CABIN COOLING AND
ANTIFOG SYSTEMS
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.
ECS RAM AIR SOLENOID
ASSEMBLY
The solenoid assembly is mounted in the duct
upstream of the actuator for the ram air scoop.
3-12
