As the aircraft climbs, the controller modulates the outflow
valve to maintain a selected cabin rate of climb and
maximum cabin pressure differential is reached. At a
cabin altitude of 12,500 feet a pressure switch mounted
on the back of the overhead control panel completes a
circuit to illuminate a red warning annunciator light, ALT
WARN, to warn of operation requiring oxygen. This light
is protected by a 5ampere breaker, placarded PRESS
Pressurization Safety Valve. Before takeoff, the
safety valve is open with equal pressure between the
cabin and the outside air. The safety valve closes on
liftoff if the CABIN PRESS CONTR switch on the copilots
subpanel is in the PRESS mode. The safety valve
adjacent to the outflow valve provides pressure relief in
the event of failure of the outflow valve. This valve is
also used as a dump valve and is opened by vacuum
which is controlled by a solenoid valve operated by the
cabin pressure dump switch adjacent to the controller. It
is also wired through the right landing gear safety switch.
If either of these switches is open, or the vacuum source
or electrical power is lost, the safety valve will close to
atmosphere except at maximum differential pressure of 6.
1 PSI. A negative pressure relief diaphragm is also
incorporated into the outflow and safety valves to prevent
outside atmospheric pressure from exceeding cabin
pressure during rapid descent.
Drain. A drain in the outflow valve static control
line is provided for removal of accumulated moisture.
The drain is located behind the lower sidewall upholstery
Flow Control Unit. A flow control unit forward of
the firewall in each nacelle controls bleed air flow and the
mixing of ambient air to make up the total air flow to the
cabin for pressurization, heating, and ventilation. The
bleed air switches located on the overhead control panel
(fig. 2-18) operates an integral electric solenoid which
controls the bleed air to the firewall shutoff valves. A
normally open solenoid operated by the landing gear
safety switch controls the introduction of ambient air flow
to the cabin on takeoff.
The unit receives bleed air from the engine
into an ejector which draws ambient air into the nozzle of
the venturi. The mixed air is then forced into the bleed air
line routed to the cabin.
Bleed air flow is controlled automatically.
When the aircraft is on the ground, circuitry from the
landing gear safety switch prevents ambient air from
entering the flow control unit to provide maximum
The bleed air firewall shutoff valve in the
control unit is a spring loaded, bellows operated valve that
is held in the open position by bleed air pressure. When
the electric solenoid_ is shut off, or when bleed air
diminishes on engine shutdown (in both cases the
pressure to the firewall shutoff valve is cut off), the
firewall valve closes.
2-61. OXYGEN SYSTEM.
Description. The oxygen system (fig. 2-19) is
provided primarily as an emergency system, however, the
system may be used to provide supplemental (first aid)
oxygen. Two 64 cubic foot capacity oxygen supply
cylinders charged with aviator's breathing oxygen are
installed in the unpressurized portion of the aircraft behind
the aft pressure bulkhead. The pilot and copilot positions
are equipped with diluter demand type regulators, which
mix the proper amount of oxygen for a given amount of
air at altitude. Also a first aid oxygen mask is provided in
the cabin. Oxygen system pressure is shown by two
gages placarded OXYGEN SUPPLY PRESSURE, located
aft of the pilot's oxygen regulator control panel. Two
pressure reducers, located in the unpressurized portion of
the aircraft behind the aft bulkhead, lower the pressure in
the system to 400 PSI, and route oxygen to the regulator
control panels. Both cylinders are interconnected, so
refilling can be accomplished through a single filler valve
located on the aft right side of the fuselage exterior. A
pressure gage is mounted in conjunction with the filler
valve, and each cylinder has a pressure gage. Table 2-4
shows oxygen duration capacities of the system.
Regulator Control Panels. Each regulator control
panel contains a blinker-type flow indicator, a 500 PSI
pressure gage, a red emergency pressure control lever
placarded EMERGENCY NORMAL TEST MASK, a white
diluter control lever placarded 100% OXYGEN -NORMAL
OXYGEN, and a green supply control lever placarded ON
Diluter control lever. The diluter control
lever selects either normal or 100% oxygen, but acts to
select only when the emergency pressure control lever is
in the NORMAL position.
When not in use, the diluter control
lever should be left in the 100%
Emergency pressure control lever. The
emergency pressure control lever has three positions.