In what scenario is the APU not utilized?

The APU, or Auxiliary Power Unit, is designed to provide supplemental power and pneumatic support to the aircraft, especially during ground operations. During flight, the main engines of the aircraft generate both electrical power and pneumatic pressure necessary for systems operation. When the aircraft is in the air, the engines are capable of meeting all of these demands, making the APU unnecessary in that context. Thus, its utilization is not required during flight when the main engines are operational.

In contrast, during ground operations for refueling, takeoff preparations, or pre-flight checks, the APU is typically engaged to supply power and bleed air. This ensures that essential systems are functional without the need for the engines to be running, allowing for efficient operations on the ground.