What is the role of the APU in an aircraft?

The Auxiliary Power Unit (APU) plays a crucial role in an aircraft by generating electricity and providing environmental control, particularly when the main engines are not running. This is important for starting the aircraft's engines, as the APU can power the engine starter and provide bleed air for engine ignition. Additionally, the APU supplies electrical power to the aircraft’s systems when on the ground, thus allowing for the operation of lighting, avionics, and other essential functions without needing the main engines to be running.

In terms of environmental control, the APU can run the air conditioning system while the aircraft is parked at the gate. This ensures a comfortable cabin environment for passengers and crew prior to departure, regardless of external weather conditions. Overall, the APU is integral for the aircraft’s functionality when it is on the ground, bridging the need for electricity and environmental control until the aircraft's engines are operational.