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-   -   Habsheim (https://www.pprune.org/tech-log/528034-habsheim.html)

Chris Scott 9th Nov 2013 19:15

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.

CONF iture 10th Nov 2013 03:52

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.

Taking advantage of the electronics it is much easier to stabilize at Alpha-Max, A/THR system being disconnected for the remainder of the flight, than trying to find Alpha-Prot without triggering Alpha-Floor.

DozyWannabe 10th Nov 2013 15:55


Originally Posted by vilas (Post 8143271)
I am sorry but it appears that you have completely misunderstood normal law and protections.

As it happens I'm pretty au fait with it. I did however phrase it somewhat badly - probably my fault for posting under the influence. I wasn't saying that protection *causes* the nose to come up, I was saying that when the protections are active, the pitch angle will invariably be nose-high, therefore putting the nose down to reduce the AoA - and disengage the protection - should be a fairly intuitive action for a pilot.

@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.

CONF iture 18th Nov 2013 09:19

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.

John,
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

john_tullamarine 18th Nov 2013 09:44

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.

390cruise 18th Nov 2013 11:32

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.

roulishollandais 19th Nov 2013 20:25

Welcome!
Could we build a list of what changed on A320 since the day before Habsheim?

CONF iture 21st Nov 2013 15:41


Originally Posted by john_tullamarine
Connection is too slow at the moment to move the posts .. will finish that task tomorrow.

John,
Here are the posts that ideally could be moved to the present thread :
  1. 8th Nov 2013, 00:23 #700 Chris Scott
  2. 8th Nov 2013, 01:00 #702 CONF iture
  3. 8th Nov 2013, 19:18 #709 DozyWannabe
  4. 9th Nov 2013, 01:01 #712 Chris Scott
  5. 9th Nov 2013, 11:13 #716 CONF iture
  6. 9th Nov 2013, 19:15 #727 Chris Scott
  7. 10th Nov 2013, 03:52 #732 CONF iture
  8. 10th Nov 2013, 15:55 #737 DozyWannabe
  9. 11th Nov 2013, 10:42 #746 CONF iture
  10. 11th Nov 2013, 13:11 #747 DozyWannabe
  11. 11th Nov 2013, 14:05 #748 Winnerhofer
  12. 11th Nov 2013, 17:37 #751 CONF iture
  13. 11th Nov 2013, 17:49 #752 Owain Glyndwr
  14. 11th Nov 2013, 18:48 #753 CONF iture
  15. 11th Nov 2013, 20:10 #754 Owain Glyndwr
  16. 12th Nov 2013, 07:35 #755 CONF iture
  17. 12th Nov 2013, 08:22 #756 Owain Glyndwr
  18. 12th Nov 2013, 14:11 #759 DozyWannabe
  19. 13th Nov 2013, 05:57 #775 CONF iture
  20. 13th Nov 2013, 08:12 #776 Owain Glyndwr
  21. 13th Nov 2013, 08:18 #777 Clandestino
  22. 13th Nov 2013, 11:29 #778 CONF iture
  23. 13th Nov 2013, 19:19 #784 Chris Scott
  24. 14th Nov 2013, 05:44 #786 CONF iture
  25. 14th Nov 2013, 10:16 #787 aircrashesandmiracle
  26. 14th Nov 2013, 16:29 #789 DozyWannabe
  27. 14th Nov 2013, 22:10 #790 Winnerhofer
  28. 15th Nov 2013, 14:21 #791 CONF iture
  29. 15th Nov 2013, 16:45 #792 DozyWannabe
  30. 15th Nov 2013, 23:06 #793 CONF iture
  31. 16th Nov 2013, 00:36 #795 DozyWannabe
  32. 16th Nov 2013, 15:01 #802 CONF iture
  33. 17th Nov 2013, 19:09 #825 DozyWannabe
  34. 17th Nov 2013, 19:37 #826 jcjeant
  35. 17th Nov 2013, 20:46 #827 john_tullamarine

For clarity, if possible, would be nice to keep that one on top.

CONF iture 26th Nov 2013 18:34

... 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.
And what is it supposed to mean ... ?


I'm not "spreading disinformation", and I challenge you to prove anything I've said recently to be wrong.
No need to go far ... http://www.pprune.org/8144775-post737.html

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.

CONF iture 28th Nov 2013 12:08


Originally Posted by 390cruise
Below 100ft they awaited for this now inhibited system.

The AoA protection is not inhibited, only Alpha-Floor is, and Alpha-Floor has to be avoided for this type of demonstration.

CONF iture 10th Dec 2013 03:51

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

No it was not.
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 ... ?

john_tullamarine 10th Dec 2013 05:37

Re moving posts, several folks have requested that their posts not be moved so I will need to proceed with consideration as time permits.

awblain 10th Dec 2013 08:00

2.5 degrees above alpha max?
 

The plane refused to deliver alpha max, it kept 2.5 deg short of it.
I'm genuinely ignorant about the details of what the aircraft would deliver in different circumstances.

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.

BOAC 10th Dec 2013 08:12

Re moving posts, John, in my day here mods could 'copy' posts to a new thread. It takes a few minutes........:p

CONF iture 10th Dec 2013 16:41


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?

The margin is already in alpha max, that's why alpha max has not been set to alpha stall. It was the Airbus responsibility to detail why, under a given altitude ... (?), and contrary to what the documentation says, alpha max cannot be delivered despite the pilot request.
It was the BEA duty to mention such characteristic.

DozyWannabe 10th Dec 2013 20:26


Originally Posted by awblain (Post 8197998)
I did find an interesting site where there's photos of someone carrying away the blackboxes...

Regardless of what the site says, no-one has ever confirmed that the flight recorder cases in that photo were the ones recovered from the aircraft. It's possible that those were just dummy cases shown to the recovery workers so that they'd know what to look for.

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.

Chris Scott 10th Dec 2013 21:11

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.

:)

DozyWannabe 10th Dec 2013 21:19

Possibly a moderator annotation explaining why the thread was created and at whose request?

john_tullamarine 10th Dec 2013 22:55

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.

DozyWannabe 10th Dec 2013 23:07

Understood John, and thanks for doing this!

CONF iture 11th Dec 2013 02:13


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 :

http://i45.servimg.com/u/f45/11/75/17/84/hud_0210.png

DozyWannabe 11th Dec 2013 02:37

How does that graphic contradict my previous post?

CONF iture 11th Dec 2013 03:02

I cannot help you on that one, still don't know what your post is supposed to mean ...

DozyWannabe 11th Dec 2013 14:32

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.

CONF iture 11th Dec 2013 15:11


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"

DozyWannabe 11th Dec 2013 16:28

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! :ok: ]

Tester78 11th Dec 2013 17:08

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.

John Farley 11th Dec 2013 17:18

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).

DozyWannabe 11th Dec 2013 17:30


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...

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.

http://i1088.photobucket.com/albums/...a330-graph.png

CONF iture 11th Dec 2013 23:16


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.

DozyWannabe 12th Dec 2013 00:41


Originally Posted by CONF iture (Post 8201500)
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).

CONF iture 12th Dec 2013 04:04


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.

HazelNuts39 12th Dec 2013 07:51

How does 'flare mode' fit into this?

DozyWannabe 12th Dec 2013 11:46


Originally Posted by HazelNuts39 (Post 8201925)
How does 'flare mode' fit into this?

It doesn't, because they weren't landing.

HazelNuts39 12th Dec 2013 12:45

They didn't go below 100 ft RadAlt?

rudderrudderrat 12th Dec 2013 12:59

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."

HazelNuts39 12th Dec 2013 13:42

Correction
 
They didn't go below 50 ft RA?

DozyWannabe 12th Dec 2013 13:46

Can't remember offhand, but if they did it wasn't for long enough to trigger flare mode.

rudderrudderrat 12th Dec 2013 14:27

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

Chris Scott 12th Dec 2013 14:38

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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