14.1 Engines for B2 — turbine, APU, piston, electric/hybrid and FADEC

Scope and exam

Module 14 Propulsion is taken by B2 and by B2L holders of the Instruments or Airframe Systems ratings. EASA paper: 32 questions (UK: 24). 14.1:

• (a) turbojet, turbofan, turboshaft, turboprop — level 1;
• (b) APUs — level 1;
• (c) piston engines — level 1;
• (d) electric and hybrid engines, energy storage and control — level 2;
• (e) electronic engine control and fuel metering (FADEC) — level 2.

Gas turbine engine types (level 1)

All work on the Brayton cycle: intake → compression → combustion (constant pressure) → expansion through turbines → exhaust.

Type	Arrangement	Output
Turbojet	All air passes through the core	Thrust from a high-velocity jet; noisy, efficient only at high speed
Turbofan	Core drives a large fan; most air bypasses the core	Thrust from fan (cold) plus core (hot) streams; bypass ratio = bypass mass flow ÷ core mass flow (modern engines 5:1 to over 10:1)
Turboprop	Turbine drives a propeller through a reduction gearbox	Mostly shaft power (plus small jet thrust)
Turboshaft	Free power turbine drives an output shaft	Shaft power (helicopters, APUs)
Geared turbofan	Gearbox between fan and LP spool	Fan and LP turbine each at optimum speed

Spools: single, twin (N1 low-pressure, N2 high-pressure) or triple spool (N1, N2, N3). The HP spool carries the accessory gearbox drive on most engines and is the one turned by the starter.

APUs (level 1)

A small gas turbine (usually tail-mounted) that provides electrical power (generator) and bleed air (load compressor or bleed from the power section) on the ground and, within limits, in flight. Controlled by an ECB/APU controller (constant speed, automatic start sequence, protective shutdowns for overspeed, low oil pressure, high EGT, fire). Started by an electric starter from the battery or from the AC system.

Piston engines (level 1)

Four-stroke Otto cycle: induction, compression, power, exhaust; two crankshaft revolutions per cycle. Horizontally opposed air-cooled engines dominate light aviation; Diesel (compression ignition, Jet-A fuel) engines are increasingly common. Fuel metering by carburettor or fuel injection; ignition by dual magnetos (self-contained, independent of the aircraft electrical system).

Electric and hybrid propulsion (level 2)

Electric propulsion chain

Energy storage (battery) → power electronics (inverter/motor controller) → electric motor → propeller/fan.

Component	Key points
Motor	Usually permanent-magnet synchronous (brushless) motors — high power density and efficiency; also induction motors. Torque is available from zero rpm
Inverter / motor controller	Converts battery DC to variable-frequency, variable-voltage AC to control motor speed and torque; IGBT or SiC switching devices; cooled by liquid/air
Battery	Lithium-ion cells in series/parallel modules (high-voltage DC bus, hundreds of volts); limited specific energy compared with fuel — the main range limitation
Battery management system (BMS)	Monitors cell voltages, temperatures and currents; balances cells; estimates state of charge (SoC) and state of health (SoH); isolates the pack on faults (contactors)
Thermal management	Cooling loops for battery, motor and inverter; thermal runaway containment
Controls and indication	Power lever → controller commands torque; displays show SoC, power, temperatures, remaining endurance

Hybrid configurations

Configuration	Principle
Series hybrid	A combustion engine drives a generator only; all propulsion through electric motors
Parallel hybrid	Combustion engine and electric motor both drive the propulsor mechanically (motor boosts at take-off/climb)
Turbo-electric	Gas turbine + generator powering distributed electric fans, little or no battery
Electric power augmentation	Electric motor adds power to a conventional engine for short periods

High-voltage safety: isolate and lock out the HV system, wait for capacitors to discharge, verify absence of voltage with rated equipment, use insulated tools and PPE; orange cabling identifies HV circuits in many designs.

Electronic engine control — FADEC (level 2)

• FADEC = Full Authority Digital Engine Control: an EEC/ECU with two independent channels (A and B), one active and one standby; they alternate between flights and cross-check each other.
• Has full authority over fuel flow (no mechanical back-up); controls the fuel metering unit (FMU) via a torque motor/servo valve and senses metering-valve position with resolvers/LVDTs.
• Own power supply: a dedicated permanent-magnet alternator on the accessory gearbox powers the EEC once the engine runs; aircraft power is used for start and as back-up.
• Inputs: thrust-lever angle (resolvers), N1/N2 speeds, temperatures and pressures (T2/P2, T3/P3, EGT), air data from the aircraft, bleed demands.
• Outputs/functions: thrust management (N1 or EPR mode), automatic start sequence with protection (hot/hung start abort on ground), limit protection (overspeed, over-temperature), variable stator vanes, bleed valves, active clearance control, oil/fuel heat management, thrust reverser control, idle selection (flight/approach/ground idle).
• Engine rating/identity plug on the engine tells the EEC the thrust rating and trim data, so the EEC can be swapped between engines.
• Faults are recorded and reported to the maintenance system; dispatch with one channel failed is defined in the MEL.

Key fact: A FADEC engine has no hydromechanical back-up — loss of both channels results in the engine going to a fail-safe condition (typically shutdown or a fixed fuel flow, type-dependent).

Exam traps

• Bypass ratio = bypass air mass ÷ core air mass.
• APU = gas turbine for electrical power and bleed air.
• FADEC = two channels, own alternator, identity plug.
• In a series hybrid the combustion engine drives only a generator.