Section 15.12 / 15.13

Air Systems, Starting & Ignition

Internal cooling/sealing air and clearance control, starters, ignition, the start sequence and start faults.

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Main ideas

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.

Solved examples

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.

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