Gas turbine engines

021 Aircraft General KnowledgeChapter 4

Basic principles

Key fact

	Piston	Gas turbine
Process	Intermittent	Continuous
Combustion	Constant volume	Constant pressure
Thermal peak	Cylinder	Combustion chamber

Static thrust = zero when aircraft stationary (V₁ = 0). Noisiest part: jet efflux. Ram air increases engine efficiency with forward speed.

Formula — Thrust and bypass ratio

T = m(VJ − VV) + A(pJ − p0)
Bypass ratio = Bypass mass flow ÷ HP compressor mass flow
HP compressor mass flow = Inlet mass flow − bypass mass flow
EPR = Exhaust pressure (Pt8) ÷ inlet pressure (Pt2)

Key fact

Turbojet: Small mass flow, large acceleration — high fuel consumption, obsolete for transport. Turbofan: Large mass flow, small acceleration — ~85% thrust from cold stream. Turboprop/turboshaft: Almost all air through core drives prop/rotor — pseudo-BPR very high.

Propulsive efficiency: Turboprop best at low speed; turbojet at high speed; high-BPR turbofan best in mid subsonic cruise (typical airline range).

Spools and terminology

Key fact

N1 = low spool (fan + LPC + LPT). N2 = high spool (HPC + HPT). Gas generator = intake + compressor + combustion chamber. Free turbine: No mechanical link to compressor; drives output shaft (prop/rotor).

Thrust setting parameters: EPR, N1, IEPR (manufacturer-dependent). Single indicator failure usually not a no-go item.

Compressors

Key fact

Axial flow: Many stages, ratio ~35:1 total (~1.2:1 per stage); small frontal area, high mass flow; expensive, FOD-vulnerable, prone to stall/surge. Centrifugal: Ratio ~12:1 (~5:1 per stage); robust, simple; large diameter, lower mass flow.

Compressor stall: Excessive AoA on rotor blades — low RPM + low inlet velocity, or rear-stage stagnation at high RPM. Indications: ↑EGT, vibration, erratic RPM, banging, yaw. Surge: Stall across all stages — flames at inlet and exhaust.

Exam trap

Variable IGV/VSV: Closed at low RPM/high OAT to maintain AoA; fewer required on 3-spool engines. Interstage bleed valves: Open on start — vent excess air. Variable bypass valve: Prevents LPC stall; opens on deceleration and thrust reverse.

Stall vs surge: Stall limited to 2–3 stages; surge is full compressor stall.

Combustion and turbine

Key fact

Primary air (~20%): Combustion at fuel nozzles. Secondary air (~80%): Cooling (~60%) + flame stabilisation (~20%). Swirl vanes: Reduce axial velocity, enhance mixing — do not increase pressure at nozzles.

Can type: Two igniters (interconnected chambers). Annular: Single flame tube, two igniters, many fuel nozzles — better thermal load distribution on HP turbine, weight saving.

Key fact

Impulse turbine nozzle: V↑ P↓ T↓. Impulse rotor: V↓ P constant T constant. Reaction rotor: V↑ P↓ T↓.

Turbine blade creep: Permanent elongation from tensile stress + high temperature over time — turbine rotors most affected. Tip shrouds increase efficiency. Active clearance control: Cools casing to maintain tip clearance.

Exhaust, reverse thrust, and accessories

Key fact

Exhaust main function: Extract energy from exhaust gas (not primarily acceleration). Convergent nozzle: Increases exit velocity; choked nozzle at cruise adds pressure thrust.

Cold stream reverser: Reverses fan airflow only — cheaper, less reverse thrust. Reverser alert: Doors unlocked with lever stowed, or stowed with lever deployed.

Accessory gearbox (AGB): Driven from HP shaft — generators/CSD, oil pumps, hydraulic pumps, HP fuel pumps, tacho. Not pneumatic thrust reverser (electric/hydraulic/bleed operated).

Engine control and fuel

Key fact

Hydromechanical FCU: Inputs include N2, CDP, CIT, TLA. FADEC: Full authority digital control — no standalone FCU; single-fault tolerant; includes overspeed and EGT protection. FADEC does NOT: modify aircraft speed, counter yaw on engine failure, auto shut-off engine.

Oil system: Scavenge pump volume greater than pressure pump (dry sump). Chip detector/magnetic plug: Impending failure warning. FCOC: Cools oil, heats fuel; internal leak → oil quantity rises; high oil temp indicates heat exchanger failure.

Starting and malfunctions

Key fact

Ground start sequence: Starter → rotation → ignition → fuel. Windmilling: Rotation → ignition → fuel. Most important indication during start: oil pressure.

Malfunction	EGT	Fuel flow	Action
Wet start	No rise	Positive	Shut down, drain, blow-out
Dry start	No rise	No flow	Shut down, maintenance
Hot start	Exceeds limit	—	Shut down
Hung start	High, RPM low	—	Shut down (may become hot start)

Exam trap

Bleed air extraction: ↑TGT/EGT, ↓EPR, ↑fuel flow, ↓thrust/performance. Flat-rated engine: Constant max take-off thrust up to flat-rated OAT; above that, EGT limits further fuel flow so thrust decreases.

Reduced thrust take-off: Low mass, low OAT, high pressure — select higher assumed temperature on FMS to reduce fatigue and maintenance cost.

Turboprop controls

Key fact

Alpha range (flight): Fine to coarse pitch — flight idle to max power. Beta range (taxi): Fine pitch, negative thrust/braking. Torque measured at reduction gearbox — primary power indication with EGT.

RPM increase (gas generator constant): free turbine RPM increases. Power increase: gas generator RPM increases, blade pitch/thrust/EGT increase.

APU

Key fact

APU on ground provides: electricity (3-phase 115 V 400 Hz), bleed air for engine start and air conditioning — not hydraulics. Can only feed bus when no other source active. Protection: high EGT, overspeed, oil pressure/temperature, fire. Substitutes on ground: GPU, air conditioning cart.