Slat Alpha/Speed Lock and Configuration Protection

There is one moment in a heavy take-off where the high-lift system quietly overrules the pilot: you call for slats up, and the slats refuse to move. That is not a fault — it is slat alpha/speed lock, a configuration protection built into the Slat and Flap Control Computers (SFCC). Retracting the slats raises the stall speed, so if your angle-of-attack is already high or your speed is still low, the system holds the slats out until the aircraft has the margin to give them up. This article takes that protection apart: exactly which slat movement it locks and why, the entry and exit thresholds and the hysteresis between them, the two cases in which it does not act, what the lock looks like inside the actuation chain, and how to tell the green, normal A.LOCK from the amber, failure-side S-LOCKED on the E/WD.

One distinction frames everything below, so it goes first:

[!warning]- Green pulsing A.LOCK and amber S-LOCKED are not two flavours of the same event. The colour is the diagnosis.

Both leave a slat "stuck" — but they are opposite situations. A.LOCK (green, pulsing) is the alpha/speed lock working as designed: the system is refusing to retract the slats because it is protecting you, and it will clear itself the instant you have accelerated. S-LOCKED (amber) is a failure: the Wing Tip Brakes (WTB) have mechanically locked the slat transmission, the slat cannot be released in flight, and you run the ECAM. Treat green as "keep flying the schedule" and amber as "this is a malfunction". Reaching for the QRH on a green A.LOCK, or waiting for an amber S-LOCKED to "clear on its own", are both wrong. The failure side is covered in High-Lift Failures; this article is the green side.

1. What the function does

The whole function is stated by FCOM in one block. Per FCOM DSC-27-30-10:

This function inhibits slats retraction at a high angle-of-attack and/or at low speed. The SFCCs use a corrected angle-of-attack (alpha), or air speed information from the ADIRUs to inhibit slat retraction. If alpha exceeds 8.5 °, or the speed goes below 148 kt, the retraction from position 1 to 0 is inhibited. The inhibition is removed when alpha goes below 8.2 °, and when the speed exceeds 154 kt. In this case, the slats automatically retract to 0. The function is not active if: ‐ Alpha exceeds 8.5 °, or the speed goes below 148 kt after the lever has been set to 0. ‐ Aircraft is on the ground, with the speed below 60 kt.

Four things are packed into that paragraph, and each gets its own section below:

• "a high angle-of-attack and/or at low speed" — the or matters. Either condition alone arms the lock; they do not both have to be true. A heavy, slow approach configuration change may trip the speed branch; an aggressive pull at marginal speed may trip the alpha branch. Two independent doors into the same protection.
• "a corrected angle-of-attack (alpha)" — the SFCC does not act on a raw vane reading. The value is already conditioned by the ADIRUs, so the threshold is referenced to a usable, corrected alpha (§5).
• "the retraction from position 1 to 0 is inhibited" — the lock guards exactly one slat movement, the last 17° back to clean (§2).
• "is not active if …" — two carve-outs that define a narrow protection window (§4), which is the most-misread part of the function.

2. Why the lock lives on the 1 → 0 step only

The function never mentions configurations 2, 3 or FULL — it names "position 1 to 0" specifically. The reason is in the slat angle schedule. Per AMM 27-51-00 (mechanical input):

Read the slat column from the bottom up. FULL → 3 leaves the slats at 24°; 3 → 2 brings them from 24° to 21°; 2 → 1 brings them from 21° to 17°. In every one of those steps the slats are still deployed afterwards — the wing keeps a leading-edge device and a high maximum lift coefficient. The only step that takes the slats from a deployed state all the way to clean is 1 → 0, the final 17°. That is the single retraction that can sharply raise the stall speed, so that is the single retraction the protection is wired to. Everything FULL→3→2→1 is either no slat movement or a reduction that still leaves the slats out, and carries no "retract into the stall" risk worth locking. (The physical-cause argument here is reasoning anchored on the FCOM "position 1 to 0" wording and the AMM angle table; the manuals state the what, this section explains the why.)

3. Thresholds and hysteresis — and why alpha has two release values

The entry and exit thresholds are deliberately not the same number. If a single threshold (say 8.5°) governed both engage and release, then small oscillations around it — 8.49° → 8.51° → 8.49° — would make the slats start, stop, start, stop, and the indication flicker. The fix is a deadband: arm on one value, release on a different value further into the safe region.

  CAS branch (slat baulk)

  BAULK ENGAGED  ───────────────┐
  (retract locked)              │◄─ engage when CAS falls to 148 kt
                                │
                                │   release when CAS rises past 154 kt
                                │                              │
  BAULK RELEASED  ──────────────┴──────────────────────────────┴────►
  (slats free to 0)
                             148 kt                         154 kt    → CAS rising
                             └────────────  6 kt deadband  ───────────┘

The speed branch is the simple one: lock at 148 kt, and do not release until the speed has climbed all the way back to 154 kt — 6 kt above the lock point. Both numbers agree across FCOM and AMM, so there is nothing to disambiguate. Per AMM 27-81-00:

The Slat Alpha Lock function prevents the slat retraction if the CAoA is more than 8.5 deg., the function resets if CAoA decreases below 7.6 deg.. The Slat Baulk function prevents the slat retraction if the CAS is less than 148 knots, this function resets if CAS increases over 154 knots.

The alpha branch needs more care, because the two manuals give two different release values — and they are not in conflict, they are describing two different event layers:

So 7.6° is the value at which the slat Alpha Lock function itself resets inside the SFCC (the AMM hardware-action layer), while 8.2° is the value at which the crew-facing inhibition is removed — the A.LOCK annunciation clears and the slats run to 0 from the pilot's point of view (the FCOM display layer). Both are verbatim manual values; they are not a contradiction and neither is a transcription error. This is the same threshold-layering discipline that recurs across the type: a displayed/field threshold in FCOM and a hardware-action threshold in AMM are different events and must be cited to their own source. The CAS branch has no such split — 148/154 hold at both layers.

Whichever layer you read, the design intent is identical: once the slats are released, they retract in a state with a real margin, not while bouncing along the threshold. Operationally you will see A.LOCK hold for a moment and then, as you keep accelerating, release cleanly and the slats run to 0 in one motion. That is the normal rhythm of the function, not a snag.

4. When it does not act — the protection window

This is the part most worth slowing down for. The function is a gate fitted to the act of selecting 0, not a standing watch over the clean configuration. FCOM lists two cases where it "is not active".

Case 1 — the trigger arrives after you have already selected 0. Per the FCOM block in §1: Alpha exceeds 8.5 °, or the speed goes below 148 kt after the lever has been set to 0. If, at the moment you move the lever from 1 to 0, alpha and speed are within limits, the slats retract normally — and a later excursion (you pull hard, alpha climbs past 8.5°) does not redeploy or re-lock them. There is no "1 → 0" left to inhibit; the slats are already at 0. AMM 27-81-00 frames the same window from the positive side: the slat/flap control lever is selected to 0 position and after selection the CAoA is < 8.5 deg. and the CAS is > 148 kts. (the slat retracts to configuration 0).

The FCTM nails the timing in plain language. Per FCTM PR-NP-SOP-120:

The slats alpha/speed lock function will prevent slat retraction at high AOA or low speed at the moment the flap lever is moved from Flaps 1 to Flaps 0. "A. LOCK" pulses above the E/WD Slat indication. The inhibition is removed and the slats retract when both alpha and speed fall within normal values. This is a normal situation at high gross weight. If alpha lock function is triggered, the crew will continue the scheduled acceleration, allowing further slats retraction.

"At the moment the flap lever is moved from Flaps 1 to Flaps 0" is the same idea as FCOM's not-active Case 1, stated as the positive trigger. The consequence is important: slat alpha/speed lock is not a substitute for stall protection. Once the slats are clean and the angle-of-attack climbs, what catches you is the flight-control law's angle-of-attack protection (AoA Protection), not this function. Two nets, each guarding its own segment — do not confuse one for the other.

Case 2 — on the ground below 60 kt. Per the FCOM block: Aircraft is on the ground, with the speed below 60 kt. On stand, taxiing, or at low ground speed, the alpha vane reading is meaningless and the airspeed is low; if the lock were live it would trap the slats out and stop normal ground retraction. Disabling it on ground with CAS below 60 kt keeps ground operation clean. As the take-off roll accelerates past 60 kt (or once airborne) the inhibit drops away and the protection is live. This is the same CAS doing two jobs at once, which the AMM input description makes explicit (§5): one ADIRU CAS feeds both the lock thresholds and the on-ground/in-flight monitoring.

[!warning]- "I moved the lever to 0, so the slats are coming up" is the trap.

Selecting 0 is a request, not a guarantee. At high gross weight you can move the lever to 0, watch nothing happen, and see A.LOCK pulse — and that is FCTM's stated "normal situation at high gross weight". The correct action is to do nothing to the lever and simply continue the scheduled acceleration; the slats retract themselves once speed and alpha are back in limits. Re-cycling the lever or chasing the indication only disturbs the sequence. The function is on the 1→0 event, so a late excursion after a clean retraction will not bring the slats back out — that case belongs to the flight-control law, not to this gate.

5. Inside the lock — the actuation chain and what "inhibit" means

The angle-of-attack and airspeed are not measured by the SFCC. They arrive from the three ADIRUs over ARINC 429. Per AMM 27-81-00:

The SFCCs receive data through the ARINC 429 busses from the ADIRU 1, 2, 3. The SFCCs receives the computed airspeed (CAS) and the correct angle of attack (CAoA). This data is used in the SFCCs for the slat alpha-lock/slat baulk function. The CAS is also used for the aircraft on ground/flight monitoring.

So the "senses" of this protection are the air-data/inertial system: the SFCC is a logic box acting on CAoA and CAS handed to it by the ADIRUs. That dependency is worth carrying into abnormals — degraded air data (unreliable airspeed, multiple ADR loss) reaches into this function as surely as it reaches the laws.

To see what "inhibit retraction" means at the hardware, start from a normal slat run. Per AMM 27-81-00:

When the SFCCs receive a command from the ADIRS the SFCCs energize the extend (retract) solenoids and the POB solenoids at both PCU valve blocks. The PCU drives the slat transmission system with a certain speed. The FPPU monitors the transmission speed. If the speed is within the limits, the SFCCs energize the high speed solenoids and the PCU drives the transmission system with high speed. When the slats get near to the commanded position, the SFCCs de-energize the high speed solenoids. The transmission system speed decreases. When the slats get to the commanded position, the SFCCs de-energize the POB and the extend (retract) solenoids. The transmission system stops.

In plain terms, a slat movement is a sequence of solenoid states across both PCU valve blocks:

• Energise the extend/retract and POB solenoids (both valve blocks). The POB (pressure-off brake) releases when energised, so the transmission is free to turn; the extend/retract solenoid sets direction. Both valve blocks are driven because the slat Power Control Unit is fed by two hydraulic supplies — the dual-motor PCU architecture of the High-Lift Overview.
• FPPU monitors speed, then high-speed solenoid. The unit starts at a controlled speed; the Feedback Position Pick-off Unit confirms it is within limits, then the SFCC switches in the high-speed solenoid. This is the source of the two-speed slat drive.
• De-energise high speed near target, then POB + direction at target. The tail of every movement is slow down, then stop: drop high speed as the slats near the commanded position, then drop the POB and direction solenoids exactly on position, which re-applies the brake and holds the slats.

  ┌──────────────┐  ARINC 429    ┌────────────────────────┐
  │ ADIRU 1/2/3  │──────────────►│ SFCC 1 / SFCC 2        │
  │  CAoA · CAS  │               │  slat alpha-lock /     │
  └──────────────┘               │  slat baulk logic      │
                                 └───────────┬────────────┘
                                             │  withholds the
                                             │  "retract to 0" command
                                             ▼
                                 ┌────────────────────────┐
                                 │ slat PCU valve blocks  │
                                 │  extend/retract + POB  │
                                 │  solenoids NOT driven  │
                                 └───────────┬────────────┘
                                             │  POB stays applied
                                             ▼
                                  slats held at position 1 (17°)
                                             │
                                             ▼
                                  E/WD: green pulsing  A.LOCK

The key insight for A.LOCK: the lock is not a brake slammed on a moving slat. When alpha/speed lock is armed, the SFCC simply does not issue the "retract to 0" command in the first place — the extend/retract solenoid is never energised, the POB stays applied, and the transmission never turns. The slats sit at position 1's 17° because nothing told them to leave. When alpha and speed return to the release values, the SFCC issues the withheld command and the slats run to 0 through exactly the sequence above. A.LOCK is "the order is being held back", not "the motion was stopped".

6. Reading the E/WD — A.LOCK versus S-LOCKED

Both events live on the same slat/flap strip of the E/WD, and the indication design draws a hard line between them by colour. Per AMM 27-85-00, the failure side first:

the white S or F indication changes to amber, - the slat and/or flap indices/index change to amber, - an amber S-LOCKED, F-LOCKED or both messages come on above the set position indication, which is below the grey wing center-section on the EWD.

and the protection side:

Alpha-Lock/Speed Baulk The slat/flap position lever in the cockpit is moved to the 0 position with the aircraft in flight. If the Corrected Angle of Attack (CAoA) is too large or the Computed Air Speed (CAS) is to low: - the flight control alpha-lock function prevents the retraction of the slats, - the flap index stays green and moves to the retracted position if it is not already in this position, - the cyan selected position dot on the flap side of the wing display goes out if it is on, - the green slat index stays at position 1 and pulses, the cyan position selected dot stays on, - the cyan 0 set position indication under the grey wing center-section stays on, - a green pulsing A-LOCK message comes on at the slat side of the indication.

  ┌──────────────────────────────────────────────────────────┐
  │  A.LOCK — normal protection (alpha-lock / speed baulk)    │
  │                                                          │
  │   slat index held at 1 and PULSING ............ GREEN     │
  │   cyan "0" selected-position dot ............... STAYS ON │
  │   flap index runs green to the retracted position        │
  │   message:  A-LOCK  (slat side) ....... GREEN, PULSING    │
  │   meaning:  protecting you — keep accelerating           │
  └──────────────────────────────────────────────────────────┘
  ┌──────────────────────────────────────────────────────────┐
  │  S-LOCKED — fault (WTB has mechanically locked the slats) │
  │                                                          │
  │   white S indication ........................... → AMBER  │
  │   slat index(es) ............................... → AMBER  │
  │   message:  S-LOCKED  ........................... AMBER    │
  │   meaning:  mechanical lock by Wing Tip Brakes — ECAM    │
  └──────────────────────────────────────────────────────────┘

The pattern to memorise: green and pulsing = the alpha/speed lock is holding the slats out to protect you, with the cyan "0" dot confirming you really did select 0; amber = the WTB have mechanically latched the transmission and the slat is a malfunction for the rest of the flight. Note also that on A.LOCK the flap index carries on green to its retracted position — only the slat is held — whereas an S-LOCKED paints the slat letter and indices amber. The two indications cannot be honestly confused once you read the colour.

7. The PFD "S" speed is not the 148 kt threshold

A common merge is to treat the green S on the PFD speed scale as the same thing as the 148 kt lock value. They are different quantities from different sources. The S symbol is the minimum slat retraction speed. Per FCOM DSC-22, it is represented by a S and appears when flap selector is in position 1, and the PFD characteristic speeds, including S, are computed by the FE (Flight Envelope) … based on the gross weight information (the FMGC also computes them for the MCDU pages). So:

• The PFD green S is a scheduled speed that moves with weight — it tells you "accelerate to here before retracting the slats".
• The 148 / 154 kt values are fixed thresholds inside the SFCC's baulk logic — they do not move with weight.

Fly the schedule and you accelerate through S well before the speed could ever fall to 148 kt, so you never meet the hard floor. The 148 kt baulk is a backstop — it catches you only if the slats are commanded up early, or if at very heavy weight the scheduled S is high and the configuration change is made on the slow side. Do not read 148 kt as "the S speed", and do not read A.LOCK as S-LOCKED.

8. Who else needs the slat position

The SFCC does not keep the slat/flap position to itself. Per FCOM DSC-27-30-10:

The SFCCs transmit flaps/slats positions to the following systems : ‐ PRIM and SEC ‐ FMGEC ‐ ADIRU ‐ EIU ‐ CIDS ‐ GPWS. Note: The ECAM system receives the position information directly from the Instrumentation Position Pick-Off Unit (IPPU). This information is used for warnings and position indications on the E/WD.

Two points matter for this article:

1. PRIM and SEC consume slat/flap position. The flight-control law's protection thresholds (the angle-of-attack limits, the speed gates) depend on configuration — clean up the wing and the stall boundary shifts, so the laws must know the current slat/flap state. This is the interface that lets the law's angle-of-attack protection take over the moment the slats are clean (the hand-off described in §4).
2. ECAM indication comes via the IPPU, not the SFCC. Position shown on the E/WD is taken directly from the Instrumentation Position Pick-Off Unit, a separate path from the SFCC's own position sensing — the same "watching is independent of controlling" philosophy seen in the FCDC chain. A reported SFCC fault therefore does not automatically blank the E/WD position picture.

9. The function across a flight

A few scenes turn the logic into a moving picture:

• Heavy take-off, slats up at S speed. After the acceleration altitude you clean up on schedule; at very heavy weight the speed at the moment you select 0 may still be below 154 kt, or the alpha may be high — green A.LOCK pulses and the slats hold at 17°. You do nothing to the lever and keep accelerating; through 154 kt / below 8.2° the inhibition clears and the slats run to 0. Textbook normal.
• High turbulence on departure. The technique is to carry extra speed margin into each retraction precisely to stay clear of the lock and the buffet boundary. Per FCTM: For takeoff in high turbulence, the flight crew must wait for the target speed +20 kt (limited to VFE-5) before retracting the slats/flaps (e.g. the flight crew must wait for F+20 kt before setting Flaps 1).
• Instinctive pull after a clean retraction. Slats already at 0, you pull and alpha climbs past 8.5°. A.LOCK does not appear — there is no 1→0 to inhibit — and the net that catches you is the law's angle-of-attack protection. This is the boundary between the two protections in the air.
• Ground retraction. On the ground below 60 kt the function is inactive, so slats retract normally for taxi and turnaround; the protection arms as the roll accelerates. "Retracts on the ground, holds in slow flight" is by design, not a contradiction.
• An amber S-LOCKED. This is the failure case, not this article's protection: the WTB have mechanically locked the slat, it cannot be released in flight, and you run the ECAM F/CTL SLATS LOCKED and the QRH, with dispatch governed by the operator MEL. Detail in High-Lift Failures.

Slat alpha/speed lock sits alongside the other high-lift configuration protections — automatic flap retraction and flap load relief — covered in ARS and FLRS; together they keep the wing's configuration inside its own safe envelope, just as the laws keep the aircraft's attitude inside theirs.

Self-test

[!note]- Q1. Which slat movement does the function lock, and why only that one?

Only the retraction from position 1 to 0 — the final 17° to clean. Across the schedule the slats are 17° at CONF 1/1+F, 21° at CONF 2, and 24° at CONF 3/FULL, so FULL→3→2→1 either leaves the slats unmoved or reduces them while they stay deployed. The single step that takes the slats from deployed to clean, and so can sharply raise the stall speed, is 1→0. That is the only retraction with a "retract into the stall" risk, so it is the only one the protection guards.

[!note]- Q2. State the engage and release thresholds. Why does alpha appear to have two release values, 7.6° and 8.2°?

Engage: alpha > 8.5° or CAS < 148 kt** (either alone). Release: CAS must climb back **> 154 kt (6 kt of deadband, the same in FCOM and AMM); for alpha there are two layers — the SFCC slat function resets below 7.6° (AMM, hardware-action layer), while the crew-facing E/WD inhibition is removed below 8.2° (FCOM, display layer). Both are verbatim manual values describing different events, not a contradiction. The offset between engage and release is hysteresis: it stops the slats chattering and ensures they only retract once a real margin exists.

[!note]- Q3. In which two cases is the function not active?

Per FCOM: (1) if alpha exceeds 8.5° or speed falls below 148 kt after the lever has already been set to 0 — the lock is fitted to the 1→0 selection, and once the slats are clean there is nothing left to inhibit (the law's angle-of-attack protection takes over from there); and (2) when the aircraft is on the ground with speed below 60 kt — so that unreliable ground alpha and low airspeed do not trap the slats out during normal ground retraction.

[!note]- Q4. What is the fundamental difference between a green pulsing A.LOCK and an amber S-LOCKED?

Colour is the diagnosis. A.LOCK (green, pulsing) is the alpha/speed lock working normally: the slat index holds at 1 and pulses, the cyan "0" selected dot stays on, and the crew simply continues the scheduled acceleration to clear it — FCTM calls this a normal situation at high gross weight. S-LOCKED (amber) is a failure: the Wing Tip Brakes have mechanically locked the slat transmission, the S letter and slat indices turn amber, the slat cannot be released in flight, and you run the ECAM/QRH. Green = protection; amber = malfunction.

[!note]- Q5. Is the green PFD "S" the same as the 148 kt threshold?

No. The PFD green S is the minimum slat retraction speed, computed by the Flight Envelope / FMGC from gross weight and therefore moving with weight — it tells you when to clean up. The 148 / 154 kt values are fixed thresholds inside the SFCC baulk logic. Flying the S schedule keeps you well clear of 148 kt, which is only a backstop for an early retraction or a very heavy, slow configuration change. Different quantity, different source.

[!note]- Q6. At the hardware level, how does the SFCC "inhibit" retraction?

It withholds the command rather than braking a moving slat. A normal retraction energises the extend/retract and POB solenoids at both PCU valve blocks (releasing the brake and setting direction), then adds the high-speed solenoid once the FPPU confirms speed, and finally de-energises them on position. When alpha/speed lock is armed, the SFCC simply never issues the "retract to 0" command: the retract solenoid is not energised, the POB stays applied, and the transmission never turns, so the slats remain at 17°. When the release values are met, the withheld command is issued and the slats run to 0.

Key takeaways

References

Per FCOM DSC-27-30-10 (Flaps and Slats — SLATS ALPHA/SPEED LOCK FUNCTION: thresholds 8.5/8.2°, 148/154/60 kt, retraction from position 1 to 0, the two not-active cases; SIGNALS TO OTHER SYSTEMS: PRIM/SEC, FMGEC, ADIRU, EIU, CIDS, GPWS, and ECAM via IPPU). Per FCOM DSC-22 (Characteristic Speeds — minimum slat retraction speed "S", FE/FMGC-computed from gross weight). Per AMM 27-81-00 (ARINC 429 input — CAoA/CAS from ADIRU 1/2/3; slat actuation sequence — extend/retract and POB solenoids at both PCU valve blocks, FPPU speed monitoring, high-speed solenoid, de-energisation on position; Slat Alpha Lock/Slat Baulk function reset values 7.6° / 154 kt and not-active conditions). Per AMM 27-51-00 (mechanical input — lever/flap/slat angle schedule). Per AMM 27-85-00 (Slat and Flap Protection — S-LOCKED/F-LOCKED amber indications versus the green pulsing A-LOCK/alpha-lock indications). Per FCTM PR-NP-SOP-120 (the alpha lock function — locks at the moment Flaps 1→0, normal at high gross weight, continue the scheduled acceleration) and FCTM (high-turbulence take-off — target speed +20 kt, limited to VFE-5, before retracting slats/flaps). Reasoning flagged in text — the physical argument that the lock lives on the 1→0 step, the layering of the 7.6°/8.2° release values as distinct event layers, and the hand-off to the law's angle-of-attack protection after a clean retraction — is integrative synthesis anchored on the quoted manual statements, to be confirmed against AoA Protection and High-Lift Overview, not verbatim manual claims.

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.