What is the primary function of an Auxiliary Power Unit (APU)?

The primary function of an Auxiliary Power Unit (APU) is to supply air for ground operations and engine starting. An APU is essentially a small turbine engine located on an aircraft that provides power when the main engines are not running. This includes supplying compressed air for starting the engines, as well as providing electrical power and bleed air for air conditioning or other systems while the aircraft is on the ground.

Using an APU eliminates the need for ground power units or external air supply, leading to increased efficiency and reduced reliance on external sources. This makes it a critical component of modern aircraft operations, especially in terms of maintaining readiness and facilitating ground operations without needing the main engines running.

In contrast, while hydraulic power for landing gear is important, it is usually supplied by the aircraft's primary hydraulic systems rather than the APU. Similarly, the APU does not enhance aerodynamic performance, nor does it manage fuel levels during flight, which are functions carried out by other systems onboard. Thus, the correct choice accurately reflects the essential and specific role of the APU in aviation.