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 3)  Climb mode operates in internal climb when valid data are not available from the FMC's. If no valid FMC cruise data is available when in internal climb, the cabin pressure controller assumes a cruise altitude of 39,000 feet and selects the rate of cabin altitude change accordingly, not to exceed 1000 SL fpm.
 4)  The forward overboard valve opens during climb when the airplane is above 25,000 feet as long as air inflow is sufficient to keep the cabin properly pressurized.
Cruise
 1)  Cruise mode is entered only from climb mode. Cruise mode is entered when the airplane is within 200 SL feet of the desired cruise altitude, or when the airplane rate of climb falls below 100 SL fpm for one minute or more.
 2)  Cruise mode uses the same pressure schedule that climb mode uses. Once cruise is set, the CPCS compares the landing altitude with the scheduled cabin altitude and takes the higher of the two for the cabin, except when destination elevation is greater than 8000 feet, in which case cabin altitude is targeted to 8000 feet. Cabin altitude is maintained at the prescribed value |25 SL feet, provided that airplane altitude does not vary by more than 500 SL feet.
 3)  The forward overboard valve is normally open in cruise as long as air inflow is sufficient for proper pressurization.
 4)  Whenever cruise mode is entered, a cruise clamp signal is generated and is sent to the ECS miscellaneous card (Ref 21-26-00). Once airplane altitude has exceeded 25,000 feet during the current flight, the ECS miscellaneous card transmits the cruise clamp signal to the packs for use by the pack control logic (Ref 21-51-00).
Descent
 1)  Descent mode is initiated when the FMC's indicate that top of descent has been reached, or when an airplane descent of 1000 |50 SL feet is detected.
 2) The forward overboard valve is driven closed upon initiation of descent mode. Normally, cabin pressure will increase at a rate of -300 SL fpm until the correct pressure for landing is reached. There are three cases where cabin pressure increase does not follow this schedule: a) The rate of change of cabin pressure may be increased up to -450 SL fpm if necessary to establish proper landing pressurization. b) The rate of change of cabin pressure may be decreased down to -250 SL fpm if necessary to protect the airplane from excessive cabin-to-ambient pressure differences.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400 MAINTENANCE MANUAL
 c)  When destination elevation is above 8000 feet, cabin pressure will rate up at 1000 SL fpm until the correct landing pressure is achieved.
 3)  In the descent mode, when the airplane levels off and ascends or descends no more than 250 feet for a two minute period, a signal is sent to the packs to switch them back to normal flow. Subsequently, another signal is sent to the packs when the airplane descends more than 250 feet to return them to high flow.
 (g)  Return to Ground
 1)  At touchdown, the forward overboard valve is opened. Cabin altitude is targeted approximately 100 SL feet below landing field altitude.
 2)  At touchdown the outflow valves are opened enough to depressurize the airplane at 500 SL fpm. If after 60 seconds the cabin is not depressurized, the rate of pressurization is increased to 2000 SL fpm.
 3)  After landing, the CPCS switches which controller is in primary control (unless the AUTO SELECT switch is not in the NORM position). The controller remains primary until the next landing, unless a controller fault is detected.
 (h)  Climb Abort
 1)  Climb abort is used for air turnback (termination of a flight shortly after takeoff and subsequent return to the airport of departure). Climb abort mode is entered if the airplane descends 1000 feet after taking off from an airport at 2500 feet or lower elevation and has not climbed to 8000 feet; or if the airplane decends 1000 feet after taking off from an airport at higher than 2500 feet elevation and has not climbed through a pressure difference of at least 2.5 psi.
 (3)  
Manual

 (a)  As a backup to automatic, the CPCS allows manual operation of the outflow valves. You can select one or both of the outflow valves to be in the manual mode. When the MAN L or MAN R switch-light is on, the applicable outflow valve will be in the manual mode. In the manual mode, the outflow valve can only be moved by the OPEN/CLOSE switch. The OPEN/CLOSE switch will command the applicable outflow valve dc motor to open or close the valve flapper doors.
　
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