Alpha Floor Protection in Airbus A320: Triggers, Logic & Inhibits

Alpha Floor protection is a critical feature in the Airbus A320’s flight envelope protection system. It automatically applies TOGA thrust when the aircraft’s angle of attack (AoA) exceeds a threshold that indicates the aircraft is approaching a stall-like condition. This function is active when the aircraft is in Normal Law, from liftoff to 100 feet RA during landing.

Table of Contents

What Triggers Alpha Floor?

Alpha Floor can be triggered in two main ways:

The angle of attack rises above the Alpha Floor threshold (between αPROT and αMAX), often due to aggressive deceleration or a steep pitch.
The pilot inputs near full aft sidestick deflection, activating pitch attitude or AoA protection.

Once triggered, Autothrust (A/THR) engages automatically—even if it was previously off—and commands TOGA thrust. This results in:

A.FLOOR displayed on the Flight Mode Annunciator (FMA), surrounded by a flashing amber box.
A.FLOOR message shown on the Engine/Warning Display (EWD).

TOGA LK (Thrust Lock)

As the aircraft accelerates and AoA decreases below the threshold, Alpha Floor mode ends, but TOGA thrust remains engaged. The FMA now displays TOGA LK (Thrust Locked), signaling that thrust is frozen. The only way to exit this mode is to manually disconnect autothrust using the instinctive disconnect pushbutton on the thrust levers.

Alpha Floor Inhibition Conditions

Alpha Floor protection is automatically inhibited in the following cases:

TCAS AP/FD mode is active
Airspeed is above Mach 0.6
Both FADEC units are in N1 mode
Single-engine operation with slats/flaps extended
Failure of both FMGCs or both FCU channels
Failure of EIU or specific SFCC/FMGC combinations
The aircraft is operating in Alternate Law or Direct Law

Example Scenario

Consider a descent scenario where the thrust levers are manually set to IDLE, and the autothrust system is off. As the aircraft decelerates, the AoA rises. Once it crosses the Alpha Floor threshold, A/THR engages automatically and applies TOGA thrust, even though the thrust levers remain at idle. After recovery, TOGA LK appears, and the flight crew must disconnect autothrust to resume manual thrust control.

Why It Matters

Alpha Floor provides a last line of protection against aerodynamic stall during high AoA events. It ensures the engines are delivering maximum thrust to aid recovery and stabilize the aircraft’s trajectory. Pilots should be familiar with its triggers, logic, and how to properly exit TOGA LK when needed.

Additional Resources

Watch: Alpha Floor in Action

For a real-world explanation, check out this excellent video:

Frequently Asked Questions

Is Alpha Floor protection available during all flight phases?

No, Alpha Floor protection is only available when the aircraft is in Normal Law, starting from liftoff up to 100 feet above ground level (radio altitude) on approach. It is not active during taxi, ground roll, or after descending below 100 ft RA during landing. Additionally, it becomes unavailable if the aircraft transitions to Alternate or Direct Law.

Does the pilot need to manually engage Alpha Floor protection?

No, Alpha Floor is fully automatic. It activates when specific flight parameters are exceeded—particularly high angle of attack or extreme sidestick input—without any pilot action. It overrides thrust lever settings and engages TOGA thrust automatically to protect against stall or loss of control due to low speed.

How does Alpha Floor protection improve flight safety?

Alpha Floor acts as a last-resort stall protection system by rapidly applying full TOGA thrust in response to dangerous pitch attitudes or excessive angle of attack. This ensures the aircraft accelerates away from the stall condition. It supports pilot recovery efforts and adds a critical layer of automation to prevent loss-of-control incidents.

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Alpha Floor

Airbus A320 Alpha Floor Protection Explained