What is one key function of an Auxiliary Power Unit (APU)?

The key function of an Auxiliary Power Unit (APU) is to provide electrical power to the aircraft when the main engines are not running. This is critical for maintaining essential systems, such as lighting, air conditioning, and avionics, while the aircraft is on the ground or during emergency situations. The APU can operate independently of the main engines, meaning that the aircraft can remain functional without needing to start the engines, which is particularly useful for ground operations.

The APU also allows for the operation of onboard systems that would otherwise require the main engines to be running, improving overall efficiency during pre-flight checks or when taxing on the runway before takeoff. This function ensures that the aircraft can be serviced and operated effectively while on the ground, enhancing both safety and convenience for crew and passengers.