How an Aircraft APU Works

Full transcript

Tucked in the tail, the APU is a small turbine engine supplying electrical power and compressed air independent of the main engines. Air enters a compressor, heats in a combustor, then expands through a turbine, whose shaft drives an accessory gearbox and generator. Combustion airflow runs only through compressor, combustor, and turbine. Separate compressor bleed air is extracted and routed through pneumatic ducts for cabin systems, engine starting, or anti-icing — a distinct path. The generator connects to aircraft buses, coordinated with external ground power. Fuel lines, sensors, control wiring, lubrication, exhaust, and fire protection all support safe operation. Starting: aircraft power energizes the controller and starter; doors open, conditions are checked, the starter spins the rotor, fuel and ignition engage, combustion self-sustains, and the starter disengages at governed speed. Interlocks monitor speed, temperature, oil pressure, with overspeed and overtemperature protection, bleed-air and generator loading, then shutdown and cooldown. This is turbine thermodynamics: compressor work raises pressure and temperature, combustion adds heat, turbine work balances the cycle. Exact limits vary by aircraft — always consult the manual.