wording question: alpha FLOOR protection

[hvogt]

Could somebody tell me what the word "floor" in "alpha floor protection" means? Is there an alpha floor, and if so, what or where exactly is it?

[Dream Land]

Try this Alpha Floor.

[hvogt]

I've done a search before posting. Thank you. Unfortunately, the thread you mention does not answer my question. I know what alpha floor protection is. I just don't understand the wording "floor".

[Dream Land]

Ok, my bad, that has never been explained to me.

[grounded27]

To the best of my knowledge pitch power protection that attempts to prevent you from flying slower than the minimum safe speed the aircraft is configured to fly at. The "floor" would be close to stall speed, I believe indicated as the lower foot on the airspeed indication of a PFD. FMS calculated FCC function.

[hvogt]

That makes sense, indeed. So, in other words the thrust kicks in as soon as you hit the floor of the airspeed tape.

[Endeavour]

Except that Alpha Floor, relates to AoA not to speed.

From A320 FCOM 1.27.20

αfloor is activated through the A/THR system, when :
– α is greater than α floor (9.5◦ in configuration 0; 15◦ in configuration 1, 2; 14◦ in configuration 3 ; 13◦ in configuration FULL), or
– Sidestick deflection is greater than 14◦ nose up, with either the pitch attitude or the angle-of-attack protection active.
The αfloor function is available from lift-off to 100 feet RA before landing.

Following a GPWS "Pull up" for example. αfloor will activate almost immediately. It doesn't wait until the speed has reached a particular value. To associate αfloor with a particularly low speed would be a mistake.

[compressor stall]

Because everyone will be picking their dinner up off the floor after it's done its thing?

[hvogt]

Thanks to everybody taking an interest in my question.

[BOAC]

It's them Froggies again - not knowing the difference between a floor and a ceiling. You only have to look at the Euro..................

[safetypee]

From the Airbus FAST Magazine #20 (refer to flight manuals for latest info).
“Alpha Floor is a low speed protection (in normal law) which is purely an autothrust mode. When activated, it provides TOGA thrust. As the aircraft decelerates into the alpha protection range, the Alpha Floor is activated, even if the autothrust is disengaged. Activation is roughly proportional to the rate of deceleration.
Alpha Floor is inhibited:

• below 100 feet radio Altitude,
• if autothrust unserviceable,
• following double engine failure on an A340 (or one engine out on the twins),
• following certain system/auto flight failures,
• above Mach 0.53.

Subject to the above, at low speeds, if a rapid avoidance manoeuvre is required 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 manoeuvring around the low speed edge of the flight envelope is virtually not possible in any conventional aircraft.”

[dixi188]

I suspect the word "FLOOR" comes from a literal translation of the French.
It is probably to do with a line on a graph, maybe at the bottom.

On the Mk 1 Airbus (A300) it is triggered by AoA. 6 degrees with slats retracted and 9.5 to 11.5 degrees with slats extended.

We sometimes get it when in a turn just above V4 when the slats are selected to retract. The N1 caption flashes on the FPI on some aircraft depending on the mod state. The power is already at the N1 limit so otherwise we would not know.

[abgd]

In an aircraft with pod-mounted engines, is it possible to get into a situation at low speed, where you are at a high angle of attack, but do not have enough elevator authority to prevent a pitch-up when adding power?

[Centaurus]

but do not have enough elevator authority to prevent a pitch-up when adding power?

That is why you have a stabiliser trim switch. Simply remove the stick force by appropriate stab trim action - ie forward.

[john_tullamarine]

Certification of the aircraft will look after pitch stability. However, it is useful to be aware that some aircraft need some help in the form of SAS. In the situation you are considering, with SAS U/S, it may be necessary to limit thrust settings during the miss to avoid pitch instability.

[Exaviator]

A matter of semantics. Floor Level means minimum or base line. Alpha equals symbol for Angle of Attack.

Hence we get Alpha Floor... Minimum Angle of Attack.

[BOAC]

ExA - refer post #10? What use is a 'minimum Angle of Attack' in terrain avoidance?

[hvogt]

I take it it's all about the notorious minimum angle of attack you need to stall the thing, but even that way an alpha ceiling protection would make more sense.

Speaking of "them Froggies", it seems there was some truth in Giscard d'Estaing's famous word about hell being a place where the French were engineers and the Germans police officers.

[Exaviator]

ExA - refer post #10? What use is a 'minimum Angle of Attack' in terrain avoidance?

Absolutely nothing!

[abgd]

That is why you have a stabiliser trim switch. Simply remove the stick force by appropriate stab trim action - ie forward.

In the light aircraft I fly trim usually just adjusts stick forces. It doesn't increase elevator authority. Is it different for big jets?

Perhaps I should explain where I was coming from. In the recent Asiana accident I had wondered whether it was possible to get into a corner where dropping the nose to gain speed would have led to an instant collision with the ground, yet adding power might have actually caused a pitch-up and stall?

John Tullimarane's answer implies that it's a theoretical possiblity, yet one that should have been designed out.