the butterfly valve to maintain the required
The cabin pressurization system also makes
provision for dumping cabin pressure in an
emergency. By setting the cabin pressure switch
on the environmental control panel to the DUMP
position, the latching solenoids on both the cabin
outflow pressure regulating valve and on the cabin
safety valve are actuated to the dump position.
In addition, the recirculation air shutoff valve will
be actuated to the full open position, provided
electrical power is available. A secondary method
of achieving cabin depressurization is to turn the
air-conditioning switch to the OFF/RESET
position and select the auxiliary vent mode. This
selection will cause the cabin outflow pressure
regulator valve to open, but it will not actuate the
cabin safety valve to the open position.
The repressurization mode of operation is used
when returning to the normal mode from the
dump mode or during a rapid descent in excess
of 4,000 feet per minute. In this mode, the
pressure regulator control modulates the rate of
cabin repressurization 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, thus allowing the
diaphragm to open the pressure regulator valve
butterfly. This reduces cabin pressure and the rate
Precautions for operating the S-3 cabin pres-
surization 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 repressuriza-
tion. Therefore, the cabin will be pressurized
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
1. Keep the cabin pressurized as in flight.
2. Set the cabin pressure switch to DUMP to
ensure a full-open pressure regulator valve and
a full-open pressure safety valve.
3. If the outside air temperature is below
80°F, turn the auxiliary vent selector to ON and
open the cabin entry door.
The S-3 cabin pressurization subsystem
consists of five primary components. Four of
them are shown in figure 3-20. The fifth
component is located in the cockpit. Each
component is discussed in the following
paragraphs. If you are to troubleshoot effectively,
it is important to know the relationship of each
component to the system as a whole.
CABIN PRESSURE REGULATOR
VALVE. The cabin pressure regulator valve is
a pneumatically actuated butterfly valve mounted
in the cabin exhaust ducting downstream of the
cabin exhaust fan. The butterfly is spring-loaded
to the closed position and diaphragm operated to
the open position. The pressure regulator valve
consists of the butterfly valve, which is actuated
by a pressure-controlled diaphragm, and a
solenoid valve to control the air pressure on the
diaphragm. The solenoid valve is electrically con-
nected to the cabin pressurization switch on the
environmental control panel (fig. 3-21).
There are three ports leading into the pressure
regulator valve diaphragm chamber (fig. 3-20).
The first port is located on the spring-loaded
closing side of the diaphragm. It admits pressure
from the cabin pressure regulator control. The
second port is the ambient vent port. It is also
located on the spring-loaded closing side of the
Figure 3-21.Environmental control panel.