Starting & Ignition Systems
Pneumatic and electric starting, high-energy capacitor-discharge ignition, the start sequence and ignition-system safety.
Starting & ignition systems — summary notes
Free- Most large turbine engines use an AIR-TURBINE (pneumatic) starter, supplied from the APU, a ground cart or a cross-bleed from a running engine; a starter control (start) valve admits the air, and the start sequence is managed by the EEC. Smaller engines and APUs use an electric starter or starter-generator.
- Turbine ignition uses a HIGH-ENERGY capacitor-discharge system: an ignition exciter charges a capacitor and dumps it as a powerful spark at the igniter plug (a surface-discharge plug), needed only to light the engine — combustion is then self-sustaining.
- There are normally two independent ignition systems (two exciters/igniters) for reliability; ignition is armed automatically for start and can be selected CONTINUOUS for flame-out protection (icing, heavy rain, take-off and landing).
- The start sequence (systems view): starter drives N2 → ignition on → fuel on at light-up speed → EGT rises → engine accelerates to idle → starter and ignition off. Faults the systems must protect against are the hot start (EGT exceedance), hung start (stalls below idle) and wet start (no light-up).
- SAFETY: the exciter stores a LETHAL electrical charge. Before handling igniter leads or plugs, switch off the system and wait the specified time (and discharge as instructed) for the capacitor to bleed down; also respect pneumatic-starter duct pressure/heat and the running/rotating hazards.
- ⚠ Exam trap: turbine ignition is HIGH-ENERGY capacitor-discharge storing a lethal charge — observe the safety wait/discharge time before touching igniters; large engines are normally PNEUMATICALLY (air-turbine) started, and ignition is switched off once the engine is self-sustaining.
Before disconnecting an igniter lead after ground runs, what safety step is essential and why?
The ignition exciter is a high-energy capacitor-discharge unit that stores a potentially lethal charge even after the system is switched off. You must switch off and de-energise the ignition system and wait the manufacturer's specified safety time (and follow any discharge procedure) so the capacitor bleeds down before touching the igniter lead or plug — otherwise a stored discharge could cause a fatal shock.
Starting & ignition concept map
FreeStarting & Ignition Systems
Starting & ignition systems quiz
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