When is the APU permitted for use?

The Auxiliary Power Unit (APU) is primarily used to provide electrical power and bleed air to the aircraft while on the ground. It's crucial for starting the main engines and powering the cabin systems when the aircraft is not connected to ground power. The APU's operation is particularly important during ground operations, as it enables the aircraft to operate independently without relying on other power sources.

In general aviation practices, the use of the APU for in-flight operations is not standard, as its design and purpose are chiefly for auxiliary support on the ground. It is also not typically permitted during takeoff and landing due to safety and performance considerations. Emergency use of the APU is limited and typically reserved for specific scenarios rather than as a routine operational choice.

Thus, the correct answer indicates that the APU is permitted for use during ground operations only, aligning with its primary function and compliance with operational protocols.