This SAE Aerospace Recommended Practice (ARP) contains guidelines and recommendations for subsonic airplane air conditioning systems an components, including requirements, design philosophy, testing and ambient conditions. The airplane air conditioning system comprises that arrangement of equipment, controls and indicators that supply and distribute air to the occupied compartments for ventilation, pressurization, and temperature and moisture control. The principal features of the system are: a. a supply of outside air with independent control valve(s); b. a means for heating; c. a means for cooling (air or vapor cycle units and heat exchangers); d. a means