Alpha Floor and TOGA Lock — the Automatic Thrust Protection

Where high-AOA protection acts on the surfaces to cap alpha, alpha floor acts on the thrust: it automatically firewalls the engines to TOGA when the aircraft gets slow at low thrust. And once commanded, the TOGA LK keeps that thrust locked on until the crew deliberately cancels it. This article covers the trigger logic, a worked operational example, the lock, the availability window, and how it fits the stall-protection and go-around picture.

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. — FCOM DSC-22_30-50-50

1. What alpha floor does, and how it completes the stall protection

Per FCOM DSC-22_30-50-50, alpha floor automatically applies TOGA thrust when AOA exceeds the alpha-floor threshold, which lies between αPROT and αMAX and depends on configuration.

[!note]- Alpha floor is the thrust half of the stall protection (integrative synthesis) The two low-speed protections work different actuators: high-AOA protection limits the elevator/THS to cap alpha at αMAX, while alpha floor commands the engines to TOGA (FCOM DSC-22_30-50-50). Its threshold sits between αPROT and αMAX — so as the PF holds back-stick into the protection, alpha floor usually fires just after entering αPROT (06), adding thrust automatically while AOA protection holds the alpha. Surfaces stop the stall; thrust flies the aircraft out — and because the AOA is already held by the surfaces, the TOGA is purely additive energy, which is why automatic full thrust is appropriate here but not in an unprotected stall recovery.

2. The trigger conditions

Per FCOM DSC-22_30-50-50, A.FLOOR activates through the autothrust when:

• α exceeds a threshold depending on configuration, ground-speed variation, and the difference between ground speed and airspeed; or
• sidestick deflection above 14° and either:
  • pitch attitude greater than 25°, or
  • AOA protection active.

[!warning]- A large pull at high pitch can trigger alpha floor before αPROT The second path matters: a sidestick pull > 14° with pitch > 25° fires alpha floor without needing the AOA threshold (FCOM DSC-22_30-50-50) — e.g. an aggressive escape manoeuvre (terrain, traffic, windshear) where the crew pulls hard with a high nose. The system reads the intent (big pull, high nose) and adds TOGA early, not only when alpha is already high. The ground-speed/airspeed terms also make it robust against windshear, where airspeed and groundspeed diverge — exactly the case where the wing is being unloaded by a changing wind and early thrust is most valuable.

When it activates, all of the following occur (FCOM DSC-22_30-50-50):

1. the A/THR auto-activates, independently of its previous engagement status;
2. the A.FLOOR mode engages;
3. the A/THR commands TOGA regardless of thrust-lever position;
4. the FMA shows A.FLOOR in a flashing amber box (Line 1, A/THR column);
5. the EWD shows A.FLOOR.

3. A worked example

Per FCTM AS-FG-10-2, a clean illustration: "The aircraft is in descent with the thrust levers manually set to IDLE. The aircraft decelerates, during manual flight with the FD off... When the speed decreases, so that the angle-of-attack reaches the ALPHA FLOOR threshold, A/THR activates and orders TOGA thrust, despite the fact that the thrust levers are at IDLE."

[!note]- It overrides idle thrust levers — the levers no longer command the engines (integrative synthesis) The example makes the key point concrete: even with the thrust levers physically at IDLE, alpha floor drives the engines to TOGA (FCTM AS-FG-10-2). The autothrust takes authority away from the lever position — the same lever-independent behaviour as a normal A/THR command, but here forced by alpha. This is why the FMA/EWD annunciation matters: the levers no longer tell you the thrust.

4. TOGA LK — the lock

If the aircraft leaves the ALPHA FLOOR conditions, the FMA displays TOGA LK with a flashing amber box on the first line of the first column. This indicates that the TOGA thrust is maintained. CAUTION: The A.FLOOR mode will disconnect, if the flight crew manually disconnects the A/THR. — FCOM DSC-22_30-50-50

Per FCTM AS-FG-10-2: "When the aircraft accelerates again, the angle-of-attack drops below the ALPHA FLOOR threshold. TOGA thrust is maintained or locked. TOGA LK appears on the FMA... The desired thrust can only be recovered by setting A/THR to off, with the instinctive disconnect pushbutton."

[!warning]- TOGA does not back off when you ease the stick — you must cancel it with the instinctive disconnect The operational trap: reducing alpha (easing forward / accelerating) clears the A.FLOOR condition but leaves thrust locked at TOGA (TOGA LK) (FCOM DSC-22_30-50-50 / FCTM AS-FG-10-2). Thrust will not spool back on its own. The crew must disconnect the A/THR with the instinctive-disconnect pb to regain thrust control — otherwise the aircraft keeps full thrust. The lock is deliberate (it guarantees the escape continues to a safe energy state), but it requires a positive crew action to end, and that action is the same instinctive-disconnect used to drop the A/THR at any time.

5. Availability and inhibition

Per FCOM DSC-22_30-50-50 / FCTM AS-FG-10-2, alpha floor is available in NORMAL LAW from lift-off to 100 ft RA in approach, if the A/THR is available, and is inhibited in some cases of engine failure.

[!warning]- Alpha floor is gone below 100 ft, in non-normal law, and in some engine-failure cases Three ways to lose it: it ceases below 100 ft RA on approach (so the late flare/landing has no automatic TOGA); it requires Normal law (lost in alternate/direct, where αfloor is inoperative); and it is inhibited in some engine-failure cases (FCTM AS-FG-10-2) — because on a real engine-out the asymmetric TOGA could be hazardous and the crew is managing thrust deliberately. AOA protection on the surfaces may still work, but the automatic thrust does not. Crews must not rely on alpha floor near the ground or single-engine.

[!note]- Alpha floor and the go-around decision (integrative synthesis) Alpha floor fires TOGA automatically, but the FCTM go-around decision-making is broader: a GPWS, TCAS or windshear alert is itself a reason to go around (FCTM). In a windshear or terrain escape the crew pulls toward αMAX and alpha floor adds TOGA — the surface protection and the thrust protection acting together give the best achievable climb performance with a single instinctive pull, which is the whole design intent.

6. Counterintuitive points

[!warning]- Alpha floor protects with thrust, alpha protection with surfaces Alpha floor fires TOGA (autothrust); high-AOA protection caps alpha (elevator/THS) — different actuators, same low-speed envelope (FCOM DSC-22_30-50-50 / DSC-27-20-10-20).

[!warning]- Leaving the condition gives TOGA LK, not a thrust reduction Thrust stays locked at TOGA until the A/THR is manually disconnected (FCOM DSC-22_30-50-50).

[!warning]- It overrides the thrust levers, and it is lost below 100 ft / non-normal law / some engine-out cases Idle levers still get TOGA; but the protection is not there in the flare, in alternate/direct, or in some engine failures (FCTM AS-FG-10-2).

7. Self-test

[!note]- Q1. What does alpha floor do, and where is its threshold? Automatically commands TOGA thrust; threshold is between αPROT and αMAX (config-dependent).

[!note]- Q2. The two trigger paths, and why the second matters? α above the threshold (config / ground-speed terms, robust to windshear); or sidestick > 14° with pitch > 25° or AOA protection active — fires early on an aggressive escape pull.

[!note]- Q3. In the descent-at-idle example, what happens and what do the levers show? The aircraft decelerates; at the threshold A/THR commands TOGA despite levers at IDLE — the levers no longer command the engines.

[!note]- Q4. What is TOGA LK and how is it cleared? After leaving alpha-floor conditions, TOGA thrust stays locked; cleared only by disconnecting the A/THR with the instinctive-disconnect pb.

[!note]- Q5. When is alpha floor available, and the three ways it is lost? Normal law, lift-off to 100 ft RA, A/THR available; lost below 100 ft, in alternate/direct law, and in some engine-failure cases.

8. Key takeaways

References

• FCOM DSC-22_30-50-50 (Autothrust — A/THR Modes / ALPHA FLOOR) — avoids low speed at low thrust; automatically applies TOGA when AOA exceeds the alpha-floor threshold (between αPROT and αMAX, config-dependent); triggers (α above config/ground-speed-dependent threshold, or sidestick > 14° with pitch > 25° or AOA protection active); on activation A/THR auto-activates, A.FLOOR mode, TOGA regardless of thrust levers, A.FLOOR on FMA (flashing amber) and EWD; TOGA LK maintains TOGA after leaving conditions, cleared by manual A/THR disconnect; available from lift-off to 100 ft RA in approach if A/THR available.
• FCTM AS-FG-10-2 (Autothrust — Alpha Floor) — worked example: descent with levers at IDLE, deceleration reaches the threshold, A/THR commands TOGA despite IDLE levers; on re-acceleration TOGA LK on FMA, thrust recovered only by A/THR instinctive disconnect; ALPHA floor available in NORMAL LAW from lift-off to 100 ft RA at landing, inhibited in some engine-failure cases.
• FCOM DSC-27-20-10-20 (Normal Law — Protections) — alpha-floor threshold between αPROT and αMAX; relationship to high-AOA protection.

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.