AIR CONDITIONING -CABIN PRESSURE CONTROL SYSTEM -FUNCTIONAL DESCRIPTION

EFFECTIVITY BEJ ALL  21-31-00 
Page 17 
D633W101-BEJ  Jan 05/2002 

AIR CONDITIONING -CABIN PRESSURE CONTROL SYSTEM -FLIGHT PROFILE

General
The air supply and cabin pressure controllers (ASCPCs) use data from the flight management computing function (FMCF) of AIMS to control cabin pressure. These are the FMCF modes that the ASCPCs use:
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Ground mode/takeoff

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Climb

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Cruise

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Descent

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Landing/ground mode.

Ground Mode/Takeoff
When the ground mode is on, the ASCPC sets the outflow valves to full open. This makes sure that the airplane does not pressurize on the ground.
When the ground mode is on more than one minute, both ASCPCs do a preflight BIT. This test does a test of the operation of the ASCPCs and the outflow valve.
When the engine N1 speed is more than 60%, and the airplane speed is above 25 knots and increasing, the ASCPC moves the outflow valves to a more closed position. This pre-pressurizes the airplane to 0.1 psi above takeoff field pressure, approximately 189 feet (58 meters) below the takeoff field. The valves continue to close as the airplane speed increases. This keeps cabin pressure surges due to the force of ram air at a minimum as the airplane rotates.
Climb
The ASCPC enables the climb mode when any of these conditions occur:
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The takeoff mode is enabled and the airplane goes from the ground to the air

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The airplane climbs more than 1000 feet (305 meters)

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The FMCF shows a climb mode.

During climb, the ASCPC has these functions:
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Calculates and sets the target value for cabin altitude at cruise

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Calculates and sets the cabin rate of climb to a value less than or equal to 500 feet/minute (153 meters/minute)

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Limits the cabin to ambient pressure differential to 8.6 psi

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Limits cabin altitude to 8000 feet (2440 meters) or less

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Adjusts the outflow valves to give a smooth change in cabin pressure.

The climb mode has two ways that it can operate:
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FMC climb mode

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Internal climb mode.

The FMC climb mode is enabled if all of these conditions occur:
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FMC data is correct

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Autopilot is on

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Vertical navigation is on.

EFFECTIVITY BEJ ALL  21-31-00 
Page 18 
D633W101-BEJ  Sep 05/2002 

AIR CONDITIONING -CABIN PRESSURE CONTROL SYSTEM -FLIGHT PROFILE

When the FMC climb mode is enabled, the ASCPC calculates and sets the target value for cabin altitude based on these inputs:
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The FMCF airplane cruise altitude data

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The FMCF landing field elevation data

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The cabin limits.

The target value is set to 0.1593 psi more that the set landing field pressure if these conditions occur:
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The cabin differential pressure does not go above its limit

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The cabin altitude does not go above its limit.

The 0.1593 psi value makes the cabin altitude approximately 300 feet (92 meters) below the landing field elevation. This helps to prevent cabin pressure surges when the airplane lands.
If the landing field is more than 8000 feet (2440 meters), the target value for cabin altitude is set to 7950 feet (2425 meters). This only occurs if the calculated cabin altitude target value is less than 8.6 psid.
The cabin altitude target value is set to a value that gives a differential pressure of 8.6 psi if the calculated target is more than 8.6 psid.
The internal climb mode is enabled when the FMC climb mode is not enabled. The internal climb mode causes these conditions to occur:
EFFECTIVITY
BEJ ALL

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The cruise altitude is set to 43,100 feet (13,146 meters)

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The target cabin altitude is set to 7950 feet (2425 meters).

If the FMC climb mode is enabled, the ASCPC uses cruise altitude data and the time to the cruise altitude from the FMCF to calculate the necessary climb schedule. The schedule makes sure the cabin reaches the target cabin altitude at the same time the airplane gets to the cruise altitude. The usual climb schedule sets the cabin rate of change to less than 0.266 psi/ minute, approximately 500 ft./min (53 m/min).
Cruise
The ASCPC enables the cruise mode if the airplane altitude does not change more than 100 feet (31 meters) for more than three minutes. The ASCPC also enables the cruise mode if the FMCF mode is cruise.
At cruise altitude the ASCPC adjusts the outflow valves to keep the cabin pressure constant at the value calculated for the cabin altitude target.
　
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