Cabin Low-Pressure Switch
the metering valve to open. This also opens a passage in
the cabin pressure safety valve head, which causes the
The low-pressure switch is installed below the
head pressure to be slightly lowered. Since the cabin
center console to sense cabin absolute pressure. The
pressure is greater than head pressure, it opens the
normally open low-pressure switch closes at 13,000
pressure-balanced main poppet to allow cabin air to be
(500) feet and reopens at 11,000 (500) feet. The
vented overboard. When the cabin pressure differential
CAB PRESS indicator light on the annunciator panel
is restored to normal, the limit control metering valve
illuminates when the low-pressure switch closes. The
closes, and the pressure safety valve returns to its
indicator light goes off when the low-pressure switch
normally closed position.
reopens.
Cabin Pressure Regulator Control
Cabin Air Pressure Sensing Filter
The pressure regulator control is a pneumatic
The air pressure-sensing filter is located in the line
control that provides four modes of cabin pressure
that connects the cabin exhaust air duct, the cabin
operation. In addition to the modes of operation, a test
pressure regulator control, and the cabin pressure
valve is included with three manually set positions
regulator valve. The replaceable filter element, which is
(FLIGHT, DIFF ON, and ALL OFF). The test valve is
connected to the air sensing tube, is mounted with
normally lockwired in the FLIGHT position for all
clamping rings on the fuselage frame. The filter
cabin pressurization modes. The DIFF ON position
element is a cylindrical plug of treated paper and fabric
permits a ground test of the normal delta-P setpoint.
in a metal housing. The clamping rings confine the air
The ALL OFF position permits a ground test of the set
entry to the dome-shaped end to trap the entry of
point of the pressure safety valve. These test are
tobacco tar and dust particles greater than 10 microns in
accomplished with pressure supplied by support
diameter.
equipment.
Four pneumatic ports are provided on the pressure
MAINTENANCE AND INSPECTION
regulator control for interfacing with various sensed
pressures and the pressure regulator valve. These ports
Very little maintenance is required on most
are different sizes to prevent improper plumbing
pressurization and ACSs other than making the
connections.
required periodic inspections and operational checks.
In most instances, a maladjusted or malfunctioning
The pressure regulator control contains an isobaric
component simply must be removed and replaced.
bellows, which is calibrated to maintain an aircraft
There are, however, certain components that require
cabin pressure of 5,000 feet while the aircraft is flying
periodic servicing, cleaning, and inspection so the
at altitudes between 5,000 and 24,000 feet. The isobaric
component will function properly and efficiently.
bellows, which modulates a control pressure, uses
Specific requirements for servicing, cleaning, and
cabin air as a pressure source and low pressure in the
inspection are listed in the daily, postflight, and
environmental control system compartment as a
special/conditional maintenance requirement card
negative pressure. Control pressure is delivered to one
(MRC) decks as well as the maintenance instruction
side of the pressure regulator valve diaphragm, and
manual (MIM) for each aircraft.
cabin pressure is connected to the opposite side.
Because control pressure is normally less than cabin
Electrical Failures
pressure, the pressure regulator valve becomes more
open to decrease cabin pressure.
Since all pressurization and ACSs have electrically
The pressure regulator control contains provisions
controlled components, maintenance of these systems
for controlling the rate of cabin re-pressurization when
must include the related electrical circuits. Although an
recovering cabin pressure after using the cabin dump
Aviation Electrician's Mate (AE) is generally called
mode, or during a rapid descent in altitude. The control
upon to locate and correct electrical troubles, the AME
pressure modulated by the isobaric bellows is further
should be able to check circuits for loose connections,
modulated by the re-pressurization diaphragm to limit
and even perform continuity checks when necessary. A
cabin re-pressurization to an equivalent 4,000 feet per
knowledge of electrical symbols and the ability to read
minute change. The pressure regulator valve is held
circuit diagrams is therefore necessary. Figure 6-14
open until normal pressure characteristics are sensed.
6-16
