Slat Alpha/Speed Lock — Keeping the Slats Out When Slow

Where the flap protections (ARS/FLRS) retract flaps to prevent overspeed, the slat alpha/speed lock does the opposite: it inhibits slat retraction (position 1 → 0) when the aircraft is slow or at high angle of attack, so the slats stay out and protect the stall margin. The cue is a green pulsing A LOCK.

This function inhibits slats retraction at a high angle-of-attack and/or at low speed... If alpha exceeds 8.5°, or the speed goes below 148 kt, the retraction from position 1 to 0 is inhibited. — FCOM DSC-27-30-10

1. The lock and unlock thresholds

Per FCOM DSC-27-30-10, the SFCCs use a corrected alpha or airspeed from the ADIRUs:

• lock (inhibit retraction 1 → 0): alpha > 8.5° or speed < 148 kt;
• unlock: alpha < 8.2°** *and* **speed > 154 kt — then the slats automatically retract to 0.

[!warning]- Lock on OR, unlock on AND — and the slats retract themselves when it clears The logic is deliberately asymmetric: either a high alpha (>8.5°) or a low speed (<148 kt) locks the slats out, but both conditions must be safe (alpha <8.2° **and** speed >154 kt) to unlock (FCOM DSC-27-30-10). The small hysteresis (8.5/8.2°, 148/154 kt) prevents chatter. When it unlocks, the slats retract to 0 on their own — you don't re-select. The green pulsing A LOCK on the EWD tells you the slats are being held out.

2. When the function is not active

Per FCOM DSC-27-30-10, the alpha/speed lock is not active if:

• alpha exceeds 8.5° or speed drops below 148 kt after the lever has been set to 0 — i.e. the lock arms only when a retraction is commanded into the danger; it does not re-extend slats already commanded away;
• the aircraft is on the ground with speed below 60 kt.

[!warning]- The lock guards the retraction command — it won't re-extend slats you already retracted A subtlety: if you set the lever to 0 first and then slow down or pitch up, the lock does not engage (FCOM DSC-27-30-10) — it only inhibits a 1 → 0 retraction commanded while slow/high-alpha. It is a guard against premature retraction, not an automatic re-extension. And below 60 kt on the ground it is disabled (taxi).

3. The mirror of the flap protections

[!note]- Slats protected against slow, flaps against fast (integrative synthesis) The high-lift protections form a complementary pair: the flap protections (ARS 1+F→1 at 200 kt, FLRS on VFE) retract flaps to prevent overspeed — a fast problem; the slat alpha/speed lock keeps slats out to prevent stall — a slow problem. Both keep the slats out either way (FLRS leaves slats extended; alpha-lock holds them at 1). So the system always biases toward keeping the slats deployed when in doubt — the slats are the stall-margin surface.

The EWD shows green pulsing A LOCK (alpha-lock active) vs amber S LOCKED (slat WTB failure, 30) — green = protecting, amber = broken.

4. Counterintuitive points

[!warning]- Locks on alpha OR speed, unlocks on alpha AND speed, then auto-retracts

8.5°/<148 kt locks; <8.2° and >154 kt unlocks and the slats retract to 0 themselves (FCOM DSC-27-30-10).

[!warning]- It guards the retraction command, not an already-retracted slat Set to 0 first, then slow → no lock; on ground <60 kt → no lock (FCOM DSC-27-30-10).

Self-test

[!note]- Q1. What does the slat alpha/speed lock do, and the lock thresholds? Inhibits slat retraction 1 → 0 when alpha > 8.5° or speed < 148 kt (keeps slats out for stall margin).

[!note]- Q2. The unlock condition and what happens then? Alpha < 8.2° and speed > 154 kt → the slats automatically retract to 0.

[!note]- Q3. When is the function not active? If alpha/speed enters the range after the lever is already at 0, or on the ground below 60 kt.

[!note]- Q4. How does it relate to the flap protections? Mirror image — flaps retract to prevent overspeed (fast), slats stay out to prevent stall (slow); the system biases toward keeping slats deployed.

Key takeaways

References

• FCOM DSC-27-30-10 (Flaps and Slats — Slats Alpha/Speed Lock Function) — inhibits slat retraction at high alpha and/or low speed using corrected alpha or airspeed from ADIRUs; retraction 1 → 0 inhibited if alpha > 8.5° or speed < 148 kt; inhibition removed (slats auto-retract to 0) when alpha < 8.2° and speed > 154 kt; not active if alpha > 8.5° or speed < 148 kt after the lever has been set to 0, or on ground with speed below 60 kt.
• FCOM DSC-27-30-20 (Controls and Indicators) — green pulsing A LOCK (alpha/speed lock active), slat box pulses green; amber S LOCKED (slat WTB applied).

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.