Turn AUX VENT selector to ON if outside air
pressure regulator valve compares this control pressure
to cabin pressure, and it positions the butterfly to
temperature is below 80F, and open the cabin entry
maintain the required differential pressure.
door to ensure an adequate supply of cooling air.
The cabin pressurization system also makes
COMPONENTS
provision for dumping cabin pressure in an emergency.
By setting the cabin pressure switch on the
The S-3 cabin pressurization subsystem consists of
environmental control panel to the DUMP position, the
five primary components (figs. 6-11 and 6-12). Four of
latching solenoids on both the cabin outflow pressure
them are shown in figure 6-11. The fifth component is
regulating valve and on the cabin safety valve are
located in the cockpit. Each component is discussed in
actuated to the dump position. In addition, the
the following paragraphs. If you are to troubleshoot
re-circulation air shutoff valve will be actuated to the
effectively, it is important to know the relationship of
full open position, provided electrical power is
each component to the system as a whole.
available. A secondary method of achieving cabin
depressurization is to turn the air-conditioning switch
Cabin Pressure Regulator Valve
to the OFF/RESET position and select the auxiliary
vent mode. This selection will cause the cabin outflow
The cabin pressure regulator is a pneumatically
pressure regulator valve to open, but it will not actuate
actuated butterfly valve mounted in the cabin exhaust
the cabin safety valve to the open position.
ducting downstream of the cabin exhaust fan. The
butterfly is spring-loaded to the closed position. The
The re-pressurization mode of operation is used
pressure regulator valve consists of the butterfly valve,
when returning to the normal mode from the dump
which is actuated by a pressure-controlled diaphragm,
mode or during a rapid descent in excess of 4,000 feet
and a solenoid valve to control the air pressure on the
per minute. In this mode, the pressure regulator control
modulates the rate of cabin re-pressurization with an
integral isobaric and differential pressure control
system. The pressure regulator control compares the
existing cabin pressure to a lagging cabin pressure
reference. If the result of this comparison exceeds the
calibrated rate, control pressure output from the
pressure regulator control is reduced. This causes the
pressure regulator valve to sense a relatively higher
pressure on the opening side of its actuating diaphragm,
allowing the diaphragm to open the pressure regulator
valve butterfly. This reduces cabin pressure and the rate
of pressurization.
Precautions for operating the S-3 cabin
pressurization subsystem on the ground, where the
elevation is 5,000 feet or higher, are required because
the cabin pressurization subsystem does not have
provisions for automatic depressurization. Therefore,
the cabin will pressurize whenever the ground elevation
is above 5,000 feet.
To ensure adequate cooling of the internal avionics
during operations at ground elevations above 5,000
feet, one of the following steps must be used:
Keep the cabin pressurized as in flight.
Set CABIN PRESS switch to DUMP to ensure a
full-open pressure safety valve.
Figure 6-11.--Cabin pressurization subsystem schematic.
6-14
