What is the purpose of an air turbine starter in engines?

• A. To provide electrical power to start electronics.
• B. To lubricate the turbine bearings.
• C. To spool the engine by mechanical energy via air expansion until self-sustaining.
• D. To measure airspeed during start.

Answer: (C)

An air turbine starter converts pneumatic energy into mechanical energy to spin the engine’s compressor until it reaches self-sustaining speed. High-pressure air from the aircraft’s bleed or an external source drives a turbine inside the starter; as the air expands through the turbine, it turns a shaft connected to the engine’s accessory gear train. This spins the engine until the compressor is fast enough for ignition and fuel to sustain operation, at which point the starter disengages. This is why it’s described as spooling the engine via air expansion until self-sustaining.