Vent-air systems provide a flow of air to the aircraft's seat or back cushions or to the ventilating air connection of the antiexposure suit when worn by the pilot and/ or crew members. The system provides a measure of personal comfort, offsetting the discomfort caused by the wearing of the antiexposure suit or heat created by cockpit equipment and resulting high-temperature ambient air. Some vent-air systems, such as the one installed in the A-4 model aircraft, supply air to only one individual and operate on a small independent motor-driven air blower. Most newer aircraft use engine bleed air that has been cooled in the aircraft's air-conditioning system auxiliary heat exchanger as a primary source of ventilation air. A schematic of the A-6 aircraft vent-air system that operates off engine bleed air is illustrated in figure 1-6. The A-6 vent-air system is supplied cooled bleed air from the refrigeration unit of the aircraft's cabin air-conditioning and pressuriza-tion system and hot bleed air from the hot bleed-air ducting just downstream of the engine bleed-air shutoff valve. The hot and cold bleed-air lines converge into one conditioned air duct that is connected directly to the pilot's and

bombardier/ navigator's flow controllers and the personnel services disconnect. Temperature control of the vent-air system is regulated between 50 F and 100 F by the vent suit temperature selector, the temperature control valve, a temperature sensor, and the cabin and vent suit temperature controller. The components of the temperature control system cause the temperature control valve to cycle between open and close. Thus, temperature control is main-tained by governing the flow of hot engine bleed air that is being mixed with cool air from the refrigeration unit. The vent-air system is turned on by the vent suit switch. This switch is an integral part of the pilot's flow controller. When the flow control valve thumbwheel is rotated slightly from the OFF position, the circuit between the temperature sensor and the cabin and vent suit controller is completed. The controller responds to signals

Figure 1-6.- Vent-air system.

from the temperature sensor and the temperature selector and supplies open and close signals, as appropriate, to the vent suit temperature control valve. The vent-air flow controllers, as the name implies, control the flow of air from the vent-air system ducting to the personnel services dis-connects and the seat cushion or the vent connection of the antiexposure suit. One controller is provided for each crew member. The controller has an inlet connector, an outlet connector, and a thumbwheel-operated flow controller. The thumbwheel shaft connects to a rotating plug, which gradually opens or closes off the outlet port as it rotates up to a maximum of 180 degrees. The flow controller will be fully open when turned to the full counterclockwise position. The temperature selector is a thumbwheel-operated potentiometer, located on the pilot's console, aft of the flow controller thumbwheel. One temperature selector serves both vent suit outlets. The thumbwheel is numbered 1 through 14, and the console is labeled HOT and COLD. Turning the thumbwheel clockwise to the lower numbered settings lowers the temperature of vent-air system air. Counterclockwise movement towards the hot position and the higher numbered settings increases the temperature. Response to temperature changes initiated by repositioning the temperature selector thumb-wheel will be noticeable at the disconnect outlet within a few seconds after making a selection change. When the aircraft is in a stabilized flight condition (maintaining a steady altitude), the temperature of vent air will be monitored and controlled within a 2 F tolerance of the temperature selected by the temperature selector thumbwheel. When the aircraft is changing altitude, the temperature is maintained within a 10 F tolerance. The thermal switch senses any abnormally high temperatures not compensated for by the temperature sensor and will provide a signal, via the cabin and vent suit temperature controller, to the temperature control valve to drive it towards the closed position. The system pressure relief valve protects the system from accidental overpressurization. The relief valve will open as necessary to prevent vent-air system ducting pressure from exceeding 10 psi. The check valve prevents conditioned air from backing up into the environmental control system ducting. Cooled air flows through the check valve, is mixed with the appropriate amount of hot bleed air, and is forced into the vent-air system ducting.