MD-11 Flight Crew Operations Manual
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Air -
Description and Operation
Air.10.7
Comfortable pressure levels are maintained by controlling discharge of cabin air
through the outflow valve. The CPCS consists of two digital Cabin Pressure
Controllers (CPC), a CABIN PRESS control panel, an outflow valve, and three
relief valves.
The CPCS has three modes of control, linked together at a common outflow valve.
The three control modes consist of two identical automatic modes and one electric
manual mode. Normal pressurization is automatic and requires no crew inputs.
The data that the CPC needs for operation can normally be received from other
aircraft systems.
The following are the interfaces that the CPC has with other aircraft systems:
• Pressure altitude, computed airspeed, and barometric correction are
received from the CADCs.
• Flight data to the CPC, such as time at top of descent, estimated time of
arrival, and landing field altitude, are received from the flight
management system (FMS). The CPCS is capable of operation without
FMS input. The system will revert to internal climb and descent rates. The
flight crew needs only to input the landing field altitude in the control
panel when this data is not available from the FMS.
The CPC outputs cabin altitude, cabin rate of change, cabin differential pressure,
outflow valve position, and system status for display on the air synoptic page.
Cabin altitude and cabin rate data are also displayed on the secondary engine page.
The outflow valve position is displayed on the CABIN PRESS control panel and
the air synoptic page.
The Centralized Fault Display System (CFDS) communicates with both CPCs.
The CPCs also send an analog pressure signal to ACCs 1 and 3. ACC 2 supplies
backup cabin altitude and rate data to the Electronic Instrument System (EIS).
All three ACCs trigger the aural warning and an alert when the cabin exceed
10,000 feet. The ACCs signal an additional alert when cabin altitude approaches
15,000 feet. The oxygen masks automatically drop.
In the automatic mode, the cabin altitude schedule is calculated as a function of
takeoff elevation, landing field elevation, aircraft altitude, and phase of flight. The
cabin altitude will remain at departure field elevation until the aircraft altitude
exceeds 5,000 feet above departure field elevation. At that time, the cabin altitude
changes according to a predetermined schedule. With the FMS data, the CPC
schedule is enhanced by setting cabin rates as a function of planned and predicted
flight altitudes.
In the automatic mode, the cabin altitude schedule is calculated by the CPCs as a
function of takeoff elevation, landing field elevation, aircraft altitude, and phase
of flight.
October 02, 2006
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