Starting & Ignition Systems
Pneumatic and electric starting, high-energy capacitor-discharge ignition, the start sequence and ignition-system safety.
Starting & ignition systems, summary notes
- 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.
Turbine engine start sequence
Starting & ignition concept map
Starting & Ignition Systems
Starting & ignition systems quiz
Starting & Ignition Systems, quiz
1. Most large turbine engines use a:
Pneumatic (air-turbine) starterHand crankHydraulic ram starter2. Turbine ignition is a:
High-energy capacitor-discharge systemLow-energy magneto systemGlow-plug system3. The main safety hazard of a turbine ignition system is:
A lethal stored electrical charge in the exciterExcess oilHigh fuel pressure only4. Before handling igniter leads you must:
Switch off and wait the specified time to dischargeDo nothing specialIncrease ignition power5. Once the engine is self-sustaining, ignition is:
Switched offLeft continuously on alwaysIncreased6. Continuous ignition is selected:
For flame-out protection (icing, rain, T/O and landing)Only during shutdown, so that any residual fuel in the combustor is burnt off safelyNever in normal service, being reserved for engine ground running and test cell work7. Turbine engines normally have:
Two independent ignition systems for reliabilityOne system onlyNo ignition system8. The start valve controls:
Air to the pneumatic starterFuel to the burnersOil to the bearings