The AP/FD TCAS Mode

The classical answer to a resolution advisory is "AP off, hand-fly the green sector". The A330's AP/FD TCAS mode turns that into "the AP flies the RA itself" — the only vertical mode in the FG catalogue ignited by a surveillance system. TCAS itself (TA/RA generation, the ATC/TCAS panel) belongs to ATA-34; this article is about how the flight guidance flies the RA. The mode is fitted on the baseline airframe of this series (article 01). Its design intent, from the ATA-34 side:

The AP/FD TCAS mode optimizes the vertical speed for a rapid and appropriate response to an RA, and minimizes the deviations from the latest ATC clearance.

Evade quickly — and cheaply: no more manoeuvre than the RA requires.

1. Chapter 34 asks, chapter 22 answers

Division of labour: the TCAS computer (ATA-34) detects the intruder and generates the TA/RA with its green/red vertical-speed sectors; the FMGEC's FG translates the RA order into a guidance law flown by the AP or FD. The lifecycle:

 TA ──► TCAS arms   (FMA vertical column, line 3, cyan)
 RA ──► TCAS engages (line 1, green — silently)
 CLEAR OF CONFLICT ──► auto-exit → V/S (or ALT family) back toward the FCU altitude

Per FCOM DSC-22_30-40-100:

When a TA is triggered, the AP/FD TCAS mode arms. When an RA is triggered, the AP/FD TCAS mode engages to provide guidance in accordance with the RA order: ‐ The AP status does not change: Remains engaged or disengaged ‐ If not previously engaged, the FDs automatically engage.

A layout detail with a purpose: the armed TCAS is written on line 3 of the vertical column — other armed modes live on line 2, so TCAS can share the screen with another armed mode (if another AP/FD vertical mode is armed, the FMA displays both modes).

2. Availability — and the silent engagement

Per FCOM DSC-22_30-40-100:

The AP/FD TCAS mode is available, when all of the following conditions are met: ‐ The aircraft is above 900 ft RA ‐ The TCAS is not failed (i.e. the ECAM did not trigger the NAV TCAS FAULT alert) ‐ TCAS is in TA/RA mode ‐ At least one AP and one FD are available ‐ The AP/FD TCAS mode is not failed (i.e. the ECAM did not trigger the AUTO FLT TCAS MODE FAULT alert) ‐ EGPWS, WINDSHEAR, or STALL warning is not triggered.

Above 900 ft RA (below it, TCAS itself inhibits RAs toward landing); switched to TA/RA; one AP and one FD available (not necessarily engaged); and none of the three higher-priority warnings shouting — EGPWS, windshear and stall outrank an RA. Arming is generous: the AP/FD TCAS mode arms, even if both APs, and both FDs are disengaged — the system posts the sentry early and recruits the crew only if the RA actually comes. (In TA ONLY, per the FCTM, the mode does not arm at all.)

Engagement is deliberately quiet:

Note: ‐ When the AP/FD TCAS mode engages, the triple click audio indicator does not sound, and the FD pitch bar does not flash.

And the FCTM gives the reason:

The FD pitch bar does not flash, and the triple click aural alert does not sound, in order to avoid to disturb the PF during the evasive maneuver.

Article 10's three reversion annunciations are all switched off here — not an omission, a courtesy: the cockpit is already full of RA audio and red/green sectors. The price: the PF must read the FMA unprompted and call the change like any other.

3. The chain reaction at engagement

Lateral: whatever was engaged or armed stays. With one addition —

If both APs and both FDs are disengaged, when the AP/FD TCAS mode engages, HDG automatically engages. The heading target is the aircraft heading at HDG engagement.

A fully hand-flown aircraft is instantly issued a complete guidance package: TCAS vertical plus HDG lateral (the third case of automatic FD engagement, after article 05's two).

Vertical: the engaged vertical mode steps down; armed modes are cancelled (with the 10 s white box), except an armed ALT/ALT CRZ compatible with the RA direction. On approach, the package splits — article 11's preview in full:

Note: ‐ If G/S / F-G/S is engaged, when the AP/FD TCAS mode engages: • G/S / F-G/S disengages • LOC / F-LOC remains engaged • The APPR pb comes off, and the LOC pb comes on. ‐ If FINAL APP is engaged , when the AP/FD TCAS mode engages: • FINAL APP disengages • Managed lateral guidance remains engaged with NAV engaged • The APPR pb comes off.

The vertical yields to TCAS; the lateral keeps watching the road — and the button lights re-arrange to say so. One warning attached:

Note: If the approach is re-armed and if the capture conditions are fulfilled, the corresponding approach mode engages. The AP/FD TCAS mode disengages (even if it is not CLEAR OF CONFLICT). A RA may be triggered again.

Re-pressing APPR before the conflict clears lets the approach modes shove TCAS aside — and the RA may simply fire again. Conflict not clear: hands off APPR.

A/THR and speed:

When the AP/FD TCAS mode engages: ‐ If the A/THR is disconnected, it becomes armed or active, depending on the thrust lever position ‐ If the A/THR is armed, it remains armed ‐ If the A/THR is active, it remains active.

The speed/Mach control becomes selected, and the speed/Mach target becomes the aircraft speed/Mach at the occurrence of the RA.

A disconnected A/THR is re-recruited (the manoeuvre wants automated speed cover), and the speed target is a snapshot — frozen at the value you carried into the conflict (a new RA takes a new snapshot).

4. The guidance law — V/S's shell, TCAS's heart

Per FCOM DSC-22_30-40-100:

Depending on the RA and the FCU selected altitude, the AP/FD TCAS mode uses the guidance law of one of the following modes: V/S, ALT*, ALT, ALT CRZ*, ALT CRZ.

No new control law is invented — the mode borrows from the V/S and ALT family according to how the RA order relates to the FCU altitude. The target:

The vertical speed target is one of the following values: ‐ The aircraft vertical speed at the engagement of the AP/FD TCAS mode, if a preventive RA is triggered, or ‐ The limit between the green and red area of the vertical speed scale, ± 200 ft/min if the aircraft should climb (descend).

Preventive RA (monitor vertical speed): hold what you have. Corrective RA: fly to the green/red boundary ± 200 ft/min — hugging the inside of the green sector, no excess manoeuvre. With built-in speed protection and an honest note about the initial bite:

L2 The AP/FD TCAS mode ensures that the aircraft speed remains between VLS - 5 kt (VLS - 2 kt in landing configuration) and VMAX. L1 Note: In order to ensure a rapid response to the RA, the initial vertical speed may temporarily exceed the vertical speed target of the TCAS. The peak of the vertical speed is reached in less than 5 s after the vertical speed enters the TCAS green area.

(These are the TCAS rows of article 10's protection panorama.) And the FCTM admits the consequence of being protected:

Note: The AP/FD TCAS mode is speed protected, i.e. it ensures that the aircraft speed remains between VLS -5 kt and VMAX. Therefore, in the case the RA is triggered when the aircraft is close to its performance limits, the RA vertical speed target may not be reached with the AP/FD TCAS mode.

The AP flies the RA less extremely than a human may. It stops at VLS−5; the hand-flying procedure is authorised down to VαMAX. If the automation cannot reach the green sector near the performance ceiling, the FCTM's caution applies: disconnect the AP, override the FD orders, and fly the vertical speed out of the red by hand.

5. The ALPHA FLOOR inhibition — and the LVR CLB special case

Per FCOM DSC-22_30-40-100:

When AP/FD TCAS mode is engaged, ALPHA FLOOR is temporarily inhibited: ‐ If all thrust levers are set to CLB detent when the RA is triggered, ALPHA FLOOR is inhibited while AP/FD TCAS mode is engaged. ‐ If at least one thrust lever is below the CLB detent when the RA is triggered, LVR CLB flashes on the FMA and ALPHA FLOOR is inhibited during 5 s, then: • If all thrust levers are set to the CLB detent, ALPHA FLOOR inhibition is maintained while AP/FD TCAS mode is engaged. • If at least one thrust lever remains below the CLB detent, ALPHA FLOOR is reactivated.

Why deliberately disarm a stall protection? Picture a DESCEND RA: if alpha floor fired TOGA mid-RA, the thrust surge would push the aircraft toward the intruder. Two bodyguards disagree — during an RA the collision bodyguard wins, with the VLS−5 speed protection standing in for the floor. The 5-second branch is the system hurrying your levers back to CL so both bodyguards become compatible again (article 17 owns ALPHA FLOOR in full). A related nicety: an RA below the thrust-reduction altitude with A/THR armed suppresses the LVR CLB flashing when the RA demands a climb above 500 ft/min (no nagging mid-evasion) — but the flashing returns the instant speed decays within VFE−20 (the flap-limit red line outranks the courtesy).

6. CLEAR OF CONFLICT — three roads home

Per FCOM DSC-22_30-40-100:

When the 'CLEAR OF CONFLICT' aural alert sounds, the AP/FD TCAS mode automatically disengages. The AP/FD provides guidance toward the previous FCU selected altitude.

The FCU window never moved during the RA — the "latest ATC clearance" is still stored there. The destination mode: inside the capture zone → ALT*/ALT CRZ*; already at the target → ALT/ALT CRZ; otherwise → V/S. And the V/S case pays back the annunciation debt:

When the AP/FD TCAS mode reverts to V/S when clear of conflict, all of the following occurs: ‐ ALT or ALT CRZ arms ‐ A triple click audio indicator sounds, and the pitch bar flashes for 10 s ‐ The vertical speed target depends on whether a preventive RA occurred, or a corrective RA occurred.

Silent going in, loud coming out — the conflict is over, it may disturb you now. The homeward rates:

Vertical Speed Target - A Corrective RA Occurred The vertical speed target becomes: ‐ 500 ft/min, if the aircraft is at, or above 30 000 ft, and the FCU selected altitude is above the aircraft ‐ ±1 000 ft/min, in all other cases.

(Preventive: the previous target is kept.) Odds and ends: a bird-flyer's note — if TRK-FPA was the flight reference before the RA, exit brings a 10 s TRK FPA DESELECTED message (the RA swapped you to HDG-V/S); and if the mode is lost to a failure mid-RA, the reversion protects the direction — a descend-RA loss reverts to V/S 0 ft/min (no continued descent), a climb-RA loss keeps the TCAS target at failure.

7. Procedures — one iron rule, two playbooks

The rule that overrides everything. Per FCTM AS-TCAS:

The flight crew must always follow the TCAS RA orders in the correct direction, even: • If the TCAS RA orders are in contradiction with the ATC instructions • At the maximum ceiling altitude with 'CLIMB, CLIMB' or 'INCREASE CLIMB, INCREASE CLIMB' TCAS RA orders • If it results in crossing the altitude of the intruder.

The intruder may be flying a coordinated RA in the opposite sense — fly against yours and you fly into him.

With the AP/FD TCAS mode (this airframe's default): at the TA, the PF calls "TCAS blue" (reading the armed FMA), has the PM engage A/THR if it's off, and makes no avoidance manoeuvre on a TA. At the RA: AP engaged — leave it engaged (it flies the RA); AP off — follow the FD bars smoothly and firmly (or have the PM engage an AP). The PM monitors the manoeuvre, does not go visual hunting for the target, and notifies ATC. After CLEAR OF CONFLICT: PM reports, PF re-establishes modes per clearance.

Without the mode (failed or unavailable) the playbook inverts. Per FCTM AS-TCAS:

‐ The PF disconnects the AP, and smoothly and firmly follows the VSI green sector within 5 s, and requests that both FDs be disconnected Note: Both FDs must be disconnected when APs are disconnected: ‐ To ensure autothrust speed mode ‐ To avoid possible confusion between FD bar orders, and TCAS aural, and VSI orders.

Both FDs off, for two reasons: it forces the A/THR into SPEED mode (article 05's FGE logic), and it removes the three-way argument between FD bars, TCAS audio and the VSI. The discriminator between playbooks: after a TA, no cyan TCAS on the FMA = mode dead or absent → the PF calls "TCAS, I have control" and prepares to hand-fly.

CLIMB RA in landing configuration. Per FCTM AS-TCAS:

If any CLIMB RA ... is generated when the aircraft is in approach in CONF 3 or FULL: ‐ The flight crew perform a go-around and follow the SRS orders ‐ The AP and FD can be kept engaged during the go-around ‐ During the go-around, the flight crew check that the vertical speed remains out of the red area of the vertical speed scale, and take over if necessary.

One hand, TOGA — SRS GA displaces the TCAS mode (article 14's fourth exit), AP/FD stay in, and the crew polices the V/S against the red sector.

Choosing TA ONLY. The FCTM's scenarios for deliberately giving up the mode: engine-out, flight with gear down, a visually acquired known neighbour, and airports the operator flags for nuisance RAs (close parallel or converging runways). And the everyday prophylactic: within the last 2000 ft of a climb or descent, keep the vertical speed at or below 1500 ft/min — gentle closure that spares the other aircraft's TCAS.

[!warning]- Four misconceptions this article corrects (1) The silent engagement is not a missing annunciation — the triple click and bar flash are deliberately suppressed mid-evasion, and repaid at CLEAR OF CONFLICT; the PF reads the FMA unprompted. (2) The AP flies the RA more conservatively than you are authorised to — it stops at VLS−5 while the manual procedure may ride to VαMAX; near performance limits, be ready to take over. (3) Re-arming the approach before CLEAR OF CONFLICT evicts the TCAS mode and can re-trigger the RA — the APPR button waits. (4) ALPHA FLOOR being inhibited during an RA is protection, not a hole — TOGA in a descend-RA would drive you at the intruder; the VLS−5 guard covers the speed meanwhile.

Self-test

[!note]- Q1. The six availability conditions — and which three warnings suppress the mode?

Above 900 ft RA; TCAS not failed; TA/RA selected; at least one AP and one FD available; the mode itself not failed; and no EGPWS, WINDSHEAR or STALL warning active — those three outrank an RA.

[!note]- Q2. Where does the armed TCAS appear on the FMA, and why can it coexist with another armed mode?

Line 3 of the vertical column, cyan — other armed modes use line 2, so both can be displayed together.

[!note]- Q3. Why is engagement silent, and when is the annunciation debt repaid?

To avoid disturbing the PF during the evasive manoeuvre (RA audio and sectors already fill the cockpit). Repaid at CLEAR OF CONFLICT: the V/S reversion arrives with the triple click and 10 s of flashing pitch bar.

[!note]- Q4. Fully hand-flown (AP and FDs off) and an RA fires — what engages automatically, and what happens to the speed target?

The FDs engage with the TCAS mode, and HDG engages on the current heading — a complete guidance package. Speed control becomes selected, frozen at the speed at RA occurrence.

[!note]- Q5. RA during an ILS approach: G/S, LOC, APPR light, LOC light? And the consequence of re-pressing APPR early?

G/S disengages, LOC remains; the APPR light goes out and the LOC light comes on. Re-arming the approach lets a capturing approach mode displace TCAS before the conflict clears — and the RA may trigger again.

[!note]- Q6. How is the corrective-RA V/S target defined — and what does the FCOM say about the initial overshoot?

The green/red boundary ± 200 ft/min in the required direction. The initial vertical speed may temporarily exceed the target for rapid response, peaking within 5 s of entering the green area.

[!note]- Q7. The two ALPHA FLOOR inhibition branches — and why inhibit at all?

Levers at CL when the RA fires: inhibited for the whole engagement. Any lever below CL: LVR CLB flashes and the inhibition lasts 5 s — maintained if the levers reach CL, otherwise ALPHA FLOOR re-arms. Reason: a TOGA burst during a descend-RA would push the aircraft toward the intruder.

[!note]- Q8. The three roads home after CLEAR OF CONFLICT — and the two corrective homeward rates?

Into ALT*/ALT CRZ* if within the capture zone; ALT/ALT CRZ if at the target; otherwise V/S. Corrective rates: 500 ft/min at or above 30 000 ft with the target above; ±1000 ft/min otherwise.

[!note]- Q9. The watershed action of the no-mode procedure — and the two reasons both FDs must go off?

PF disconnects the AP and flies the VSI green sector within 5 s, with both FDs off: to force the A/THR into speed mode, and to remove confusion between FD bars, TCAS aurals and VSI orders.

[!note]- Q10. "CLIMB, CLIMB" in CONF FULL on approach — the standard answer?

Go around and follow the SRS orders (TOGA — SRS GA replaces the TCAS mode); AP/FD may remain engaged; monitor that the V/S stays out of the red sector and take over if necessary.

Key takeaways

References

Availability, arming/engagement behaviour, FMA layout, the engagement chain reaction (HDG auto-engagement, approach-package split, A/THR states, selected-speed snapshot), guidance-law borrowing, targets and speed protection, ALPHA FLOOR inhibition, LVR CLB special case and the CLEAR OF CONFLICT reversion set per FCOM DSC-22_30-40-100; design purpose and lifecycle per FCOM DSC-34-20-60. Iron rule, both playbooks, the silence rationale, the performance-limit note, TA-selection scenarios, the 1500 ft/min technique and the CONF 3/FULL climb-RA procedure per FCTM AS-TCAS. Mode fit per one representative airframe's options list. The bodyguards framing is an integrative synthesis.

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.