OUTFLOW VALVES
Two outflow valves (one primary and one sec-
ondary) are located on the forward pressure
bulkhead (Figure 12-3). The outflow valves are
controlled by the CPC (automatic or manual
mode) to modulate the outflow of air from the
pressurized fuselage.
The valves are pneumatic poppet-type valves
and are spring-loaded closed. Ambient pres-
sure, cabin pressure, spring pressure, and vac-
u u m ( n eg a t ive p r e s s u r e ) a r e a l l fa c t o r s
affecting the position of the outflow valve,
but the vacuum source is used for control of
the valves. A vacuum source is provided
through a bleed-air jet ejector pump.
The vacuum control pressure in the outflow
valves is achieved through electropneumatic
torque motors located on each outflow valve.
The torque motor on the primary outflow valve
receives command signals from the left chan-
nel of the CPC and the torque motor on the sec-
ondary outflow valve receives command
signals from the right channel of the CPC, but
only one channel of the CPC can be active at
a time. The control chambers of the two out-
flow valves are connected with an open line
so that one of the valves is always being con-
trolled by the active channel of the controller.
Normally the left channel of the CPC is active.
The outflow valves are designed to keep pres-
sure in the cabin and will not hold pressure out
if atmospheric pressure becomes higher than
cabin altitude. For example, should the crew
mistakenly set the landing altitude too high,
the CPC would attempt to control the cabin to
that altitude during descent, but when the air-
plane descended below that altitude, the out-
flow valves would be forced open by the
atmospheric pressure being higher than the
cabin pressure. The valves open when the at-
mospheric pressure is 0.5 psid greater than
cabin pressure.
Cabin-to-atmospheric differential pressure
(Delta P) relief valves are located on each out-
flow valve to limit cabin pressure in the event
a system malfunction causes overpressuriza-
tion (Figure 12-3).
The Delta P limiters operate pneumatically
and are independent of all other CPC operat-
ing modes. They will limit differential pres-
sure (Delta P) to 9.7 psid. In the event that one
of the Delta P limiters fails, the remaining
Delta P limiter will limit differential pressure
to slightly higher than 9.7 psid.
The outflow valves each incorporate a cabin
altitude limiter (Figure 12-3). In the event the
cabin reaches 13,700 feet for whatever reason,
the altitude limiters pneumatically close the
outflow valves in an attempt to keep the cabin
altitude from climbing any higher. They may
not prevent the cabin from climbing, however,
if the supply of bleed air for pressurization has
been interrupted or if there is a leak in the
pressurized vessel.
VACUUM JET PUMP
The vacuum jet pump (Figure 12-3) is located
in the tail section of the aircraft and is driven
by bleed air from either engine. It provides a
source of vacuum (low pressure) to the torque
motor valve on the primary and secondary out-
flow valves for automatic and manual operation.
12-7
FOR TRAINING PURPOSES ONLY
L E A R J E T 4 5
P I L O T T R A I N I N G M A N U A L
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