FAA Aviation Mechanic Powerplant (AMP)Ignition and Starting SystemsMedium
A turbine engine uses a combined starter-generator unit on the accessory gearbox. How does this component function during a normal engine start and after start?
- AIt functions as a starter throughout the entire flight and is disengaged only for shutdown
- BIt functions as a starter to crank the engine, then automatically converts to a generator once the engine reaches self-sustaining speed
- CIt functions only as a starter and a separate generator is engaged once the engine is running
- DIt functions as a generator only, with a separate pneumatic starter used for cranking
Show answer & explanationAnswer & explanation
Correct answer: B. It functions as a starter to crank the engine, then automatically converts to a generator once the engine reaches self-sustaining speed
A starter-generator is a single dual-purpose unit: during start it receives electrical power to motor the engine, and once the engine reaches self-sustaining RPM the same unit automatically switches roles to generate electrical power for the aircraft.
Why the other options are wrong
- A. It does not remain in starter mode after the engine self-sustains.
- C. A separate generator is not needed since the same unit converts function.
- D. Starter-generators combine both functions in one unit, not two separate units.
Starter-Generator
A single electrical machine mounted on the accessory gearbox that cranks the turbine engine during start and automatically switches to generate electrical power once the engine reaches self-sustaining speed.
- Saves weight versus separate starter and generator units
- Switch-over is typically automatic via a start control unit
- Common on many turboprop and turbojet/turbofan engines
Memory trick: Starter-generator = 'Two jobs, one motor' — cranks then charges