Typical air conditioning systems provide bleed air from two engines or the APU (a separate gas turbine engine that provides power and air when the engines are not running). The bleed air from the engine is drawn from the medium-pressure or high-pressure compressor. Because of the compressor, it is very hot. Before entering the air-conditioning system, it must be pre-cooled by a precooler, and then sent to the air-conditioning system.

Aircraft ground air conditioning (hereinafter referred to as PCA) is an air conditioning unit that provides fresh air suitable for temperature and humidity environment and physiological needs for passengers and crew members parked on the boarding bridge at the airport, which provides a comfortable environment during ground maintenance.