High-Speed Protection — Nose-Up Bias, Spiral Stability to Wings Level

High-speed protection is the mirror image of high-AOA protection: instead of guarding the slow edge of the envelope, it recovers the aircraft from the fast edge. Above VMO/MMO it freezes pitch trim, biases the aircraft nose-up, reduces nose-down authority, and rolls the wings level on their own — so a high-speed dive self-recovers. This article covers the mechanism, the speed-threshold cascade, and the overspeed-prevention technique the crew uses before the protection ever has to act.

The aircraft automatically recovers following a high speed upset... the High Speed Protection is activated at/or above VMO/MMO. When it is activated, the pitch trim is frozen. Positive spiral static stability is introduced to 0° bank angle (instead of 33° in normal law)... The bank angle limit is reduced from 67° to 45°. — FCOM DSC-27-20-10-20

1. The design intent — full authority without overstress

Per FCTM AOP-10, the second of the ten Airbus cockpit design requirements is that the flight crew "can exercise their full authority by performing intuitive actions, while aiming at eliminating the risks of overstress or overcontrol."

[!note]- High-speed protection is this philosophy applied to a dive (integrative synthesis) The high-speed protection embodies that requirement: the PF retains full authority to perform a steep-dive escape via a reflex push, while the protection prevents the overspeed from becoming unrecoverable (FCOM DSC-27-20-10-20, FCTM AOP-10). Just as high-AOA protection lets you pull full aft safely, high-speed protection lets you push to escape while guaranteeing recovery — intuitive action, no overstress.

2. What changes when it activates

Per FCOM DSC-27-20-10-20, the protection activates at or above VMO/MMO (depending on flight conditions — high acceleration, low pitch attitude). On activation:

• pitch trim is frozen;
• positive spiral static stability is introduced to 0° bank — with the stick released, the aircraft returns to wings level (instead of the 33° of normal law);
• the bank-angle limit is reduced from 67° to 45°;
• as speed rises above VMO/MMO, nose-down authority is progressively reduced and a permanent nose-up order is applied to aid recovery.

[!warning]- Hands-off, the jet rolls wings-level and pitches up to recover The two automatic biases work together: a high-speed upset is usually a banked dive, and bank accelerates the descent. So the protection rolls toward 0° bank on its own (spiral stability to wings level, not 33°) and adds a permanent nose-up order (FCOM DSC-27-20-10-20). Release the sidestick and the aircraft levels its wings and pulls out — the recovery is built in.

[!note]- This is the mirror of high-AOA protection (integrative synthesis) Where high-AOA protection reduces nose-up authority near the stall, high-speed protection reduces nose-down authority near VMO/MMO — each progressively limits the input that would push the aircraft further past the threatened edge, while leaving full authority to escape toward the safe side. Together they bracket the speed envelope, and at altitude they can meet: the high-speed nose-up bias may itself raise alpha into AOA protection.

3. The dive behaviour and the escape

Per FCOM DSC-27-20-10-20, in a dive:

• no sidestick input → the aircraft slightly overshoots VMO/MMO and flies back toward the envelope;
• sidestick held full forward → it significantly overshoots; at approximately VMO+16 / MMO+0.04 the nose-down authority smoothly reduces to zero (which does not mean the aircraft stabilises there).

[!warning]- Full nose-down authority remains for an escape — until ~VMO+16/MMO+0.04 The PF keeps full authority for a high-speed / steep-dive escape via a reflex push (FCOM DSC-27-20-10-20) — the protection does not block a deliberate descent. But push-over authority fades to zero near VMO+16/MMO+0.04: beyond there the system will not let you command further nose-down. The protection assists escape without permitting an unrecoverable overspeed dive.

4. The speed-threshold cascade

Per FCOM DSC-27-20-10-20, the events stack up with speed:

The protection deactivates when speed drops below VMO/MMO, restoring normal control laws. At high altitude, the nose-up bias may itself trigger the AOA protection.

[!warning]- The AP quits before nose-down authority is gone, and high altitude can flip to AOA protection Two subtleties: the autopilot disconnects at VMO+12/MMO+0.03 (FCOM DSC-27-20-10-20) — earlier than the authority cutoff — so the PF is hand-flying through the deepest part of the protection. And at altitude the permanent nose-up order can raise alpha into AOA protection — the coffin-corner case where the high-speed and high-AOA protections meet, and AOA protection's priority then governs.

5. The overspeed-prevention technique — before the protection acts

Per FCTM PR-AEP-MISC (OVERSPEED PREVENTION), if the aircraft sees significant speed variations near VMO/MMO:

• keep the AP and A/THR engaged — this holds the intended flight path while thrust reduces to idle if needed;
• select a lower target speed to increase the margin to VMO/MMO — but not below Green Dot (the minimum recommended in-flight speed);
• monitor the PFD speed-trend arrow — its length indicates the acceleration rate; if the aircraft keeps accelerating and the trend arrow approaches/exceeds VMO/MMO, use speedbrakes appropriately to the acceleration rate.

[!warning]- Manage the speed trend before the protection has to — speedbrakes are certified for the whole envelope The protection is the last line; the first is technique: select a lower speed (not below Green Dot), watch the speed-trend arrow (its length = acceleration rate), and deploy speedbrakes if the trend reaches toward VMO/MMO (FCTM PR-AEP-MISC). Speedbrakes are an efficient deceleration means certified for the whole flight envelope. A pilot who flies the trend arrow rarely lets the protection engage — the protection exists for the upset, not routine speed management.

6. Counterintuitive points

[!warning]- Spiral stability changes to wings-level Normal law returns to 33° bank hands-off; high-speed protection returns to 0° bank and caps bank at 45° (FCOM DSC-27-20-10-20) — because a banked dive accelerates.

[!warning]- Pitch trim freezes and a permanent nose-up order is added The system stops trimming and biases nose-up (FCOM DSC-27-20-10-20) — it actively works to recover, not just limit.

[!warning]- The protection is the last line — fly the speed-trend arrow first Lower the target (not below Green Dot) and use speedbrakes per the trend arrow (FCTM PR-AEP-MISC) before VMO/MMO.

7. Self-test

[!note]- Q1. When does high-speed protection activate, and what three pitch/roll changes occur? At/above VMO/MMO: pitch trim frozen, spiral stability to 0° bank (returns wings-level), bank limit 67°→45°, plus nose-down authority reduced + permanent nose-up order.

[!note]- Q2. What happens hands-off vs full-forward stick in a dive? Hands-off → slight overshoot, returns toward envelope; full forward → significant overshoot, nose-down authority to zero at ~VMO+16/MMO+0.04.

[!note]- Q3. The speed-threshold cascade? VMO/MMO (protection) → +4/+0.006 OVERSPEED → +12/+0.03 AP disconnect → ~+16/+0.04 nose-down authority zero.

[!note]- Q4. The overspeed-prevention technique before the protection? Keep AP/A/THR, select a lower target speed (not below Green Dot), monitor the speed-trend arrow, use speedbrakes per the acceleration rate.

[!note]- Q5. What can happen at high altitude? The nose-up bias may trigger AOA protection (which then has priority).

8. Key takeaways

References

• FCOM DSC-27-20-10-20 (Normal Law — High Speed Protection) — automatic recovery from high-speed upset, activates at/above VMO/MMO; pitch trim frozen; positive spiral static stability to 0° bank (returns wings-level, vs 33°); bank-angle limit reduced 67°→45°; nose-down authority progressively reduced + permanent nose-up order above VMO/MMO; hands-off slight overshoot returns toward envelope, full-forward significant overshoot with nose-down authority to zero ~VMO+16/MMO+0.04; PF retains full authority for steep-dive escape; deactivates below VMO/MMO; OVERSPEED warning VMO+4/MMO+0.006, autopilot disconnects VMO+12/MMO+0.03; at high altitude may trigger AOA protection.
• FCTM PR-AEP-MISC (Overspeed Prevention) — near VMO/MMO keep AP/A/THR engaged, select a lower target speed (not below Green Dot), monitor PFD speed-trend arrow (length = acceleration rate), use speedbrakes per acceleration rate; speedbrakes certified for the whole flight envelope.
• FCTM AOP-10 (Design Philosophy) — flight crew can exercise full authority via intuitive actions while eliminating overstress/overcontrol risk.

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.