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pressure.
Two outflow valves on the aft pressure bulkhead
control the outflow of cabin air. The outflow
valves vary between open and closed to
increase or reduce outflow.
When the outflow valves are more open, inflow
air escapes the cabin rapidly, reducing
cabin pressure. This increases cabin altitude
(the cabin “climbs”).
When the outflow valves are more closed, inflow
air remains in the cabin longer, increasing
cabin pressure. This decreases cabin
altitude (the cabin “dives”).
On the ground, the outflow valves open fully,
putting the cabin at ambient pressure. This
ensures that doors open easily and prevents
sudden pressure changes when they are
opened.
The outflow valves are controlled pneumatically.
Automatic control requires normal DC
power and is not available during emergency
electrical power operations. Additionally,
safety valves operate regardless of the availability
of other systems.
Selecting AUTO mode generates an automatic
cabin altitude schedule (autoschedule) based on:
• Departure field elevation
• Maximum altitude reached
• Pilot input of destination elevation
(DEST ELV)
After takeoff, the pressurization system attempts
to maintain a cabin altitude equal to departure
elevation until cabin differential
pressure reaches 8.3 psid. Then the pressurization
system reverts to the autoschedule.
The pressurization controller has two modes of
operation, normal and high altitude. High-altitude
mode is discussed later in this chapter.
DESCRIPTION
The pressurization system includes controlling
pressurized cabin air outflow and the associated
valves, controls, indications, pneumatic
lines, and electrical circuits (see Figure 12-1).
The pressurization outflow system includes the
following components:
• Outflow valves (with associated control
and safety valves)
• Pressurization controller
• Pressurization system controls and
display
To provide system redundancy, each outflow
valve contains a maximum differential pressure
limiter valve and a maximum cabin altitude
limiter valve. These valves ensure the cabin
pressure remains within safe limits. Outflow
valves, operated manually by the CABIN DUMP
switch and automatically by the pressurization
controller, provide control of the pressurization
system. The system authority in descending
order of control is as follows:
• Maximum cabin altitude limiter
• Maximum differential pressure limiter
• CABIN DUMP switch
• Pressurization controller
COMPONENTS
OUTFLOW VALVES
The outflow valves release air from the cabin
at a regulated rate. This establishes the pressure
balance between inflow and outflow, and
resulting cabin pressure (“cabin altitude”).
The outflow valves are controlled by various
regulating valves that connect to (or are built
into) the outflow valves (see Figure 12-1).
Outflow Valve Basics
Two outflow valves regulate the outflow of
pressurized cabin air through two openings in
the aft cabin pressure bulkhead. This regulated
outflow controls cabin pressure differential
(Delta-P) and resulting “cabin altitude.” Each
outflow valve opens or closes its respective
cabin opening with a flexible diaphragm. The
diaphragm is on the back of a hollow control
chamber (Figure 12-2). When the outflow
valve control chamber inflates, the diaphragm
on the back side of the valve expands, reducing
the cabin opening. This restricts airflow
between the cabin and outside air.
When the outflow valve control chamber deflates,
the diaphragm pulls back from the cabin
opening and exposes more of it, allowing more
air to move through the opening between the
cabin and the outside (tail cone). Outflow
valves normally are partially open, providing
a limited outflow of cabin air, and a controlled
cabin pressure differential, resulting in a controlled
cabin altitude.
The outflow valves synchronize with each
other through a pneumatic link. The control
chambers of the two outflow valves always
have the same pressure because they are pneumatically
connected. Consequently, each outflow
valve duplicates the action of the other.
This feature balances the cabin air outflow of
the two valves (see Figure 12-1).
Outflow Valve Regulation
Multiple regulating valves are built into each
outflow valve. These valves regulate the air
pressure in the outflow valve control chamber
and thereby control the diaphragm position.
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