Relationship between alpha prot/max/floor

[b773]

Hello everyone, I'm trying to get a better understanding of how these three relate to each other. Based on other related topics here, I gather that alpha floor causes the engines to deliver max thrust irrespective of the position of the thrust levers. Does it do anything else? Also this is a result of the aircraft having reached alpha max and therefore initiated by the alpha prot system. Is this correct?

If after reaching alpha max, one were to release the sidestick, would the aircraft automatically lower the nose to level attitude similar to where if the aircraft were overbanked it would automatically roll back to 33 degrees of bank?

Thanks in advance for the replies.

[bobrun]

I'll give it a try...

Alpha floor is triggered at a predetermined angle of attack threshold that depends on the aircraft configuration, weight, ground speed variation and the difference between ground speed and airpseed, or can be activated by sidestick input (more than 14 degrees) under certain conditions. Alpha floor activation logic is done by the PRIM, and the auto thrust system then commands max thrust. It usually activates somewhere below alpha prot while approaching alpha max, depending on the conditions, but usually before alpha max.

Once the speed is below alpha prot, the sidestick controls the angle of attack directly instead of doing the normal pitch control. With full back stick held, the speed will go to alpha max and stay there. If the side stick is then released, the speed will increase to alpha prot and stay there until the low speed protection mode is left, usually by pushing forward on the side stick.

In normal law you can't stall the airplane since it won't let you go below alpha max, but it doesn't mean you can't be in a very low energy state. You won't be stalled, but you'll be descending like a brick!

[CONF iture]

b773, If you have not seen that one Airbus A330 C-GGWD, Airbus A340 TC-JDN (http://www.aaib.dft.gov.uk/sites/aaib/cms_resources/dft_avsafety_pdf_501275.pdf) very instructive reading in that only 9 pages report.

Five seconds after the autopilot disengaged, the thrust levers were closed and then the autothrust was disconnected, probably by the handling pilot in an effort to prevent another overspeed condition. Ten seconds after the autopilot disengaged, the corrected or phase-advanced angle of attack (a computed parameter which is not recorded but can be calculated by Airbus Industrie from the DFDR data) reached the 'alpha prot' value. This angle of attack excursion beyond alpha prot caused a change in the pitch flight control law from normal law (NZ law) to angle of attack protection law (AoA law). If both sidesticks are at neutral, the AoA protection law seeks to hold the angle of attack constant at alpha prot until a sidestick pitch command is made. If the stick is pulled fully aft then the angle of attack increases to alpha max. If the sidestick is not moved aft, AoA protection law remains active until a nose-down command greater than half forward travel is made or until a nose down sidestick input has been applied for more than one second. The first recorded sidestick input was made at 14:22:08 which was some 28 seconds after the commencement of the Master Warning.

Angle of Attack protection law
Once AoA law is active, rearward movement of the sidestick controls angle of attack between alpha prot (neutral sidestick) and alpha max (full aft sidestick). Forward movement of the sidestick disengages AoA protection law and the system reverts to normal pitch law. However, there is no aural or text message which informs a crew that AoA protection law has been invoked. If the sidestick is not moved from its neutral position, the pitch flight control system is programmed to capture alpha prot and not the airspeed that corresponds to alpha prot in 1g flight. Consequently, in
turbulence the speed scale will probably be oscillating, the aircraft pitch angle could also be oscillating, and the change from normal pitch law to AoA protection law could be difficult to detect.
The commander's reported sighting of an 'Alpha Lock' message was probably an alpha floor warning on the flight mode annunciator portion of the PFDs. Alpha floor is an autothrottle function which applies full thrust, irrespective of the position of the thrust levers, if the airspeed is likely to reduce to a value approaching alpha max. In this incident, the A340's calibrated airspeed decreased from around 270 kt before the turbulence encounter to 205 kt at the apogee of the climb.

In normal law you can't stall the airplane since it won't let you go below alpha max, but it doesn't mean you can't be in a very low energy state. You won't be stalled, but you'll be descending like a brick!
Correct, but only if Alpha Floor has been either automatically inhibited or manually inhibited.

Once the speed is below alpha prot, the sidestick controls the angle of attack directly instead of doing the normal pitch control. With full back stick held, the speed will go to alpha max and stay there.
And AoA will reach alpha max and be maintained at that value ... but it never did at Habsheim !?

[bobrun]

And AoA will reach alpha max and be maintained at that value ... but it never did at Habsheim !?

In fact, from the accident report, the aoa reached alpha max without going any higher, as it is designed to do, and the airplane did not stall. Alpha Floor didn't activate, as it is inhibited below 100 feet. The AoA protection do not prevent the aircraft from being in a very low energy state.

[CONF iture]

In fact, from the accident report, the aoa reached alpha max without going any higher
No Sir !
Alpha Max was 17.5 deg but airplane never reached more than 15 !?
Airplane did not deliver what it was supposed to.

Please read again 1.16.1.2. Lois de pilotage de l'Airbus A 320

[Selfloading]

No Sir !
Alpha Max was 17.5 deg but airplane never reached more than 15 !?
Airplane did not deliver what it was supposed to.



Just to satisfy the curiosity of a non pilot, what is it that the Airplane did not deliver that it was supposed to ?

[CONF iture]

Just to satisfy the curiosity of a non pilot, what is it that the Airplane did not deliver that it was supposed to ?
AoA stopped at 15 (Alpha Prot) when the request was for 17.5 (Alpha Max)
Pitch flight controls moved in contradiction with sidestick request.
But in the mean time, speed was increasing … !?

All the data are available for the one willing to take time to analyze on his own … and not blind fully repeating the accepted but biased conclusions.

Would it have prevented the accident ?
Maybe not …

The first thing to avoid that accident was not to recklessly pretend to present a pax loaded airliner at great AoA (Alpha Max in the flight preparation) at a planned 100 feet.

No one will argue on that, but this is not acceptable to use that fact in order to hide the technical malfunction.

[Selfloading]

AoA stopped at 15 (Alpha Prot) when the request was for 17.5 (Alpha Max)
Pitch flight controls moved in contradiction with sidestick request.


But would this have been caused by entering "flare mode" on passing 50ft RA, I'm not arguing with you by the way, it just seems so hard to get to the facts of this particular accident :confused:

Been reading this, found it interesting
1990 | 1070 | Flight Archive (http://www.flightglobal.com/pdfarchive/view/1990/1990%20-%201070.html)

[CONF iture]

But would this have been caused by entering "flare mode" on passing 50ft
That would be a logical possibility … but according to the BEA on page 14 Alpha Prot was active before 30 feet RA was passed, therefore Alpha Prot had priority and the flare mode that usually brings the attitude to minus 2 degrees position in 8 seconds when passing 30 feet could not have been activated.

But the question remains :

More than 4 seconds before the trees (Generated Time 331 sec) the pilot increased the pitch up request but the flight controls refused to follow the order limiting the attitude between 13 and 14 deg and the AoA between 14 and 15 deg when that one should have gone to 17.5 deg (Alpha Max) with a full back stick demand ?

Been reading this, found it interesting
The FI article is interesting as it gives an idea of the French accident report which has, to my knowledge, never been translated.
But Mr Learmount certainly does not try to question further and ask why the AoA never increased more than Alpha Prot ... ?

[bobrun]

Just read the "official" report. I'm no expert on that case, but it doesn't say anywhere that : flight controls refused to follow the order limiting the attitude between 13 and 14 deg and the AoA between 14 and 15 deg when that one should have gone to 17.5 deg (Alpha Max) with a full back stick demand ?

Rather, the report states that:" had the elevator control been brought to full aft sooner or faster, an aoa greater that 15 degrees could have been reached before impact" It then goes on to say that "however, without a sooner application of go-around thrust, such a maneuver would not have prevented impact with the trees"

Looking at the timeline, engines were at idle, even when at 32 feet above the ground and 122 kts (6 secs before impact), with TOGA selected four secs before impact and with not enough time to accelerate to full power.

At one sec before impact, the sidestick is now at full aft with N1 at 67% and 112 kts.

[CONF iture]

You are correct, the text of the official report doesn’t say that, but was it their intention to dig deeper ?
They obviously took great care to avoid mentioning the elevators response to the stick inputs :

From TGEN 331 which is a full 5 seconds before the trees the stick request is increasing but the elevators up displacement is decreasing.

It was clearly not in the computers mind to let the AoA to increase further than Alpha Prot and / or the attitude to increase further than 13 or 14 deg (?)

Page 37 in the report but the quality is so poor … (?) I did transcript the concerned data in the following table

http://i75.servimg.com/u/f75/11/75/17/84/hab_0210.gif (http://www.servimg.com/image_preview.php?i=17&u=11751784)