*
Checks if ground test is selected
*
Software compatibility (left ASCPC to right ASCPC).
Preflight BIT
The ASCPCs do a preflight BIT when the airplane is on the ground and one or the two outflow valve switches are in AUTO.
When the preflight BIT mode is on, the ASCPCs do internal checks and an operational check of the outflow valves. During these checks, the outflow valves move. If there are faults, flight deck indications show.
Automatic Mode
When the outflow valve switches are in AUTO, the OPAS sends a signal to the ASCPCs. This sets the outflow valves for automatic operation.
Usually, the left ASCPC controls the left motor in each outflow valve through the left VCU channels. If, because of system failures, the left ASCPC cannot control both valves, the control configuration changes. This is the control sequence:
*
L ASCPC is in control if it can control both valves
*
R ASCPC is in control if it can control both valves
*
L ASCPC is in control if it can control the aft valve
*
R ASCPC is in control if it can control the aft valve
*
L ASCPC is in control if it can control the forward valve
*
R ASCPC is in control if it can control the forward valve.
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AIR CONDITIONING -CABIN PRESSURE CONTROL SYSTEM -FUNCTIONAL DESCRIPTION
Usually the ASCPC uses the VCUs to set the positions of the valves so that 20 percent of the outflow is through the forward valve and 80 percent is through the aft valve. This changes if one of the ASCPCs cannot control both valves. In that condition, each valve permits 50 percent of the outflow. This helps prevent loss of cabin pressure control if one valve fails so that it can not be controlled by the left or right ASCPC.
The same change occurs if you arm the forward or aft cargo fire switches. This keeps smoke from the cargo compartment out of the cabin area and keeps a sufficient concentration of extinguishing agent in the cargo compartment.
The ASCPCs use the VCUs to control cabin pressure and cabin pressure rate of the change. The VCUs get speed commands from the ASCPC. The speed commands can make the VCU move the outflow valve slowly or fast. The valve can move fully open or close in less than 10 seconds.
The ASCPCs use this data to control cabin pressure:
SIGNAL PRI SOURCE BU SOURCE
Cabin Pressure Active ASCPC Other ASCPC
Ambient pressure ADIRU EEC
Ground speed ADIRU
Landing altitude AIMS (FMCF) Ldg Alt Selector
Differential press AIMS (FMCF)
SIGNAL PRI SOURCE BU SOURCE
Cruise altitude AIMS (FMCF)
Time for climb AIMS (FMCF)
Time for descent AIMS (FMCF)
ETA top of descent AIMS (FMCF)
Barometric correction AIMS (PDF)
Vertical nav on/off AFDC
Air or ground mode WOW
Cabin pressure
The controllers monitor cabin pressure from an internal sensor. The sensors also give an output on the ARINC 629 buses.
The remote cabin pressure sensor sends an analog pressure signal to the electrical load management system (ELMS). The ELMS puts this signal on the ARINC 629 buses. The ASCPCs use this data to do a BITE check of the internal cabin pressure sensors.
The remote sensor also gives an analog pressure output to the AIMS. The AIMS uses this signal if the cabin pressure data is not available from the ASCPCs or from ELMS.
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AIR CONDITIONING -CABIN PRESSURE CONTROL SYSTEM -FUNCTIONAL DESCRIPTION
Ambient Pressure
The ASCPCs usually get ambient pressure data from the ADIRU. If that data is not available or is not valid, the ASCPCs use ambient pressure data from the electronic engine control (EEC).
Landing Altitude
In usual operation, the flight management computing function (FMCF) of the AIMS sends landing altitude data to the ASCPCs. Alternatively, the flight crew manually sets the data with the landing altitude selector on the bleed air/pressurization panel.
Manual Mode
When an outflow valve switch is in the MAN position, the ASCPC releases control of that outflow valve. The outflow valve manual switch sends open or close signals to the left and right channels of the VCU. The VCU opens or closes the outflow valve at a fixed speed. In the manual mode the valve can open or close fully in approximately 27 seconds.
Training Information Point
You can use the MAT to do a ground test of the cabin pressure control system. The ground test is the same as the preflight BIT.
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