During what phase of operation may the APU be used for engine starting?

The APU (Auxiliary Power Unit) is primarily utilized to provide power and compressed air for starting the aircraft's engines, among other functions. The correct phase of operation during which the APU can be used for engine starting is before departure and after landing.

Before departure, the APU is started to ensure that there is sufficient bleed air available to initiate the engine starts without relying entirely on the main engines or an external power source. This allows for a more efficient and controlled operation.

After landing, the APU can also be employed to start the engines for a subsequent flight if the ground operations and conditions support this process. The APU provides the necessary power, allowing for a smoother transition between phases of operation without unnecessary delays.

In contrast, using the APU during taxi, cruise, or descent does not align with standard operational procedures, as these phases generally involve the main engines for significant thrust and operational efficiency. Therefore, the APU is not utilized for engine starting during these times.