What is an Auxiliary Power Unit (APU) primarily used for?

An Auxiliary Power Unit, or APU, is primarily used to provide auxiliary power when the aircraft's main engines are shut down, which is essential for several ground operations. This includes supplying electrical power for lighting, air conditioning, and other systems that require energy when the engine is not running. Additionally, the APU can be utilized to start the main engines, offering a reliable source of compressed air necessary for engine ignition without relying solely on the aircraft's main engines.

This functionality is especially crucial during pre-flight preparations on the ground or in situations where the main engines are not operational, allowing the crew to maintain essential systems and onboard comfort. The APU thus plays a vital role in ensuring operational efficiency and passenger convenience while the aircraft is not in flight.