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 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 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.