Each member of the aircrew of an advanced fighter airplane wears a complex, multilayered ensemble that imposes a significant thermal burden by inhibiting normal transport of heat from the body. The thermal burden may be aggravated by the use of nuclear, biological, and chemical (NBC) protective clothing, insolation through the canopy, psychological mission stress, and, in the future, positive-pressure breathing equipment used in COMBAT EDGE (Combined Advanced Technology Enhanced Design "G" Ensemble) equipped aircraft. The inability of the body to rid itself of excess heat can, at the very least, cause discomfort, but can also degrade mission effectiveness and pose a serious threat to safety and health. The aircrew personnel environmental control system (APECS) was designed to satisfy the need to remove excess heat from the body under these conditions. The APECS can also be used to keep pilots relatively cool when waiting for long times on the ground in hot weather, as well as in flight.

Figure 1. A Heat-Transfer Liquid Is Circulated between heat exchangers and a liquid vest to cool the wearer.

The APECS is a liquid-circulation cooling system (see Figure 1) that comprises two basic subassemblies — (1) the crew side, which is donned by each aircrew member, and (2) the aircraft side, which is permanently installed in each aircraft. The crew side (see Figure 2) consists of a cooling garment worn under the flight suit, tubing, and a quick-disconnect assembly for emergency egress. The aircraft side contains a network of plumbing, two heat exchangers, and a pump to chill and circulate the cooling fluid. The fluid consists of 70 percent distilled water and 30 percent denatured alcohol, plus a trace amount (150 parts per million) of sodium nitrate to inhibit corrosion in the pump. In future versions of this system, the pump will contain polytetrafluoroethylene gears, eliminating the need for sodium nitrate.

Figure 1 schematically depicts the APECS. The two heat exchangers in the aircraft side of the APECS are tied into the environmental control system (ECS) of the aircraft for cooling of the fluid. The fluid passes to thermal contact with the crewmember through tough nylon tubes worn close to the skin and, via thermal conduction, removes excess heat from the crewmember. The cycle is completed by pumping the heated fluid back to the heat exchangers for cooling. The temperature of the fluid pumped to the crewmember can be controlled manually by adjusting the cockpit air temperature via the ECS. During the winter, the heat-exchange process can be reversed by increasing the cockpit air temperature via the ECS, allowing warm fluid to pass through the APECS. The system includes a bypass that enables the crew member to elect not to use the APECS while keeping cooled or heated fluid circulating in a standby mode.

Figure 2. The APECS Garment and Quick-Disconnect Assembly are parts of the crew side of the APECS. The garment is worn under the flight suit. The quick-disconnect assembly is needed so that the APECS will not impede emergency egress.

The APECS hardware was originally designed and tested in the laboratory by Foster-Miller, Inc., of Waltham, Massachusetts, under contract with the United States Air Force Flight Stress Protection Division of Armstrong Laboratories at Brooks Air Force Base. Human physiological testing to verify that the APECS would provide acceptable protection against heat stress was also conducted at Brooks Air Force Base. Having been proven in the laboratory, the APECS was then provided to NASA Dryden Flight Research Center for installation in an aircraft for field testing.

NASA elected to use the F-16XL aircraft as the test bed, inasmuch as this aircraft is flown by NASA as well as visiting Air Force Flight Test Center pilots. This gives the APECS exposure to a wide cross section of crewmembers with different physiological characteristics. To date, flight tests have proven the success of the APECS. Comments from aircrew members who have flown with the APECS indicate that the system works extremely well, is very comfortable, and responds quickly to changes in cockpit temperature. The system has operated during flight at high accelerations and during simulated air combat maneuvers, with no loss of cooling and without noticeable effect on the mobility of the pilot under acceleration load.

Although the APECS is now installed in the F-16XL only, work is in progress to put the APECS in a two-seat F-15 aircraft as well as in a single-seat F-18 aircraft at NASA Dryden Flight Research Center. Plans for the future also provide for the incorporation of a preflight cooler to help control body temperature on the ground and in the cockpit prior to connection of the crew side of the APECS with the aircraft side.

This work was done by Nick Kiriokos and Mark Collard of Dryden Flight Research Center and Dan Fischbach of Foster-Miller, Inc. No further documentation is available. DRC-98-86

NASA Tech Briefs Magazine

This article first appeared in the April, 1999 issue of NASA Tech Briefs Magazine.

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