Flight Recorders and the ACMS

The recording domain is the chapter's last piece — it does two things: the black box for accident investigation (DFDRS) and the aircraft-health monitoring ACMS. Both share one brain: a single FDIMU with two internal processors, one half recording (FDIU part), one half monitoring (DMU part). This article covers that "one box, two jobs" architecture, the DFDR's storage and protection, when the recording system runs automatically, and how the ACMS does trend monitoring for the operator.

1. FDIMS / FDIMU — one box, two processors, two jobs

The whole recording system is FDIMS, controlled by one FDIMU, split internally in two. Per AMM 31-33-00:

"The FDIMU-part puts together the function of the former FDIU (System: DFDRS) and the DMU (System: ACMS) into a single Line-Replaceable-Unit (LRU). Inside the FDIMU two separate processor-units, which operate independently from each other do the two functions, referred to as FDIU-part and DMU-part. An internal ARINC-429 data-bus does the data-transmission from the FDIU-part to the DMU-part."

The architecture in a line: one FDIMU, two processors, the FDIU part running the black box, the DMU part running the health monitoring. Combining them into one saves weight and eases maintenance, but the two jobs are internally independent — a recording fault does not affect monitoring, and vice-versa. This is the basis for the MEL splitting "the data management (DMU) may be inoperative, but the DFDR must work".

2. DFDRS — FDIMU + DFDR + tri-axial accelerometer

The black box (recording part) has three components. Per FCOM DSC-31-60:

"The Flight Data Recording System, which records the mandatory parameters, consists of the following components: - A Flight Data Interface and Management Unit (FDIMU) - A Digital Flight Data Recorder (DFDR) - A three-axis Linear Accelerometer (LA). The FDIMU collects and processes parameters from the SDACs, DMCs, FWCs, FCDCs, BSCU, EIVMUs, FCU, the DFDR event pushbutton, the GND CTL pushbutton and the Clock. It stores the mandatory flight parameters in the DFDR. The DFDR can store the last 25 h data, at least. It stores this data on a fireproof and shockproof device. An underwater locator beacon is attached to the DFDR."

Three facts to memorise: the DFDR stores ≥ 25 hours, is fireproof and shockproof, and carries an underwater locator beacon (ULB) — the hard requirements of investigation. Data is stored in ARINC-717 frames. The FDIMU's parameter sources are wide (nearly every main computer), which is why it is the aircraft's data confluence. The LA (tri-axial accelerometer) has a detail worth noting: it sits between Frame 46 and 47 under the cabin floor (the aircraft CG), outputs analogue → first to the SDAC to digitise → then over ARINC-429 to the FDIU → stored on the DFDR. So acceleration data reaches the recorder via the SDAC. The FDIU also sends one thing to the CVR: an audio signal with encoded GMT time (AMM 31-33-00) — so the voice recording and the data recording share a time reference. The CVR itself belongs to ATA-23; ATA-31 only controls its start/stop through the RCDR GND CTL.

3. Automatic operation — three conditions to start, off five minutes after shutdown

The recording system needs no attention; it runs automatically. Per FCOM DSC-31-60:

"The recording system is automatically active: - On ground, during the first 5 min after the aircraft electric network is energized. - On ground, after the first engine start. - In flight (whether the engines are running or not). On the ground, the recording system automatically stops 5 min after the second engine shuts down. On the ground, the crew can manually start the recording system by pressing the GND CTL pushbutton."

Memorise the three-plus-one: records automatically in the first 5 minutes after ground power-up, after the first engine start, and throughout flight; stops automatically 5 minutes after the second engine shuts down on the ground. Why "the first 5 minutes after power-up"? To leave a recording window for ground self-test / brief operations. The GND CTL button is a ground manual force-start (e.g. for maintenance to test the recorder, or to press CVR test).

4. RCDR GND CTL and the DFDR event button

Two controls. Per FCOM DSC-31-60:

"(1) RCDR GND CTL pb-sw — The CVR, DFDR, and QAR automatically operate (AUTO), according to the logic... [ON] The flight crew manually starts CVR, DFDR and QAR. After engine start, the ON light goes off and the recorders automatically operate."

A DFDR event button (on the centre pedestal): a momentary press sets an event mark on the DFDR — a moment the crew judges worth reviewing later (turbulence, an anomaly), for easy locating on playback.

5. QAR and CVR — same data, easier to access / don't confuse the ownership

Per FCOM DSC-31-60:

"The QAR is a recorder that stores the same data as the DFDR. However, the QAR is more accessible for the maintenance crew."

The QAR is "the easy-access DFDR" — the same data, but where the DFDR is the fireproof/shockproof black box (for investigation, hard to remove), the QAR is for routine download and trend analysis. On the CVR ownership again: the CVR is ATA-23's cockpit voice recorder; ATA-31's RCDR GND CTL merely starts/stops it (alongside the DFDR/QAR); its recorded content, channels and protection are in ATA-23, and the MEL's RECORDER CVR FAULT also refers to ATA-23 (see the MEL article).

6. ACMS — aircraft health monitoring and reports

The ACMS, run by the DMU part, is a different line — not for accidents, but for routine health monitoring. Per FCOM DSC-31-60:

"The ACMS is used to monitor various aircraft system parameters in order to make maintenance easier and to allow formulating operational recommendations. The ACMS can generate system reports... The system may be programmed using the MCDUs... The Printer prints the flight phase programmed reports or any report selected on the MCDU... The ACMS may send automatic reports via ACARS. An optional Digital Recorder may be installed to extend the recording capacity."

The ACMS's value is trend monitoring: long-term tracking of engine and APU parameters to pre-empt unscheduled maintenance away from base; also specific troubleshooting (a hard landing). It stores reports on the DMU part's internal solid-state mass memory (SSMM), and optionally on a Smart ACMS Recorder (SAR), a PCMCIA card, or an external Digital ACMS Recorder (DAR). To the crew the ACMS is mostly invisible — an aircraft "health-check" instrument for maintenance and engineering; the crew touch it only to press ACMS PRINT or select a report on the MCDU.

Self-test

[!note]- Q1. What are the two internal parts of the FDIMU, and why combine them? The FDIU part (DFDRS) and the DMU part (ACMS), two independent processors. Combined into one LRU for weight and maintenance; the two jobs remain internally independent.

[!note]- Q2. What three components make up the recording system, and give the DFDR's three facts. FDIMU, DFDR, tri-axial accelerometer. DFDR: ≥ 25 h storage, fireproof and shockproof, with an underwater locator beacon.

[!note]- Q3. Do the QAR and DFDR store the same data, and what is the difference? Yes. The DFDR is the fireproof/shockproof black box; the QAR is the same data but more accessible for maintenance.

[!note]- Q4. When does the recording system run automatically, when does it stop, and what does GND CTL do? Automatically in the first 5 min after ground power-up, after the first engine start, and throughout flight; stops 5 min after the second engine shutdown on the ground. GND CTL is a ground manual force-start.

[!note]- Q5. Which chapter owns the CVR, and how is ATA-31 related to it? ATA-23 owns the CVR. ATA-31 only starts/stops it via RCDR GND CTL, and the FDIU supplies its encoded GMT time reference.

Key takeaways

References

• FCOM DSC-31-60 — recording-system components, DFDR 25 h/protection/ULB, QAR, automatic operation, GND CTL, ACMS.
• AMM 31-33-00 — FDIMS/FDIMU two parts, FDIU→CVR encoded GMT, LA via the SDAC.
• AMM 31-36-00 — ACMS trend monitoring, DMU/SSMM/SAR/DAR.
• AMM 31-30-00 — recording-system block overview.

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.