Ignition
Ignition tests the high-energy system that lights the combustor during start and provides relight capability in flight. Helicopter turbines usually have two igniter plugs and a high-energy exciter.
Core rules
- The exciter stores energy and discharges a high-voltage pulse to the igniter plug.
- Igniter plugs are designed for high-energy, high-pressure operation, not automotive spark plug conditions.
- Ignition is normally selected for start and for continuous ignition in rain, snow, icing or autorotation relight conditions.
- Dual igniters improve light-off reliability; one may be sufficient for start.
- A fouled or worn plug can spark at low pressure but fail at high pressure.
- Ignition checks must follow the maintenance manual; never handle leads with the system live.
Worked example
During start, fuel flow rises, N1 begins to accelerate, but TGT stays low and there is no light-off. The starter and fuel system appear normal. The logical split is: is fuel reaching the nozzles, and is there a spark? Check the igniter plug for fouling and gap, then the exciter output with an approved tester. If the plug sparks outside the engine but not installed, the higher pressure at light-off may be the difference. Replace or clean per the manual, then retest.
Traps
- Assuming both igniters must spark for every start.
- Testing ignition with fuel vapour present.
- Over-gapping or damaging the plug electrode.
- Leaving continuous ignition selected unnecessarily.
- Blaming the exciter before checking the plug.