What does the APU do on an aircraft?

The Auxiliary Power Unit (APU) is an important component of an aircraft that provides auxiliary power primarily when the engines are not running. Its primary function is to supply electrical power and bleed air for starting the main engines and for various other needs when the aircraft is on the ground. This allows the aircraft systems to remain operational without relying on the main engines, which is especially useful during boarding and while parked at the gate.

While controlling cabin temperatures and powering cockpit instruments are important functions on an aircraft, those tasks are typically handled by different systems that work in conjunction with the APU, especially during flight. The APU itself does not generate thrust for takeoff, which is a critical aspect solely managed by the aircraft's main engines. Thus, the role of the APU as a source of auxiliary power stands out as its primary and most critical function.