Alpha Floor Protection Explained (Including Examples)

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  • Post last modified:July 28, 2021

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Alpha floor protection is an Airbus low speed protection available in Normal Law which will command full power from the engines if the system detects the aircraft is dangerously close to a stall.

“…at low speeds, if a rapid avoidance manoeuvre is re- quired to avoid terrain, windshear or another aircraft, it is safe to rapidly pull the sidestick fully aft and/or bank and hold it there. The aircraft will pitch up to max Alpha, engage TOGA thrust and climb away. Such precise ma- noeuvring around the low speed edge of the flight envelope is virtually not possible in any conventional aircraft. “

Airbus FAST Publication Issue 20

Alpha Floor Protection is a function of the AutoFlight Flight Envelope Protection and on the Airbus A320 Alpha Floor Protection is provided by two FACs (Flight Augmentation Computers).

When alpha floor protection is triggered it automatically commands TOGA thrust on both engines, regardless of the position of the thrust levers.

“When ALPHA FLOOR is activated, regardless of the initial status of A/THR and the position of the thrust levers, the A/THR activates”

Airbus A320 FCOM (DSC-22-30-90)

Airbus Alpha Floor Protection

Alpha Floor Indications

When this protection is active, “A FLOOR” will be displayed in green (surrounded by a flashing amber box) on the FMA (Flight Mode Annunciator) on the PFD (Primary Flight Display) in front of both pilots. It will also be displayed in amber on the E/WD (Engine Warning Display).

What is TOGA LK on Airbus Aircraft?

“TOGA Lock” (TOGA LK) indicates to the pilots that TOGA (Take Off Go Around) thrust has been set on the engines automatically and is “locked”.

This will be the active auto thrust mode when the flight crew leave the conditions that originally triggered the Alpha Floor warning.

“TOGA LK” will be displayed in green on the FMA (surrounded by a flashing amber box).

Book and aircraft.

The 5 Best Aviation Audiobooks (Chosen by a Pilot) A list of the best aviation audiobooks chosen by a current airline pilot. From fascinating stories at the dawn of aviation to modern day aircraft accidents.

How to exit TOGA LK

Once the aircraft has been returned to a safe flight path and the high angle of attack condition no longer exists, it will then be possible to cancel TOGA lock.

To exit TOGA LK the pilots have to move the thrust levers to a sensible power setting and press either of the two “instinctive disconnect” (I/D) buttons – these are small red buttons on either side of the thrust levers.

When is Alpha Floor available?

Alpha Floor Protection is available from lift-off until the aircraft reaches 100 feet RA (Radio Altitude – i.e. 100 feet above the ground) on the approach to landing.

When is Alpha Floor triggered?

Alpha Floor Protection is triggered when the Angle of Attack is greater than a pre-determined threshold depending on the configuration of the aircraft.

Alpha Floor (αfloor) is activated through the Auto Thrust (A/THR) system when: α is greater than αfloor:

  • Clean: 9.5 degrees
  • Conf 1 or 2: 15 degrees
  • Conf 3: 14 degrees
  • Conf Full: 13 degrees

Alpha Floor will also trigger in the event of:

  • Sidestick deflection greater than 14 degrees nose-up, when either Pitch Attitude Protection, or Angle-of-Attack (AoA) Protection are active.

Note: in the case of deceleration the Low Energy Aural Warning (“SPEED SPEED SPEED”) will trigger before Alpha Floor, unless Alpha Floor Protections have been triggered by sidestick deflection.

The amount of time between the two warnings will depend on the rate of deceleration. (FCOM:DSC-22-40-30)

Alpha Floor and Speedbrakes: Speed brake extension is inhibited in the case of Alpha Floor activation.

“If an inhibition occurs when the speedbrakes are extended, they retract automatically and stay retracted until the inhibition condition disappears and the flight crew reset the [speedbrake] lever.”

A320 FCOM DSC-27-10-20

Book and aircraft.

The 5 Best Aviation Audiobooks (Chosen by a Pilot) A list of the best aviation audiobooks chosen by a current airline pilot. From fascinating stories at the dawn of aviation to modern day aircraft accidents.

Airbus A330 cockpit in flight showing instruments and systems

When is Alpha Floor inhibited?

On the Airbus A320, Alpha Floor Protection is inhibited in the following cases:

When is Alpha Floor lost?

Alpha Floor Protection is lost with one of the following combinations of failures:

  • SFCC1 and FAC2
  • SFCC2 and FAC1
  • Both FCU channels
  • 1 EIU
  • Both FMGC’s lost
  • In Alternate Law
  • In Direct Law
  • With an engine out when the flaps or slats are deployed

How to exit Alpha Floor Protection

To cancel Alpha Floor the pilots have to disconnect the Auto Thrust.

To do this they will move the thrust levers to an appropriate setting and press either of the two i/d (“instinctive disconnect”) pushbuttons on either side of the thrust levers.

Alpha Floor Activation – An Example

Sometimes considering an example helps us to understand a system.

If you imagine that the aircraft is being flown manually by one of the pilots, with the autothrust manually set to idle. If they were to level off the speed would slowly decay.

In order to maintain level flight the pilots would be forced to increase the pitch by slowly pulling back on the side stick – this will increase the angle of attack.

Once the Alpha Floor protection limit is exceeded the auto thrust will become active and demand TOGA thrust from both engines (regardless of the fact that the thrust levers are still at idle). The pilots will be alerted by indications on the PFD and the E/WD (Engine Warning DIsplay).

When the aircraft has accelerated enough and the angle of attack reduces below the Alpha Floor trigger, “TOGA LK” will replace the “A FLOOR” warning on the PFD indicating to the pilots that the thrust has been “locked” at TOGA power. To exit this protection the pilots simply have to disconnect the auto thrust by pressing one of the “instinctive disconnect” pushbuttons on either side of the thrust levers.

Video: Alpha Floor and TOGA LK Explained

Related Reading

Airbus FAST Publication Homepage
Airbus Fast 20: Airbus Fly-By-Wire Aircraft at a Glance
Airbus Fast 23: Training Philosophy for Protected Aircraft in Emergency Situations

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