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MD-11 Central Air Data Computers (CADCs) - Operation and Overview
The MD-11 is equipped with 2 Central Air Data Computers (CADCs), which
both receive inputs from the following sources: static air pressure from the static system,
total air pressure from the pitot system, TAT resistance from the TAT
probes, synchro signals from the angle-of-attack (AOA) sensors, digital
baroset signals from the glareshield control panel (GCP) baroset controls,
wing tip tank fuel quantity digital signals from the fuel quantity data
control unit, TAT probe heater status signal from the miscellaneous systems
controller (MSC), and control signals from the gear down switches.
The CADCs process these inputs to supply: total air pressure, static air
pressure, impact air pressure, captain's baroset corrections, first
officer's baroset corrections, computed airspeed, true airspeed, mach number
speed, maximum speed, overspeed discreet, altitude, captain's
baroset-corrected altitude, first officer's baroset-corrected altitude,
altitude rate, and total air temperature.
CADC #1 is powered by the Left Emergency AC Bus. CADC #2 is powered by the
Right Emergency AC Bus.
- The Flight Management Computers (FMC) use airspeed, altitude, air pressure,
and air temperature to calculate the aircraft performance.
- The Flight Control Computers (FCC) use airspeed, altitude, and altitude
rates to automatically control the flight of the aircraft.
- The Electronic Instrument System (EIS) Digital Electronics Units (DEU) supply the visual display (through
the display units) for the CADC-calculated data. The airspeed and altitude
are shown to the flight crew on the primary flight display (PFD).
- The Inertial Reference Units (IRU) use altitude, true airspeed, and
altitude rate to help calculate aircraft motion.
- The Full Authority Digital Engine Controls (FADECs) use altitude, TAT, and
total air pressure to help set engine performance and limits.
- The Air Traffic Control (ATC) transponders transmit altitude data to the
air traffic control receivers on the ground.
- The Ground Proximity Warning Unit (GPWU) uses altitude rate, altitude, and
computed airspeed to calculate the ground proximity warning modes.
- The Central Aural Warning System unit (CAWS) uses computed airspeed and overspeed
data to make altitude, stall, and windshear aural warning messages on the
flight deck speakers.
- The cabin pressure controllers use altitude and air pressure data to keep
the cabin pressure at the correct level.
- The Digital Flight Data Acquisition Unit (DFDAU) uses altitude and airspeed
to make flight data records. These records are sent to the Digital Flight
Data Recorder (DFDR)("Black Box"), the Quick Access Recorder (QAR), and the ARINC
Communications and Reporting System (ACARS).
- The Data Management Unit (DMU) gets digital inputs of altitude and airspeed
from the CADCs. The DMU sends this data to the digital flight data recorder
(DFDR).
The CADCs have a continuous self-test function that supplies failure data to
the EIS. The system display supplies an alert message when there is a CADC
fault. There is also a ADC FAIL indicator on the front side of the CADC.
This indicator becomes yellow when the CADC self-test function finds a
fault. Also, a CADC failure causes the EIS to remove the incorrect data from
the display. The EIS replaces the data with red "X's".
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