What is the primary function of the APU on an aircraft?

The primary function of the Auxiliary Power Unit (APU) on an aircraft is to power onboard electrical systems. This small engine is crucial as it provides electrical power and pneumatic pressure when the main engines are not running, particularly during ground operations.

During pre-flight procedures, the APU enables critical systems such as lighting, environmental control systems, and avionics to function without needing external power sources. When the aircraft is on the ground and not connected to ground power, the APU serves as an independent power source, making it essential for maintaining the operations of key systems essential for safety and functionality before takeoff.

This capability allows crews to perform checks, run systems, and maintain comfort inside the cabin while preparing the aircraft for flight, illustrating the significance of the APU in overall aircraft operations.