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Old 11th Dec 2013, 01:13
  #21 (permalink)  
 
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Originally Posted by Dozy
Alpha Max is an internal designation - it is not a universally recognised variable. Airbus never claimed that Alpha Prot would deliver a max AoA equivalent to the edge of stall, just that it would maintain an AoA short of stall while providing as much of the demanded pitch attitude within the safe boundary as it could.
Do you only understand your own dialect ... ?

Let me stick to this ... by Airbus :

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Old 11th Dec 2013, 01:37
  #22 (permalink)  
 
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How does that graphic contradict my previous post?
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Old 11th Dec 2013, 02:02
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I cannot help you on that one, still don't know what your post is supposed to mean ...
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Old 11th Dec 2013, 13:32
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It's not difficult - to the best of my knowledge, Airbus never claimed that Alpha Prot would hold the aircraft at Alpha Max or near Alpha CL Max (1g stall boundary), just that it would keep the AoA inside the envelope with sufficient pitch authority.
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Old 11th Dec 2013, 14:11
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Originally Posted by Dozy
It's not difficult - to the best of my knowledge, Airbus never claimed that Alpha Prot would hold the aircraft at Alpha Max or near Alpha CL Max (1g stall boundary), just that it would keep the AoA inside the envelope with sufficient pitch authority.
So forget about your knowledge and go straight to the Airbus documentation where "pulling the sidestick all the way back will maintain alpha max"
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Old 11th Dec 2013, 15:28
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Right - so having looked at a couple of online (probably obsolete) FCTM documents:

A330 A340 Flight Crew Training Manual

(p. 2.60.5)

http://www.737ng.co.uk/A320%20321%20...g%20Manual.pdf

(p. 10-12)

The first document relates to the A330/340 and states:

In level flight, if the A/THR is disengaged and thrust set to idle, the aircraft decelerates until the auto-trim stops. This occurs at a predetermined angle of attack called Alpha Prot. The speed that equates to Alpha Prot (Va PROT) is displayed as the top of a black and amber strip on the PFD speed scale. If no input is made on the sidestick, the aircraft will descend to maintain its current AOA (Va PROT). To maintain the flight path, the pilot must increase the backpressure on the sidestick, which also provides a tactile indication that auto-trim has stopped. At Va PROT, AOA protection becomes active and, if the sidestick is released to neutral and no thrust applied, the aircraft will gently descend maintaining Va PROT. When AOA protection is active, the speed brakes retract automatically, if previously extended, and the bank angle limit is reduced from 67 to 45.

If the pilot maintains the backpressure, Alpha Floor (covered below) will activate. If the pilot disconnects the A/THR while maintaining full back stick, Alpha Max may be reached. The speed which equates to Alpha Max (Va MAX) is displayed as the top of the red strip on the PFD speed scale. Alpha Max is close to, but short of the 1g stall. When flying at Va MAX, the pilot can make gentle turns if necessary. In turbulence, airspeed may fall temporarily below Va MAX without significant effect.
So what the FCTM is saying is that when holding full back stick in High AoA Protection mode Alpha Max *may* be reached, but it says nothing about the aircraft being held there precisely.

The second (A320 series) doc states:

The PF will notice if the normal flight envelope is exceeded for any reason, because the autopitch trim will stop, the aircraft will sink to maintain its current AOA (alpha PROT, strong static stability), and a significant change in aircraft behavior will occur.
If the PF then pulls the sidestick full aft, a maximum AOA (approximately corresponding to CL Max) is commanded. In addition, the speedbrakes will automatically retract, if extended.
...
HIGH AOA protection: Provides maximum aerodynamic lift
So it also makes no claim that Alpha Max (as illustrated on the graph you posted) will be held precisely.

Both documents indicate that the system will hold the AoA at a level calculated to provide maximum lift, but given the constantly shifting variables behind that calculation it makes sense that the value will actually lie somewhere between Alpha Prot and Alpha Max (the range indicated on the PFD by alternating amber lines on the speed tape) while the stick is held aft. Releasing the stick commands the aircraft to maintain Alpha Prot.

Of course, while these documents are probably obsolete, they've undoubtedly changed somewhat since 1988, so you'd need data of that vintage to be sure.

[EDIT : I think I've actually learned stuff today! ]

Last edited by DozyWannabe; 11th Dec 2013 at 16:11.
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Old 11th Dec 2013, 16:08
  #27 (permalink)  
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Dozy and CONF, you're both correct in a way but Dozy is closest to the truth.

'Alpha Max' is an Airbus term to describe the maximum AoA that the system will command in Normal Law. It is less than the AoA for true CL/alpha max, to avoid stall, as shown in CONF's graph.

In a calm atmosphere, and with a smooth deceleration, Alpha Max will be accurately captured and then maintained at full aft stick. In real life, especially after a sudden pull (GPWS pull-up, for example) some gentle variation will occur until everything settles down, as Airbus describes in the FCTM quotes provided by Dozy.
 
Old 11th Dec 2013, 16:18
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Alpha at the stall

CONF

Is the diag you posted earlier this morning really from Airbus? And all of it? If so they need to get their stuff proof read by somebody who knows the subject - and they have plenty of those in the company.

The peak of the curve should be labelled Stall as the alpha values apply at any value of g at speeds below serious compressibilty issues (say 200kt).
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Old 11th Dec 2013, 16:30
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Originally Posted by Tester78 View Post
In a calm atmosphere, and with a smooth deceleration, Alpha Max will be accurately captured and then maintained at full aft stick. In real life, especially after a sudden pull (GPWS pull-up, for example) some gentle variation will occur until everything settles down...
Indeed - in the case of AF296, what you had was a rushed approach (caused in part by inadequate preparation and briefing materials on the part of the airline) coupled with poor thrust and speed management, likely because of the rushed approach and the decision to do so instead of circling and trying again. The poor thrust management then led to poor altitude management (descending below 100ft RA), and with the stick all the way back throughout the flypast to maintain the altitude they had, there was no way of getting more lift until the engines spooled back up.

@John Farley - in the A330 FCTM linked above (from 2005), the peak is simply labelled "Stall", as you suggest.

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Old 11th Dec 2013, 22:16
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Originally Posted by dozy
So what the FCTM is saying is that when holding full back stick in High AoA Protection mode Alpha Max *may* be reached, but it says nothing about the aircraft being held there precisely.
So it also makes no claim that Alpha Max (as illustrated on the graph you posted) will be held precisely.
No input on the stick and alpha prot is maintained, full back stick and alpha max is commanded and maintained. Hopefully you will come up with valuable argumentation to justify that the airplane had no intention to deliver anything closer than 2.5 degrees of alpha max ... ?
and with the stick all the way back throughout the flypast to maintain the altitude they had
Is it again disinformation or just ignorance on the report ?

Originally Posted by Tester78
In real life, especially after a sudden pull (GPWS pull-up, for example) some gentle variation will occur until everything settles down, as Airbus describes in the FCTM quotes provided by Dozy.
Yes, some gentle variation around alpha max which allows the protected aircraft to outperform the unprotected one.
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Old 11th Dec 2013, 23:41
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Originally Posted by CONF iture View Post
full back stick and alpha max is commanded and maintained.
No. Read the extracts again. "Alpha Max may be reached" - not "will be reached" or "will be commanded and held", but "may be reached". The second document refers to commanding "a maximum AoA (approximately corresponding to CL Max)" - it doesn't refer to "Alpha Max" (as in the vertical line on the graph) at all. This is because the calculated AoA to provide optimum lift performance will vary depending on the conditions.

Hopefully you will come up with valuable argumentation to justify that the airplane had no intention to deliver anything closer than 2.5 degrees of alpha max ... ?
I still don't know where you're getting this 2.5deg figure from - a reference would be handy.

(Though if Alpha Prot is 2.5 degrees less than theoretical Alpha Max, then that's consistent with the FCTMs - but ordinarily I'd expect the optimum AoA to be closer to Max than Prot)

Regardless, if the aircraft behaves as the FCTM describes, then the calculated "optimum lift" AoA will always be somewhere between Alpha Prot and Alpha Max (up to Alpha Max itself). The FCTMs explicitly use terms such as "may be reached" and "approximately", which clearly indicate that achieving Alpha Max is not guaranteed if the conditions aren't right.

Is it again disinformation or just ignorance on the report ?
I don't have a translation of the full report, just extracts. I'm working on the assumption that significant back-stick would be required during the flypast to maintain altitude, as releasing the stick commands a return to Alpha Prot (and a shallow descent).

Last edited by DozyWannabe; 12th Dec 2013 at 00:05.
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Old 12th Dec 2013, 03:04
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Originally Posted by dozy
The second document refers to commanding "a maximum AoA (approximately corresponding to CL Max)" - it doesn't refer to "Alpha Max" (as in the vertical line on the graph) at all.
Full aftstick is associated to alpha max, as simple as that, but good luck in your attempt to redefine what is the High AoA Protection.

I still don't know where you're getting this 2.5deg figure from
From the Report you don't know but still think you are in a position to comment ...

The day you have a translation of the full report, not just extracts, come back with facts not your assumptions.
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Old 12th Dec 2013, 06:51
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How does 'flare mode' fit into this?
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Old 12th Dec 2013, 10:46
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Originally Posted by HazelNuts39 View Post
How does 'flare mode' fit into this?
It doesn't, because they weren't landing.
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Old 12th Dec 2013, 11:45
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They didn't go below 100 ft RadAlt?
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Old 12th Dec 2013, 11:59
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Hi HazelNuts39,
They didn't go below 100 ft RadAlt?
Are you are confusing 30'?

"The flight mode changes to flare mode when the aircraft passes ​50 ft RA as it descends to land.
The system memorizes the attitude at ​50 ft, and that attitude becomes the initial reference for pitch attitude control.
As the aircraft descends through ​30 ft, the system begins to reduce the pitch attitude, reducing it to ​2 nose down over a period of ​8 s. This means that it takes gentle nose-up action by the pilot to flare the aircraft."
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Old 12th Dec 2013, 12:42
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Correction

They didn't go below 50 ft RA?
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Old 12th Dec 2013, 12:46
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Can't remember offhand, but if they did it wasn't for long enough to trigger flare mode.
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Old 12th Dec 2013, 13:27
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Hi,
Would it not be easier if you all read the accident report?
They didn't go below 50 ft RA?
Page 17 http://www.bea.aero/docspa/1988/f-kc...f-kc880626.pdf or,
ASN Aircraft accident Airbus A320-111 F-GFKC Mulhouse-Habsheim Airport
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Old 12th Dec 2013, 13:38
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Re. the postulated activation of Flare Mode

Guys,

Haven't got the BEA report to hand (does anyone have a link) **, but I imagine the data published in the Airbus report of 1995 would be in accord with them. The Airbus report supplies the following -

(1) About 20 secs before "impact", there was a momentary ramp of RA (as previously commented somewhere by you, Dozy) to 24 ft, presumably due to some tree or other obstruction on the airfield boundary inbound.

(2) For about the last 16 secs before "impact", the RA was below 50 ft.

(3) During those last 16 secs, however, the RA remained above 30 ft until approximately 2 secs before "impact". Specifically, it shows an RA of 24 ft approximately 1 sec before "impact".

Just thought you three might like some figures to chew over in relation to the possible engagement (and maybe disengagement) of Flare mode. I've got to rush out...


** Thanks r-r-rat, but that link's not working for me right now.
[Edit (1606Z) Got it now, thanks: must have been a very slow download.]

PS
It would be remarkable if Airbus had not made some amendments to the FBW logic, as well as the FMGC logic (which they definitely did), in the first few years of A320 ops. So I think we have to be cautious when quoting from any documentation that is dated 1989 or later.

Last edited by Chris Scott; 12th Dec 2013 at 15:06. Reason: PS added. "Edit (1606Z)" added.
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