Auxiliary Power Unit (APU) is a small engine on an aircraft that provides essential power and pneumatic support when the main engines are off. During flight, the APU is typically not used for primary power but can serve as a backup emergency power source if needed, ensuring continued operation of critical systems in the event of a main engine failure. On the ground, it supplies electricity to the aircraft’s systems, and the air conditioning in the cabin through air compressing and condensing.

PCA is a specialized air conditioning device on the ground designed to provide fresh air that has been filtered, pressurized, dehumidified, and cooled (or heated) for aircraft cabins parked on the ground. These devices provide a comfortable cabin environment for passengers and crew members from the time the aircraft docks at the boarding bridge to the time it leaves the bridge.

PCA devices provide a comfortable cabin environment for passengers and crew members from the time the aircraft docks at the boarding bridge to the time it leaves the bridge. Compared to APU, it provides the air conditioning to the aircraft when after landed and waiting for the boarding, which has high social and economic benefits.