During what circumstances does the APU need to provide cooling air?

The APU needs to provide cooling air primarily during high-temperature ground operations. In these situations, the aircraft's environmental control system relies on the APU to supply necessary airflow to maintain optimal temperature conditions within the cabin and other critical areas of the aircraft. High temperatures on the ground can lead to overheating and potential operational issues if adequate cooling is not maintained.

This functionality is crucial, especially when the aircraft is parked on a tarmac under direct sunlight or when its engines are not running. The APU serves as an essential unit to regulate the cabin environment, prevent heat buildup, and ensure the comfort of passengers and crew before takeoff.

Standard flight operations do not typically require the APU to provide cooling air since the main engines and other systems can manage temperature control during flight. Pre-flight inspections usually focus on checking systems and equipment rather than demanding cooling air. During storage phases, the aircraft is typically not actively in use, reducing the need for cooling air as systems are either powered down or in maintenance mode. Thus, high-temperature ground operations are the key circumstance requiring the APU's cooling air.