During what phase of flight is the APU most commonly used?

The correct answer is that the APU is most commonly used during ground operations on the tarmac. The Auxiliary Power Unit (APU) provides essential electrical and pneumatic power while the aircraft is on the ground, which is crucial for pre-flight checks, starting engines, and running their systems without needing external power sources or the main engines to be running.

During takeoff and landing phases, the main engines provide the necessary power and thrust, while descent primarily requires the aircraft's systems to be managed with minimal power needs. Therefore, while the APU may have a role in these phases for certain systems, its primary purpose is to support ground operations, including cabin conditioning, powering onboard systems, and starting the engines. This highlights the important role of the APU in ensuring operational efficiency and safety during ground handling of the aircraft.