The FCU and EFIS Control Panels

Article 01 established the operating principle: short-term orders go through the FCU, long-term orders through the MCDU. This article is about the short-term interface itself — one box on the glareshield wearing three faces, the grammar of its knobs (turn, pull, push), what its windows show when the automation degrades, and why a system full of redundancy still treats the FCU as a no-dispatch item. Per FCOM DSC-22_10-10:

The FCU on the glareshield is the short-term interface between the flight crew and the FMGEC.

1. One box, three faces, two channels

Per FCOM DSC-22_10-40-20:

The Flight Control Unit (FCU) is located on the glareshield and is constituted of three control panels: One for the automatic flight controls and two for the Electronic Flight Instrument System (EFIS). The FCU has two channels, each of which can independently command the central panel. If one channel fails, the other channel can control all the functions.

The centre panel commands the AFS; the two outboard panels command the left and right EFIS. As hardware, per AMM 22-81-00:

Characteristics : - dimensions : 568 mm X 103 mm X 258 mm - weight : < or equal to 7.4 kg - power : < 25 W - connectors : two 61-pin connectors and two 19-pin connectors.

Channel 1 (side 1) is fed from the 28 VDC ESS bus, channel 2 from DC BUS 2. The green engagement bars in the pushbuttons are driven through the overhead ANN LT switch (DIM/BRT/TEST — TEST lights every window and button lamp), while the digital windows and the integral panel lighting have their own two potentiometers below the FCU.

Notice what "two channels" does not mean: two boxes. Both channels, both power supply units, live in the same FCU housing. That is why AUTO FLT FCU FAULT is a no-dispatch item in the MEL (article 01, article 32) — the FCU is one of the AFS's rare single-point components. A maintenance aside that prevents a false conclusion: after both FCU circuit breakers are pulled, the displays keep glowing for a while — residual polarisation of the liquid crystals, not evidence of a live supply.

2. The window language — managed, selected, and the altitude exception

Per FCOM DSC-22_10-40-20:

When the aircraft uses target quantities from the FMGES (managed guidance), the FCU windows display dashes and the white dots next to those windows light up. When the aircraft uses target quantities that the flight crew enters (selected guidance), the windows display the selected numbers and the white dots do not light up.

In order to arm or engage managed guidance for a mode, the flight crew pushes in the associated knob. If, for example, they push in the HDG knob, this engages or arms the NAV mode.

So the grammar is: pull — I take it (selected, the aircraft obeys the FCU value); push — it takes it (managed, the aircraft obeys the FM); dashes plus a lit white dot mean the FM owns the target. One window refuses to speak this language:

The altitude window always displays an altitude that the flight crew selects (never dashes).

The altitude is a clearance quantity. Whoever is providing guidance, the ATC-cleared altitude must remain visible and must be set by hand — that is the level-bust design floor. "Managed altitude" therefore shows up not as dashes but as whether the FM respects its constraints against the FCU-selected level (article 07, article 08).

What if you turn a knob but do not pull it? Per FCOM DSC-22_10-40-20:

If the flight crew turns and does not pull a knob, it displays a value that is the sum of the current target and the turn action value. The value remains displayed 45 s before the dashes reappear. This rule does not apply to the ALT knob/window.

A wording note for the careful reader: this general rule starts the count from the current target, while the HDG and SPD window sections (section 5 below) describe the window as first synchronising with the current aircraft value and then following the manual selection. Both wordings coexist in the FCOM. Teach the behaviour, not the starting value: turn without pull is provisional and evaporates after 45 seconds — to make it real, pull (selected) or push (give it back to the FM).

3. The AFS panel — knobs, ranges, and the engagement area

Per AMM 22-81-00:

The selector knobs are shaped differently to avoid any confusion by the crew. They have three positions : - rotate action for display (30 clicks per turn) - pull action for engagement of selected control - push action for engagement of managed control.

Ranges and steps:

On the altitude knob's outer ring, per AMM 22-81-00:

This knob incorporates a concentric outer knob to modify the rotational detent: one click corresponds to 100 ft. in one case, 1000 ft. in the other case.

Three monostable toggle pushbuttons complete the panel: SPD↔MACH, HDG-V/S↔TRK-FPA (which switches the whole guidance pair), and the metric-altitude button (adds metres on PFD/ECAM). A dispatch pattern worth noting here and developed in article 32: in some operators' MEL the three selector knobs (SPD-MACH, ALT, HDG-TRK) carry no relief — all must work — while the V/S-FPA knob is dispatchable when not operationally required, because a level-off has alternatives (push the ALT knob, push the V/S knob) whereas the three primary knobs have none.

The AP/A-THR engagement area holds the AP1, AP2 and A/THR pushbuttons. Per FCOM DSC-22_10-40-20:

The flight crew uses this pushbutton to arm, activate, or disconnect the autothrust (A/THR). This button illuminates green if the A/THR is armed or active.

Read that carefully: the AP pushbutton lights mean one thing (lit = engaged), but the A/THR green light is deliberately ambiguous — armed and active look identical. To know whether the A/THR is actually commanding thrust, read FMA column 5 (article 06) and the thrust levers, not this lamp. The mode pushbuttons with green bars (LOC, ALT, APPR) carry the same ambiguity: the bar lights whether the mode is armed or active.

4. The EFIS control panels

Each pilot has a mirror-image EFIS panel. The barometric reference knob follows the same three-action grammar. Per AMM 22-81-00:

This selector knob used for baro setting features three possible actions : - rotate action for the display (1 turn = 30 clicks) - pull action for the display of Std - push action for barometric pressure and QNH or QFE selection according to the QFE pin programming wiring.

Range 745–1050 (or 1100) hPa / 21.99–31.00 (32.48) inHg, one hPa or 0.01 inHg per click, with the outer ring selecting the unit. On the representative airframe of this series the QFE option is active (article 01), so pushing the knob genuinely toggles QNH/QFE rather than doing nothing. Some operators' MEL treats the inner baro knob like the primary knobs — both must work — while the unit-selection outer ring may be inoperative provided both panels can display the unit planned for the flight.

The rest of the panel: the FD pushbutton (green bars, lit = that side's FD orders displayed), the LS pushbutton (LS deviation scales on that side's PFD), the ND mode selector with five positions — ARC, PLAN, ENG, and ROSE (NAV · VOR · LS). The odd one out, per AMM 22-81-00:

ENG (in case of ECAM system failure, this mode provides engine data on the ND)

The range selector offers 10/20/40/80/160/320 NM. Five option pushbuttons (CSTR, WPT, VOR.D, NDB, ARPT) overlay data on the ND — with a catch, per AMM 22-81-00:

It is to be noted that only one function can be selected at a time: the last option selected is the one which is taken into account.

Two ADF-OFF-VOR switches select the sources for ND bearing pointers 1 and 2. Dispatch of the ND mode/range selectors drags a chain of PBN restrictions behind it (a failed selector on the PF side can bar the whole RNAV/RNP family — article 27, article 34).

5. What the windows show when things degrade

The HDG/TRK window displays a value in four situations. The everyday one, per FCOM DSC-22_30-40-30-10:

The flight crew turns the HDG/TRK knob. The value in the window first synchronizes with the current HDG/TRK, then displays the manual selection. It remains displayed for 45 s, then vanishes if the flight crew does not pull the knob (except in HDG preset)

And the degraded one:

AP/FD is lost. The value becomes that of the aircraft current heading or track.

The other two: while HDG or TRK mode is engaged (current or manually selected target), and during a preset (continuously displayed — the 45 s rule does not apply). Switching HDG↔TRK converts the displayed value to its equivalent, so the aircraft's path intention is preserved across the format change.

The SPD/MACH window has four special cases of its own. Per FCOM DSC-22_30-60-40:

If the FMGES is powered up in flight, the synchronized speed/Mach value is the current aircraft speed or Mach

If no V2 is entered at takeoff, the V/S mode engages 5 s after lift-off (no SRS TO). The FCU speed target is the speed at V/S mode engagement.

If both FMGECs fail, the displayed speed is the last aircraft speed acquired before the failure.

The fourth is the pleasant one: a speed/Mach preselected on the PERF page for the next phase engages and displays automatically on phase entry (article 26). Notice what the three degraded cases have in common with the HDG story above: when the guidance brain restarts or dies, the windows always hand you a flyable number — the current value or the last known value, never a blank or a zero. In degradation, the FCU windows quietly become your manual-flying reference card.

6. Channels B and C — reconfiguration without a flicker

Per AMM 22-81-00:

In normal operating conditions each computation channel performs a specific function as follow: - channel B : EIS LEFT and AFS - channel C : EIS RIGHT In the event of a failure of one channel, there is reconfiguration on the remaining channel.

The two channels run the same software — their identities set by pin programming — each with its own power supply unit (PSU 1 → B, PSU 2 → C) plus a common 21 VDC output for shared circuits and the displays. Recall the contrast drawn in article 01: the FMGEC's channels run dissimilar programs because the FMGEC makes guidance decisions; the FCU's channels run identical software because the FCU selects and relays. Different stakes, different philosophy.

Why does the takeover not show on the panel? Per AMM 22-81-00:

In order to avoid display modifications during reconfigurations, the channel non-active on a given function synchronize on the values computed by the channel active on this function.

There is also synchronization, with aircraft in flight, after long power cuts. In this case, the data used are those stored in the RAM (Random Access Memory) prior to power cutoff. The displays are thus maintained.

The standby channel is continuously copying the active channel's answers over the loop-back buses, so at the moment of takeover it already holds the current values — and after a long power cut the pre-cutoff RAM contents restore the displays. This is the component-level mechanism behind article 01's system-level statement that the FCU retrieves its selected data after interruptions of up to 5 minutes.

Health monitoring is layered. Each channel has a watchdog; the EIS outputs are self-compared via loop-back; and the FMGEC provides the outside pair of eyes. Per AMM 22-81-00:

the active FMGEC which compares, for the channel active on the AFS function, the data received from the FCU with its transmitted data.

In the event of a fault, the faulty channel disconnects itself (FCU HLTY/A, B or C discrete signal).

The order-giver checks the messenger — the same philosophy as the thrust-target read-back in article 01. A channel caught misbehaving takes itself off line and says so on the FCU HLTY discretes.

7. The engagement pushbuttons — hardware all the way

Per AMM 22-81-00, on the AP1/AP2/A-THR engagement controls:

Those data are issued directly from the engagement pushbutton switches without software processing. Each pushbutton switch contains two contacts in order to duplicate the output signals.

Even with the FCU software dead, pressing AP1 still reaches the FMGECs: button → hardwired discrete (duplicated contacts, both FMGECs) → the engagement logic that article 01 showed is implemented in FMGEC hardware. Engaging and disengaging the automation is the crew's last manual power, and the design lets no software stand in that doorway.

The FCU's digital outputs number six ARINC buses: EIS CP-L and CP-R, plus four AFS buses carrying identical content (AFS-1, AFS M-1, AFS-2, AFS M-2) — quadruplicated for engine segregation (M-1 towards FMGEC 1/EIVMU 1, M-2 towards FMGEC 2/EIVMU 2), with AFS M-1 and the EIS buses looped back into both channels for synchronisation and monitoring. These four same-message buses are the physical substance of the thrust chain FMGEC → FCU → EIVMU → FADEC from article 01.

8. Power-up tests and resets

Per AMM 22-81-00:

The FCU power up tests are initiated automatically on the channels concerned (B/PS 1, C/PS 2) when the following conditions are met: - aircraft on ground (configuration confirmed by two data) - restoration of electrical power after power cutoff longer than 5 s.

Action on one of the RESET circuit breakers located in the cockpit is equivalent to a power cutoff longer than 5 s.

So the cockpit FCU RESET breakers are, to the FCU, indistinguishable from a long power cut. The QRH then adds two usage rules (the reset system as a whole is article 29's subject). Per a representative operator QRH, computer-reset table:

In flight: ‐ When PART FCU is displayed as INOP SYS, do not reset the FCU ‐ When FCU is displayed as INOP SYS, check FCU targets after the reset.

Following a total FCU reset either in flight or on ground, FCU targets are FCU entries (selected modes).

The mechanics behind both rules follow from section 6. PART FCU means one channel has already failed and the survivor is doing everything — resetting now gambles your only good channel, so in flight you don't. And after a total reset the RAM's managed relationships are gone: the system is back to "fly what the windows say" (selected). First action after any total FCU reset: check all four window targets, or you may be flying pre-reset numbers.

9. Using it — and losing it

On discipline, per FCTM AOP-20:

The FCU, the EFIS CP and the MCDU must be used in accordance with specific rules, in order to ensure: - Safe operation (correct entries made) - Effective inter-pilot communication (knowing each other's intentions).

The task-sharing that follows (article 30): engaging/disconnecting AP and A/THR belongs to the PF; with the AP off, the PF does not touch the FCU — the PM operates it on the PF's command; with the AP on, the PF operates it himself; each pilot manages his own EFIS panel.

And the total loss. FCOM PRO-ABN (AUTO FLT FCU FAULT) lists the consequences; three lines carry the teaching weight:

With all FCU channels failed, the barometer reference goes to 1 013 hPa. Use standby altimeter to change this to the actual barometer setting.

Do not insert the MDA (MDH) value on the MCDU PERF APPR page (because the PFD altitude is referenced to STD, not to the correct barometric value).

A/THR, AP 1+2 and FD 1+2 not available. (except in LAND or GO AROUND mode where only A/THR is lost)

The LAND/GO AROUND exception is an architecture lesson in one parenthesis. Once the common LAND mode is established, the guidance targets are already locked inside the FMGEC — no further FCU relay needed — so a dying FCU cannot stop the AP finishing the landing. The A/THR command, by contrast, physically travels through the FCU (section 7's four AFS buses): FCU dead, thrust chain cut. The same procedure also costs altitude alerting, the TRK/FPA and MACH selections, fixes the ND to ARC 20 NM with no weather radar image, and forces bearing pointer 1 to VOR 1 and pointer 2 to ADF 2. In the electrical emergency configuration the FCU is on a single channel to begin with (article 31).

The MEL pattern across the panel (article 32): controls with no alternative path are strictest (the three primary knobs, the inner baro knobs — all must work); controls with an alternative are relieved (V/S-FPA knob, metric button); display windows may be inoperative provided the corresponding indications work on both PFDs and NDs (the information has a second exit); and a failed APPR pushbutton leaves NAV/FPA, TRK/FPA, LOC/FPA (if the LOC pushbutton works) and raw-data manual approaches — capability capped at CAT 1.

[!warning]- Six misconceptions this article corrects (1) "Two channels" does not mean two boxes — both FCU channels share one housing, which is why FCU FAULT is a no-dispatch item despite full internal redundancy. (2) The altitude window never shows dashes — altitude is a clearance quantity that must stay visible and hand-set; do not wait for a "managed altitude" display format that doesn't exist. (3) The A/THR pushbutton's green light does not tell you the A/THR is controlling thrust — armed and active light identically; read FMA column 5. (4) Turning a knob without pulling changes nothing permanently — the number is provisional and vanishes after 45 s (ALT window excepted, HDG preset excepted). (5) FCU channel health is not self-declared only — the active FMGEC re-reads what the FCU transmits and compares it against what it sent. (6) In FCU FAULT, LAND/GA keeping the AP while losing A/THR is not arbitrary — guidance targets are already inside the FMGEC, but the thrust command physically routes through the FCU.

Self-test

[!note]- Q1. What do rotate, pull and push mean on an FCU knob? How long does a turned-but-not-pulled value survive, and which window is exempt?

Rotate = display a value (30 clicks per turn); pull = engage selected guidance (the aircraft obeys the FCU); push = engage/arm managed guidance (the aircraft obeys the FM). A turned value not confirmed by a pull vanishes after 45 s (dashes return). The rule does not apply to the ALT knob/window, and a HDG preset is also exempt.

[!note]- Q2. Why does the altitude window never display dashes, and what are the ALT knob's two click values?

Altitude is the ATC-clearance quantity: it must always be visible and always be set by the crew, whatever guidance is active — the design floor against level busts. The concentric outer ring selects 100 ft or 1 000 ft per click.

[!note]- Q3. What do channels B and C each do normally? If B fails, who runs the AFS panel — and why doesn't the display jump at takeover?

B runs the left EIS plus the AFS function; C runs the right EIS. Either channel can run everything, so C takes over all functions. No jump because the non-active channel continuously synchronises on the active channel's computed values via the loop-back buses — it already holds the answers when it takes over.

[!note]- Q4. When you press AP1, what path does the signal take to the FMGEC? Does an FCU software failure block it?

Directly from the pushbutton contacts as hardwired discretes — no software processing, duplicated contacts, to both FMGECs — into engagement logic that is itself implemented in FMGEC hardware. Software death does not block the path.

[!note]- Q5. STATUS shows PART FCU in flight — may you reset the FCU? And what are the FCU targets after a total reset?

No — PART FCU means one channel is already dead and the survivor is doing everything; a reset gambles the only good channel. After a total FCU reset (flight or ground), targets are FCU entries — selected modes — so check all four windows before trusting them.

[!note]- Q6. In FCU FAULT, why can LAND/GA keep the AP but not the A/THR? And what happens to the baro reference?

LAND/GA guidance targets are already locked inside the FMGEC, needing no FCU relay; the A/THR command physically routes through the FCU's four AFS output buses, so it dies with the FCU. The barometric reference is forced to 1013 hPa — set the actual setting on the standby altimeter, and do not insert MDA/MDH on the PERF APPR page (PFD altitude is referenced to STD).

[!note]- Q7. What does the SPD window show if both FMGECs fail? And what happens at takeoff with no V2 entered?

Both FMGECs failed: the last aircraft speed acquired before the failure. No V2 entered: no SRS takeoff mode — V/S engages 5 s after lift-off, with the FCU speed target frozen at the speed at engagement. Degraded cases always leave a flyable number in the window.

Key takeaways

References

FCU definition and window philosophy per FCOM DSC-22_10-10 and DSC-22_10-40-20 (three panels/two channels, managed dashes and white dots, altitude-window exception, push-to-manage, the 45 s rule, A/THR pushbutton light). Window synchronisation cases per FCOM DSC-22_30-40-30-10 (HDG/TRK window) and DSC-22_30-60-40 (SPD/MACH window). Hardware characteristics, knob actions and ranges, EFIS panel controls, channel split and reconfiguration, passive synchronisation and RAM restoration, monitoring and HLTY discretes, pushbutton discretes, ARINC output buses, and power-up test conditions per AMM 22-81-00 (with the discrete list per AMM 22-85-00 and bus detail per AMM 22-86-00; power and lighting per AMM 22-84-00). Reset usage rules per a representative operator QRH computer-reset table; FCU FAULT consequences per FCOM PRO-ABN AUTO FLT; usage discipline per FCTM AOP-20. Dispatch patterns reflect some operators' MEL practice and vary by operator. The mechanism explanations connecting the LAND/GA exception, the reset rules and the four-bus thrust chain are integrative syntheses of the sources above. Maintenance-layer detail (connector pinouts, BITE menus, test sequences) 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.