Closing the auxiliary power unit air intake after it is turned off on a Boeing 787-8 aircraft

Let me remind you, the auxiliary power unit (APU) is a compact gas turbine engine usually located in the tail section of the aircraft. Unlike the main engines, it does not produce thrust; its purpose is to power onboard systems when the aircraft is on the ground and external power is unavailable.

One of the key functions of the APU is to start the main engines. To do this, it supplies compressed air or electricity needed to spin them up. This allows the engines to be started independently, without connection to ground power sources. Additionally, while the aircraft is parked, the APU provides electrical power and supports the operation of air conditioning systems.

In some cases, the APU is also used in flight. This usually happens in the event of failure of one or more engines or in other situations where energy is needed to maintain vital system operation.