What is the primary use of the auxiliary power unit (APU) in aircraft?

The auxiliary power unit (APU) is primarily used to supply electrical power and pneumatic air for air conditioning while the aircraft is on the ground. This is crucial for maintaining the aircraft's systems and comfort when the main engines are not running.

When the APU is operational, it provides the necessary power for onboard systems, ensuring that vital electrical components remain functional. Additionally, it generates compressed air, which is essential for starting the main engines and for providing cabin air conditioning. This capability is especially important during ground operations, where the aircraft may be parked for extended periods without access to ground power units or other external sources of energy.

In contrast, other options do not correctly describe the primary function of the APU. Control of flight navigation systems is managed by different avionics components, backup hydraulic pressure is typically provided by hydraulic systems or pumps, and while the APU does assist with starting the engines, it is not directly involved in takeoff procedures. Thus, the role of the APU in supplying electrical power and conditioning air is fundamental to its function in aircraft operations on the ground.