What is the primary function of the aircraft's auxiliary power unit (APU)?

The auxiliary power unit (APU) serves a critical role in aircraft operations, primarily focused on generating electrical power and pneumatic pressure needed for ground operations. The APU is essentially a small turbine engine located at the rear or in the engine compartment of the aircraft, which can operate independently of the main engines.

When the aircraft is on the ground and not connected to an external power source, the APU provides the electricity necessary to power lights, instrumentation, and other systems within the cabin and cockpit. Additionally, it supplies pneumatic pressure for starting the main engines and for air conditioning, which is crucial for passenger comfort before takeoff and after landing.

By performing these functions, the APU enhances ground operations and ensures that various systems are operational without relying on the main engines, thereby improving efficiency and functionality while the aircraft is stationary. This makes it an essential component for modern aircraft operations, particularly in customer service and maintenance contexts.