EASA Module 14 cheat sheet

Brayton cycle: compression, constant-pressure combustion, expansion, heat rejection; thermal efficiency rises with pressure ratio and turbine entry temperature.

Thrust (turbojet/turbofan): F = ṁ(Vj − V0) + (pj − p0)Aj; the second term is pressure thrust, negligible when the nozzle is correctly expanded.

Specific fuel consumption: TSFC = fuel mass flow / thrust (kg·N⁻¹·h⁻¹); lower is better and it falls as bypass ratio increases at subsonic speeds.

Compressor surge is a reversal of flow often triggered by a stall on one or more blades; symptoms are bang, rising EGT, falling RPM and fluctuating fuel flow.

Compressor ratio per stage is typically 1.2–1.4; axial stages are limited by blade tip speed, while centrifugal compressors give a high single-stage pressure ratio.

Turbine blades operate above the creep limit of the alloy and rely on cooling air from the HP compressor plus thermal barrier coatings; HP turbine entry temperatures exceed 1500 °C in modern engines.

Nozzle behaviour: convergent nozzles choke at pressure ratio ≈1.89; a convergent-divergent nozzle is needed to expand fully above that ratio.

Engine station numbering: 1 inlet, 2/25 compressor entry/exit, 3 combustor exit, 4 turbine entry, 5 turbine exit, 8 exhaust; EGT is measured at station 4.5 or 5.

Four-stroke cycle: intake, compression, power, exhaust - two crankshaft revolutions per cycle; a two-stroke fires once per revolution.

Compression ratio r = (swept + clearance volume) / clearance volume; thermal efficiency ≈ 1 − r^(1−γ) for the ideal Otto cycle.

Volumetric efficiency = actual air inducted / theoretical swept volume; falls with throttle restriction, high RPM and high intake temperature.

Detonation (knock) is spontaneous ignition of end gas; causes are high compression, lean mixture, high inlet temperature and low octane. Pre-ignition occurs before the spark and is not the same phenomenon.

Mixture ratio: stoichiometric for petrol ≈ 14.7:1 by mass; best power ≈ 12:1, best economy ≈ 16:1; excessively lean raises CHT and can burn valves.

Power = P × L × A × N for a four-stroke (N = cycles per second = RPM/120); brake mean effective pressure links torque to displacement.

Magneto timing is set in degrees before top dead centre; both magnetos must be checked individually during run-up because a dead magneto hides in the BOTH position.

Diesel compression ratios are 14:1 to 23:1 because ignition relies on compression temperature exceeding the fuel's auto-ignition point.

Fuel system: LP pump → filter → HP pump → fuel control unit / hydro-mechanical or FADEC metering → fuel nozzles; the FCU schedules fuel against N2 and throttle position.

FADEC is a dual-channel digital control with full-authority scheduling of fuel and variable geometry; it provides overspeed and over-temperature protection.

Oil system functions: lubrication, cooling, cleaning and corrosion protection; a dry-sump engine uses a pressure pump plus scavenge pumps and a separate tank.

Ignition: high-energy capacitor discharge igniters (typical 15–20 kV) are used for turbine start; only one igniter is normally needed in flight.

Starting sequence interlocks: ignition on, HP fuel cock open, starter engaged; a hot start is indicated by EGT rising beyond the limit before light-up, a hung start by stagnation below idle N2.

Bleed air is taken from HP or LP compressor stages for cabin pressurisation, anti-icing and starting; excessive bleed reduces thrust and raises EGT.

Fire detection uses resistance or gas-filled continuous loops; a fire bottle discharge is a one-shot action, so the correct bottle and timing must be selected.

Thrust reversers: cascade, bucket-target and cold-stream types; a mechanical or hydraulic lock plus EEC logic prevents in-flight deployment.

Blade element theory: thrust and torque come from lift and drag on each section; a section's angle of attack is the blade angle minus the helix (advance) angle.

Geometric pitch is the theoretical forward distance in one revolution; effective pitch is less because of slip; slip = geometric pitch − effective pitch.

Blade angle increases from root to tip so that every section meets the relative airflow at a similar angle of attack, since tangential speed rises with radius.

Fixed-pitch propellers are a compromise: coarse pitch suits cruise, fine pitch suits take-off and climb; a constant-speed unit varies pitch to hold selected RPM.

Constant-speed propeller: a governor uses flyweights and a speeder spring to port oil to or from the pitch-change mechanism; loss of oil pressure usually drives blades to fine or feathered, depending on design.

Feathering reduces drag after engine failure; reverse pitch increases blade angle past the fine pitch stop to give negative thrust on the ground.

Propeller balancing: static balance corrects the centre of gravity, dynamic balance corrects vibration in rotation; a tracking check compares blade tip paths.

Propeller efficiency = thrust power / shaft power; it peaks around a blade angle of attack of 2–4° and falls at very high tip Mach numbers due to compressibility.