How does an APU generally support the aircraft's systems?

The auxiliary power unit (APU) primarily supports aircraft systems by supplying power and pneumatic pressure. The APU is a small engine that operates independently of the main engines and is designed to provide electrical energy and compressed air. This capability is crucial during ground operations, as it powers essential systems such as lighting, avionics, and other electronic components when the aircraft's main engines are not running.

Moreover, the APU provides pneumatic pressure that is used to start the main engines and can also be utilized for air conditioning and cabin pressurization while the aircraft is on the ground. This makes it vital for maintaining passenger comfort and ensuring operational readiness without relying on external power sources or the main engines.

Other options like providing additional thrust or assisting in navigation do not align with the primary functions of an APU, as it is not designed for those roles. Its main purpose revolves around supplying necessary energy and air pressure rather than performing propulsion or navigation tasks. Similarly, the APU does not contribute to cooling the main engines, which is managed through other systems within the aircraft.