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AIR CONDITIONING -CABIN PRESSURE CONTROL SYSTEM -FLIGHT PROFILE

During descent, the ASCPC adjusts the outflow valves to give a smooth change in cabin pressure. The descent mode sets the target cabin altitude to a cabin pressure value of 0.1593 psi more than landing field pressure.
For descent, the landing altitude can be equal to the airport that the airplane departed from. It can also be equal to the destination airport. The ASCPC uses the destination airport if any of these conditions occur:
*
More than 400 nautical miles from departure airport

*
More than halfway to the destination airport

*
Two minutes or less before start of FMC planned descent.

The descent mode has two ways that it can operate:
*
FMC descent mode

*
Internal descent mode.

When the FMC descent mode is enabled, the ASCPC uses landing altitude data from the FMCF to calculate the necessary descent schedule. The descent schedule makes sure the cabin pressure gets to the target for the cabin altitude when the airplane gets to the landing altitude. The usual schedule for the FMC descent mode sets the cabin rate of change to 0.159 psi/ minute, approximately 300 ft./min (92 m/min) or less.
If the internal descent mode is enabled, the cabin pressure changes at a maximum rate until one of these conditions occurs:
*
The target cabin altitude is reached

*
The 8.6 psi cabin differential pressure limit controls cabin pressure.

If the landing field elevation is above the cabin altitude, the cabin rate of changes is 0.266 psi/minute, approximately 500 ft./ min (153 m/min). If the landing field elevation is below the cabin altitude, the cabin rate of change is 0.239 psi/minute, approximately 451 ft./min (138 m/min).
If the landing altitude is more than 8000 feet (2440 meters), the descent schedule lets the cabin altitude go above the usual limit of 8000 feet (2440 meters).
If landing altitude data from the FMCF is incorrect, the flight crew manually sets the landing altitude. If there is no landing altitude data given, 1000 feet (305 meters) is the default value.
Landing/Ground Mode
When the airplane lands, the ASCPC in control puts the outflow valves in the full open position. The valves move at a rate that makes the cabin pressure change 0.2655 psi/minute, approximately 500 ft./min (153 m/min). If the valves are not full open after 45 seconds, the rate changes to 2000 feet/minute (610 m/min.). One minute after landing, the ASCPCs do a preflight BIT.
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AIR CONDITIONING -CABIN PRESSURE CONTROL SYSTEM -FLIGHT PROFILE

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777 AIRCRAFT MAINTENANCE MANUAL

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Page 22 D633W101-BEJ Jan 05/2002

AIR CONDITIONING -CABIN PRESSURE CONTROL SYSTEM -FLIGHT PROFILES

General
These flight profiles give examples of the relation between airplane altitude and cabin altitude from takeoff to landing. They are based on these FMC modes: climb, cruise, and descent. The modes are active if all of these conditions occur:
*
FMCF supplies correct data

*
Autopilot is on

*
Vertical navigation is on.

New York (USA) to Quito (Ecuador)
This shows a flight profile where the landing field elevation is above the 8,000 feet (2440 meters) cabin altitude limit. The ASCPC sets the target for cabin altitude to 7,950 feet (2425 meters).
The ASCPC makes sure the cabin altitude gets to the target altitude when the airplane gets to cruise altitude. Scheduled step climbs are included in the calculation of the cabin rate of climb.
The ASCPC holds the cabin altitude at the target value when the airplane is in the cruise mode. When the airplane descends more than 1000 feet (305 meters), the ASCPC changes the target value for the cabin pressure. The ASCPC sets the cabin pressure value to 0.1593 psi more than the landing field pressure. The cabin altitude climbs at a rate that lets the cabin get to its target altitude at the same time the airplane lands.
EFFECTIVITY
BEJ ALL

Seattle (USA) to Denver (USA)
This shows a flight profile from a low altitude airport to a high altitude airport. In this example, the target for the cabin altitude can not be set to the landing field elevation. if the landing field elevation were selected, cabin differential pressure would go above the 8.6 psid limit. Therefore, the ASCPC sets the target for the cabin altitude to the pressure limit of 8.6 psid.
　
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