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
access panel in the baggage section of the aft
Flow Control Unit. A flow control unit, located
forward of the firewall in each engine 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. An integral electric solenoid firewall
shutoff valve is controlled by the ENVIRO & PNEU
BLEED AIR valve switches on the overhead control
panel (fig. 2-15). A solenoid, operated by the right
landing gear safety switch, controls the introduction of
ambient air to the cabin upon takeoff. Both the ambient
air flow control valve and the bleed air flow control valve
are motor driven.
The unit receives bleed air from the engine into an
ejector which draws ambient air into the venturi of the
nozzle. 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 heating. 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. 2-26) is
provided primarily as an emergency system; however,
the system may also be used to provide supplemental
(first aid) oxygen. Two 70 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's
and copilot's positions are equipped with diluter demand
type regulators, which automatically mix the proper
amount of oxygen for a given amount of air at altitude. A
first aid oxygen mask is also provided in the cabin.
Oxygen system pressure is shown by two gages,
placarded OXYGEN SUPPLY PRESSURE, located on
the pilot's and copilot's oxygen regulator control panels.
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, allowing refilling to be accomplished
through a single filler valve located in 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 flow planning
rates vs. altitude. Table 2-5 shows oxygen duration
capacities of the system in liters per minute (LPM) per
mask at normal temperature and pressure, dry (NTPD).
Regulator control panels.
The rapid loss of oxygen pressure
when the aircraft is above 12,500 feet
is a valid cause for alarm. Should
this condition arise, descend as
rapidly as possible to an altitude
which does not require the use of
When the oxygen system is not in
use, the diluter control lever should
be left in the 100% OXYGEN position
to prevent regulator contamination.
Each regulator control panel contains a blinker-type
flow indicator, a 500 PSI pressure gage, a red
EMERGENCY - NORMAL - TEST MASK, a white diluter
control lever placarded 100% OXYGEN - NORMAL
OXYGEN, and a green supply control lever placarded
ON - OFF. The diluter control lever selects either normal
or 100% oxygen when the emergency pressure control
lever is in the NORMAL position.
The emergency pressure control lever has three
positions. Two positions control oxygen consumption for
the individual using oxygen, and the remaining position
serves for testing hose and mask integrity. In the
EMERGENCY position, the control lever causes 100%
oxygen to be delivered at a safe, positive pressure. In
the NORMAL position, the lever allows delivery of
normal or 100% oxygen, depending upon the selection
of the diluter control lever. In the TEST MASK position,
100% oxygen at positive pressure is delivered to check
hose and mask integrity.