What are the two primary systems the C-5 APU supports?

The auxiliary power unit (APU) of the C-5 aircraft primarily supports the pneumatic and electrical systems. The APU provides a reliable source of electrical power and supplies compressed air for the aircraft's pneumatic systems, which are essential for functions such as starting the main engines and operating environmental control systems.

The pneumatic system's operation is vital for various pre-flight checks and systems, ensuring the aircraft is ready for takeoff. Additionally, the electrical system support allows for operation of essential systems when the main engines are not running, providing convenience and operational efficiency on the ground and during emergencies. Understanding the APU's critical role in these areas is key to understanding its impact on the overall functionality and safety of the C-5 aircraft.

The other systems mentioned do not primarily rely on the APU for their operation, as the C-5's flight control, landing gear, and fuel management systems primarily function through different means and do not rely as heavily on the APU.