Ignition and Starting Systems
What this domain tests
Starting questions test the sequence, igniter and exciter operation, starter types, and recognition of hot, hung, wet and no-light-off starts.
Core rules
- Ignition uses high-energy spark igniters fed by an exciter box. Some engines have two igniters, one for start and both for continuous ignition in rain or turbulence.
- Starters are air turbine (bleed air), electric, or cartridge. The starter turns N2 (or N1 on some engines) through the accessory gearbox.
- Start sequence: dry motoring to purge and establish airflow, ignition on, fuel on, light-off, EGT peak, acceleration to idle, starter cut-out.
- Malfunctions: hot start (EGT exceeds limit, often from too much fuel or low airflow), hung start (N2 stabilises below idle), wet start (fuel without light-off), no light-off (ignition or fuel problem).
Worked example
During a start, EGT peaks at 820°C against a limit of 800°C, then falls. The start is a hot start exceedance. Record peak EGT, duration, OAT and altitude, shut down if EGT continues rising, and report to maintenance. Reasoning: EGT exceedance can damage turbine blades even if the engine later runs normally, so it is a reportable event, not just a gauge observation.
Traps
- Leaving continuous ignition on for normal cruise; it shortens igniter life.
- Confusing a hung start (N2 stalls) with a hot start (EGT overshoots).
- Cranking too long and overheating the starter; respect duty cycles.
- Assuming starter cut-out means the start is complete.
Key points
- High-energy igniters and an exciter box; continuous ignition for rain or turbulence.
- Start sequence: motoring, ignition, fuel, light-off, EGT peak, idle.
- Hot start means EGT overshoot; hung start means N2 stalls below idle; wet start means fuel without light-off.
- Respect the starter duty cycle.
- Record and report EGT exceedances.