Learning Objective: Recognize the com-ponents and functions of cabin cooling and antifog systems.

The F-18 aircraft cabin cooling and antifog system (fig. 3-10) controls and transports conditioned air to the cabin. Conditioned air from the air cycle ACS is transported to the cabin flow valve where cabin inlet airflow is controlled by signals from the cabin airflow/ temperature sensor through the ACS temperature/ flow controller. Air leaving the cabin flow valve is mixed with hot air from the cabin add heat valve through signals from the suit/ cabin temperature control, cabin airflow/ temperature sensor, and the ACS temperature/ flow controller. The conditioned air is divided between cabin air and wind-shield defog air by the cabin defog plenum distribution valve through the control handle and linkage and push-pull control. Cabin cooling and antifog components are discussed in the following paragraphs.

CABIN FLOW VALVE

The cabin flow valve is a normally open, electropneumatically controlled modulating valve. The valve responds to signals from the cabin airflow/ temperature sensor and the ACS temperature/ flow controller to maintain cabin inlet airflow at a differential pressure above cabin pressure. A control circuit in the ACS temperature/ flow controller compares the signal from the cabin airflow/ temperature sensor with a preset reference and produces the required signal to modulate the cabin flow valve to satisfy the flow requirements. A position indicator is visible on the valve body.

CABIN ADD HEAT VALVE

The cabin add heat valve is an electro-pneumatic, butterfly, modulating valve. The valve modulates hot airflow into the cabin inlet duct in response to electrical signals from the cabin airflow/ temperature sensor through the ACS temperature/ flow controller and suit/ cabin temp control. As cabin inlet air temperature decreases to below the required level, as selected by the suit/ cabin temp control, the cabin airflow/ temperature sen-sor and ACS temperature/ flow controller provide an electrical signal to the valve torque motor, allowing regulated air pressure to open the valve. When electrical current to the valve is below a minimum, the valve is held closed by spring pressure. If a cabin air supply overtemperature occurs, the cabin air overtemperature sensor vents control pres-sure from the valve, allowing it to close. A position indicator is visible on the valve body.

CABIN AIR OVERTEMPERATURE SENSOR

The cabin overtemperature sensor is a pneumatic bleedoff type. It is installed in the cabin distribution duct downstream of the cabin add heat valve. At a preset temperature, the expansion bellows unseats a ball. Control head pressure is vented overboard allowing the cabin add heat valve to close.

ECS RAM AIR ACTUATOR ASSEMBLY

The actuator part of the ram air system supplies mechanical force to extend the ram air scoop. The actuator is connected to the ram air scoop through linkage. It is held closed when air pressure is applied and the ECS ram air solenoid is de-energized. The actuator opens when there is a loss of air pressure or when the solenoid is energized.