FAA Aviation Mechanic Powerplant (AMP)Ignition and Starting SystemsMedium
A turbine engine uses annular-gap igniter plugs instead of the massive-electrode plugs common on reciprocating engines. What design feature allows these igniters to fire reliably even when fouled with carbon or fuel residue?
- AA semiconductive material between the electrodes allows a small ionizing current to establish a conductive path before the capacitor fully discharges
- BA massive tungsten center electrode resists oxidation and contamination
- CThe plug operates on continuous low-voltage AC current instead of a pulsed discharge
- DA wide air gap reduces the voltage required to produce a spark
Show answer & explanationAnswer & explanation
Correct answer: A. A semiconductive material between the electrodes allows a small ionizing current to establish a conductive path before the capacitor fully discharges
Annular-gap igniter plugs used with capacitor discharge exciters have a semiconductive material bridging the gap. A small ionizing current flows through this material, creating a low-resistance path that triggers a very high-intensity spark discharge from the capacitor even through fuel or carbon contamination.
Why the other options are wrong
- B. Reciprocating spark plugs, not turbine igniters, typically use massive electrodes.
- C. Turbine igniters use pulsed capacitor discharge, not continuous AC.
- D. A wide gap would increase, not decrease, required firing voltage.
Annular-Gap Igniter Plug
A turbine engine igniter with a semiconductive material bridging its electrode gap, allowing a high-intensity capacitor discharge spark even when contaminated.
- Used with capacitor discharge (CD) exciters
- Semiconductive material ionizes to trigger discharge
- Produces very high energy, short-duration spark
- Resistant to fouling from fuel/carbon deposits
Memory trick: Semi-conductor = 'semi-open door' letting the spark jump through grime.