Pressurization Outflow Schematic

MAIN OUTFLOW VALVE 
DIFFUSER, OVERHEAD OUTLETS AND CARGO  DIFFUSER OUTLET  pressurization system. -Controlled by the 

AFT LOOKING VIEW
Auto Mode Operation (CPCS)
In AUTO, the pressurization control panel is used to preset two altitudes into the pressure controller:
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FLT ALT (flight or cruise altitude).

.
LAND ALT (landing or destination airport altitude).

Takeoff airport altitude (actually cabin altitude) is input into the pressurization controller at all times when on the ground.
Copyright . The Boeing Company. See title page for details.
2.40.6  June 7, 2002

Air Systems -Pressurization System Description
737 Operations Manual
The air/ground safety sensor signals whether the airplane 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 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 airplane and cabin altitude. By increasing the cabin altitude at a rate proportional to the airplane climb rate, cabin altitude change is held to the minimum rate required.
When the aircraft is operated at low gross weights it is capable of sustaining rates of climb to cruising altitude greater than the pressure controller logic can reduce cabin pressure. This may result in reaching the maximum pressure differential and causing the relief valves to open and vent the cabin.
An amber OFF SCHED DESCENT light illuminates if the airplane 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.
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 7.8 psid above 28,000 feet or 7.45 psid below 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 7.8 psid above 28,000 feet and 7.45 psid below 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 to 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.
Taxiing in, the controller drives the main outflow valve slowly to full open when the FLT/GRD switch is positioned to GRD, thereby depressurizing the cabin. Having the main outflow valve full open also prevents the equipment cooling fan from depressurizing the airplane to a negative pressure.
Copyright . The Boeing Company. See title page for details.
December 3, 2004  2.40.7
Air Systems -Pressurization System Description 737 Operations Manual

An amber AUTO FAIL light illuminates if any one of these conditions occurs:
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Loss of AUTO AC power

.
Excessive rate of cabin pressure change (1890 sea level feet/minute)

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Excessive differential pressure

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High cabin altitude (13,875 feet). With illumination of the AUTO FAIL Light, the pressure controller automatically

trips to STANDBY mode; however, the pressurization mode selector remains in AUTO. Positioning the mode selector to STBY extinguishes the light.
Flight Path Events – Auto Mode

CABIN ALTITUDE VS. AIRPLANE ALTITUDE
Auto Mode Operation (DCPCS)
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:
　
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