensor, the purpose of T2, and the response to sensor failures (e.g., TLA, LVDT). The concept of dual-channel control and the response to a channel failure.
Troubleshooting Logic: The correct first step in a troubleshooting procedure (e.g., checking wiring, verifying hydraulic pressure, following the AMM). The importance of comparing findings against the EMM's allowable damage limits.
Safety Procedures: The critical safety checks before actuating a thrust reverser (e.g., ensuring mechanical locks are engaged) and the importance of bonding and grounding during fuel system maintenance.
Maintenance Practices: The correct action for an open igniter plug lead, the correct torque for igniter plugs, and the need to configure a new EEC with the correct software.
Specific Systems: The operation of a continuous loop pneumatic fire detector, the purpose of a fuel heater, and the function of the HPV.