DMCs, DUs and EIS Reconfiguration

The overview fixed the normal configuration: six screens, three brains, "1 drives the captain, 2 the first officer, 3 the centre". This article is what happens when that configuration breaks — reconfiguration, the most hands-on of the EIS's three pillars. It is the part of the chapter to drill to reflex, because reconfiguration is done by hand, and the wrong hand kills a good screen along with the bad one.

Only two kinds of component are reconfigured: the drawing brain (DMC) and the screen (DU). So the whole article has two threads: source-switch a failed brain, transfer a failed screen. Between them sits a set of screens watching one another (the CHECK messages), and an emergency-power trade that keeps only three screens alive.

1. The DMC and the DU — the screens watch themselves

The three DMCs are identical and interchangeable, their role set by teleloading. Per AMM 31-60-00:

"The 3 DMCs are identical and interchangeable. The DMCs are teleloaded to perform the EFIS function or the ECAM function."

Physically a DMC is a 6-MCU box (brown, 5.5 kg) that ingests up to 108 discretes, 99 ARINC 429 and 8 ARINC 629 channels; it validates each input — switching to another source if invalid — then drives the DUs over ARINC 629. The six DUs are identical 6.25 × 6.25-inch full-colour LCDs, each generating symbols on an X/Y matrix behind the screen. The DU's most important trick is to watch its neighbours. Per AMM 31-60-00:

"Each DU monitors two of the remaining five DUs by comparing the critical parameters value at the DMC relevant input and the value of the same parameters returning on the ARINC 629 feedback bus. When a discrepancy is detected, a message related to the faulty DU is displayed on the monitoring DU and a warning is sent to the FWC."

This is the origin of the CHECK messages of §7: "what was sent" and "what is displayed" disagree means something drew it wrong. Every screen is watched by two others, driving the undetected-failure probability very low.

2. Two reconfiguration rules — only on real failure, manual first

The AMM collapses reconfiguration into two iron rules. Per AMM 31-68-00:

"A reconfiguration takes place only when: - the DMC which drives a display unit is faulty - a DU is faulty (or switched off). In all cases manual switching has priority over automatic switching."

Their meaning: (1) the system reconfigures only on a DMC or DU failure — not on a whim; (2) once your hand intervenes it overrides the automatics — which is both the capability and the trap (§9, "not allowed" configurations).

3. Single DMC failure — INVALID DATA and switch to 3; DMC 3 is the exception

The FCOM states single-DMC failure crisply. Per FCOM DSC-31-05-50:

"If a DMC fails, the corresponding DU displays the INVALID DATA message. The flight crew can replace DMC 1 or 2 with DMC 3 by turning the EFIS DMC selector, on the EFIS DMC panel, to 3. In case of DMC 3 failure, DMC 1 automatically takes over and supplies the ECAM DUs, provided the ECAM SWITCHING selector is in the AUTO position."

As crew actions:

• DMC 1 fails (CAPT PFD/ND show INVALID DATA) → the captain turns his EFIS DMC selector to 3.
• DMC 2 fails (F/O PFD/ND show INVALID DATA) → the first officer turns his selector to 3.
• DMC 3 fails (E/WD/SD briefly show INVALID DATA) → DMC 1 takes over automatically (provided ECAM SWITCHING at AUTO); the crew confirm by turning the ECAM SWITCHING/DMC selector to 1.

Only DMC 3 is automatic — it drives the shared ECAM and cannot wait for a reaction; DMC 1/2 drive per-side EFIS and are switched by hand. This asymmetry (EFIS manual, ECAM automatic) is the sharpest discriminator in the article.

4. Dual / triple DMC failure — COPY mode and the last-ditch standby

When a second DMC falls, the system enters COPY mode — all four EFIS screens on one DMC's single EFIS channel. Per AMM 31-68-00:

"The COPY mode is the configuration in which the four EFIS displays are driven by a single EFIS part of one valid DMC; this copy mode takes place only when one single DMC is valid. When two DMCs are available, the CAPT side and the F/O side are driven independently to keep the most redundant situation."

Entering COPY mode, a single-source message appears on the PFD. Per AMM 31-68-00:

"Whenever this COPY mode takes place, a message comes into view on the PFD... EFIS SINGLE SOURCE 2... This means that the CAPT and the F/O PFDs are identical and display the data of the side 2 sources such as ADIRU2, FMGEC2, radio altimeter 2. The CAPT and the F/O NDs are also identical... the ranges and the modes displayed are selected by the First Officer only."

Read EFIS SINGLE SOURCE X in two layers: (1) X is the surviving DMC's side — SINGLE SOURCE 2 means everything now comes from side 2 (DMC2 + ADIRU2 + FMGEC2), SINGLE SOURCE 1 from side 1; (2) who is in control — with SINGLE SOURCE 1 the captain selects ranges/modes, with SINGLE SOURCE 2 the first officer does ("the surviving side rules"). The hazard follows directly: both PFDs become identical and fed from one side — cross-check is gone; if that side's ADIRU or RA drifts, both screens drift together. When all three DMCs fail, the crew fall back to the last resort. Per AMM 31-68-00:

"In case of triple DMC failure, the PFDs and NDs are no longer available. The pilots use the standby indicators: - Standby airspeed indicator - Standby altimeter - Standby horizon indicator - DDRMI (if installed) - Standby compass - Angle of attack indicator (optional). No ECAM is available."

On aircraft with an integrated standby instrument system (ISIS), the first three separate indicators are combined into a single ISIS, and where the DDRMI is not fitted it simply does not appear. So after a triple DMC failure the picture is ISIS plus the standby compass, and ECAM entirely blank — attitude, speed and altitude survive (ISIS has its own baro and GPS-altitude backup), but systems monitoring and warning text are gone, and the drill is on paper and memory items only.

5. DU failure and PFD/ND transfer — first, recognise the dead screen

A failed screen must first be recognised — in one of three appearances. Per FCOM DSC-31-05-60:

"If a DU fails, the flight crew may find one of the following displays: - A blank screen with a "F" letter in amber, or - A distorted display, or - A blank screen with the "INVALID DISPLAY UNIT" message in amber."

Distinguish carefully: INVALID DATA is "the DMC did not give me valid data" (brain dead — switch the DMC); INVALID DISPLAY UNIT is "this screen itself is dead" (screen dead — transfer the image). A failed PFD is rescued automatically. Per FCOM DSC-31-05-60:

"If a PFDU fails, the system automatically transfers the PFD to the NDU."

This is the EFIS REVERSE mode: the PFD has priority over the ND, so a failed PFD DU makes the DMC swap its PFD and ND outputs and draw the PFD on the good ND DU (your ND image is temporarily displaced). To recover the ND, push the PFD/ND XFR pushbutton. A failed ND is the reverse: push PFD/ND XFR to put the ND image on the PFD DU. One discipline: a REVERSE entered automatically does not revert automatically when the failed DU recovers — a manual request is needed.

6. ECAM DU failure and single display — one screen, two pages

ECAM has only two screens, E/WD and SD; losing one enters SINGLE DISPLAY mode — one screen rotating two pages. Per FCOM DSC-31-05-60:

"If the upper ECAM display fails, or is switched off: - The engine/warning page automatically replaces the system/status page on the lower ECAM DU. The flight crew can display the system/status page by: - Using the "ECAM/ND XFR" switch, on the ECAM SWITCHING panel, to move it to a navigation display unit (NDU), or - Pushing and holding (for a maximum of 3 min) the related system page pushbutton, on the ECAM control panel..."

The lower ECAM DU is treated symmetrically. Note the single-display trade: the E/WD is displayed in priority, the SD needing the related system-page pushbutton held to show for up to three minutes. Hence with an ECAM DU failed you see the engine page by default and must call the synoptic deliberately. Transferring a system page to an ND shows "ECAM ON ND".

7. Feedback monitoring — screens watch, and CHECK

Each screen is watched by two others (§1). When that watch catches a discrepancy it writes a CHECK message. The first kind is a feedback message — "this screen drew something the DMC did not intend". Per FCOM DSC-31-05-60:

"The DU displays the following messages in amber when the DMC detects a discrepancy between the parameters obtained by the DMC and the operational parameters displayed on the DU: - "CHECK CAPT PFD"("CHECK F/O PFD")... - "CHECK CAPT ND"("CHECK F/O ND")... - "CHECK EWD"... - "CHECK SD"..."

On the ground a DU NOT MONITORED message appears when only one (not two) DMC provides feedback — indicating a DMC test in progress or an EIS failure (the latter needing maintenance). The second kind compares the two sides directly. Per FCOM DSC-31-05-60:

"CHECK ATT: Attitude data displayed on both PFDs differ by more than 5 ° in pitch and/or roll. CHECK ALT: Altitude data displayed on both PFDs differ by: - More than 250 ft when the flight crew selects a QNH different from STD, or - More than 500 ft when the flight crew selects a QNH STD. CHECK HDG: Heading data displayed on both PFDs and NDs differ by more than 5 °."

Memorise the three figures: ATT 5°, ALT 250/500 ft, HDG 5°. One easy-to-miss footnote: when the two pilots' baro references are deliberately set differently, CHECK ALT and its caution disappear — an altitude difference from different QNH settings is not a fault (which is why CHECK ALT has separate QNH/STD thresholds). And if both NDs are in PLAN mode, CHECK HDG is not displayed. A DU restarting briefly shows green SELF TEST IN PROGRESS and/or WAITING FOR DATA — normal self-test, not a fault.

8. Emergency power — only three screens, plus the DMC bus customers

After total loss of main generation, the DUs do not degrade evenly — only three survive. Per AMM 31-60-00:

"Each DU has its independent 115VAC/400HZ power supply. The DUs are supplied from various buses (NORMAL and EMERGENCY A/C network) to ensure redundancy in case of emergency electrical configuration. In that case, only the CAPT PFD DU, the CAPT ND, and EWD DU are supplied."

Why exactly these three? The captain keeps one PFD+ND (someone can fly and navigate) and the centre keeps the E/WD (engine parameters and red warnings must not be lost); the first officer's screens and the SD give way to save power. A point easily overlooked: the DMC output bus feeds more than screens. Per AMM 31-68-00:

"The general EFIS output bus... is issued to control: - the FWC1 and the FWC2 - the CMC1 and the CMC2 - the DMU - the FDIU... In addition the DMC1 ECAM output bus is directly wired to the FDIU for redundancy."

So the DMC also feeds flight warning (FWC), maintenance (CMC), data management (DMU) and the flight-data interface (FDIU, the recorders). This is why switching a DMC source re-sources not only the screen in front of you but also which DMC's data the recorder writes.

9. EFIS SWTG NOT ALLOWED — don't switch into an illegal configuration

Manual priority (§2) is a capability and a trap. The trap: even with all three DMCs healthy, you can turn the selectors into an "each side on a different DMC" oddity. Per AMM 31-68-00:

"The pilot can switch his EFIS or ECAM DUs to another source, even when the three DMCs work correctly. These configurations are not allowed and are indicated to the crew by a message on the PFD: EFIS SWTG NOT ALLOWED."

Captain on 2, first officer on 1 — cross-feeding each other's DMC — raises EFIS SWTG NOT ALLOWED. Not a fault, but "your hand made a configuration it should not have; put it back". The last discipline of reconfiguration: don't switch if nothing is broken; if you see the message, return to NORM — the price of manual priority is responsibility for your own hand.

Self-test

[!note]- Q1. DMC 1, DMC 2, DMC 3 each fail — what does the crew do, and why is only DMC 3 automatic? DMC 1/2: the affected side turns its EFIS DMC selector to 3 (manual). DMC 3: DMC 1 takes over automatically (ECAM SWITCHING at AUTO), crew confirm to 1. Only DMC 3 is automatic because it drives the shared ECAM.

[!note]- Q2. INVALID DATA versus INVALID DISPLAY UNIT — which failure, which action? INVALID DATA = the DMC gave no valid data (brain) → switch the DMC. INVALID DISPLAY UNIT = the screen itself failed → transfer the image.

[!note]- Q3. When does COPY mode appear, what does the "2" in EFIS SINGLE SOURCE 2 mean, and what is the hazard? When only one DMC remains for EFIS. The "2" is the surviving DMC's side — both PFDs show side-2 data (F/O selects ranges/modes). Hazard: both PFDs identical from one side — cross-check is lost.

[!note]- Q4. Which three screens survive on emergency power, and why those three? CAPT PFD, CAPT ND, E/WD. The captain keeps a fly-and-navigate pair; the centre keeps the engine parameters and red warnings. The F/O screens and SD give way to save power.

[!note]- Q5. What are the CHECK ATT / ALT / HDG thresholds, and why does CHECK ALT disappear when the two baro references differ? ATT 5° pitch/roll, ALT 250 ft (QNH) / 500 ft (STD), HDG 5°. With different QNH settings, the altitude difference is expected, not a fault — so the message is suppressed.

Key takeaways

References

• FCOM DSC-31-05-50 — DMC reconfiguration: INVALID DATA, switch to 3, DMC 3 auto-takeover.
• FCOM DSC-31-05-60 — DU reconfiguration, PFD/ND transfer, DU-failure appearances, feedback messages, CHECK ATT/ALT/HDG, DU reset.
• AMM 31-60-00 — DMC/DU internals, DU cross-monitoring, emergency supply (three DUs).
• AMM 31-68-00 — reconfiguration rules, COPY/REVERSE/single-display, single/dual/triple DMC failure, EFIS SINGLE SOURCE, DMC output buses, SWTG NOT ALLOWED.

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.