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

The Auxiliary Power Unit (APU) is primarily responsible for providing electrical power and air conditioning to the aircraft when it is on the ground and not connected to external power sources. When an aircraft is parked at the gate, especially during boarding or maintenance, the APU supplies the necessary power to support various systems, including cabin air conditioning and lighting without relying on the main engines or ground power units.

This is particularly important for passenger comfort and operational efficiency while the aircraft is not in flight. The APU enhances the flexibility of ground operations and contributes significantly to overall passenger comfort before takeoff. Additionally, it enables systems to run for maintenance checks and pre-flight preparations.

Other functions of the APU may include providing bleed air for engine start-up, which is crucial for starting the main engines. However, its primary role addressing the question is its capacity to sustain electrical power and climate control on the ground.