What is the primary function of the C-5 APU?

The primary function of the C-5 Auxiliary Power Unit (APU) is to provide power for engine starting, air conditioning, and electrical power. The APU is a crucial component of the aircraft's ground support systems. It supplies necessary electrical power when the main engines are not running, ensuring that essential systems can operate, such as lights and avionics. During engine start-up, the APU delivers the required pneumatic power to start the main engines and provide the air conditioning system with a source of air before the main engines are operational.

In contrast, assisting with navigation systems is not a primary role of the APU; navigation systems rely on other aircraft components and systems. Improving fuel efficiency during flight is also not a function of the APU; its utility is generally associated with ground operations rather than in-flight performance. Furthermore, enabling in-flight refueling capabilities pertains to different systems designed for that specific task, rather than the APU's functions. Therefore, the definition of the APU's role aligns closely with providing power and support for the aircraft when it is on the ground.