AP and FD Engagement, Disengagement, and Priority

Engagement and disengagement are the most fundamental — and the most hardware-hardened — layer of the autoflight system. Article 01 noted that engagement logic is one of the very few things implemented in FMGEC hardware. The whole chain, in one line: FCU pushbutton (hardwired, duplicated contacts — article 02) → FMGEC hardware engagement logic → AP ENGD discretes out to the FCPCs, the FCU, the opposite FMGEC and the FWC. Software supplies the conditions; hardware performs the engagement. This article covers how the AP and FD come on, how they go off, and who holds the power to take them off. What they fly once engaged starts in article 06; A/THR engagement is article 16.

1. The FD — engagement and display

The FDs engage themselves in two families of cases. First, at power application. Per AMM 22-11-00:

The two FDs engage if the internal monitoring channel of the FMGEC does not detect a failure of the FMGEC, and if the FD pushbutton switches on the FCU are lighted.

In flight The safety tests of the AFS are not performed. The two FDs engage in Vertical Speed (V/S) and Heading (HDG) modes.

Power restored in flight: no safety tests, and the FDs come up in V/S and HDG — basic modes referenced to what the aircraft is currently doing. Second, on FMGEC request:

(b) After engagement request from the FMGEC 1 After engagement of the GO-AROUND mode 2 If the AP is lost when the ROLL-OUT mode is engaged.

Go-around engages the bars for you (article 14); and if the AP drops during ROLL OUT, the FD steps up to keep giving rollout guidance. (Honeywell-configured airframes additionally allow manual FD engagement from the EFIS FD pushbutton, and their on-ground auto-engagement waits for the ADIRS to initialise before the FMGEC.) If one FMGEC fails, both PFDs switch to the valid side's FD — the FMA writes it as 1FD1 or 2FD2 (article 06).

What the bars mean — and what the bird does not. Per FCOM DSC-22_30-20:

The pitch, roll, and yaw bars respectively indicate the pitch, roll, and yaw FD guidance orders.

Independently from FD display, the FPV can be displayed on the PFDs. The FPV does not indicate guidance orders.

The FPV is a flight-path vector — a statement of what the aircraft is currently doing, not a command. Close to the runway a third bar appears:

The PFD displays a yaw bar that replaces the roll bar in green below 30 ft radio height if a localizer signal is available during: ‐ Takeoff in RWY mode, or ‐ Landing in FLARE and ROLL OUT modes.

— centre-line guidance through the rudder, for RWY-mode takeoffs and FLARE/ROLL OUT landings. In TRK-FPA presentation the FD becomes the FPD symbol (superimpose the bird on it and you are on the ordered path); and when SRS engages, a previously selected FPV/FPD presentation reverts automatically to the cross bars — dynamic manoeuvres read better on bars (article 14 makes the same point about attitude as the go-around reference).

Three grades of "abnormal FD display", each with its own meaning. Disappearance — stop following:

The FD bars disappear, when one of the following occurs: ‐ The aircraft bank angle exceeds 45 °, or ‐ The aircraft pitch exceeds 25 ° up or 13 ° down.

Flashing — it has something to tell you:

The pitch bar (or FPV) flashes for 10 s and remains steady, if one of the following occurs: ‐ An automatic reversion to V/S (or FPA) mode occurs, or ‐ All APs and FDs are previously disengaged and the flight crew engages the FD.

(In approach, an MMR unable to compute vertical deviation above 100 ft RA or lateral deviation above 15 ft RA makes the corresponding bar flash continuously — article 13.) And the flag — systemically broken:

If the guidance orders are not available while the FD is engaged, a FD flag appears on the PFD and the FD bars disappear.

Flashing = a message. Disappeared = out of the FD's working domain, don't chase it. Red flag = failure. Three levels, three different callout responses (article 06).

2. Clearing — and the system's right to drop your FD

Per-axis clearing is done by the FMGEC setting the pitch/roll/yaw order labels to no-computed-data: no mode on an axis, no bar on that axis — while the FD pushbutton stays lit (the light means "FD engaged", not "bars showing").

And one case where the system takes the FD away from you entirely. Per AMM 22-11-00:

The FGE requests the FCU to disengage the FD if: - Aircraft speed is not in the flight envelope (VLS, VMO, VFE), and - The AP is disengaged, and - The FD is in OPEN CLB/DES or CLB/DES modes. When the FD is disengaged, the A/THR changes to SPEED mode to bring the aircraft speed back in the flight envelope.

Decode it with the mode-pairing rule from article 16: in CLB/DES-type modes the A/THR is in THRUST mode — thrust fixed, speed belongs to the FD. Hand-flying without following the bars leaves speed owned by nobody; the system's backstop is to kill the FD so the A/THR can flip to SPEED and recover the envelope. The FCTM turns the backstop into discipline. Per FCTM AS-FG:

‐ If not using FD or FPD orders, set the FDs to OFF. It is strongly recommended to turn OFF the FDs to ensure that the A/THR is in SPEED mode if the A/THR is active.

Not following the FD? Turn it off — don't leave your airspeed hanging on a safety net.

3. AP engagement — capability, limitation, procedure

The attitude/speed hard gate first. Per FCOM DSC-22_30-30-20:

The AP engages only if the aircraft pitch, bank angle, and speed are within an acceptable range.

The AMM supplies the numbers:

AP engagement is inhibited when abs(phi) > 40° . AP is automatically disengaged when abs(phi) > 45°.

AP engagement is inhibited when theta < -10° or theta > +22° . AP is automatically disengaged when theta < -13° or theta > 25°.

AP engagement is inhibited in flight when CAS < VLS or CAS > (VMO/MMO or VLE/VFE).

(Plus: FCPC reporting pitch protection active inhibits engagement.) Notice the engagement gates are tighter than the disengagement gates (40° vs 45°, +22° vs +25°) — hysteresis against boundary chatter, and directly relevant to when you may re-engage after an upset recovery.

Then the timing question, which has three different answers depending on who is speaking:

"Five seconds" is whether the system is willing; "100 feet and 5 seconds" is whether the rules allow. Limitation always outranks capability. On the ground there is a testing convenience with its own tripwire. Per AMM 22-11-00:

On the ground, with the engines stopped, the AP can be engaged in any mode. But, when an engine is started, the AP disengages.

Dual AP belongs to exactly two situations. Per FCOM DSC-22_30-30-20:

However, the second AP can engage only when one of the following conditions is applicable: ‐ The following approach modes are armed or engaged: • LOC is armed, or LOC* or LOC is engaged, and • G/S is armed, or G/S* or G/S is engaged ‐ Go-around is engaged.

When both APs are engaged, AP1 is active, and AP2 is on standby, regardless of the order of AP engagement.

The AMM adds the succession rule: the FCPCs use AP1's commands and switch to AP2 if AP1 drops; and when the approach or go-around modes are deselected, AP2 disengages by itself — back to the single-AP world. Press AP2 first and AP1 second: AP1 is still the active one. Number is priority (the side-1 philosophy of article 01), not first-come-first-served.

4. The qualification checks — common conditions

Beyond the attitude gate, engagement and retention pass through two families of common conditions. The AP/FD family (AMM 22-11-00, six items):

The FMGEC must receive data from at least two valid Air Data and Inertial Reference Units (ADIRU). When two ADIRUs are lost, the IR/ADR condition disengages the AP/FD.

The FCU must always be seen valid by the FMGEC except in LAND TRACK or GO AROUND modes.

— there, verbatim, is the mechanism of article 02's FCU FAULT exception: in LAND TRACK or GO AROUND, FCU validity is no longer a condition, so the AP finishes the landing.

One of the RA must be valid during the approach phase. The condition is inhibited in ROLL OUT mode to reduce the risk of AP loss in this phase.

Total loss of ILS information at LAND ARM or LOC ARM selection results in AP/FD disengagement (except in the event of glide deviation loss below 100 ft, or LOC deviation below 15 ft).

Monitoring of the deviation between runway heading and magnetic track delivered by the ADIRS: deviation must be less than 7°, with AP engaged between 700 and 100 ft in LOC and GLIDE modes.

That 7° check between 700 and 100 ft is how the flight envelope section confirms the memorised runway heading really is the runway's — the precondition of ROLL OUT (article 13).

If FINAL DESCENT mode is not armed, the loss of lateral or longitudinal flight plans leads to reversion in HDG and V/S modes (AP remains engaged).

And the AP/FD/A-THR family (two items):

Power must be supplied to the FMGEC for more than 3 seconds.

If the two FM sections of the FMGECs 1 and 2 are lost, the AP/FD can be engaged only by using selected modes.

Read the family as a whole and a pattern appears: FM dead does not kill the AP (selected modes remain — article 19), and the closer to the ground, the fewer peripherals the AP depends on (FCU exempted in LAND/GA, ILS-loss exempted at low height, RA condition inhibited in ROLL OUT). The design deliberately makes the automation hardest to knock offline exactly when you need it most.

5. Who may disconnect the AP

You — by design, three ways. The sidestick takeover pushbutton (the standard action, also the dual-input priority arbiter), the FCU AP pushbutton (press while lit), and force. AP engagement energises locking coils that raise the feel thresholds; overpowering them is itself a disconnection order. Per AMM 22-11-00:

The pitch load threshold changes from 0.5 daN to 5 daN. The roll load threshold changes from 0.5 daN to 3.5 daN. Any load on the side stick controller which exceeds these values, results in AP disconnection (wired discrete from the FCPCs).

The load threshold changes from 10 to 30 daN when the AP is engaged.

(That second line is the rudder pedals; overriding them on the ground likewise disconnects.)

The FCPCs — the inner loop's veto. The FCPCs feed an AP ENGD ENABLE discrete to each FMGEC channel; opening it repossesses the AP. Per AMM 22-11-00, the list includes:

- or the Alpha protection is active - or high speed protection is reached

- or the pilot takes control: . by overriding the side stick control . or by moving the rudder pedals on ground . or by moving the THS wheel

— yes, turning the pitch-trim wheel disconnects the AP. The remainder of the automatic-disconnection ledger (same section): computation-channel validity lost, abnormal law engaged (attitude/speed/Mach/alpha beyond limits), bank over 45°, loss of critical surfaces (THS above 200 ft, one elevator, yaw damper), trim-position readback disagreeing with the FMGEC command while AP is not yet engaged, loss of the FMGEC–FCPC ARINC interface, aileron preset, all three IRs lost — and the AP also auto-disconnects when no rudder trim is available except in LAND TRACK below 200 ft.

The ground rules. Per AMM 22-11-00:

On the ground, the AP disengages when the GO AROUND mode is engaged or when the throttle control levers are positioned above the MCT gate. The AP also disengages when the aircraft rolls on the taxi-way in ROLL OUT mode.

And the power supply. Per AMM 22-11-00:

When a Long Power Failure (LPF) is detected by the power unit of the FMGEC. In case of a short power failure, the AP engage signal maintains its pre-cutoff state.

— the engagement flip-flop rides through a sub-200 ms blink (article 01's SPF tier), and drops on anything longer. Across the whole list, notice whose vote weighs most: the FCPCs'. Protections, law degradations, surface losses — the inner loop disconnects the outer loop unilaterally, because the surfaces answer to the inner loop. The FCPC is the AP's final judge.

6. Two shapes of warning — "I did it" versus "it did it"

Disconnect with the takeover pushbutton — the normal act — and the warnings are a time-boxed acknowledgement. Per AMM 22-11-00:

The red MASTER WARN lights on the glareshield flash for 3 seconds - The red AP OFF message is displayed for 9 seconds in the MEMO section of the Engine/Warning Display (EWD) - The cavalry charge aural warning sounds for 1.5 seconds.

All of it self-clears. Any other disconnection — failure, force override, even pressing the FCU AP pushbutton — is treated as an event:

The red MASTER WARN lights flash - The red AUTO FLT AP OFF message is displayed on the EWD - The cavalry charge aural warning sounds - The CLR key on the ECAM control panel comes on.

Nothing self-clears: cancel with the takeover pushbutton or MASTER WARN, clear the message with CLR. Alongside either shape, the FCU AP light extinguishes and AP1/AP2/AP1+2 vanishes from FMA column 5. The ECAM procedure for AUTO FLT AP OFF is a single line — crew awareness (article 31); losing the AP below 200 ft in automatic landing escalates into the red AUTOLAND light system (article 13). There is also a static path: the FMGEC continuously synthesises its own validity (FM, ADR, IR, FCU, rudder trim, FCPC authorisation) into an AP INOP signal for the FWC — AP1/AP2 in the INOP SYS list on the SD. The design lesson in the asymmetry: the standard way to disconnect the AP is the takeover pushbutton — everything else, including the FCU button, is booked as an incident.

7. How firmly does the AP fly?

The AP's outer-loop orders carry hard limits — the quantified basis of "the AP flies more gently than you". Per AMM 22-11-00:

AP pitch control laws are directly targeted in vertical load factor according to the active mode: - 0.3g max in Take off/Go Around; glide capture or track; land track - 0.15g max in climb-descent transition; vertical path; V/S-FPA protection; with airbrakes out; in level off

(Altitude capture and OP CLB/DES run at 0.1 g; the rest at 0.05 g.) Roll:

The roll order limitations in the FMGEC are as follows: - 30° in LOC capture - 8° if LOC track and GLIDE track have been engaged for 15 seconds or are in go-around mode, - 25° up to 270 kts, slowing to 19° at 450 kts, without engine failure

(Engine-out limits shrink from 25° towards 15° as speed margin narrows.) When a selected V/S–FPA target is more than the guidance will honour:

With AP engaged, when V/S-FPA target is not held by the AFS because it is excessive, the AP speed protection is activated: a triple click aural indication and a flashing amber box around V/S or FPA mode and target on the FMA are commanded by FMGEC.

— the triple click and the flashing amber box are article 10's territory. On the inner-loop side, the FCPCs cap the FMGEC's roll authority at 45° bank and ±10°/s roll rate (±20°/s in approach); each FMGEC channel computes its own command, and the pair is voted (smaller absolute value wins) and smoothed before use, with takeoff/landing/go-around outer-loop orders computed in both channels for double cover.

8. Operating wisdom

Why the FCTM tells you to fly first and engage second. Per FCTM AS-FG:

Engaging the AP while large orders are required to achieve the intended flight path may result in an AP overshoot of the intended vertical and/or lateral target. This situation can surprise the flight crew due to the resulting large pitch/roll changes and thrust variations.

The standard three steps: put the aircraft on the intended path; check the FMA (FD engaged, modes correct — set them first if not); centre the FD bars; then press AP. Section 7 explains the why: the AP's load-factor caps make it slow at repairing large errors — hand it a stabilised aircraft. And the certification honesty, per FCTM AOP:

The AP has not been certified in all configurations, and its performance cannot be guaranteed.

In unusual configurations, stay suspicious — if the AP deviates from the expected or safe path, disconnect immediately. One audio detail worth pocketing: holding the takeover pushbutton about 1 second is the audio-cancel position for autoflight aurals. Dispatch interlocks (article 32, some operators' MEL practice): AP1 inoperative bars ETOPS and RNP AR; both APs bar ETOPS, RNP AR, CAT 2/3 and RVSM; the takeover priority function carries no relief at all; one failed takeover disconnect function caps at CAT 1 with the working button given to the PF; a failed AP-disconnect warning means the AP must not be engaged; and a failed stick/pedal locking coil (unlocked) bars autoland and CAT 3 — the coils are section 5's force thresholds in hardware.

[!warning]- Six misconceptions this article corrects (1) "The AP can engage 5 seconds after lift-off" is a system capability, not permission — the limitation is 100 ft AGL and 5 s; limitation outranks capability. (2) Dual AP is not "more is better" in cruise — the second AP engages only with LOC+G/S armed/engaged or in go-around, AP1 is active regardless of button order, and AP2 self-disengages when approach modes are deselected. (3) FM death does not drop the AP — selected modes remain; and approaching the ground the AP sheds dependencies (FCU exempt in LAND/GA, ILS-loss exempt at low height, RA exempt in ROLL OUT). (4) Moving the THS trim wheel disconnects the AP — as does ground pedal input and stick force above 5 daN pitch / 3.5 daN roll. (5) Pressing the FCU AP button is not the standard disconnect — only the takeover pushbutton gives the self-clearing 3 s/9 s/1.5 s warning set; everything else demands manual cancel and CLR. (6) A flashing FD bar, a disappeared bar and an FD flag are three different messages — reversion notice, out-of-domain, and failure — not one.

Self-test

[!note]- Q1. Power is restored in flight — what modes do the FDs auto-engage in? And on the ground at power-up?

In flight: V/S and HDG, with no safety tests. On the ground: both FDs engage after the AFS safety tests, provided the FMGEC's monitoring channel sees no failure and the FCU FD buttons are lit.

[!note]- Q2. What is the earliest the AP can engage after lift-off, what does the limitation say, and how do the layers rank?

Capability: 5 s after lift-off (current FD modes, or HDG+V/S with no FD). Limitation: not below 100 ft AGL and not before 5 s. SOP: above 100 ft as required. Limitation outranks capability, always.

[!note]- Q3. Preconditions for dual-AP engagement? If you press AP2 then AP1, which is active — and what happens to AP2 when APPR is deselected?

LOC and G/S armed or engaged (i.e. APPR pressed), or go-around engaged. AP1 is active and AP2 standby regardless of engagement order — number is priority. Deselecting the approach modes auto-disengages AP2.

[!note]- Q4. After AP engagement, what do the stick and pedal force thresholds become, and what happens beyond them?

Pitch 0.5 → 5 daN, roll 0.5 → 3.5 daN, pedals 10 → 30 daN. Exceeding them is a disconnection order — the FCPCs report the override on a wired discrete and the AP disengages.

[!note]- Q5. Name five cases where the FCPCs repossess the AP. Does turning the THS wheel disconnect it?

Any five of: alpha protection active, high-speed protection reached, abnormal law engaged, bank > 45°, loss of THS above 200 ft / an elevator / yaw damper, sidestick override, ground pedal input, FMGEC–FCPC ARINC loss, all three IRs lost. And yes — moving the THS wheel is "the pilot takes control": AP off.

[!note]- Q6. Why does the AP survive FCU FAULT in LAND TRACK? Would total ILS loss at 50 ft drop the AP?

Because the AP/FD common condition "FCU must be seen valid" is explicitly waived in LAND TRACK and GO AROUND. And no — the ILS-loss disengagement carries exceptions below 100 ft (glide) and 15 ft (LOC); low-height ILS trouble is handled by the AUTOLAND warning system instead.

[!note]- Q7. How do the warnings differ between a takeover-button disconnect and an FCU-button disconnect — and which is the standard action?

Takeover button: MASTER WARN flashes 3 s, red AP OFF memo 9 s, cavalry charge 1.5 s — all self-clearing. FCU button (or failure/override): continuous MASTER WARN and cavalry charge, red AUTO FLT AP OFF, ECAM CLR lit — cancel and clear by hand. The takeover pushbutton is the standard action.

[!note]- Q8. Maximum AP bank after 15 seconds of LOC and GLIDE track — and why so small?

8°. Established on the beam, only small corrections are legitimate; the tight cap keeps the AP gentle exactly where passengers and stabilised-approach criteria demand it (compare 30° allowed during LOC capture).

Key takeaways

References

FD engagement/display/clearing, AP engagement envelope and timing capability, common conditions (AP/FD and AP/FD/A-THR families), disconnection logic and warnings, locking-coil thresholds, and AP authority limits per AMM 22-11-00 (with FCPC discretes and voting per AMM 22-10-00). FD bars/FPV/yaw bar, disappearance/flashing/flag behaviour per FCOM DSC-22_30-20; AP engagement envelope and dual-AP rules per FCOM DSC-22_30-30-20; aural table per FCOM DSC-31-10. Engagement technique, FD-off discipline and the uncertified-configuration caution per FCTM AS-FG and AOP; takeoff minimum-use height per FCOM LIM-AFS (article 33) and normal procedures. Dispatch interlocks reflect some operators' MEL practice. The "three doors/three hands" framings are integrative syntheses. Maintenance-layer detail (discrete pin lists, BITE) 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.