Part-66 Module 15: gas turbine engine essentials

Module 15 is 92 multiple-choice questions in 115 minutes, covering turbojets, turbofans, turboprops, turboshafts and APUs.

Fundamentals and performance

• Gas turbines follow the Brayton cycle: compression, combustion at nearly constant pressure, expansion, exhaust.
• Net thrust = mass flow × (jet velocity − flight velocity). Gross thrust ignores ram drag; the two are equal only when stationary.
• High-bypass fans move a lot of air slowly: high propulsive efficiency, low noise. TSFC = fuel flow ÷ thrust.
• Thrust falls with higher temperature, altitude and bleed extraction. Flat rating holds take-off thrust up to a set temperature. EGT margin shrinks as the engine deteriorates.

Inlet and compressor

• Subsonic intakes are divergent, turning velocity into pressure.
• An axial stage (rotor + stator) gives about 1.1–1.3:1; the annulus narrows towards the rear. Centrifugal stages give about 4–5:1 but are large in diameter.
• Stall is blade-level flow breakdown; surge is flow reversal through the whole compressor. Prevented by bleed valves, variable vanes and multiple spools.
• FOD nicks are blended within limits; compressor washing restores EGT margin.

Combustion, turbine and exhaust

• Combustors are can, can-annular or annular. Only about a quarter of the air burns; the rest cools and dilutes. Faulty nozzles cause hot streaks.
• NGVs accelerate and turn the gas onto the blades. Turbine blades use fir-tree roots, nickel superalloys, internal air cooling and thermal barrier coatings.
• Creep stretches blades; discs are life-limited in cycles because their failure cannot be contained.
• Subsonic nozzles are convergent. Reversers: cascade with blocker doors (high bypass), clamshell (turbojets), pivot doors, all with multiple locks.

Bearings, oil and fuel

• Each spool has one ball bearing (thrust) and roller bearings (radial load, allow expansion), sealed by air-pressurised labyrinth, carbon or brush seals.
• Oil is synthetic ester; never mix brands unless approved. Scavenge pumps out-pump the pressure pump. The FCOC cools oil and warms fuel.
• Monitoring: filter bypass indicators, chip detectors, SOAP and consumption trends. Check level within the set time after shutdown and secure the cap.
• Jet A-1 freezes at about −47 °C; FSII prevents ice crystals.

Fuel control, air and starting

• FADEC: two channels, its own alternator, a rating plug on the engine. It schedules fuel for acceleration (no surge) and deceleration (no flameout) and limits speed and temperature.
• The HP fuel pump is a gear pump; surplus fuel returns to its inlet. Fuel often powers VSV and bleed actuators.
• The internal air system cools turbines, seals bearings and balances thrust loads. Service bleed passes a pre-cooler and PRSOV.
• Large engines use air turbine starters; fuel is introduced at about 20–25% N2. Hot start: EGT over limit. Hung start: lights but stops accelerating. Wet start: no light; dry-motor to clear. Ignition units store lethal energy.

Indication, augmentation, turboprops

• EGT: chromel-alumel thermocouples in parallel. N1/N2: magnetic speed sensors. Thrust is set by N1 or EPR; turboprops use torque.
• Afterburners use spare oxygen in the exhaust with flame holders and a variable nozzle. Water/methanol injection adds mass flow.
• Turboprops: free turbine (PT6, reverse flow) or fixed shaft (NTS drives towards feather). Beta range is ground pitch control.

APU, installation, fire, monitoring

• The APU runs at governed speed and shuts down automatically for overspeed, overtemperature, low oil pressure or fire.
• Firewalls contain fire; fuse pins let the engine break away without rupturing the tank.
• Fire detection: dual continuous loops, often in AND logic. The fire handle shuts off fuel, hydraulics and bleed and arms the squibs. Red disc = thermal discharge, yellow = normal discharge.
• Borescope, trend monitoring, power checks and preservation keep engines on condition.