A/THR Modes, Limits, and Disconnection

Article 16 put the butler on duty; this article is the work rulebook and the ways off duty: the mode families in detail, where the speed target comes from (including the complete ground-speed-mini machinery), which lights flash when, three ways to stand down — and one freeze trap.

When active, the A/THR operates in one of four mode types: THRUST, SPEED/MACH, RETARD, and ALPHA FLOOR (FCOM DSC-22_30-50). The first two are assigned by the vertical mode (article 16); RETARD belongs to automatic landing only; ALPHA FLOOR is the protection that jumps every queue.

Reading the A/THR on the E/WD (EPR gauges on the baseline engines): the EPR COMMAND circle is the A/THR's target when active; the transient arc shows the gap between commanded and current (thrust "in transit"); and the blue circle is the lever — per FCOM DSC-70-90-40, in automatic mode, the thrust (EPR value) that corresponds to the position of the associated thrust lever — the letter of authority made visible (article 16). Reverse selection removes the blue circle; both engines at idle in flight pulses an IDLE legend for 10 s then steadies. The monitoring trio (FCTM): FMA column 1 + PFD speed and trend + E/WD command circles.

1. The THRUST family — six faces

The FCOM's table, key rows verbatim:

THR CLB All of the following conditions are applicable: ‐ SRS TO, CLB, OP CLB, or SRS GA is engaged, and ‐ The most forward thrust lever is set to CL detent, and ‐ The derated CLB is not selected on the FMS PERF TAKEOFF page.

(Derated climb selected → THR DCLB instead.)

THR IDLE DES, OP DES, or the A/THR RETARD mode is engaged. IDLE … THR DES All of the following conditions are applicable: ‐ DES is engaged in idle path, and ‐ The FMS requests a thrust slightly higher than the idle thrust. Slightly higher than IDLE

THR DES is the idle path's fine-trim face (article 08): when the wind runs friendlier than forecast, the FMS asks for a whisker above idle to stay on its profile. The remaining two: THR MCT (engine-out, live lever at MCT) and THR LVR (levers below CL, or parked between CL and MCT with a climb mode engaged — thrust clamped at the lever position; the mode name behind the LIMITED alert of section 5).

2. SPEED/MACH — and the four speed fences

The attachment list (which vertical modes buy SPEED): V/S–FPA, the whole ALT family, DES on geometric segments, the G/S family, FINAL APP, the AP/FD TCAS mode — and with no AP/FD engaged at all, an active A/THR is in SPEED/MACH only (the mechanism behind article 05's "turn the FDs off to guarantee SPEED"). The fences:

When in SPEED or MACH mode, the A/THR does not allow speed excursions beyond the following limits, regardless of the target speed or Mach number: ‐ As a maximum speed VMAX in high lift configuration, VMAX - 5 kt in clean configuration ‐ As a minimum speed: ‐ For a selected speed target, VLS ‐ For a managed speed target, the maneuvering speed of the current slats/flaps configuration (VAPP, F, S, GD).

Note the managed floor sits higher than the selected floor — the manoeuvring speed of the current configuration, not bare VLS (article 10's A/THR fence, complete). Gear-changing: climbing through the crossover altitude swaps the target from speed to Mach automatically (descent mirrors it); in selected, the FCU SPD/MACH pushbutton swaps by hand.

3. Where the speed target comes from — the managed table and ground-speed mini

The managed-target table (FCOM DSC-22_30-60, summarised): on-ground SRS TO = V2; airborne SRS TO = V2+10 (the magenta triangle stays drawn at V2); takeoff engine-out = memorised speed clipped V2…V2+15 (article 07); SRS GA = memorised speed with the 700-ft-memorised VAPP as floor, capped min(VLS+25 — engine-out +15 —, VMAX−5) (article 14); approach phase (or G/S/LAND/FLARE/ROLL OUT engaged) = VAPP plus the ground-speed-mini correction; otherwise the FMS profile speed — with constraints honoured only while NAV is engaged (article 09's chain, speed edition); and before landing configuration the floor is green dot/S/F by configuration. Two memory clauses:

VAPP is memorized at 700 ft RA, so if the FMS loses the VAPP below 700 ft RA, the managed speed target will still be computed. … If the VAPP computed by the FMS is not available, the VAPP is VLS + 5 kt.

(Article 11 met the 700 ft memory from the AMM side; here is the FCOM's own sentence.) Now the star of the section — the ground-speed-mini function, in the FCOM's words:

The Ground Speed Mini function takes advantage of the aircraft inertia when the wind varies during the approach in order to provide an appropriate indicated target speed... When the flight crew flies this indicated target speed, the energy of the aircraft is maintained above a minimum level that ensures standard aerodynamic margins versus the stall. The minimum energy level is the energy level the aircraft will have at touchdown with an indicated airspeed equal to VAPP, and with the wind equal to the tower reported wind as inserted in the PERF APPR page.

The ground speed itself is never displayed; the function is active whenever the speed is managed in the approach phase, and it does not correct green dot/S/F. The arithmetic:

VAPP is the highest of the following computations: ‐ VAPP = VLS + 1/3 x TWR HEADWIND COMPONENT, or ‐ VAPP = VLS + 5 kt. Note: '1/3 of the TWR HEADWIND COMPONENT' has 2 limits: ‐ 0 kt as the minimum value (no wind or tailwind) ‐ +15 kt as the maximum value.

‐ Above 400 ft RA: Managed speed target =VAPP + (CURRENT HEADWIND COMPONENT - TWR HEADWIND COMPONENT). ‐ Below 400 ft RA: Managed speed target = VAPP + K x (CURRENT HEADWIND COMPONENT - TWR HEADWIND COMPONENT) 'K' factor reduces from 1 to 0.33 in 25s. The managed speed target has the following limits: ‐ VAPP, as the minimum value ‐ VFE next, in CONF 0, 1, 2, 3, VFE - 5 kt in CONF FULL, as the maximum value.

Read the parenthesis carefully: the correction is the headwind difference — current minus tower-reported. The tower wind is already banked inside VAPP (the 1/3 term, clipped 0…+15); ground-speed mini only adds what the air is doing beyond the report. So a magenta triangle dancing in gusts is the function working, not failing — fly it. (And per the FCTM, do not "stabilise" it by switching to selected: that is precisely how you throw the energy protection away — article 30.) Below 400 ft the K factor bleeds the correction from 1 to 0.33 in 25 s: the closer to the ground, the deafer the target to gusts, so touchdown happens near VAPP.

Dual-FM failure, three speed branches (same section): on approach with LOC+G/S engaged below 700 ft — the memorised VAPP holds and the speed stays managed; in go-around — the GA memorised speed, still managed; anywhere else — reversion to selected at the speed at failure. And the selected rulebook: seven doors in (FCU pull — earliest 5 s after lift-off; all AP/FD off except in approach phase; FM target lost except takeoff/LAND/GA below 700 ft; preselection activating; FMGES powered up in flight; no-V2 takeoff; TCAS RA), selected outranks managed, and the only automatic selected→managed transition is go-around engagement. Out-of-envelope selected targets are clamped to VLS/VMAX(−5) by the FMGES.

4. SOFT mode, the deliberate deafness at 150 ft, and RETARD

Per FCOM DSC-22_30-50:

The A/THR SOFT Mode activates if all the following conditions are applicable: ‐ The A/THR is active in SPEED or MACH mode ‐ The current speed is within a range of ± 3 kt of the target speed ‐ The AP is engaged in ALT CRZ mode ‐ The speed brakes are retracted ‐ All engines are operative ‐ The Mach target is above 0.65. Note: No message indicates that the A/THR soft mode is active.

Six conditions — including ALT CRZ, the cruise-identity reward from article 07 — and no annunciation whatsoever: light chop with the speed wandering a couple of knots and the thrust not chasing it is design, not defect (exit on larger deviations or VMAX, possibly with a momentary thrust change). Low on approach, the opposite tuning:

Below 150 ft RA, the A/THR behavior is optimized to be less responsive to speed variations. This logic exists to avoid large thrust increases during the lower segment of the approach that may lead to destabilization and possible long flare.

Deliberately sluggish below 150 ft — the vaccination against the long flare that article 13's AUTOLAND light would otherwise catch at 1000 m. And RETARD, the autoland-only face:

The A/THR RETARD mode is only available during an automatic landing. … 3. When the aircraft reaches approximately 50 ft RA, the A/THR RETARD mode automatically engages. The FMA displays THR IDLE and the A/THR sets the thrust to idle 4. A RETARD callout triggers at 10 ft RA, asking the flight crew to set the thrust levers to the IDLE detent 5. The flight crew sets the thrust levers to IDLE detent: The A/THR disconnects, and the RETARD mode disengages. If the last AP disengages when RETARD is engaged, RETARD disengages, and SPEED engages. Note: In manual landing, the RETARD callout triggers at 20 ft RA.

RETARD at ~50 ft, the callout at 10 ft (20 ft manual — and it calls regardless of lever position), and levers-to-idle is the clean standard disconnection. Lose the last AP mid-RETARD and the A/THR falls back to SPEED — still guarding your speed.

5. LIMITED and the flashing family

Per FCOM DSC-22_30-50:

The A/THR limits the engine thrust at the thrust levers position when: ‐ All the thrust levers are set below the CL detent (AEO), or ‐ The thrust lever of all operative engines are set below FLX-MCT detent (OEI). When the thrust is limited, the AUTO FLT A/THR LIMITED alert triggers. L2 The AUTO FLT A/THR LIMITED alert is repeated every 5 s, until the flight crew moves the thrust levers to the CL detent (AEO) or FLX-MCT detent (OEI).

The system's most persistent nag — every five seconds until the levers go home. The prompts:

L2 LVR CLB flashes on the FMA, when all the following conditions are applicable: ‐ All engines are operative (AEO) ‐ The position of the thrust levers are not appropriate: • Both thrust levers are below the CL detent, or • One thrust lever is above CL detent. ‐ The aircraft is above 100 ft RA ‐ No windshear is detected.

Note the last condition: in detected windshear, LVR CLB stays silent — no one nags you to reduce thrust in shear. (LVR MCT is the engine-out twin, with an extra branch past the thrust-reduction altitude.) And the asymmetry catcher:

LVR ASYM flashes on the FMA if all the following conditions are satisfied: ‐ The A/THR is armed or active, and ‐ At least one thrust lever is set to CL detent or FLX-MCT detent , and ‐ One thrust lever is not in the same position, and ‐L2 The aircraft is above 100 ft RA. L1 Note: ... LVR ASYM will not be displayed in the case of OEI or if the flight crew sets one thrust lever to the IDLE detent.

Two exemptions — engine-out (asymmetry is the situation) and one lever deliberately at IDLE (read as intentional).

6. Disconnection — two dignities and a freeze

Standard disconnection (the dignified exit): the instinctive-disconnect pushbutton with the thrust target already at the lever position, or all levers to IDLE. The warnings are self-clearing — amber A/THR OFF memo (up to 9 s), MASTER CAUT (up to 3 s), single chime — and below 50 ft, levers-to-idle disconnects in complete silence (nobody scolds a landing thrust reduction; the tail of article 13's RETARD).

Non-standard disconnection (the incident exit): the FCU A/THR pushbutton, an A/THR failure, or loss of an arming prerequisite. Warnings are continuous (AUTO FLT A/THR OFF + MASTER CAUT + chime, cleared by hand or by re-arming) — and it brings the freeze:

The thrust lock function activates if an A/THR non-standard disconnection occurs. As long as the thrust lock function is active, all the following are applicable: ‐ The engines maintain the thrust at the value set before the A/THR disconnection ‐ The FMA displays the flashing THR LK message on Line 3 of the 'A/THR modes and Messages' column ‐ The ECAM alert ENG THRUST LOCKED appears every 5 s. The thrust lock deactivates as soon as the flight crew moves the thrust levers.

During thrust lock, the lever position is a lie — thrust is frozen at the pre-disconnection value while the levers sit at CL. It is the one window where the non-back-driven philosophy's promise ("manual thrust = lever position") is suspended; moving the levers restores the truth and your manual thrust. The FCTM accordingly classifies the FCU pushbutton as an involuntary A/THR off command — a failure-case tool, not a technique. And the red line:

CAUTION If the flight crew presses, and holds one A/THR instinctive disconnect pb for more than 15 s, the A/THR will disconnect for the remainder of the flight. All A/THR modes, including A.FLOOR, will be lost. The A/THR will be recovered at the next FMGEC power-up on the ground.

Fifteen seconds on the button fires the butler and the bodyguard — ALPHA FLOOR included — for the rest of the flight; only a ground power-up brings them back (article 16's fourth arming prerequisite, in full).

7. The reversion — and ALPHA FLOOR

The THRUST→SPEED self-rescue (article 10's A/THR row, verbatim):

THRUST SPEED/MACH The aircraft does not follow the speed target: ‐ The aircraft speed is below VLS - 2 kt when DES or OP DES is engaged, or ‐ The aircraft speed is above VMAX + 4 kt when CLB or OP CLB is engaged. Note: If the FD is engaged, the FD also disengages A triple click audio indicator sounds.

Fixed thrust losing the speed → the A/THR swaps itself to SPEED at VLS−2 (descent) or VMAX+4 (climb), taking the FD down with it (its pitch target is no longer trustworthy) — triple click, and the OVERSPEED warning on top where applicable.

And the last fence of all. Per FCOM DSC-22_30-50:

ALPHA FLOOR avoids flying at low speed with a low thrust. ALPHA FLOOR automatically applies TOGA thrust if the aircraft angle of attack exceeds the ALPHA FLOOR threshold (between αprot and αmax) that depends on the aircraft configuration.

(A second trigger path: sidestick deflection above 14° with pitch above 25° or the AOA protection already active; and the threshold computation blends ground-speed variation and the ground-speed/airspeed difference — a windshear-precursor sensor.) When it fires:

If A.FLOOR activates, all the following events occur: 1. The A/THR automatically activates, independently of the previous A/THR engagement status 2. The A.FLOOR mode of the A/THR engages 3. The A/THR commands TOGA thrust, regardless of the thrust lever positions 4. The FMA displays A.FLOOR with a flashing amber box on Line 1 of the 'A/THR Messages and Modes' column 5. The EWD displays A.FLOOR

Item 3 is the letter of authority's single exception (article 16): survival outranks authorisation. When the angle-of-attack condition clears, the thrust stays locked at TOGA — TOGA LK — released only by manually disconnecting the A/THR (instinctive pushbutton), with the printed caution that the manual disconnection also kills A.FLOOR itself. Availability and the five inhibitions:

ALPHA FLOOR is available from the lift-off to 100 ft RA in approach if the A/THR is available. ALPHA FLOOR is inhibited if one of the following is applicable: ‐ A/THR is failed, or ‐ The aircraft altitude is below 100 ft during landing, or ‐ The aircraft speed is above M 0.53,or ‐ One engine is failed with the flaps extended or with Derated Takeoff selected, or ‐ AP/FD TCAS mode engages

Engine-out with flaps extended: no alpha floor on a single-engine approach — worth saying aloud in training. The TCAS inhibition is article 15's two-bodyguards story. And one trap:

CAUTION The system may consider an engine as failed if the corresponding Thrust Lever Angle (TLA) is below 5° and another TLA is above 5°. The system may therefore inhibit ALPHA FLOOR. In manual thrust control, the flight crew should check that all the thrust levers are correctly aligned above 5° to avoid an undue inhibition of ALPHA FLOOR.

Misaligned levers in manual thrust — one below 5°, one above — can masquerade as an engine failure and quietly switch the last energy fence off. Keep the levers aligned above 5°.

8. Operating wisdom

Switching the A/THR off, in order of grace (FCTM):

If the I/D pb is pressed when the thrust levers are in CL detent, thrust will increase to MAX CL. This will cause an unwanted thrust increase and may destabilize the approach. Therefore, the recommended technique for setting A/THR to off is: ‐ Return the thrust levers to approximately the current thrust setting, by observing the TLA symbol on the thrust gauge ‐ Press the I/D pb

Technique ①: match the levers to the current thrust (watch the blue TLA circle on the E/WD) then press the instinctive disconnect — because disconnection means "obey the levers", and levers at CL mean MAX CL thrust (article 16's revolving door). Technique ②: levers to IDLE (descent/landing — the standard disconnection). Technique ③: the FCU pushbutton, only when both instinctive buttons have failed (thrust lock is the price).

Use philosophy (FCTM summary): A/THR recommended for the entire flight, usable in most failures — engine failure included, single-engine autoland included (article 13), abnormal configurations included; prefer manual thrust when autotrim is lost (large thrust changes with no trim following); monitor via FMA + PFD speed/trend + E/WD commanded thrust. Housekeeping: after a non-standard disconnection (say a failed master FMGEC), thrust lock → move the levers to take over → consider re-arming the other side. And the ground-speed-mini input lives on the PERF APPR page: the tower wind entry is a computation input, not decoration (article 30).

[!warning]- Five misconceptions this article corrects (1) The magenta approach target moving around in gusts is ground-speed mini doing its job — it adds only the headwind difference over the tower report; do not flatten it with selected speed. (2) SOFT mode has no annunciation — cruise speed drifting ±3 kt with the thrust not chasing is design; expect a momentary thrust change on exit. (3) During THRUST LOCK the lever position does not equal the thrust — the one suspension of the non-back-driven promise; move the levers to restore truth. (4) LVR CLB not flashing in windshear is deliberate — the system never asks you to reduce thrust in shear. (5) ALPHA FLOOR is not always on duty: failed A/THR, below 100 ft landing, above M 0.53, engine-out with flaps out (or derated takeoff), TCAS mode — and misaligned levers below/above 5° can fake that engine-out.

Self-test

[!note]- Q1. THR DES versus THR IDLE — which descent segments own them?

THR IDLE: DES/OP DES (and RETARD) at idle thrust. THR DES: only in DES on an idle path when the FMS requests slightly more than idle — the fine-trim face for staying on profile in better-than-forecast wind.

[!note]- Q2. The four SPEED-mode fences?

Maximum: VMAX in high-lift configuration, VMAX−5 clean. Minimum: VLS for a selected target; the current configuration's manoeuvring speed (VAPP/F/S/green dot) for a managed target — the managed floor is higher.

[!note]- Q3. The VAPP formula with its limits — and how does the magenta target converge below 400 ft?

VAPP = the higher of VLS + ⅓ × tower headwind component and VLS + 5 kt, the wind term clipped between 0 and +15 kt. Below 400 ft the gust correction is multiplied by K, bleeding from 1 to 0.33 in 25 s — target ≈ VAPP by touchdown, bounded VAPP…VFE next (VFE−5 in FULL).

[!note]- Q4. Both FMs fail — the speed behaviour on approach below 700 ft, in cruise, and in go-around?

Approach with LOC+G/S engaged below 700 ft: the memorised VAPP holds, speed stays managed. Go-around: the GA memorised speed, managed. Anywhere else: reversion to selected at the speed at failure.

[!note]- Q5. SOFT mode's six conditions — and is there any annunciation?

SPEED/MACH active; within ±3 kt of target; AP in ALT CRZ; speedbrakes retracted; all engines operative; Mach target above 0.65. No message indicates it is active.

[!note]- Q6. The four LVR CLB conditions — does it flash in windshear? And LVR ASYM's two exemptions?

All engines operative; levers inappropriate (both below CL, or one above); above 100 ft RA; no windshear detected — so no, silent in shear. LVR ASYM exempts engine-out and one lever deliberately at IDLE.

[!note]- Q7. Three standard and three non-standard disconnections, with the warning difference — and when is levers-to-idle silent?

Standard: instinctive pushbutton (levers matched), all levers to IDLE — self-clearing warnings (memo ≤ 9 s, caution ≤ 3 s, chime). Non-standard: FCU pushbutton, A/THR failure, arming-prerequisite loss — continuous warnings plus thrust lock. Below 50 ft, levers to idle disconnects with no warning at all.

[!note]- Q8. THRUST LOCK — trigger, indications, release; does the lever equal the thrust meanwhile?

Any non-standard disconnection: thrust frozen at the pre-disconnection value, THR LK flashing on FMA line 3, ENG THRUST LOCKED every 5 s. Released the moment the levers move. Meanwhile the lever position does not represent the thrust.

[!note]- Q9. Hold the instinctive disconnect for 15 seconds — what is lost, and how does it come back?

The A/THR for the remainder of the flight, all modes including A.FLOOR. Recovery only at the next FMGEC power-up on the ground.

[!note]- Q10. ALPHA FLOOR's five inhibitions, the TLA 5° trap, and the TOGA LK release?

Inhibited when: A/THR failed; below 100 ft in landing; above M 0.53; one engine failed with flaps extended (or derated takeoff selected); AP/FD TCAS mode engaged. The trap: one lever below 5° with another above can read as an engine failure and inhibit the floor — align the levers above 5° in manual thrust. TOGA LK releases only by manual A/THR disconnection (instinctive pushbutton).

Key takeaways

References

Mode families, THRUST/SPEED tables and fences, SOFT mode, the 150 ft behaviour, RETARD, LIMITED and the flashing family, disconnection types and warnings, thrust lock, the 15-second caution, the THRUST→SPEED reversion and the complete ALPHA FLOOR clauses per FCOM DSC-22_30-50; managed-target table, VAPP memory and fallback, ground-speed-mini principle and arithmetic, dual-FM speed branches, selected-speed rules and crossover behaviour per FCOM DSC-22_30-60. Disconnect techniques, the involuntary-command classification and the use summary per FCTM AS-FG-10-2; E/WD thrust-gauge reading per FCOM DSC-70-90-40 (baseline-engine presentation). The energy-savings-account framing is an integrative synthesis. Soft GA and GLS-related wording is quoted as printed where it appears; those functions are not installed on the baseline airframe of this series.

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.