position until duct temperature returns to normal,
System Operation
then automatic control resumes.
The cabin duct thermal switch provides an
As you read this section, refer to figure 4-25.
Placing the cockpit switch to ON causes contact
additional override to the cabin temperature
sensor. This temperature-sensitive switch closes
3 to remove power from the cabin bleed air
when the duct inlet temperature exceeds 121.1C
shutoff valve. This valve opens, allowing hot air
or the dual temperature sensor malfunctions.
to flow to the dual temperature control valve.
During either of these conditions, 120-volt ac from
Simultaneously, a second portion of the cockpit
the ENVIRONMENT/SEATS CB goes through
switch, ganged to the first, connects the center tap
the HOT side of the cabin duct thermal switch
of the MAN/RAM AIR switch through energized
and contact 4 of the cockpit switch to the control
contact of the air-conditioning control relay, the
winding of the dual temperature control valve.
COLD side of the cabin duct thermal switch, and
The valve drives to the full cold position. When
contact 4 of the COCKPIT SWITCH to the
cabin duct inlet temperature falls to 121.1C, the
control phase of the dual temperature control
switch goes to the cold position and automatic
valve. Contact 1 of the cockpit switch powers the
temperature control again resumes.
coil of the air-conditioning control relay when the
In the automatic mode, positioning the
cockpit switch is in the ON or OFF position.
AUTO TEMP CONT thumbwheel selects cockpit
Cockpit switch contact 2 (in the ON or OFF
temperature. This fixes the output voltage of the
position) supplies 120 volts to the close winding
bridge circuit in the cabin and ventilation suit
of the cabin ram air valve. A phase shifting
controller. The controller amplifies the signal
capacitor powers the open winding of the same
valve, which drives the two-phase induction motor
and transmits through contact 4 of the air-
conditioning relay to the AUTO position of the
to the full close position.
In the air-conditioning mode (cockpit switch
MAN/RAM AIR switch. The signal travels back
through contact 3 of the air-conditioning relay to
ON), hot engine bleed air flows through the
the COLD side of the cabin duct thermal switch.
cabin shutoff valve to the dual temperature
The signal then goes through the cockpit switch
control valve, where it mixes with refrigerated,
to the control winding of the dual temperature
dehumidified air. The cabin temperature control
control valve.
system automatically maintains the cabin at a
To gain manual control of the cabin air-
selected temperature by continuously modulating
the dual temperature control valve. The modu-
conditioning system, momentarily position the
lating signals are cabin temperature errors that the
MAN/RAM AIR switch at COLD or HOT. In
cabin temperature sensor translates into error
the COLD position, power from the ENVIRON-
voltages. The cabin and ventilation suit tempera-
MENT/SEATS CB goes through contact 1 of the
air-conditioning control relay, the MAN/RAM
ture controller modulates and amplifies these
signals. Then, they go to the control winding of
AIR switch, through contact 3 of the air-
the dual temperature control valve. The hot side
conditioning control relay, the COLD contacts of
of the valve starts to close and the cold side opens
the cabin duct thermal switch, and contact 4 of
until the selected cabin temperature is achieved.
the cockpit switch to the control winding of the
dual temperature control valve. This voltage lags
To prevent temperature hunting by the cabin
the fixed-phase voltage by 900 and drives the two-
temperature sensor, the cabin duct dual tempera-
phase induction motor. This allows more
ture sensor anticipates changes in cabin tempera-
ture by sensing changes in the duct inlet
refrigerated air and less hot air to enter the cabin.
The valve will continue to move until the switch
temperature.
is released. After the switch is released, it returns
The dual temperature sensor transmits error
to HOLD and the valve remains fixed.
signals to the cabin and ventilation suit
When the MAN/RAM AIR switch is toggled
temperature controller when cabin air supply
to the HOT position, power from the ENVIRON-
temperature changes. The size of the transmitted
MENT/SEATS CB goes through the reversing
signals depends on the rate of temperature
transformer, contact 2 of the air-conditioning
changes. When the cabin duct inlet temperature
control relay, the MAN/RAM AIR switch,
exceeds 121.1C (250F), the overtemperature
contact 3 on the air-conditioning control relay,
sensor portion of this sensor transmits a fixed
the COLD contacts on the cabin duct thermal
signal. The signal goes through the temperature
switch, and contact 4 of the cockpit switch to the
controller to the dual temperature control valve
control winding of the dual temperature control
control winding. The valve drives to the full cold
