This ARP discusses design philosophy, system and equipment requirements, environmental conditions, and design considerations for helicopter environmental control systems (ECS). The helicopter ECS comprises that arrangement of equipment, controls, and indicators which supply and distribute dehumidified conditioned air for ventilation, cooling and heating of the occupied compartments, and cooling of the avionics. The principal features of the system are: a A controlled fresh air supply b A means for cooling (air or vapor cycle units and heat exchangers) c A means for removing excess moisture from the air supply d A means for heating e A temperature control system f A conditioned air distribution system The ARP is applicable to both civil and military helicopters where an ECS is specified; however, certain requirements peculiar to military applications, such as nuclear, biological and chemical (NBC) protection, are not covered.