FCDC and Interfaces — the Data Bridge to the PFD, SD, Warnings and Recorder

The PRIM and SEC computers fly the aircraft; the two FCDCs (Flight Control Data Concentrators) make what they are doing visible and recorded. Every surface position on the SD F/CTL page, every protection-speed band on the PFD, every flight-control ECAM warning, and the flight-control data on the recorder passes through the FCDC. Lose an FCDC and you lose display and monitoring, not control.

The Flight Control Data Concentrator (FCDC) serves as an interface between the flight control computers (FCPCs and FCSCs) and the display system (EIS, FWC), the maintenance system (CMS) and the recording system (FDIU). It achieves the data concentration functions, the warning transmission and performs the flight control system maintenance. — AMM 27-95-00

1. Two FCDCs, three functions

Per AMM 27-95-00, there are two FCDCs (FCDC-1, FCDC-2) sitting between the FCPCs/FCSCs and the rest of the aircraft, with three functions:

1. Data concentration — transmit selected data to the DMCs (displays), the FDIU and the flight recorder, chosen from multiple sources by validity and priority;
2. Warning — report flight-control failure status to the FWC and DMCs;
3. Maintenance — isolate and memorize flight-control failures and interface with the CMC.

[!warning]- The FCDC is a data bridge, not a flight-control computer (integrative synthesis) This is the key distinction: the FCDC does not fly the aircraft. It concentrates the PRIM/SEC output for display, warning and recording. So an FCDC failure degrades your F/CTL page, PFD bands and ECAM annunciation — the information layer — while the surfaces stay controlled by the PRIM/SEC. Two FCDCs provide redundancy for that information layer.

2. What the FCDC feeds the displays

Per AMM 27-95-00, the concentrated data, by destination:

[!warning]- The PFD protection-speed bands come through the FCDC (integrative synthesis) The amber and red strips on the PFD speed scale — Alpha Prot, Alpha Max, stall-warning speeds — are computed by the FCPCs and reach the PFD via the FCDC (AMM 27-95-00). Likewise the estimated sideslip (the sideslip target "blue beta") and the sidestick-priority arrows. So the protection cues you fly to are FCDC-concentrated FCPC data — the protection logic and its display are two different boxes.

[!note]- Spoiler position is binary on the SD (integrative synthesis) Unlike the analogue elevator/aileron/rudder positions, spoiler position is sent as extended-or-retracted (AMM 27-95-00) — the F/CTL page shows spoilers as discrete arrows, not a continuous deflection, because that is all the FCDC concentrates for them.

3. Warnings and maintenance

Per AMM 27-95-00, the FCDC reports flight-control failure status to the FWC (which raises the ECAM F/CTL alerts) and DMCs, and on the maintenance side it isolates and memorizes failures for the CMC. Each FCDC engages when the electrical network is energised; after a power interrupt longer than 6 s on the ground it runs a ~3 s self-test, and a watchdog forces a reset if the program stalls (limited to a few automatic restarts).

[!note]- The warning path is separate from the control path (integrative synthesis) When you see an ECAM F/CTL caution, the chain is: FCPC/SEC detects → FCDC transmits failure status → FWC raises the alert (AMM 27-95-00). The same fault that reconfigures a law is annunciated through this independent data path — which is why the F/CTL page and the law-reversion message are consistent.

4. Counterintuitive points

[!warning]- Losing an FCDC costs you display/monitoring, not control The FCDC is a concentrator between FCPC/SEC and the displays/recorder (AMM 27-95-00); the surfaces stay flown by the PRIM/SEC.

[!warning]- The PFD speed-scale protection bands are FCDC-delivered FCPC data Alpha Prot / Alpha Max / stall-warning speeds and estimated sideslip reach the PFD through the FCDC (AMM 27-95-00).

Self-test

[!note]- Q1. What does the FCDC do, and what does it not do? It concentrates data for displays/recorder, transmits warnings to the FWC, and handles flight-control maintenance — it does not fly the aircraft (PRIM/SEC do).

[!note]- Q2. Which protection cues on the PFD come from the FCDC? Alpha Max, Alpha Prot, stall-warning speeds, target/estimated sideslip, sidestick movement — FCPC-computed, FCDC-concentrated.

[!note]- Q3. How is spoiler position shown on the SD, and why? Binary (extended/retracted) — that is the form the FCDC concentrates for spoilers.

[!note]- Q4. The ECAM F/CTL warning chain? FCPC/SEC detects → FCDC transmits failure status → FWC raises the alert.

Key takeaways

References

• AMM 27-95-00 (Flight Control Data Concentrator — Description and Operation) — two FCDCs (FCDC-1/2) interface FCPCs/FCSCs with display (EIS, FWC), maintenance (CMS/CMC), recording (FDIU); three functions — data concentration (to DMCs/FDIU/recorder by validity and priority), warning (failure status to FWC and DMCs), maintenance (isolate/memorize failures, CMC interface); SD data (elevator/THS/aileron/rudder positions, spoilers binary, rudder trim, travel-limitation value, defect statuses); PFD data (sidestick movement, target/estimated sideslip, Alpha Max/Alpha Prot/stall-warning speeds); FDIU data (surface deflections, rudder travel limit/trim, sidestick/pedal position, sidestick priority, speedbrake); engage at network energization, ~3 s self-test after >6 s ground power interrupt, watchdog reset.
• AMM 27-97-00 (EFCS ARINC Buses and Discretes Interfaces) — computer-to-computer and computer-to-system interfaces (covered under 27-90-00 Interface).

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.