What command is used when there is a requirement for the air supply unit?

The command that indicates a requirement for the air supply unit is focused on ensuring that there is a clear request for supply of the necessary air pressure or ventilation that the aircraft may need, particularly during ground operations or engine starts. The phrase "Want air supply unit" explicitly communicates the need for the air supply unit to be activated or monitored, thereby facilitating proper operations in scenarios where external air supply is essential. It emphasizes the requirement to ensure that the aircraft systems have access to the necessary air resources.

In contrast, the other provided options relate to different operational functions or scenarios. For instance, closing external doors or retracting the airstair is connected to securing the aircraft before departure, while deactivating the airstart unit pertains to stopping an auxiliary unit that assists with engine starting. The command for "engine fire" refers to a critical emergency situation requiring immediate action but does not address the specific need for air supply. Therefore, the selected phrase accurately captures the operational requirement for the air supply unit.