.
LAND ALT (landing or destination airport altitude).

Takeoff airport altitude (actually cabin altitude) is fed into the auto controller at all times when on the ground.
Copyright . The Boeing Company. See title page for details.
2.40.8  December 3, 2004

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 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 when N1 is greater than 60%, N2 is greater than 89% and after a time delay of 1.5 seconds. 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 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 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.
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.
Copyright . The Boeing Company. See title page for details.
December 3, 2004  2.40.9
Air Systems -Pressurization System Description 737 Operations Manual

While taxiing in, the controller drives the outflow valve slowly to the 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 airplane to a negative pressure.
An amber AUTO FAIL Light illuminates if any of the following conditions occurs:
.
Loss of DC power

.
Controller fault

.
Outflow valve control fault

.
Excessive differential pressure ( > 8.30 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 automatic operation.

Flight Path Events – Auto Mode

CABIN ALTITUDE VS. AIRPLANE ALTITUDE
Copyright . The Boeing Company. See title page for details.
2.40.10  December 3, 2004

Air Systems -Pressurization System Description
737 Operations Manual
Standby Mode Operation (CPCS)
A green STANDBY light will be illuminated when the pressure controller is in the
STANDBY mode. On the ground, the GRD position of the FLT/GRD switch drives the main outflow valve full open. The FLT position drives the main outflow valve to attempt to pressurize the cabin to the selected CAB ALT. CAB ALT should be set 200 feet below the takeoff airport altitude to pressurize the cabin properly when the FLT/GRD switch is placed to FLT prior to takeoff.
　
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