What is one of the primary functions of the APU?

The APU's primary function is to provide electrical power for various onboard systems when the aircraft is on the ground and not connected to external power sources. This capability is crucial for powering essential systems such as lighting, avionics, air conditioning, and other electrical loads without needing to rely on the aircraft's main engines. The APU operates independently, allowing for these systems to function while the engines are off, thereby supporting pre-flight checks and maintenance activities.

While generating hydraulic power and serving as a backup for the main engines may be functionalities associated with some auxiliary power units, they are not the primary purpose of an APU. The APU is specifically designed to deliver electrical power reliably, making it an integral component for the aircraft's ground operations. Improving aerodynamics is also not a function of the APU, as its design and operation focus on power generation rather than aircraft performance characteristics.