An Energy Efficient Environmental Control System (EEECS) for a patrol-type aircraft has been developed and tested. Benefits of the system include not only substantial fuel savings, but also significant increases in avionics reliability through reduced cooling air temperatures supplied to aircraft electronics by the system. Testing was conducted in a laboratory to demonstrate energy efficiency throughout a wide range of operating conditions and in a Navy P-3C aircraft to demonstrate flight-worthiness of the system. This paper discusses the results of both the laboratory and flight tests.The heart of this vapor-cycle system is an electrically-driven, variable-speed centrifugal compressor that operates at speeds from 30,000 to 70,000 RPM. The compressor uses oil-less, tilt-pad, hydrodynamic bearings for maintenance-free operation. The compressor motor is of the permanent-magnet type and is located between the two compression stages. 270 Volt DC power supply technology makes operation at these elevated speeds possible, thereby increasing the efficiency and reducing the size and weight of the compressor. Energy savings are achieved by continuously monitoring aircraft cooling requirements at the evaporator heat exchanger and adjusting compressor speed accordingly, thereby supplying only the amount of cooling required at any given flight condition. Conventional systems, on the other hand, operate continuously at the operating point established by some worst case condition, even though the worst case cooling condition is experienced during only a small percentage of the total operating time.Additional energy efficiency is derived through the use of a binary non-azeotropic refrigerant mixture and subcoolers, which ultimately reduce the level of compression necessary for the refrigeration cycle. Even greater energy savings are realized when the evaporator heat load and the sink temperature are low enough such that operation of the compressor is not necessary and circulation of the refrigerant through the evaporator and condenser heat exchangers is sufficient to satisfy cooling requirements. A refrigerant pump circulates the refrigerant in this “free cooling” mode of operation.The system was designed to operate in a closed air loop, with fresh air being supplied only for pressurization and make-up, whereas the existing P-3 ECS utilizes an open-loop air-cycle system. The closed-loop design not only introduces additional inherent energy efficiency, but also facilitates the addition of a simpler and lighter-weight Chemical, Biological, and Radiological protection system. Due to cost restrictions, the system was not flight tested in a true closed-loop arrangement.