ment panel (fig. 2-29) above the cabin rate-of-climb
integral electric solenoid which controls the bleed air
indicator. The longer needle indicates aircraft alti-
to the firewall shutoff valves. A normally open sole-
tude in thousands-of-feet on the outside dial. The
noid operated by the landing gear safety switch con-
shorter needle indicates pressure differential in PSI
trols the introduction of ambient air flow to the
on the inner dial. Maximum differential is 6.1 PSI.
cabin on takeoff.
f. Outflow Valve. A pneumatically operated
outflow valve, located on the aft pressure bulkhead,
maintains the selected cabin altitude and rate-of-
climb commanded by the cabin rate-of-climb and
altitude controller on the copilots instrument panel.
As the aircraft climbs, the controller modulates the
outflow valve to maintain a selected cabin rate of
climb and increases the cabin differential pressure
until the 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 opera-
tion requiring oxygen. This light is protected by a
5-ampere breaker, placarded PRESS CONTR.
(1.) 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 automati-
cally. When the aircraft is on the ground, circuitry
from the landing gear safety switch prevents ambi-
ent air from entering the flow control unit to pro-
vide maximum heating.
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 instrument panel 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 sole-
noid valve operated by the cabin pressure dump
switch adjacent to the controller. It is also wired
through a landing gear safety switch. If either of
these switches is open, or the vacuum source or elec-
trical 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.
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.
2-60. OXYGEN SYSTEM.
Drain. A drain in the outflow valve static
control line is provided for removal of accumulated
moisture. The drain is located behind the lower side-
wall upholstery access panel in the baggage section
of the aft compartment.
Flow Control Unit. A flow control unit for-
ward 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. 2-12) operates an
a. Description. The oxygen system (fig. 2-19)
is provided primarily as an emergency system, how-
ever, the system may be used to provide supplemen-
tal (first aid) oxygen. Two 70 cubic foot capacity
oxygen supply cylinders charged with aviators
breathing oxygen are installed in the unpressurized
portion of the aircraft behind the aft pressure bulk-
head. 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 PRES-
SURE, located aft of the pilots 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 pan-
els. Both cylinders are interconnected, so refilling
can be accomplished through a single tiller 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 flow planning rates vs alti-
tude. Table 2-5 shows oxygen duration capacities of