What does the Auxiliary Power Unit (APU) do?

The Auxiliary Power Unit (APU) serves as a vital power source for an aircraft when its main engines are not running. Specifically, one of its primary functions is to provide electrical power and pneumatic energy to the aircraft systems while on the ground or when the main engines are shut down. This allows the APU to function as a generator, supplying electricity for cabin lights, avionics, climate control, and other electrical systems, ensuring the aircraft remains operational and comfortable even when the engines are off. Moreover, the APU can also provide bleed air for engine starting and environmental control systems.

The other choices indicate functions that do not directly relate to the core purpose of an APU. For example, while starting the engines is an important task in aircraft operation, it is primarily accomplished through the engines' own starter systems in conjunction with the APU providing the necessary bleed air. Supplying fuel to the engines is managed through the aircraft's fuel system rather than the APU itself. Control of hydraulic systems is typically handled by the main engines or dedicated hydraulic systems, not directly by the APU.