heat valve to maintain the selected temper- ature. MANUAL TEMPERATURE CONTROL Manual temperature control is selected by setting the ECS mode switch to MAN. In the manual mode, adjusting the cabin knob of the suit/cabin temp control determines the control signal sent to the cabin add heat valve. EQUIPMENT COOLING SYSTEMS Learning Objective: Recognize the source for avionics cooling air; identify com- ponents of avionics cooling systems and the function of each component. As electronics and avionics in naval aircraft became more common and more sophisticated, it was discovered that the heat generated by this equipment was becoming excessive for proper operation. A means of keeping the avionics at a proper operating temperature was needed. To overcome this problem, some aircraft have. separate ACS for equipment cooling, while others use conditioned air diverted from the environmental ACS. If the equipment ACS is an air-to-air type, its operation is the same as the environmental ACS discussed at the beginning of this chapter. Only operating temperatures will be different. The other type of equipment air conditioning (vapor cycle) operates on the same principal as a home ACS and will be discussed later in this chapter. AIR-TO-AIR SYSTEM The F-18 aircraft avionics cooling system (fig. 3-11, a foldout at the end of this chapter) controls and transports conditioned air to the various avionics packages and equipment bays. Conditioned air from the air cycle ACS is transported to the avionics flow valve where inlet pressure and airflow is controlled in response to signals from the avionics flow/temperature sensor through the ACS temperature/flow controller. Airflow rates are matched to the individual cooling requirements of each equip- ment package or bay. Ground cooling of the avionics equipment bays is provided by an avionics ground cooling fan. The cockpits avionics equipment cooling is done by four cockpit avionics cooling fans, two in the forward cockpit and two in the rear cockpit. Emergency ram air cooling for essential avionics equipment is provided by an emergency ram air scoop and avionics ram air valve. The avionics cooling components are discussed in the following paragraphs. Avionics Airflow Valve The avionics airflow valve is a normally open, pneumatically actuated and controlled pressure regulating valve. The valve is in the avionics supply line and maintains a pressure differential of 1.5 psi between an externally sensed upstream duct pressure and an externally sensed cabin duct pressure. Actuator spring pressure holds the valve butterfly to full open. Cabin pressure is sensed on one side of the control diaphragm and upstream valve pressure is sensed on the other side. If no pressure is applied to the control diaphragm, the feedback spring holds the control nozzle closed. Any external supply pressure applied under this condition overrides the actuator spring pressure and closes the valve. Avionics Flow/Temperature Sensor The avionics flow/temperature sensor is made up of a flow sensor and temperature sensor enclosed in a single unit. The sensor is in the supply line downstream of the avionics airflow valve. The sensor provides signals to the flow sensing and anti-ice temperature sensing bridges in the ACS temperature/flow controller for automatic operation of the flow modulating system pressure regulator valve and anti-ice add heat valve. Avionics Ram Air Valve The avionics ram air valve is a normally open, pneumatically closed, remote-controlled shutoff valve. It is between the secondary heat exchanger ram air inlet and the avionics supply line. The valve provides ram air augmentation for avionics cooling if cooling airflow drops below an established limit. The valve is closed by 3-15