h. Cabin Rate-Of-Climb Indicator D2 T . An
indicator, placarded CABIN CLIMB, is installed just
ahead of the control quadrant. It is calibrated in
thousands-of-feet per-minute change in cabin altitude.
i. Cabin Altitude Indicator C D1. An indicator,
placarded CABIN ALT, is installed in the instrument
panel, Figure 2-17, above the cabin rate-of climb
indicator. The longer needle indicates aircraft altitude
in thousands-of-feet on the outside dial. The shorter
needle indicates pressure differential in psi (pounds-
per-square-inch) on the inner dial. Maximum
differential is 6.1 psi, Figure 2-17.
j. Cabin Altitude Indicator D2 T . An
indicator, placarded CABIN ALT, is installed just
ahead of the control quadrant. The longer needle
indicates aircraft altitude in thousands -of-feet on the
outside dial. The shorter needle indicates pressure
differential in psi (pounds-per-square-inch) on the
inner dial. Maximum differential is 6.1 psi D2 T1 and
6.5 ± psi T2 .
k. 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. 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, placarded ALT WARN, to
warn of operation requiring oxygen.
l. Safety Valve. Before take-off, the safety
valve is open with equal pressure between the cabin
and the outside air. The safety valve closes on lift off if
the CABIN PRESS DUMP switch is in the PRESS
mode. The safety valve adjacent to the outflow valve
provides pressure relief in the event of a 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 PRESS 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 electrical
power is lost, the safety valve will close to atmosphere
except at maximum pressure differential of 6.1 psi D2
T1 and 6.5 ± psi T2 .
A screen in the safety valve should be cleaned at
1000-hour intervals. 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.
m. 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 compartment.
n. 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. This unit is fully pneumatic except for
an integral electric solenoid firewall shutoff valve
controlled by the BLEED AIR VALVE switches on the
overhead control panel, Figure 2-15, and a normally
open solenoid operated by the landing gear safety
switch which controls the introduction of ambient air
flow to the cabin on take off.
(1) 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.
(2) Bleed airflow 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
(3) 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.
2-69. OXYGEN SYSTEM C .
Do not smoke while oxygen is in use.
a. Description. The oxygen system provides a
sufficient supply of oxygen for the pilot, copilot, and
nine cabin outlets to permit a descent from 31,000 feet
down to 13,000 feet pressure altitude. The system is a
constant flow-type for the nine passenger outlets. The
pilot and copilot utilize diluter-demand 100% masks. A
49-cubic foot lightweight supply cylinder is installed
behind the aft pressure bulkhead. The oxygen gauge
on the copilot's subpanel gives direct readout of
cylinder pressure. The pressure regulator and control
valve are adjacent to the supply cylinder and are
activated by a remote push-pull knob located in the
cockpit, immediately aft of the overhead circuit breaker
panel. Pulling the SYSTEM / PULL ON / CREW
READY knob supplies oxygen to the pilot, copilot, and