High-AOA Protection — Alpha Prot to Alpha Max and Stick-as-AOA-Demand

The high angle-of-attack protection is the one that makes the aircraft stall-proof in Normal law: full back-stick stabilises it at an AOA close to but below the 1 g stall, never beyond. To understand why this protection is the cornerstone of the Airbus envelope and how it changes the way the sidestick works, this article covers four things: the characteristic speeds that mark it on the PFD, the stall aerodynamics it exists to prevent, the αPROT→αMAX demand-change mechanism, and the operational contrast with the unprotected (alternate/direct) case where a stall must be recovered by hand.

In normal law, the aircraft is protected against stall, in dynamic maneuvers or gusts. When the current angle-of-attack becomes greater than αPROT, the high angle-of-attack (AOA) protection activates... When the High AOA protection is activated, the normal law demand is modified and the side stick input is an angle-of-attack demand, instead of a load factor demand. — FCOM DSC-27-20-10-20

1. The aerodynamics it exists to prevent

Per FCTM PR-AEP-MISC, a stall is "a condition in aerodynamics where the Angle of Attack (AOA) increases beyond a point such that the lift begins to decrease." The lift coefficient (CL) rises linearly with AOA up to the point where the airflow begins to separate from the upper wing surface; if AOA increases to AOAstall, CL reaches its maximum CLMAX, and beyond AOAstall the airflow separates and CL decreases — the stall.

The stall will always occur at the same AOA for a given configuration, Mach number and altitude.

Two warnings the crew may see approaching the stall (FCTM PR-AEP-MISC):

• buffeting — depends on slat/flap configuration and increases at high altitude due to high Mach;
• a pitch-up effect — mainly for swept wings and aft CG, which further increases the AOA (a vicious circle).

[!warning]- The stall is an AOA phenomenon, not a speed phenomenon — which is exactly why the protection is on AOA A given wing stalls at the same AOA regardless of weight or bank (FCTM PR-AEP-MISC) — the speed at which that AOA is reached varies with load. So a load-factor (g) demand alone cannot guarantee stall protection, but an AOA limit can: capping AOA at αMAX (< AOAstall) makes the wing physically unable to stall in Normal law. The swept-wing pitch-up tendency (which would drive AOA even higher) is the reason an unprotected aircraft can depart so quickly — and the reason the Airbus puts a hard AOA ceiling there.

2. The characteristic speeds and the PFD bands

Per FCOM DSC-22_10-50-40, VαPROT, VαMAX and VSW are computed by the PRIM from aerodynamic data and displayed on the PFD speed scale:

[!note]- The speed scale tells you instantly which law you are in (integrative synthesis) In Normal law the low-speed end shows the black-amber (VαPROT) and red (VαMAX) strips — the protection is alive. The instant normal law is lost, those vanish and a VSW barber pole appears instead (12) — the FCDC delivers the PRIM's computed values to the PFD. So "do I still have alpha protection?" is answered by the shape of the bottom of the speed scale: amber-and-red strips = protected; VSW barber pole = not. Because the speeds are derived from AOA, they move with weight, configuration and load factor — VαPROT in a turn is higher than in level flight.

3. The αPROT → αMAX mechanism

Per FCOM DSC-27-20-10-20:

• when AOA exceeds αPROT, the protection activates;
• with no crew input, the computers hold AOA = αPROT;
• the crew can increase AOA up to αMAX — and no further, even at full back-stick;
• in protection, the sidestick commands AOA, not load factor.

The angle-of-attack will not exceed αMAX, even if the flight crew gently pulls the sidestick all way back... the aircraft stabilizes at an angle-of-attack close to but less than the 1 g stall... When flying at the αMAX, the PF can make gentle turns, if necessary. If the flight crew releases the sidestick, the angle-of-attack returns to αPROT and stays there.

[!warning]- Full back-stick gives αMAX and no more — the aircraft cannot be stalled in Normal law This is the headline protection: hold the sidestick fully aft and the aircraft flies to αMAX and stabilises just below the 1 g stall (FCOM DSC-27-20-10-20). Gentle turns are still possible at αMAX. Release the stick and AOA returns to αPROT and stays there — the aircraft "perches" at αPROT hands-off. This is what lets a PF instinctively pull full aft to escape a threat (terrain, traffic, windshear) with no risk of stalling — the single most valuable property of the protected law.

[!note]- The demand changes from g to alpha, continuously at αPROT (integrative synthesis) Outside protection the stick is a load-factor demand; cross αPROT and the same stick becomes an AOA demand (FCOM DSC-27-20-10-20). The two laws are continuous at αPROT — the load-factor protection hands the aircraft to AOA protection as the PF keeps pulling, which is why the FCOM says load-factor protection "enhances" high-AOA protection. The full-aft escape therefore cascades automatically: 2.5 g (load factor) → αMAX (AOA) → alpha-floor TOGA thrust with no mode selection by the crew.

4. The trim inhibit and the PFD entry

As the aircraft enters the protection at the amber and black strip (αPROT), the system inhibits further nose-up trim beyond the point already reached. The nose-down trim remains available, if the flight crew pushes the stick forward.

[!warning]- At αPROT the THS cannot trim further nose-up — so recovery never fights an out-of-trim stabiliser Entering αPROT inhibits further nose-up trim — the THS is frozen at its current setting (nose-down still available if the stick is pushed forward) — exactly the auto-trim limit noted in Normal-law pitch (THS limited to 2° nose-down in AOA protection). The system will not trim toward the stall, so when the PF eases forward to leave the protection there is no nose-up THS to fight — a crucial difference from a runaway-trim stall scenario on a conventional aircraft.

5. Priority, high-altitude buffet, and takeoff

Per FCOM DSC-27-20-10-20:

• High AOA protection has priority over all other protections;
• it also protects against the buffet boundary at high level — but here the αPROT alpha is reduced as a function of Mach, so it is not the same alpha as at low speed; consequently the aircraft may climb with the stick free when leaving a turn after entering αPROT;
• at takeoff, αPROT = αMAX for 8 s (a transient preventing the protection biting during rotation).

[!warning]- High-altitude αPROT is a Mach-reduced alpha — and the buffet is part of the cue At altitude αPROT guards the buffet boundary with a Mach-reduced alpha, not the low-speed alpha (FCOM DSC-27-20-10-20). The FCTM (PR-AEP-MISC) notes buffeting increases at high altitude due to high Mach — so near the coffin corner the protection engages at a lower alpha while the airframe buffets. After entering αPROT in a turn, releasing the stick can leave the aircraft climbing on its own as it holds that reduced alpha — the pilot must manage it actively, not assume hands-off level flight.

6. Leaving the protection

Per FCOM DSC-27-20-10-20, the aircraft leaves AOA protection when the sidestick is:

• pushed more than 8° forward, or
• pushed more than 0° forward for at least 1 s, when α < αMAX, or
• at neutral or forward for at least 0.5 s, when α < αPROT.

The PF should leave αPROT as soon as other considerations allow, by easing forward to reduce alpha below αPROT while adding power (if alpha floor has not been triggered or has been cancelled). Between αPROT and αMAX, αfloor may automatically command go-around thrust (07).

7. Protected vs unprotected — and why the technique differs

In Normal law, full-aft is safe to αMAX. But in alternate or direct law (12/13) the high-AOA protection is lost and the aircraft can be stalled — so a different, conventional technique applies. Per FCTM PR-AEP-MISC STALL RECOVERY:

• "The immediate key action is to reduce AOA" — apply a nose-down pitch order on the sidestick for an immediate response; if pitch-down authority is lacking, reduce thrust;
• simultaneously level the wings (reduces the lift, hence the AOA, required for flight);
• "minimizing the loss of altitude is secondary to the reduction of the AOA" — regaining lift is first priority;
• only then increase energy: add thrust smoothly (not immediate max — engine spool-up is slow and, for under-wing engines, thrust produces a pitch-up that opposes the AOA reduction), and ensure the speedbrakes are retracted.

[!warning]- In a protected law you pull; in an unprotected law you push — know which you are in The defining operational reversal: in Normal law the stall escape is full aft to αMAX (the protection guarantees it); in alternate/direct the stall recovery is reduce AOA — nose-down — first, then energy (FCTM PR-AEP-MISC). Pulling full aft in an unprotected law would cause the stall, not escape it. This is why reading the PFD speed scale (§2) — αPROT/αMAX strips vs VSW barber pole — is a life-or-death scan: it tells you whether back-stick is your friend or your enemy.

[!note]- Why immediate full thrust is wrong even in recovery (integrative synthesis) The FCTM warns against slamming to max thrust on stall recognition: engine spool-up is slow so the speed (and AOA reduction) it buys is delayed, and under-wing engines pitch the nose up as thrust rises — directly opposing the needed AOA reduction (FCTM PR-AEP-MISC). Contrast the protected case, where alpha floor does command TOGA automatically — because there the AOA is already held by the surfaces, so the thrust is purely additive energy, not a recovery tool.

8. Counterintuitive points

[!warning]- The stick stops being a g-demand inside the protection Between αPROT and αMAX it is an AOA demand (FCOM DSC-27-20-10-20); full aft = αMAX, release = back to αPROT.

[!warning]- αPROT at altitude is a different (Mach-reduced) alpha The high-level buffet αPROT is lower than the low-speed αPROT, and the aircraft may climb stick-free (FCOM DSC-27-20-10-20).

[!warning]- A stall is an AOA event, so the protection — and the recovery — are about AOA, not speed Cap AOA (Normal law) or reduce AOA (recovery in alternate/direct) — never chase speed first (FCTM PR-AEP-MISC).

9. Self-test

[!note]- Q1. What does the sidestick command between αPROT and αMAX, and what is the limit? An AOA demand; AOA will not exceed αMAX even at full back-stick (stabilises just below the 1 g stall).

[!note]- Q2. What happens if the stick is released in the protection? AOA returns to αPROT and stays there (the aircraft perches at αPROT hands-off).

[!note]- Q3. How do VαPROT, VαMAX and VSW appear on the PFD, and what does each mean? VαPROT = top of a black-and-amber strip (protection active point); VαMAX = top of a red strip (max AOA in normal law); VSW = red-and-black barber pole shown when normal law is inoperative (stall warning).

[!note]- Q4. Why is a stall an AOA phenomenon, and how does that drive technique? A wing stalls at the same AOA for a given config/Mach/altitude (FCTM); so Normal law caps AOA (full aft safe), while alternate/direct recovery reduces AOA first (nose-down), energy second.

[!note]- Q5. What does entry at αPROT do to the trim, and what is its priority? Inhibits further nose-up trim (THS frozen, nose-down still available); High AOA protection has priority over all other protections.

[!note]- Q6. How is αPROT different at high altitude? It guards the buffet boundary with a Mach-reduced alpha (not the low-speed value); the aircraft may climb stick-free after a turn, and buffet accompanies it.

10. Key takeaways

References

• FCOM DSC-27-20-10-20 (Normal Law — High Angle-of-Attack Protection) — activates when AOA > αPROT; computers hold αPROT with no input, crew can increase to αMAX (not beyond, even full back-stick), sidestick becomes AOA demand instead of load-factor demand; aircraft stabilises just below 1 g stall, gentle turns at αMAX, release → returns to αPROT and stays; entry at amber-and-black strip inhibits further nose-up trim (nose-down available); priority over all other protections; high-level buffet protection with Mach-reduced αPROT (may climb stick-free); αPROT = αMAX for 8 s at takeoff; exit conditions (stick > 8° fwd / > 0° fwd ≥ 1 s when α<αMAX / neutral or fwd ≥ 0.5 s when α<αPROT); αfloor between αPROT and αMAX.
• FCOM DSC-22_10-50-40 (Protection Speeds) — VαPROT/VαMAX/VSW computed by the PRIM from aerodynamic data; VαPROT = AOA-protection-active speed (top of black-and-amber PFD strip), VαMAX = maximum AOA in pitch normal law (top of red strip), VSW = stall warning speed (red-and-black strip when normal law inoperative), VMAX = high-speed limit (bottom of red-and-black strip).
• FCTM PR-AEP-MISC (Stall — Definition and Recovery) — stall = AOA beyond the point where lift decreases; CL linear to AOAstall/CLMAX then separation; buffeting (config/Mach dependent, worse at altitude), swept-wing/aft-CG pitch-up; STALL RECOVERY: reduce AOA first (nose-down, reduce thrust if needed), level wings, altitude loss secondary; then increase energy smoothly (not immediate max thrust — spool-up lag and under-wing-engine pitch-up), retract speedbrakes.

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.