This ARP discusses design philosophy, system and equipment requirements, environmental conditions, and design considerations for helicopter environmental control systems (ECS). The heliocpter 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; and f. a conditioned air distrubtion 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. The integration of NBC protection into aircraft ECS is discussed in AIR4362. The purpose of this ARP is to provide the guidelines and requirements for analysis, design, and installation of helicopter ECS.