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foot level grills, down around the aft cargo compartment, where it provides
heating, and is discharged overboard through the main outflow valve.
AIR SYSTEMS
737 Sec. 6.2 Page 41
Flight Manual Continental Rev. 11/15/02 #41
PRESSURE RELIEF VALVES
A negative relief valve prevents external atmospheric pressure from exceeding
internal cabin pressure.
>>@ Two pressure relief valves provide maximum safety pressure relief by
limiting the differential pressure to a maximum of 8.65 psi (max certified ceiling
37,000).
BCD Two pressure relief valves provide maximum safety pressure relief by
limiting the differential pressure to a maximum of 9.1 psi (max certified ceiling
41,000).
CABIN PRESSURE CONTROL SYSTEM (AIRCRAFT WITH CPCS)
>@ Outflow Valves
The main outflow valve can be actuated by either an AC or a DC motor. The
AC motor is used during AUTO and MAN AC operation. The DC motor is used
during STANDBY and MAN DC operation.
The forward outflow valve closes automatically to assist in maintaining cabin
pressure when the main outflow valve is almost closed or when the recirculation
fan is operating.
Electronic Cabin Pressure Controller
The pressurization system controls cabin altitude in any one of four modes as
selected by the pilot:
AUTO - Automatic, the normal mode of operation. Uses AC motor.
STBY - Semi-automatic; a standby system in the event of AUTO failure.
Uses DC motor.
MAN AC - Manual control of the system using AC motor.
MAN DC - Manual control of the system using DC motor.
In the automatic mode of operation, aircraft altitude is sensed directly from the
static ports. In the standby mode, aircraft altitude is sensed electrically from the
Air Data Computer (ADC). Barometric corrections to these pressures come
from the Captain’s altimeter in AUTO and the First Officer’s altimeter in STBY.
The controller receives additional information from the air / ground safety
sensor and cabin pressure altitude sense port.
AIR SYSTEMS
Sec. 6.2 Page 42 737
Rev. 11/15/02 #41 Continental Flight Manual
>@ CABIN PRESSURE CONTROL SYSTEM (CPCS) SCHEMATIC
AIR SYSTEMS
737 Sec. 6.2 Page 43
Flight Manual Continental Rev. 11/15/02 #41
>>@ PRESSURIZATION OUTFLOW SCHEMETIC
AIR SYSTEMS
Sec. 6.2 Page 44 737
Rev. 11/15/02 #41 Continental Flight Manual
AUTO MODE OPERATION (AIRCRAFT WITH CPCS >@)
In AUTO, the pressurization control panel is used to preset two altitudes into the
pressure controller:
· FLT ALT (flight or cruise altitude)
· LAND ALT (landing or destination airport altitude).
Takeoff airport altitude (actually cabin altitude) is fed into the pressurization
controller at all times when on the ground.
The air / ground safety sensor signals whether the aircraft is on the ground or in
the air. On the ground, the FLT/GRD switch is used to keep the cabin
depressurized by driving the main outflow valve full open when the switch is in
the GRD position. With the switch in the FLT position, the controller modulates
the main outflow valve towards close, slightly pressurizing the cabin. This
ground pressurization of the cabin makes the transition to pressurized flight
more gradual for the passengers and crew, and also gives the system better
response to ground effect pressure changes during takeoff.
In the air, the pressure controller maintains a proportional pressure differential
between aircraft and cabin altitude. By climbing the cabin altitude
“proportional” to aircraft climb rate, cabin altitude change is held to the
minimum rate required.
Approximately 1000 feet below flight altitude a cruise relay trips. The
controller schedules a constant cabin altitude during cruise using a 7.45 psi
differential (7.80 psi differential with Flt Alt >28,000 ft.) between flight and
cabin altitudes.
An amber OFF SCHED DESCENT light illuminates if the aircraft begins to descend
without having tripped the cruise relay; for example, a flight aborted in climb
and returning to the takeoff airport. The controller programs the cabin to land at
the takeoff field elevation without further pilot inputs. If the Flight Altitude
Indicator is changed or the Flight Altitude Selector is depressed during climb,
the automatic cabin abort capability to the original takeoff field elevation will be
lost.
During isobaric cruise, minor aircraft excursions from flight altitude may cause
the pressure differential to go as high as 7.90 psid to maintain a constant cabin
altitude.
The controller programs the cabin altitude at cruise to slightly below the
selected LAND ALT if the differential (between the corresponding ambient
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