DIGISTAR: Swissair Flight 111 Web Site: MD11 Air Data System

Supplimentary Web Site for Swissair SR111 Crash:

MD-11 ADS (Air Data System) computers and displays:

Flight Control Computers (FCCs) - Operation and Overview:
The AutoFlight System (AFS) receives signals from two air data computers, the inertial reference system, surface position sensors (flaps, slats, ailerons, elevators, horizontal stabilizer) and navigation systems. The AFS uses the input data from these sensors and systems to transmit control signals (through the FCC) to actuators. These signals control the pitch, roll, yaw and thrust of the aircraft during the full flight envelope, through ground rollout. The signals are also sent to the electronic instrumentation system (EIS) to supply the flight crew with flight guidance information.
The AFS includes three primary software functions supplied by the FCCs, which are: Autopilot/Flight Director, Flight Augmentation, and the Autothrottle/Engine Thrust Trim System.
The signal outputs to the DEU and CAWS for visual and aural alerts are to alert the pilots of any source failures which may affect the accurate presentation of flight data on their displays, and which may affect the availability and dependability of the autoflight system.
The FCCs contain 2 channels each called A and B. Each of these channels controls a surface in the roll axis, yaw axis, and the pitch axis.
FCC #1 receives power to channel A from DC Bus 1, Channel B from Left Emergency DC Bus, and there is also a back up power feed from the Battery Direct Bus. FCC #2 receives power to channel A from DC Bus 3, Channel B from Right Emergency DC Bus, and there is also a back up power feed from the Battery Direct Bus.

Central Air Data Computers (CADCs) - Operation and Overview
The 2 CADCs receive inputs from: 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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