What is one of the key benefits of having an APU on large aircraft like the C-5?

Having an Auxiliary Power Unit (APU) on large aircraft such as the C-5 offers significant operational benefits, particularly in enhancing flexibility during ground operations. The APU provides necessary electrical power, pneumatic air, and hydraulic pressure while the aircraft is stationary on the ground, enabling a range of critical systems to function without the need for the main engines to be running. This capability allows for tasks such as starting the main engines, running onboard systems, and powering air conditioning or lights while the aircraft is not in the air.

By utilizing the APU instead of relying on external power sources or the main engines, ground handling can be conducted more efficiently and independently. This operational flexibility is crucial in various scenarios, such as maintaining systems during delays or ensuring quick turnaround times between flights, thereby optimizing ground operations.

The other aspects mentioned, such as increased passenger capacity, reducing the weight of the aircraft, or improving fuel efficiency in flight, do not accurately reflect the primary function and advantages of an APU, as they pertain to different technical and operational considerations that are not directly influenced by the presence of an APU.