The MCDU and the Multifunction Concept

The MCDU is the AFS's long-term order interface (article 01: short-term through the FCU, long-term through the MCDU) — but it is not the FM's private keyboard. It is a multi-tenant terminal: flight management, the maintenance system and the communication systems all talk to you through it. This article is about the device — screen, keys, annunciators, the multifunction switching, and the substitution logic; how to use the FM pages is article 26, and backup navigation as a navigation function is article 19.

Per FCOM DSC-22_10-10 and AMM 22-82-00:

Three MCDUs are installed on the pedestal for flight crew loading and the display of data.

The Multipurpose Control and Display Units (MCDU) provide access to the following : - FM1 or FM2 /Flight Management function - ACMS /Aircraft Condition Monitoring System (optional) - CMS /Central Maintenance System - SAT /Satellite Communication (optional) - ATSU /Air Traffic Services Unit (optional) - NAV B/UP /Standby Navigation.

Think of the MCDU as a building's front desk, with FM merely the highest-priority tenant. The consequence is a pair of independencies that unlocks every failure scenario in this article: FM dead does not kill the MCDU (it still serves CMS, ATSU, backup navigation), and MCDU dead does not kill the FM (the flight plan lives in the FMGEC — article 01).

1. The hardware and the port hierarchy

Each unit weighs no more than 4.5 kg and runs on 115 VAC: MCDU 1 from AC ESS SHED, MCDU 2 from AC BUS 2, and MCDU 3 from the AC ESS/static-inverter bus — which is why MCDU 3 is the survivor on batteries alone (article 01). The design follows ARINC 739.

Identity is not software configuration: each MCDU reads two pin-programming pins at long power-up (MCDU 1 = OPEN/GND, MCDU 2 = GND/OPEN, MCDU 3 = GND/GND). Per AMM 22-82-00:

A MCDU which has both pins in an OPEN state is considered as having failed the power up self-test and is disabled from functioning.

Subsystems connect on eleven ARINC 429/739 ports: ports 1 and 2 are high-speed and belong to FM1 and FM2; ports 3–7 (low-speed) carry the non-FM subsystems; priority equals port number (1 highest). Addressing uses the MCDU address labels (octal 220/221/222) and subsystem address labels (FM1 = 300, FM2 = 301, ACMS = 302, CMS = 303, ATSU = 304, SATCOM = 307).

2. Screen, scratchpad, and the colour language

Per FCOM DSC-22_10-40-10:

The Multipurpose Control and Display Unit (MCDU) is a Liquid Crystal Display (LCD) with 14 lines, of 24 characters each

One title line, six pairs of label/data lines (matching line-select keys 1L–6R), and the scratchpad at the bottom. Data entry is deliberately a two-step act:

The flight crew enters data by typing it into the scratchpad on the MCDU. Next, pressing the line select key (LSK) will load the data from the scratchpad into the desired field. An error message displays if the data are out of range or not formatted correctly.

And behind the format check sits a sanity check. Per FCOM DSC-22_10-10:

If the flight crew enters data in the MCDU that is not logical, or is beyond the capabilities of the aircraft, it is disregarded or it generates an advisory message.

The colour language (same family as the FMA's in article 06 and the ND's in article 27):

Note the inversion of everyday intuition: on the MCDU, blue is yours to change and green means "in use — hands off". Yellow belongs to one thing only. Per FCOM DSC-22_10-40-10:

PRIMARY F-PLN GREEN WAYPOINTS, WHITE LEGS TEMPORARY F-PLN YELLOW WAYPOINTS, WHITE LEGS SECONDARY F-PLN WHITE WAYPOINTS AND LEGS

Yellow waypoints mean a temporary flight plan is pending confirmation (article 21).

The function keys, briefly (contents of each page are article 26's subject): DIR (direct-to), PROG (progress — OPT/REC MAX altitudes, position update, bearing/distance to any point), PERF (performance by phase, opening on the current phase), INIT (initialisation, including INIT FUEL PRED and IRS alignment — the INIT A page is accessible only in preflight, airborne you can only revise the alternate plan, and INIT FUEL PRED closes at engine start), DATA, F-PLN (A and B pages — left keys for lateral revisions, right keys for vertical), RAD NAV, FUEL PRED, SEC F-PLN, plus ATC COMM and MCDU MENU. One key hides a tour:

The first push of the AIRPORT key displays the destination. Successive pushes show the alternate, the origin, and the destination again.

3. Eight annunciators — and their precise triggers

The FCOM's quick version, per FCOM DSC-22_10-40-10:

FM 1 and FM 2 (amber) : The onside FM is failed. IND (amber) : The onside FM detects an independent mode of operation while both FM are healthy. RDY (green) : MCDU has passed its power up test after it was turned to OFF using its DIM key.

The AMM sharpens the triggers. Per AMM 22-82-00:

The FM1 failure light on MCDU1 and/or MCDU2 comes on (amber) if FM1 is the selected FM, the MCDU BRT knob is at ON and the FM1 subsystem identifier word (label 172) is not received for three seconds by the MCDU.

Additionally, a MCDU never has both its FM failure annunciators on.

This annunciator comes on (amber) when the selected FM detects an independent operation (loss of dual mode) while both FMs are healthy. If either FM is failed, the annunciator is not on, regardless of the state of the intersystem bus.

Hold on to the asymmetry: the FM lights answer "is it dead?"; the IND light answers "are the two still talking?". IND lit means both FMs are alive but each is working alone — time to cross-check entries (article 19, INDEPENDENT operation). If an FM actually dies, you get its FM light instead, never IND. The IND light is the cockpit outlet of article 01's architectural fact that FM has no monitoring channel and relies on dual-FM cross-comparison.

Three more annunciators sit at the screen sides: FAIL (amber — the MCDU itself has failed), MCDU MENU (white — a non-FM subsystem requests the display), and FM (white — the FM, while not the active subsystem, has an amber-level important message waiting). With a spare position that makes eight.

4. The MENU page — who owns the display

Per FCOM DSC-22_20-20-10 (Thales chapter):

This page lists the various systems to which the pilot can gain access through the MCDU. It also allows activation of the navigation backup mode.

Selection-state language:

The pilot selects a system by pressing the key adjacent to the name of that system. The screen shows the name of the selected system in green, all others in white. If the MCDU cannot establish communication with the selected system, it displays “TIME OUT”.

The Honeywell chapter adds the in-between state:

‐ Blue, associated with the (SEL) indicator, until communication is established. The scratchpad displays “WAIT FOR SYSTEM RESPONSE”.

And the service bell:

When a system calls for pilot attention, the MCDU displays “REQ” next to the system name, and the “MCDU MENU” annunciator lights up.

Two automatic rules of the display state-machine prevent "the page jumped by itself" confusion. Per AMM 22-82-00:

An auto-return occurs to the active subsystem if 60 seconds elapse without any line select key selections. Auto-returns are inoperable if no active subsystem exists.

When the selected FM assumes control of the MCDU display via the FM line select key, a manual return or automatic return, the last known page displayed by the FM is redisplayed. If the page is unknown, the A/C Status page is displayed.

So "the MCDU ended up on the A/C STATUS page" after a reset or switching is not a quirk — it is the FM's default homepage when it has forgotten where it was (article 29 meets the same behaviour). The Honeywell chapter also advises the way back:

When the MCDU is communicating with a system other than the FMGEC, the pilot should use the MCDU MENU page to revert to the FMGEC system.

(In practice, pressing any FM function key also jumps you straight back.)

5. MCDU 3 — the substitute, not a hot standby

Per AMM 22-82-00:

If MCDU1 is in the OFF position, regardless of the MCDU2, MCDU3 functions as MCDU1 and selects the FM subsystem as described previously.

In addition, the failed MCDU must be in the OFF position for 5 seconds continuously.

The full rule set: MCDU 3 substitutes for number 1 in preference to number 2 (protecting the FM1 channel); when number 1 comes back ON, MCDU 3 exits the substitution immediately (then checks whether number 2 has been OFF ≥ 5 s, else returns to its own role). How does MCDU 3 know the others are off? Each MCDU broadcasts a KBRTON discrete (to MCDU 3, both FMGECs and both CMCs). On the ground you can watch it work: switch MCDU 1 (or 2) off and after about 5 seconds the FM1 (FM2) entry appears on MCDU 3's MENU page, disappearing as soon as the unit is back ON.

The FCOM's operational version (FCOM DSC-22_20-40, loss of one MCDU):

SELECT the failed MCDU off. The MCDU 3 automatically replaces it.

3 IRSs, 2 FMGECs, 2 MCDUs are still available

— and the procedure lists the limitation as none. But note what "automatic" conceals: the substitution is not a hot standby. It waits for you to put the failed unit to OFF (and hold it there 5 s). A dark screen is therefore not a cue to pull breakers: try the QRH reset first (section 7), and only if that fails switch the unit OFF and let number 3 take the seat.

6. FM source switching — and why MCDU 3 can never do backup navigation

The SWITCHING/FM selector on the pedestal decides which FM port each MCDU listens to. NORM: MCDU 1/2 each listen to their own side. Per AMM 22-82-00:

If the SWITCHING/FM selector switch is at BOTH ON 1 the MCDU will listen to the port connected to FM1 (i.e., port 1 for MCDU1 and MCDU3 ; port 2 for MCDU2).

BOTH ON 2 mirrors it, and an invalid selector position is treated as NORM. The system-level effect: at BOTH ON 1, ND 2 displays the same picture as ND 1 (and vice versa) — the switch that gives both pilots the healthy FM's map after a single FM failure (article 19).

Backup navigation has a hard availability rule. Per AMM 22-82-00:

Standby Navigation is available on MCDU1 and MCDU2 only if the SWITCHING/FM selector switch is at NORM. It is unavailable at all other times. Standby Nav is never available on MCDU3 at any time, even when it operates as a backup to MCDU1 or MCDU2.

The FCOM says it in one line — MCDU 3 is not able to achieve the Back Up navigation. — and the hardware explains it: the KNAVBKUPSEL discrete (+18 V to the onside FMGEC) exists only on MCDU 1 and 2. On the MENU page:

Pressing the [1R] key selects the NAV B/UP function and the field displays DESELECT NAV B/UP. If the NAV B/UP is inoperative, the field is blank.

Not greyed out — gone. And a trap for the dual-FM-failure day (article 19): wanting backup navigation, your reflex to move the FM selector to BOTH ON is exactly wrong — NAV B/UP requires NORM, and switching away removes the prompt. The FCTM makes this warning explicit in the dual-FMGEC-failure guidance.

7. Overheat blanking, power-up, and resets

A blank screen has an innocent explanation. Per AMM 22-82-00:

This mode causes the screen to blank if a MCDU button push has not occurred within the last two minutes. The scratchpad message MCDU OVERHEATED is displayed for approximately 15 seconds after the screen is blanked, after which it is removed from the display.

The FCOM registers MCDU OVERHEATED as a Type II message — the unit is hot and idling in power-saving mode. Blank is not dead: communication continues behind the dark screen (the FM and MCDU MENU annunciators still announce incoming traffic), any keypress re-lights it, and it is a thermal characteristic, not a failure — no component swap, no reflex reset.

Power-up behaviour: after a long power interruption the MCDU self-tests and, if it passes, goes straight to work — no RDY light. RDY belongs to the turn-off path: hold the brightness knob to OFF (more than 3 s) and a passing self-test parks the unit with the green RDY lit. Per AMM 22-82-00:

Further MCDU processing is suspended until the BRT key is pressed, after which the MCDU extinguishes the RDY annunciator and commences its normal operations.

The QRH reset for a blank or frozen unit, per a representative operator QRH:

‐ Set MCDU (affected) to OFF ‐ Wait 10 s ‐ Set MCDU (affected) to ON.

If reset unsuccessful set MCDU (affected) to OFF. MCDU 3 will automatically replace affected MCDU 1 or 2.

8. Discipline and dispatch

Input discipline (developed in article 30). Per FCTM AOP-20:

Below 10 000 ft, entries should be restricted to those that have an operational effect

— PERF APPR data, DIR TO, navaids, a late runway change, activating SEC, ENABLE ALTN. Time-consuming entries go to the PM (on the PF's command), or the PF hands over control first. Head-down time is attention spent; this rule and the two-step scratchpad→LSK entry are the MCDU's human-factors defences.

Failure and dispatch ledger (article 31, article 32): a single MCDU failure costs nothing after substitution — three IRS, two FMGEC, two MCDU remain, limitation none. In some operators' MEL practice: MCDU 1 inoperative is dispatchable with ETOPS prohibited; MCDU 2 or 3 inoperative is dispatchable without condition; two MCDUs inoperative is still dispatchable where the flight plan needs only one MCDU — but bars RNP AR, RNP 4, RNAV 10 and oceanic/remote RNP 2 (combinations that include MCDU 1 also bar ETOPS). The deactivation procedure in every case: set the failed unit's brightness knob to OFF. The brightness-knob transfer function and the NAV B/UP function are separately dispatchable (placarded) — the latter becomes a hard restriction generator when stacked with FM-failure dispatch (article 32). In the electrical emergency configuration the complement is "MCDU 1 only" (emergency generator) or, on the ground on batteries, "MCDU 3 only" via the static inverter.

[!warning]- Seven misconceptions this article corrects (1) The MCDU is not the FM's screen — it is a multi-tenant terminal; FM death doesn't kill it, and its death doesn't kill the FM. (2) Green data is not "good to edit" — green means active/in-use (hands off); blue is what you may change; yellow means an unconfirmed temporary flight plan is pending. (3) IND lit does not mean an FM has failed — the opposite: both must be healthy for IND to light at all; a dead FM lights FM1/FM2 instead. (4) MCDU 3 is not a hot standby — the failed unit must be selected OFF (5 s continuously) before it substitutes. (5) MCDU 3 can never provide backup navigation, even while substituting — the enabling discrete exists only on MCDU 1/2, and NAV B/UP also requires the FM selector at NORM (BOTH ON removes the prompt). (6) A blank screen with MCDU OVERHEATED is power-saving, not failure — communication continues; press a key. (7) After a reset or display switch, landing on the A/C STATUS page is the FM's amnesia default, not a malfunction.

Self-test

[!note]- Q1. Which MCDU data colour can you modify — green or blue? And what do yellow waypoints mean?

Blue is modifiable/selectable; green is active and not modifiable. Yellow waypoints (with white legs) are the temporary flight plan — a revision awaiting confirmation. Primary plan: green waypoints, white legs; secondary: all white.

[!note]- Q2. What are the three precise conditions for the FM1 light, and why can a single MCDU never show both FM lights?

FM1 is the selected FM + the BRT knob is ON + the FM1 subsystem identifier word (label 172) not received for 3 seconds. Both lights can never be on together because an MCDU listens to only one FM port at a time.

[!note]- Q3. What does IND lit tell you? Would IND light if FM2 failed?

The selected FM has detected independent operation — loss of dual mode — while both FMs are healthy: they are alive but no longer cross-talking, so crew cross-checking must substitute. If either FM is failed, IND stays off regardless of the intersystem bus — you would see the FM2 light instead.

[!note]- Q4. The complete condition chain for MCDU 3 replacing MCDU 1 — and can it activate NAV B/UP while substituting?

MCDU 1 selected OFF and continuously OFF for 5 s (signalled by the KBRTON discrete) → MCDU 3 functions as MCDU 1, with priority to number 1 over number 2; MCDU 1 back ON ends the substitution immediately. NAV B/UP: never on MCDU 3, substitution or not — no KNAVBKUPSEL discrete — and on MCDU 1/2 only with the FM selector at NORM.

[!note]- Q5. On the MENU page, what do REQ, (SEL) and TIME OUT each mean? And where does the display go after 60 idle seconds?

REQ = that subsystem requests attention (MCDU MENU annunciator lights). Blue with (SEL) = selection made, communication being established (scratchpad: WAIT FOR SYSTEM RESPONSE). TIME OUT = communication could not be established. After 60 s without a line-select action the display auto-returns to the active subsystem (no auto-return if none exists).

[!note]- Q6. The screen goes dark and MCDU OVERHEATED flashes in the scratchpad — first reaction? And the QRH reset?

Recognise power-saving: the unit blanked after 2 minutes without a keypress because it is hot; communication continues; press any key. If genuinely frozen: OFF → wait 10 s → ON; if unsuccessful, leave it OFF and MCDU 3 substitutes.

[!note]- Q7. MCDU 1 alone fails — is ETOPS still available? And with two MCDUs out?

Per some operators' MEL practice: MCDU 1 inoperative bars ETOPS (MCDU 2 or 3 inoperative carries no condition). Two inoperative remains dispatchable where one MCDU suffices for the planned flight, but bars RNP AR, RNP 4, RNAV 10 and oceanic/remote RNP 2 — and combinations including MCDU 1 also bar ETOPS.

Key takeaways

References

MCDU role, entry protections and backup-navigation statements per FCOM DSC-22_10-10 and DSC-22_20-40 (loss of one MCDU); screen format, data entry, colours, function keys and annunciator quick reference per FCOM DSC-22_10-40-10; MENU page behaviour per FCOM DSC-22_20-20-10 (Thales and Honeywell variants) and the MCDU OVERHEATED Type II message per FCOM DSC-22_20-20-20. Hardware characteristics, pin programming, port and address architecture, precise annunciator triggers, display state-machine, substitution logic, overheat mode and power-up/turn-off behaviour per AMM 22-82-00 (discretes per AMM 22-85-00; power supplies per AMM 22-00-00; the 5-second substitution observation per the AMM operational test). Reset procedure per a representative operator QRH computer-reset table; entry discipline per FCTM AOP-20; dispatch consequences per some operators' MEL practice (operator-specific — verify your own). The front-desk/tenant framing is an integrative synthesis. Maintenance-layer detail (connector pinouts, BITE, address-label tables beyond identification) is intentionally excluded.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.