Memory (off-topic) [This post has been copied from "AF447 Thread No. 11"]
Quote from Conf_iture, re the A320 Habsheim accident of 1988:
"Sorry Chris but I just can't understand your thinking here as part of the procedure to present the Airbus at high AoA is specifically to inhibit A/THR to prevent Alpha Floor to spoil the demonstration. How can you suggest the guy was waiting for Alpha-Floor to kick in when his initial intention was to prevent it to interfere in the first place ?" My off-topic reference to Habsheim was merely to illustrate that speed greater than M0.53 is not the only inhibition criterion for Alpha-Floor. What you say suggests the game plan was even more cavalier than I remembered, and my memory of the tortuous, much-criticised investigation has faded. That accident was 25 years ago, in our first summer of A320 ops. We fellow A320 pilots were naturally riveted by the excellent camcorder footage of the a/c descending gently into the treetops as the engines spooled up. Much speculation followed, but it was clear to us that the a/c had stabilised safely at Alpha-MAX, but lacked enough thrust to maintain its height. I assumed the plan had been to stabilize at about Alpha-Prot, maintain height at that speed by increase of manual thrust while passing in front of the crowd, and then go-around. It never occurred to me at the time that the crew would have been reckless enough to attempt to stabilise at Alpha-MAX, as you seem to believe, disabling Alpha-Floor to enable that. I presumed that Alpha-Floor was being retained as a back-up, but that its inhibition below a certain height had been overlooked. |
Chris, more than anything, why Habsheim has not a thread on its own ...?
Originally Posted by Chris Scott
I assumed the plan had been to stabilize at about Alpha-Prot, maintain height at that speed by increase of manual thrust while passing in front of the crowd, and then go-around. It never occurred to me at the time that the crew would have been reckless enough to attempt to stabilise at Alpha-MAX, as you seem to believe, disabling Alpha-Floor to enable that.
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Originally Posted by vilas
(Post 8143271)
I am sorry but it appears that you have completely misunderstood normal law and protections.
@CONF - I've invited you on several occasions to start your own Habsheim thread in AH&N (the logical place to have it, as the incident is over two decades old) - but you have not as yet done so. @Chris - I think we do know that Alpha Floor was not a consideration for the pilot of AF296, as he *disabled* A/THR (and thus A. Floor) by holding down the disconnect switches to perform the flypast. |
Habsheim
Originally Posted by john_tullamarine
Folks,
We've had a few complaints about introducing Habsheim in this thread. If folks see a need to discuss, please raise a second thread to keep the two easier to follow. It is not clear why pprune did not let me start a thread on Habsheim in the past, but as the offer is now formulated, sure I can proceed. There's a lot to say on the technical side, stuff that may help to understand how the Airbus works. Maybe you would like to transfer here what we wrote lately regarding Habsheim in the AF 447 Thread No. 11 |
Can't speak to the modding history but I guess so long as we keep the thread on a tech mindset and don't get into any operator or personality aggro then things should go fine.
Connection is too slow at the moment to move the posts .. will finish that task tomorrow. |
100 ft. Rad Alt
Guys,
My understanding was that this crew intended to slow towards alpha protection during the fly past as some others had done at previous displays. Unaware the others stayed above 100ft rad alt to ensure protection was available.Below 100ft they awaited for this now inhibited system. |
Welcome!
Could we build a list of what changed on A320 since the day before Habsheim? |
Originally Posted by john_tullamarine
Connection is too slow at the moment to move the posts .. will finish that task tomorrow.
Here are the posts that ideally could be moved to the present thread :
For clarity, if possible, would be nice to keep that one on top. |
... Then maybe I'll resume from where we left :
#825 from Dozy Because to the best of my knowledge nothing in the documentation ever implied that Alpha Prot, or even Alpha Max, were the equivalent of Critical AoA. I'm not "spreading disinformation", and I challenge you to prove anything I've said recently to be wrong. I think we do know that Alpha Floor was not a consideration for the pilot of AF296, as he *disabled* A/THR (and thus A. Floor) by holding down the disconnect switches to perform the flypast. |
Originally Posted by 390cruise
Below 100ft they awaited for this now inhibited system.
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http://www.pprune.org/8197677-post10.html
Originally Posted by SMOC
the plane didn't climb over the trees because it was already at max alpha for the slow fly-by
The plane refused to deliver alpha max, it kept 2.5 deg short of it. Did the BEA Report actually mention that the airplane was flying at alpha max ... ? |
Re moving posts, several folks have requested that their posts not be moved so I will need to proceed with consideration as time permits.
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2.5 degrees above alpha max?
The plane refused to deliver alpha max, it kept 2.5 deg short of it. Did it deliver alpha that was 2.5 deg less than alpha max in order to avoid pushing too close to the limit? I.e. is that 2.5 degrees a safety margin in the software? I can find the BEA report in French, and can't translate accurately enough to tell. I did find an interesting site where there's photos of someone carrying away the blackboxes - AirDisaster.Com: Investigations: Air France 296. The site claims that the white stripes on the DFDR box in pictures No. 3 and No. 4 are different, when in fact they look identical to me. |
Re moving posts, John, in my day here mods could 'copy' posts to a new thread. It takes a few minutes........:p
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Originally Posted by awblain
Did it deliver alpha that was 2.5 deg less than alpha max in order to avoid pushing too close to the limit? I.e. is that 2.5 degrees a safety margin in the software?
It was the BEA duty to mention such characteristic. |
Originally Posted by awblain
(Post 8197998)
I did find an interesting site where there's photos of someone carrying away the blackboxes...
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. |
This thread
Quote from john_tullamarine:
Re moving posts, several folks have requested that their posts not be moved so I will need to proceed with consideration as time permits. Quote from BOAC: Re moving posts, John, in my day here mods could 'copy' posts to a new thread. I've no objection to any posts of mine that may have some relevance to Habsheim being duplicated here, although I'm not sure there are any. Any editing could result in comments finding themselves out of context. Would also prefer them not to be deleted or edited in their original threads, for the same reason. As for finding myself involuntarily and nominally the original poster of a thread on the infamous Habsheim accident, I am sanguine. However, it may be worth placing on record that it was not my idea, and I was not consulted. :) |
Possibly a moderator annotation explaining why the thread was created and at whose request?
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Without going back into the history and dotting is and crossing ts ...
(a) the other thread saw some folk requesting the Habsheim discussion be calved into a separate thread - easy enough to sort out (b) some folk have requested that they NOT be involved with the Habsheim thread - easy enough to sort out but requires a bit of care to honour the relevant undertakings in that regard Nothing sinister anywhere along the way .. just trying to keep the maximum number of folks happy at the same time. With this sort of exercise, I am not interested in any editing but Chris's comment on maintaining context is valid and not always easily sorted out. |
Understood John, and thanks for doing this!
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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.
Let me stick to this ... by Airbus : http://i45.servimg.com/u/f45/11/75/17/84/hud_0210.png |
How does that graphic contradict my previous post?
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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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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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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.
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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. 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 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! :ok: ] |
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. |
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). |
Originally Posted by Tester78
(Post 8200858)
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...
@John Farley - in the A330 FCTM linked above (from 2005), the peak is simply labelled "Stall", as you suggest. http://i1088.photobucket.com/albums/...a330-graph.png |
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. and with the stick all the way back throughout the flypast to maintain the altitude they had
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.
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Originally Posted by CONF iture
(Post 8201500)
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 ... ? (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 ? |
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.
I still don't know where you're getting this 2.5deg figure from The day you have a translation of the full report, not just extracts, come back with facts not your assumptions. |
How does 'flare mode' fit into this?
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Originally Posted by HazelNuts39
(Post 8201925)
How does 'flare mode' fit into this?
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They didn't go below 100 ft RadAlt?
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Hi HazelNuts39,
They didn't go below 100 ft RadAlt? "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." |
Correction
They didn't go below 50 ft RA?
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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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Hi,
Would it not be easier if you all read the accident report? They didn't go below 50 ft RA? ASN Aircraft accident Airbus A320-111 F-GFKC Mulhouse-Habsheim Airport |
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. |
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