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BCD The air data computer in earlier B737 models has been replaced by
Air Data Inertial Reference Units (ADIRUs). The ADIRUs provide ambient
static pressure, baro corrected altitude, non-corrected altitude and calibrated
airspeed to both automatic controllers. The ADIRUs receive barometric
corrections from the Captain’s and First Officer’s Barometric Reference
Selectors.
The controllers receive additional information from the air / ground safety
sensor, cabin pressure altitude sensing ports and the stall management
computers.
AIR SYSTEMS
Sec. 6.2 Page 54 737
Rev. 11/15/02 #41 Continental Flight Manual
>@ DIGITAL CABIN PRESSURE CONTROL SYSTEM (DCPCS)
SCHEMATIC
AIR SYSTEMS
737 Sec. 6.2 Page 55
Flight Manual Continental Rev. 11/15/02 #41
BCD DIGITAL CABIN PRESSURE CONTROL SYSTEM (DCPCS)
SCHEMATIC
AIR SYSTEMS
Sec. 6.2 Page 56 737
Rev. 11/15/02 #41 Continental Flight Manual
BCD PRESSURIZATION OUTFLOW SCHEMATIC
AIR SYSTEMS
737 Sec. 6.2 Page 57
Flight Manual Continental Rev. 11/15/02 #41
AUTO MODE OPERATION (AIRCRAFT WITH DCPCS >@BCD)
In the AUTO or ALTN mode, 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 and at lower power settings, the cabin is depressurized
by driving the main outflow valve to the full open position. The cabin begins to
pressurize on the ground when N1 is greater than 60%, N2 is greater than 89%
and after a delay of 1.5 seconds. The controller modulates the main outflow
valve toward 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 at a rate
proportional to the aircraft climb rate, cabin altitude change is held to the
minimum rate required.
Approximately 1,000 feet below flight altitude (when outside air pressure is
within .25 psi of the pressure that will exist when the aircraft is at selected FLT
ALT) the cruise mode is activated. The controller schedules a constant cabin
altitude differential between flight and cabin altitudes during cruise as follows:
· < 28,000 ft. - 7.45 psid (>@BCD)
· > 28,000 ft. - 7.80 psid (>@)
· 28,000 ft. to 37,000 ft. - 7.80 psid (BCD)
· > 37,000 ft. - 8.35 psid (BCD).
An amber OFF SCHED DESCENT light illuminates if the aircraft begins to descend
without having reached the planned cruise altitude, 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, the automatic cabin abort capability to the
original takeoff field elevation will be lost.
AIR SYSTEMS
Sec. 6.2 Page 58 737
Rev. 11/15/02 #41 Continental Flight Manual
During isobaric cruise mode, minor aircraft excursions from flight altitude may
cause the pressure differential to go as high as 7.90 psid (>@) or 8.45 psid
(BCD) to maintain a constant cabin altitude.
Beginning descent, approximately 1,000 feet below aircraft cruise altitude, the
cabin begins a proportional descent to 300 feet below the selected LAND ALT.
The controller programs the cabin to land slightly pressurized, so that rapid
changes in altitude during approach result in minimum cabin pressure changes.
While taxiing in, the controller drives the main outflow valve slowly to full open
position depressurizing the cabin when N1 is less than 50%. Having the main
outflow valve full open also prevents the equipment cooling fan from
depressurizing the aircraft to a negative pressure.
An amber AUTO FAIL light illuminates if any of the following conditions occur:
· Loss of DC power
· Controller fault
· Excessive rate of cabin pressure change (+ 2,000 sea level feet / minute)
· High cabin altitude (15,800 feet)
With illumination of the AUTO FAIL light, the pressure controller automatically
transfers to the ALTN mode.
Moving the pressurization mode selector to the ALTN position extinguishes the
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