Air Systems, Starting & Ignition
Internal cooling/sealing air and clearance control, starters, ignition, the start sequence and start faults.
Air systems, starting & ignition, summary notes
- Internal air systems use compressor bleed air for engine services: cooling turbine discs and blades, pressurising/sealing bearing sumps, balancing axial thrust loads on the bearings, and anti-icing the nose cowl and inlet.
- Active clearance control (ACC) sprays fan/compressor air onto the turbine casing to shrink it and keep blade-tip clearances small for efficiency, opening the clearance during transients to avoid rubs.
- Starting spins the HP compressor up to a self-sustaining speed while fuel and ignition are introduced. Starters are usually air-turbine (pneumatic, fed from an APU, ground cart or cross-bleed) or electric on smaller engines.
- Ignition uses high-energy igniter plugs fired by an ignition exciter, needed for start and selectable as continuous for protection against flame-out (icing, heavy rain, take-off/landing). Once lit, combustion is self-sustaining and ignition is switched off.
- The start sequence is: starter → rotation (N2) → ignition on → fuel on at light-up speed → engine accelerates to idle → starter and ignition off. Faults include the hot start (EGT exceedance), hung start (stagnates below idle) and wet/no start (no light-up).
- ⚠ Exam trap: once lit, a gas turbine's combustion is self-sustaining, so ignition is normally switched OFF after start; a HOT start is an EGT over-temperature during start (too much fuel / too little airflow), a HUNG start stalls below idle RPM.
During start the EGT rises rapidly toward its limit while N2 hangs well below idle. What is happening and what is the immediate action?
This is a hot/hung start, the engine has lit but is not accelerating (weak starter, low starting air, or a fuel-schedule fault), so combustion heat is not being carried away by enough airflow and EGT climbs. The immediate action is to close the HP fuel cock to cut fuel and abort the start, motoring the engine to cool and clear it before investigating.
Air, starting & ignition concept map
Air Systems, Starting & Ignition
Air systems, starting & ignition quiz
Air Systems, Starting & Ignition, quiz
1. During a gas turbine start, the starter is disengaged:
Once the engine is self-sustaining above idle-approach speedImmediately after light-upOnly after the engine reaches maximum RPM2. A hung start is when the engine:
Lights up but stagnates below idle RPMFails to light at allExceeds its temperature limit rapidly3. Internal cooling and sump-sealing air is taken from the:
Compressor (bleed air)ExhaustOil tank4. Active clearance control (ACC) adjusts turbine-tip clearance by:
Spraying air on the casing to shrink/grow itMoving the bladesAdding fuel5. A common gas-turbine starter is the:
Air-turbine (pneumatic) starterHand crankHydraulic ram6. Once the engine is lit, ignition is normally:
Switched off (combustion is self-sustaining)Left on alwaysIncreased to maximum7. Continuous ignition is selected:
For protection against flame-out (icing, heavy rain, T/O)Only while the aircraft is on the ground, since airborne relight is handled automaticallyNever in normal service, being reserved for engine ground running and test cell work8. A HOT start is:
An EGT over-temperature during startA start below freezingA normal start