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

The Auxiliary Power Unit (APU) serves a crucial role in aircraft operations, particularly when the aircraft is on the ground. Its primary function is to provide electrical power and pneumatic pressure for starting the main engines as well as for powering various systems and services during ground operations. This includes supplying power to cabin lights, air conditioning, and other essential systems before the main engines are running. The APU allows the aircraft to operate independently of ground support systems and power sources, enhancing efficiency and safety.

While other functions like navigation, cabin cooling, and engine performance enhancement are critical in different contexts, they are either performed by other components of the aircraft or become relevant mainly during flight rather than on the ground. Thus, recognizing the APU's role in providing essential power and support during ground operations helps clarify its importance in overall aircraft functionality.