CDU/MCDU Interface: Data Entry Logic, EXEC Workflow, and Redundancy

022-11-04Pablo Asensio Martínez2026-03-272 min

Overview

The Control Display Unit (CDU), or Multipurpose Control and Display Unit (MCDU) on Airbus aircraft, is the primary interface between the flight crew and the Flight Management Computer (FMC).

It allows the crew to:

  • Enter navigation and performance data.
  • Monitor flight progress.
  • Manage automatic flight functions.
  • Send non-FMS commands (e.g., ACARS, Datalink) on modern MCDUs.

Interface Components

The unit consists of a display screen and a keyboard.

Display Layout

  • Title Line: Displays the name of the current page (e.g., INIT, LEGS, PROG).
  • Label Lines: Small text identifying the data field below it.
  • Data Lines: The actual values (computed data or pilot entries).
  • Scratchpad: The bottom line of the screen.
    • Displays alphanumeric characters typed by the pilot before they are inserted into a data field.
    • Displays system messages (e.g., CHECK GW, GPS PRIMARY).

Controls

  • Line Select Keys (LSK): Buttons on the left (1L-6L) and right (1R-6R) of the screen used to:
    • Move data from the scratchpad to a field.
    • Select items or prompts (e.g., <ACTIVATE>).
  • Keyboard: Alpha-numeric keypad for data entry.
  • EXEC Key: The "Execute" button (often illuminated) acts as the "Enter" or "Confirm" key to make a modified flight plan active.

Data Symbology

  • Amber Boxes ([][][]): Indicate Mandatory data entry required for the FMS to function (e.g., ZFW, Cruise Flight Level).
  • Blue/Green Dashes (-----): Indicate Optional data entry (e.g., Wind data, Temp deviation).

Pre-Flight Setup Sequence

A standard logical flow is used to initialize the FMS:

  1. IDENT: "Who am I?"
    • Start-up identifying Aircraft Model, Engine Rating, and Navigation Database validity.
  2. POS INIT: "Where am I?"
    • IRS Alignment.
    • Entry of Present Position (Latitude/Longitude).
  3. RTE (Route): "Where am I going?"
    • Origin and Destination airports.
    • Route definition (Airways, Waypoints, Procedures).
  4. PERF INIT: "How/When will I get there?"
    • Zero Fuel Weight (ZFW): Critical mandatory input from the loadsheet to calculate Gross Weight.
    • Fuel: Block Fuel entry for planning (FMS usually reads actual FOB from sensors).
    • Cost Index: Economic strategy.
    • Cruising Altitude: Planned level.

Operational Redundancy

  • Dual Configuration: In a dual FMS installation, data entered on one CDU is typically crosstalked to the other FMC.
  • Independent Display: While databases are shared, each pilot can select different pages on their respective CDU (e.g., Pilot A views LEGS, Pilot B views PROG).
  • Cross-Side Operation: In failure cases (e.g., Failure of FMGC 1 and MCDU 2), the remaining MCDU 1 can often communicate with FMGC 2 to maintain control.