Indicating/Recording Overview and the EIS

If ATA-34 is the aircraft's senses — where the numbers come from — then ATA-31 is where those numbers come out, and where the flight is written down. Everything the sensors and flight-control computers compute has to be put in front of the crew (indicating); when a system misbehaves, the aircraft has to shout it out and print the drill on the screen (warning); and every frame of the flight has to be sealed in a fireproof box for later investigation (recording). Three jobs — display, warning, recording — are the whole chapter.

Draw the boundary at the outset. The airspeed tape, the ILS deviation, the engine N1 — those numbers belong to their source chapters (ATA-34/22/27/70). ATA-31 owns the display system that carries them (how six screens are driven, how they reconfigure when one dies), the warning system that shouts (ECAM/FWS), and the recorders. In other words, other chapters make the numbers; ATA-31 decides whether they reach the screen, whether they raise a voice, and whether they leave a trace.

This first article maps the whole chapter, then erects the skeleton of the largest single system in it — the Electronic Instrument System (EIS): six display units, three display management computers, two system data acquisition concentrators, two flight warning computers, and the pair of lights under each glareshield.

1. How ATA-31 divides — five subsystems

The maintenance manual states the chapter's scope plainly. Per AMM 31-00-00:

"The indicating and recording systems are those system which display (for the indicating systems) or store (for the recording systems) all sorts of flight parameters."

It then lists five subsystems, and the list is worth memorising because the following articles are built on it. Per AMM 31-00-00:

"These systems include: - the Clock (indicating system) - the Recorders (recording system) - the Attitude Monitor (indicating system) - the Flight Warning system (indicating system) - the Electronic Instrument system (indicating system)."

Four of the five are indicating; only the recorders store. The EIS is the principal system — the main instrument at the crew's disposal to check the aircraft, covering both navigation display and aircraft-systems monitoring. The Flight Warning System (FWS) follows close behind. Together they carry the first eleven articles. The clock and the recorders take one article each; the Attitude Monitor is merely a spirit level used on the ground (see §7).

ATA-31 Indicating / Recording
├─ Display: Electronic Instrument System (EIS)   → this article + reconfiguration / controls / PFD / ND
│   ├─ 6 × DU (2 PFD + 2 ND + EWD + SD, full-colour LCD)
│   ├─ 3 × DMC (drive the DUs over ARINC 629)
│   └─ reconfiguration: DMC source / PFD-ND transfer / ECAM-ND transfer / single display
├─ Warning: Flight Warning System (FWS)          → ECAM / attention-getters / E/WD / SD / ECP / operation
│   ├─ 2 × SDAC (feed the DMCs and the FWCs)
│   ├─ 2 × FWC (compute alerts, aural warnings, synthetic voice)
│   └─ outputs: E/WD + SD (via the DMCs), MASTER WARN/CAUT lights, loudspeakers
├─ Indicating instrument: the Electrical Clock   → clock article
└─ Recording: the Recorders (FDIMS)              → recorders article
    ├─ DFDRS (FDIU part of the FDIMU + DFDR + tri-axial accelerometer)
    └─ ACMS (DMU part of the FDIMU + DAR / QAR / printer)
    [+ Attitude Monitor: a single ground spirit level — §7]

2. The EIS — two display functions

The EIS presents everything on a common set of screens. Per FCOM DSC-31-05-10:

"The electronic instrument system (EIS) presents data on six identical display units (DUs): - The electronic flight instrument system (EFIS) displays mostly flight parameters and navigation data on the primary flight displays (PFDs) and navigation displays (NDs) - The electronic centralized aircraft monitor (ECAM) presents data on the engine/warning display (E/WD) and system display (SD)..."

The split between these two functions is worth stating in one line, because it decides who the screen is for. Per AMM 31-60-00:

"The Electronic Flight Instrument System display function is shared in two, one for CAPT and one for F/O. Each crew member has at his disposal two displays presenting the whole FLIGHT information (for safe flight and navigation): the PFD and the ND."

"The Electronic Centralized Aircraft Monitoring display function is unique for both crew members. The ECAM display function presents the whole AIRCRAFT information via two displays: the EWD and the SD."

Fix the rule: PFD/ND = my flight (per side, each sees their own); E/WD/SD = the aircraft's state (one copy, seen together). So the two PFDs show attitude and speed from their own side's sensors — they may differ, which is exactly what the unreliable-airspeed drill cross-checks (ATA-34); whereas the centre E/WD and SD hold a single aircraft truth. This division also underlies the two reconfiguration logics later: EFIS switches per side, ECAM switches on its own.

• PFD (outer screen): short-term flight information — used to fly the aircraft (see PFD).
• ND (inner screen): medium-term information relative to navaids, map and flight plan, plus weather radar (see ND).
• E/WD (centre upper): engine primary indications, slat/flap position, warning/memo messages — short-term aircraft information (see E/WD).
• SD (centre lower): system synoptic pages plus status messages, switched automatically by aircraft configuration (see SD).

3. The three design pillars — redundancy, segregation, reconfiguration

The entire EIS architecture is built around three words. Per AMM 31-60-00:

"The Electronic Instrument System (EIS) architecture is designed to cope with requirements in regard to: - REDUNDANCY (2 PFDs, 2 NDs, 1 EWD, 1 SD, 3 DMCs), - SEGREGATION (4 types of symbologies, PFD, ND, EWD and SD), - RECONFIGURATION capabilities for minimization of operational consequences and maximization of A/C availability in case of any EIS component failure."

• Redundancy — two PFDs and two NDs, plus three DMCs of which any two keep all four EFIS screens alive. Numerical margin.
• Segregation — the information is cut into four distinct symbologies (PFD, ND, EWD, SD), each drawn and computed independently, so a drawing fault in one family cannot pollute another. A DMC is teleloaded to perform the EFIS function or the ECAM function — the same box, different software, a different symbology family. Functional isolation.
• Reconfiguration — any failed part is bypassed by switching devices plus software in the DMC and DU. The whole of the next article.

Together these three are why one screen, one computer, even several failing together, still leave the critical information on display — the reason a glass cockpit can replace a wall of electromechanical instruments and be more reliable.

4. Six DUs and three DMCs — one computer, four screens

Only two kinds of hardware perform the display: the DMC (the drawing brain) and the DU (the screen). Per FCOM DSC-31-05-30:

"Three identical Display Management Computers (DMCs) obtain data from the different sensors and computers, and send it to the display units. The display units then generate and display the applicable images. Each DMC has a single DMC channel, and can simultaneously supply one PFD, one ND, and both ECAM display units (E/WD and SD)."

Note the capacity in the last sentence: one DMC can feed up to four screens at once. Read as a redundancy figure now, it is a lifeline in the reconfiguration article: lose two of three, and the survivor drives four screens in COPY mode. The normal allocation is simple and symmetric. Per AMM 31-60-00:

"In normal operation, each DMC drives two display units as follows: - the DMC 1 drives the CAPT PFD DU and the CAPT ND DU, - the DMC 2 drives the F/O PFD DU and the F/O ND DU, - the DMC 3 drives the EWD DU and the SD DU."

"1 drives the captain, 2 the first officer, 3 the centre (ECAM)" is the single most useful sentence in the chapter — every reconfiguration departs from it. DMC-to-DU traffic runs on ARINC 629 (with a separate ARINC 453 for weather radar). The six DUs are identical and interchangeable; hardware detail (DMC size, DU screen, how DUs monitor one another) is the subject of the next article.

5. Two SDACs and two FWCs — the warning line

The warning line does not run through the DMCs; it has its own two stages: acquisition by the SDAC, computation by the FWC. The SDAC is the data porter. Per FCOM DSC-31-05-30:

"The two identical SDACs acquire data, then generate signals. Some of these signals go to the three DMCs, which use them to generate displays of system pages and engines parameters. Others go to the flight warning computers which use them to generate ECAM messages and aural alerts."

The computation of alerts is done in the FWC, which acquires along two paths. Per FCOM DSC-31-05-30:

"The two identical FWCs generate alert messages, memos, aural alerts, and synthetic voice messages. For this purpose they acquire data: - directly from aircraft sensors or systems to generate red warnings - through the SDACs to generate amber cautions."

This is a crucial design point: red warnings (level 3, immediate action) do not pass through the SDAC — the FWC acquires them directly; only amber cautions (level 2) go via the SDAC. Life-critical warnings (fire, overspeed, stall) must not depend on one more box that can fail — one fewer dependency, one fewer single point. This "red direct, amber via SDAC" is the physical reason a single-SDAC failure is dispatchable while the red-warning chain is preserved (see MEL), and why losing both SDACs costs you amber cautions and system pages, not red warnings. The FWC also does three tasks that touch "warning" but are in fact independent functions. Per FCOM DSC-31-05-30:

"The FWCs also generate: - radio height callouts - decision height callouts - landing speed increments."

The "TWO HUNDRED" and "MINIMUM" you hear on approach, and the landing speed increment (VAPP correction), are the FWC's work. So the FWC is not only an alarm — it is also a callout announcer and an arithmetician; hence the MEL clause "dual FWC failure — no dispatch" loses more than alerts.

6. Attention-getters and the loudspeaker

A warning cannot live only on the screen; it must also grab the crew's eyes and ears. The eyes, by two lights. Per FCOM DSC-31-05-30:

"The FWCs also drive the attention-getters. Each pilot has a set of these on the panel under the glareshield. They are : - a master warning light that flashes "MASTER WARN" in red for red warnings - a master caution light that illuminates "MASTER CAUT" in amber for amber cautions."

Fix the action difference: the red MASTER WARN flashes, the amber MASTER CAUT is steady. A flash catches the eye harder — red is more urgent. The ears, by the loudspeaker, with a counter-intuitive detail. Per FCOM DSC-31-05-30:

"The communications loudspeakers announce aural alerts and voice messages, and do so even when they are turned off."

The loudspeaker announces alerts even when "off". Not a bug, but design: a crew may turn the communications volume right down for quiet, but fire and stall alerts must never be silenced with it — so the alert audio takes a path not fully governed by the volume knob. This is the classic "I turned the speaker off, why is it still sounding?" trap for new students.

7. The Attitude Monitor — a ground spirit level

The most inconspicuous of the five, worth knowing because it is often mistaken for some electronic attitude sensor. Per AMM 31-00-00:

"The Attitude Monitor is used to check the A/C attitude, on ground. A single spirit-level (similar to the one used for refuel operations) installed behind an access door of the A/C lower part allows this check from the ground."

It is a spirit level — used to level the aircraft during maintenance (aircraft on jacks) — with nothing to do with the cockpit attitude display. Knowing "the attitude monitor is a spirit level, not an attitude sensor" spares a conceptual mix-up.

8. Configuration baseline

Optional-equipment content in this material follows the configuration summary that some operators carry in the QRH operational data, cross-checked against the FCOM general options list and AMM applicability. Relevant to this chapter:

Self-test

[!note]- Q1. Into which five subsystems does the AMM divide ATA-31, and which one is the principal system? Clock, Recorders, Attitude Monitor, Flight Warning System, Electronic Instrument System. Only the Recorders store; the rest indicate. The EIS is the principal system, the FWS close behind.

[!note]- Q2. What is the essential difference between the PFD/ND pair and the E/WD/SD pair? By who it is for: PFD/ND present flight information split per side (each crew member sees their own side's data, which may differ); E/WD/SD present aircraft information, one copy shared by both.

[!note]- Q3. In normal operation which DMC drives which screens, and how many screens can one DMC drive at most? DMC 1 → CAPT PFD+ND, DMC 2 → F/O PFD+ND, DMC 3 → E/WD+SD. One DMC can drive up to four screens (1 PFD + 1 ND + E/WD + SD).

[!note]- Q4. How do the FWC's red-warning and amber-caution acquisition paths differ, and why? Red warnings are acquired directly; amber cautions come through the SDAC. Life-critical red warnings must not depend on one more box — fewer dependencies, fewer single points. So loss of both SDACs costs amber cautions and system pages, not red warnings.

[!note]- Q5. Does the loudspeaker still sound alerts when turned off? What else does the FWC produce besides alerts? Yes — alert audio bypasses the volume-off state so life-critical warnings cannot be muted. The FWC also produces radio-height and decision-height callouts and landing speed increments.

Key takeaways

References

• FCOM DSC-31-05-10 — EIS six-DU presentation.
• FCOM DSC-31-05-30 — DMC, SDAC, FWC, attention-getters, loudspeaker.
• AMM 31-00-00 — five-subsystem list, Attitude Monitor.
• AMM 31-60-00 — three design pillars, EFIS/ECAM function, normal DMC drive.

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.