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 B.  The 3-position AUTO SELECT switch allows the flight crew to choose which of the 2 cabin pressure controllers is in primary control of the CPCS. When the switch is in the NORM position, the choice between controller A or controller B is made automatically. When the switch is in the A or B position, the corresponding controller is specifically assigned to be in primary control.
 C.  The LDG ALT switch allows manual selection of landing altitude as a backup to the normal automatic selection of landing altitude via the flight management computers.
 D.  The two MAN-L and MAN-R pushbutton switches enable each outflow valve to be under manual control. The OPEN-CLOSE toggle switch can then be used to drive each outflow valve more open or more closed.
 E.  Two position indicators on the selector panel display outflow valve position in all modes of operation.
 3.  Cabin Pressure Controller (Fig. 2)
_________________________
 A.  Two identical automatic cabin pressure controllers control and perform fault detection of the CPCS in all flight modes. The controllers are located on the E1-3 and E2-3 electronics shelves in the main electrical equipment center.
 B.  Each pressure controller receives inputs from the following sources:
 (1)
Left and Right Flight Management Computers (FMC)

 (a)
Time for Climb

 (b)
Time for Descent

 (c)
Time to Start of Descent

 (d)
Cruise Altitude

 (e)
Landing Altitude

 (f)
Autopilot Mode

 (g)
VNAV Mode

 (h)
FMC Master

 (i)
Groundspeed > 65 knots

 

 (2)
Left and Right Air Data Computers (ADC)

 (a)
Ambient (Static) Pressure

 (b)
Barometric Correction Factor

 

 (3)
CPCS Selector Panel

 (a)
Primary Air/Ground (from air/ground relay system)

 (b)
Alternate Air/Ground (from air/ground relay system)

 (c)
Manual Landing Altitude

 

 (4)
Left and Right Interface Control Units (ICU)

 (a) Outflow Valve Position

 (5)
Central Maintenance Computer System

 (a) Ground Test Initiate

 (6)
Opposite Cabin Pressure Controller

 (a) Controller Status
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MAINTENANCE MANUAL

 C.  Both controllers continuously receive input data, but one waits on standby while the other acts as the primary controller to position both outflow valves. Normally, primary control is alternately assigned to controller A and controller B on alternate flights. If the primary controller fails, the system will switch controllers automatically.
 D.  Each pressure controller has 2 internal cabin pressure sensors which sense cabin pressure through a port on the front of the controller. The digital sensor in each controller is used for computations. The analog sensor in each controller provides backup cabin pressure information for display on EICAS when information from the digital sensor is not available.
 E.  The controller has a continuous self-test feature which isolates faults in the CPCS and transmits them to the EIU's for annunciation on EICAS and for fault isolation by the central maintenance computer system. A ground test of the system may be performed with the central maintenance computer system.
 F.  When manual control of one or both outflow valves is chosen on the CPCS selector panel, the cabin pressure controllers disable their control outputs and cannot change outflow valve position.
 4.  Interface Control Unit (Fig. 2)
______________________
 A.  Two identical Interface Control Units (ICU's) are located next to the outflow valves. Each ICU receives digital command data from the primary automatic pressure controller and converts the data into an electrical output to position the outflow valve more open or more closed.
 B.  Each ICU receives command data from and transmits status data and outflow valve position data to both automatic pressure controllers over RS422 data busses. ICU A operates the left outflow valve and ICU B operates the right outflow valve.
 C.  If cabin altitude rises above 11,000 |500 feet, a high altitude switch in each ICU trips and both outflow valves are automatically driven fully closed. The switch does not trip if the cabin pressure control system is in manual mode.
　
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