pneumatic pressure and is opened by pneumatic signals from a remotely located avionics ram air servo. Pressure is supplied from the servo to hold the valve closed. When externally supplied pressure from the servo drops below a specified value, the valve opens and supplies ram air. ECM Cooling Air Control Valve The ECM cooling air control valve is a two- position solenoid controlled valve. The valve provides a high flow of cooling air to the electronic countermeasure (ECM) package when de-energized and a low flow when energized. The valve is energized when the ECM mode switch on the ECM control panel assembly is selected to OFF or STBY. Avionics Ground Cooling Fan The avionics ground cooling fan is an electrically driven axial flow fan that provides a flow of ambient air for direct avionics cooling during aircraft ground operation, taxi, takeoff, and landing. The fan is in the nose wheelwell and is powered by an induction motor. Avionics Ground Cooling Fan Check Valve The avionics ground cooling fan check valve is made up of two spring-loaded flappers hinged around a central shaft. The check valve is located between the upper and lower plenums, and it functions to prevent cooling air from escaping through the avionics ground cooling fan during flight. Avionics Fan Control Pressure Switch The avionics fan control pressure switch is a pneumatically operated switch in the ducting downstream of the primary heat exchanger. This switch controls the avionics ground cooling fan operation to provide supplemental air for avionics equipment cooling. Emergency Ram Air Scoop The emergency ram air scoop is a spring- loaded scoop on the forward fuselage. The scoop is held closed by a solenoid, which releases the scoop when energized. A linkage arrangement, connected to the scoop, closes a flapper on the avionics cooling plenum and directs ram air to the essential avionics when the scoop is opened. The 3-16 FCS cool switch, on the right vertical console, activates the scoop release solenoid when set to EMERG. Avionics Undercool Warning Temperature Sensor The avionics undercool/warning temperature sensor is made up of two temperature sensing elements, one is self-heated. On detecting an undercool condition of avionics cooling air, the sensor sends a signal to the signal data converter, which displays the AV AIR HOT caution message on the left digital display indicator. Avionics Cooling Fans The two avionics cooling fans in the cockpit are axial flow fans driven by electric motors. Both fans operate continuously with any ground power switch to A ON or B ON or flt cent switch to ON. Individual cockpit fan operation can be verified by setting the fan test switch on the fan test control panel assembly to A LEFT or B RIGHT. Rear Cockpit Avionics Cooling Fans The rear cockpit avionics cooling fans are identical in design and function to the cockpit avionics cooling fans. Individual rear cockpit fan operation can be verified by setting the fan test switch on the fan test control and utility light panel assembly to A LEFT or B RIGHT. VAPOR CYCLE AIR-CONDITIONING SYSTEM Vapor cycle systems make use of the scientific fact that a liquid can be vaporized at any temperature by changing the pressure above it. Water at sea level barometric pressure of 14.7 psi will boil at 212°F. The same water in a closed tank under a pressure of 90 psi will not boil at less than 320°F. If the pressure is reduced to 0.95 psi by a vacuum pump, the water would boil at 100°F. If the pressure is reduced further, the water would boil at a still lower temperature; for instance, at 0.12 psi, water will boil at 40°F. Water can be made to boil at any temperature if the pressure corresponding to the desired boiling temperature can be maintained. Liquids that boil at low temperatures are the most desirable for use as refrigerants. Com- paratively large quantities of heat are absorbed when liquids are evaporated; that is, changed to