sealing compounds. Access and removable doors
and hatches have integral seals. Canopies are
constructed with inflatable seals. The pressurizing
air is the air from the aircraft ACS.
The S-3 aircraft incorporates a cabin pres-
surization subsystem. This subsystem regulates the
outflow of air from the cabin to control the cabin
pressure according to a predetermined schedule.
Cabin air is drawn through the internal avionics
racks by the cabin exhaust fan and is modulated
by the cabin pressure regulator valve. A cabin
pressure regulator control provides the pres-
surization schedule.
System Operation
The cabin pressurization subsystem is
governed by the pressure regulator control, which
provides five modes of operation: unpressurized,
isobaric, differential cabin-to-ambient pressure,
dump, and repressurization.
Below 5,000 feet, the cabin is normally un-
pressurized. Between 5,000 and 25,000 feet, the
cabin altitude will remain at 5,000 feet. Maximum
cabin pressure-to-ambient differential is 6.7 ± 0.1
psi. Table 3-1 displays various cabin pressure
differentials and cabin altitudes for different flight
levels.
During the unpressurized mode of operation,
the pressure regulator control directs low-pressure
air to the pressure regulator valve to command
it to the full open position. This mode of
operation occurs at all altitudes below 4,350 feet.
In this mode, cabin pressure is maintained at a
near ambient pressure. The pressure is slightly
above ambient because of the duct pressure losses,
the quantity of air flowing into the cabin, and the
pressure across the internal avionics ventilation
subsystem.
During flight operations between 5,000 and
24,000 feet, the isobaric mode maintains the cabin
altitude between 4,350 and 5,000 feet. The
pressure regulator control, using the sensed
ambient pressure as a low-pressure source and the
sensed cabin pressure as the high-pressure source,
modulates the pressure regulator open or closed
to maintain cabin pressure at the specific altitude.
The differential mode of operation overrides
the isobaric mode of operation when the aircraft
is flying at altitudes in excess of 24,000 feet. As
cabin-to-ambient differential pressure reaches
6.7 ± 0.1 psi, a spring-loaded diaphragm in the
pressure regulator control positions a poppet valve
to supply this differential pressure as a control
pressure to the pressure regulator valve. The
pressure regulator valve compares this control
pressure to cabin pressure, and it positions
Table 3-1.—Cabin Altitude versus Flight Altitude Schedule
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