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OPERATING MANUAL
PSP 606
PRESSURIZATION (Figures 6, 7 and 8)
A. General
The pressurized area of the aircraft extends from the bulkhead immediately
forward of the windshield to the pressure bulkhead at the rear of the
cabin, and includes the underfloor area. The pressure in this area is
controlled by two outflow valves in the rear pressure bulkhead. The
outflow valves are controlled by an automatic electronic selector with a
manual pneumatic selector provided as a backup. The pressurized area of
the aircraft is maintained to a maximum altitude of 8000 feet, and the
cabin differential pressure is maintained between maximum positive and
negative differentials of +9.35 and -0.5 psi.
B. Outflow Valves
There are two outflow valves, located side by side in the rear pressure
bulkhead. One of the valves is designated the electropneumatic outflow
valve and contains a torque motor controlled by the automatic selector on
the CABIN PRESSURIZATION panel. The other valve is designated the
pneumatic outflow valve and is controlled and operated pneumatically.
The torque motor in the electropneumatic valve receives electrical signals
from the automatic selector on the cabin pressurization control panel and
is positioned to modulate vacuum pressure to a diaphragm chamber in the
valve. This vacuum causes a low pressure internally that opens the valve
which is spring-loaded closed. When the torque motor is commanded by the
automatic selector to reduce the valve opening it reduces the vacuum and
allows the valve to close. Opening the valve reduces cabin pressure,
closing the valve increases cabin pressure.
In the manual operating mode, the pneumatic valve operates in a similar
manner to the electropneumatic valve, except that the torque motor is
replaced by a pneumatic relay. The pneumatic relay is operated by the
manual regulator on the cabin pressurization control panel.
The outflow valve diaphragm chambers are connected by a pressure equalizing
line so that the valves operate together to regulate cabin pressure. In
the automatic operating mode, the operation of the pneumatic outflow valve
is slaved to that of the electropneumatic outflow valve. In the manual
operating mode, the slaving sequence is reversed.
In the manual mode, the system requires occasional adjustment to maintain a
selected cabin pressure, while the automatic mode is self-monitoring.
The vacuum required for the operation of the outflow valves is generated by
an air jet pump in the rear fuselage equipment bay. When pressurized air
from the bleed air manifold is forced through the jet pump, an integral
venturi creates a vacuum in an adjoining tee-fitting which connects with
the outflow valves.
SECTION 2
Page 13
Mar 01/85
OPERATING MANUAL
PSP 606
Cabin Pressurization Control Panel (Figure 6)
The cabin pressurization control panel is located in the pilot's overhead
panel and contains the components described below:
(1) Automatic Selector
The automatic selector has ALT, RATE and BARO SET controls for
selection of cabin altitude, cabin altitude rate of change and
airfield barometric pressure. A pressure transducer and associated
circuits in the selector detect the cabin pressure and measure its
rate of change. The selector then powers the torque motor on the
electropneumatic outflow valve to achieve the desired cabin altitude
and selected rate of change.
(2) Manual Regulator
The manual regulator is a needle valve assembly that meters jet pump
vacuum and cabin pressure to the pneumatic relay of the pneumatic
outflow valve. The needle valve is located behind the CABIN
PRESSURIZATION panel and is controlled by the rotary MAN REG switch.
To ensure that the pneumatic outflow valve is fully slaved to the
electropneumatic outflow valve during operation of the system in the
automatic mode, the MAN REG selector must be turned to the full down
(DN) position.
(3) Emergency Depressurization Switch/light
The emergency depressurization switch/light, EMER DEPRESS, is used to
depressurize the aircraft rapidly. This is achieved by supplying
power to the outflow valve torque motor which causes the outflow valve
to open to vent the aircraft pressure.
(4) Emergency Pressurization Switch/light
The emergency pressurization switch/light, EMER PRESSN, is used to
open a normally closed shutoff valve to permit the entry of warm air
from the footwarmer and demist system. When the switch/light is
operated, the left footwarmer and demist system pressure regulator and
shutoff valve also open, and air from the footwarmer and demist system
enters the conditioned air crossover duct which supplies air to the
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