Pressurization Outflow Schematic
Copyright . The Boeing Company. See title page for details.
2.40.4 June 1, 2003
Air Systems -Pressurization System Description
737 Flight Crew Operations Manual
Auto Mode Operation
The AUTO system consists of two identical controllers, with one controller alternately sequenced as the primary operational controller for each new flight. The other automatic controller is immediately available as a backup.
In the AUTO or ALTN mode, the pressurization control panel is used to preset two altitudes into the auto controllers:
.
FLT ALT (flight or cruise altitude).
.
LAND ALT (landing or destination airport altitude).
Takeoff airport altitude (actually cabin altitude) is fed into the auto controllers at all times when on the ground.
The air/ground safety sensor signals whether the airplane is on the ground or in the air. On the ground and at lower power settings, the cabin is depressurized by driving the outflow valve to the full open position.
The cabin begins to pressurize on the ground at higher power settings. The controller modulates the 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 auto controller maintains a proportional pressure differential between airplane and cabin altitude. By increasing the altitude at a rate proportional to the airplane climb rate, cabin altitude change is held to the minimum rate required.
An amber OFF SCHED DESCENT light illuminates if the airplane begins to descend without having reached the preset 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 FLT ALT indicator is changed, the automatic abort capability to the original takeoff field elevation is lost.
Copyright . The Boeing Company. See title page for details.
Air Systems -Pressurization System Description 737 Flight Crew Operations Manual
The cruise mode is activated when the airplane climbs to within 0.25 psi of the selected FLT ALT. During cruise the controller maintains the cabin altitude slightly below the selected LAND ALT, if the differential pressure between the selected LAND ALT and FLT ALT is less than or equal to 8.35 psid above 37, 000 feet, 7.80 psid with the FLT ALT between 28,000 and 37,000 feet, and 7.45 psid with FLT ALT less than 28,000 feet. If the differential pressure between the selected LAND ALT and FLT ALT is greater than these values, the controller maintains a pressure differential of 8.35 psid above 37,000 feet, 7.80 psid with the FLT ALT between 28,000 and 37,000 feet, and 7.45 psid with FLT ALT less than 28,000 feet. Deviations from flight altitude can cause the pressure differential to vary as the controller modulates the outflow valve to maintain a constant cabin altitude.
The descent mode is activated when the airplane descends 0.25 psi below the selected FLT ALT. The cabin begins a proportional descent to slightly 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 outflow valve slowly to the full open position depressurizing the cabin.
An amber AUTO FAIL light illuminates if any of the following conditions occurs:
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Loss of DC power
.
Controller fault
.
Outflow valve control fault
.
Excessive differential pressure (> 8.75 psi)*
.
Excessive rate of cabin pressure change (±2000 sea level feet/minute)*
.
High cabin altitude (above 15,800 feet).* *If controller is not responding properly
With illumination of the AUTO FAIL light, the pressure control automatically
transfers to the other auto controller (ALTN mode).
Moving the pressurization mode selector to the ALTN position extinguishes the
AUTO FAIL light, however the ALTN light remains illuminated to indicate single
channel operation.
Copyright . The Boeing Company. See title page for details.
2.40.6 June 1, 2003
Air Systems -Pressurization System Description
Flight Path Events – Auto Mode
*Note: Deviations from cruise alt can cause
P to vary.
CABIN ALTITUDE VS. AIRPLANE ALTITUDE
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