What is the main advantage of the APU for military aircraft like the C-5?

The main advantage of the Auxiliary Power Unit (APU) for military aircraft, such as the C-5, is that it allows various systems to operate without the need for external power. The APU generates electrical power and provides compressed air, which is essential for starting the main engines, operating environmental control systems, and powering other onboard systems while the aircraft is on the ground or during flight when the engines are not running. This capability is vital for military operations, where readiness and self-sufficiency are crucial, especially in remote or austere environments where ground support may not be readily available. The ability to maintain power and system functionality ensures that the aircraft can remain operational and respond quickly to mission requirements.