FAA Aviation Mechanic Powerplant (AMP)Turbine EnginesMedium
A ground crew connects an external air cart to start a turbine engine. Which statement best describes how a pneumatic (air turbine) starter functions?
- AIt uses aircraft hydraulic system pressure to rotate the low-pressure spool during start
- BIt converts compressed air energy into mechanical rotation through a small air turbine, which drives the engine spool through reduction gearing and an automatic disengaging clutch
- CIt uses direct current supplied by the aircraft battery to spin an electric motor coupled to the compressor shaft
- DIt ignites a small fuel-air charge in an internal combustion chamber to directly rotate the starter shaft
Show answer & explanationAnswer & explanation
Correct answer: B. It converts compressed air energy into mechanical rotation through a small air turbine, which drives the engine spool through reduction gearing and an automatic disengaging clutch
A pneumatic starter uses high-pressure air (from an APU, ground cart, or cross-bleed) directed onto a small turbine wheel. The turbine's rotation is geared down and coupled to the engine's N2 spool through a clutch that automatically disengages once the engine reaches self-sustaining speed.
Why the other options are wrong
- A. Pneumatic starters use air pressure, not hydraulic pressure.
- C. This describes an electric starter, not a pneumatic starter.
- D. This describes a combustion (cartridge) starter, not a pneumatic starter.
Pneumatic Starter
A turbine engine starting device that uses compressed air to spin a small turbine, geared down to drive the engine's high-pressure spool through an automatic disengaging clutch.
- Air source can be APU, ground cart, or engine cross-bleed
- Reduction gearing lowers turbine speed to usable starter output speed
- Clutch automatically disengages at self-sustaining engine speed
Memory trick: Pneumatic starter = a tiny turbine spun by a gust of borrowed air