Habsheim

[Chris Scott]

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]

Chris, more than anything, why Habsheim has not a thread on its own ...?

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]

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]

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 (http://www.pprune.org/tech-log/511119-af-447-thread-no-11-a.html)

[john_tullamarine]

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]

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]

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

[CONF iture]

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 :

8th Nov 2013, 00:23 #700 Chris Scott
8th Nov 2013, 01:00 #702 CONF iture
8th Nov 2013, 19:18 #709 DozyWannabe
9th Nov 2013, 01:01 #712 Chris Scott
9th Nov 2013, 11:13 #716 CONF iture
9th Nov 2013, 19:15 #727 Chris Scott
10th Nov 2013, 03:52 #732 CONF iture
10th Nov 2013, 15:55 #737 DozyWannabe
11th Nov 2013, 10:42 #746 CONF iture
11th Nov 2013, 13:11 #747 DozyWannabe
11th Nov 2013, 14:05 #748 Winnerhofer
11th Nov 2013, 17:37 #751 CONF iture
11th Nov 2013, 17:49 #752 Owain Glyndwr
11th Nov 2013, 18:48 #753 CONF iture
11th Nov 2013, 20:10 #754 Owain Glyndwr
12th Nov 2013, 07:35 #755 CONF iture
12th Nov 2013, 08:22 #756 Owain Glyndwr
12th Nov 2013, 14:11 #759 DozyWannabe
13th Nov 2013, 05:57 #775 CONF iture
13th Nov 2013, 08:12 #776 Owain Glyndwr
13th Nov 2013, 08:18 #777 Clandestino
13th Nov 2013, 11:29 #778 CONF iture
13th Nov 2013, 19:19 #784 Chris Scott
14th Nov 2013, 05:44 #786 CONF iture
14th Nov 2013, 10:16 #787 aircrashesandmiracle
14th Nov 2013, 16:29 #789 DozyWannabe
14th Nov 2013, 22:10 #790 Winnerhofer
15th Nov 2013, 14:21 #791 CONF iture
15th Nov 2013, 16:45 #792 DozyWannabe
15th Nov 2013, 23:06 #793 CONF iture
16th Nov 2013, 00:36 #795 DozyWannabe
16th Nov 2013, 15:01 #802 CONF iture
17th Nov 2013, 19:09 #825 DozyWannabe
17th Nov 2013, 19:37 #826 jcjeant
17th Nov 2013, 20:46 #827 john_tullamarine


For clarity, if possible, would be nice to keep that one (http://www.pprune.org/8159591-post4.html) on top.

[CONF iture]

... Then maybe I'll resume from where we left :
#825 from Dozy (http://www.pprune.org/8158812-post825.html)

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]

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]

http://www.pprune.org/8197677-post10.html
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]

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]

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 (http://www.airdisaster.com/investigations/af296/af296.shtml#blbox). 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]

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

[CONF iture]

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]

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]

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]

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

[john_tullamarine]

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]

Understood John, and thanks for doing this!

[CONF iture]

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 (http://www.servimg.com/image_preview.php?i=115&u=11751784)

[DozyWannabe]

How does that graphic contradict my previous post?

[CONF iture]

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

[DozyWannabe]

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]

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]

Right - so having looked at a couple of online (probably obsolete) FCTM documents:

A330 A340 Flight Crew Training Manual (http://www.smartcockpit.com/aircraft-ressources/A330-A340_Flight_Crew_Training_Manual.html)

(p. 2.60.5)

http://www.737ng.co.uk/A320%20321%20FCTM%20Flight%20Crew%20Training%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]

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]

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]

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/i331/turricaned/a330-graph.png

[CONF iture]

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 ?

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]

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]

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]

How does 'flare mode' fit into this?

[DozyWannabe]

How does 'flare mode' fit into this?

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

[HazelNuts39]

They didn't go below 100 ft RadAlt?

[rudderrudderrat]

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]

They didn't go below 50 ft RA?

[DozyWannabe]

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

[rudderrudderrat]

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-kc880626/pdf/f-kc880626.pdf or,
ASN Aircraft accident Airbus A320-111 F-GFKC Mulhouse-Habsheim Airport (http://aviation-safety.net/database/record.php?id=19880626-0)

[Chris Scott]

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.

[DozyWannabe]

@Chris Scott:

I don't know for certain, but all the credible sources I've encountered state that flare mode was not a factor in the aircraft's behaviour. The oft-repeated claim that "the aircraft thought it was landing and wouldn't let the pilot pull up" was a complete fabrication of the press*.

Thinking about how the systems were designed to interact, I suspect that the engagement of flare mode would only happen if the descending "ramp" from the RA sensor was consistent over time, as any system responding instantly to transient changes would not be appropriate in an aircraft.

Airbus have changed code many times over the years (in fact the ability to fine tune aircraft qualities without requiring mechanical work was one of the factors in favour of civil airliner FBW), but even by 1988 they had a pretty good handle on this kind of basic behaviour - having tooled their modified A300 around for several years.

* - And also the opening salvo in terms of a lot of people misunderstanding FBW as a concept, by erroneously conflating it with autoflight. Given the usual short shrift accredited to journalists among pilots, it's surprising how long that particular rumour has lasted and how many were willing to believe it.

[rudderrudderrat]

Hi DW,
was a complete fabrication of the press
I agree with you there.
The baulked landing procedure (Select TOGA power and rotate to 10 degs pitch) allows you to fly away from any attempted landing - even after touch down.

Some pilots seem to forget that the only thing that makes an aircraft climb is thrust (when you have no more kinetic energy to swap for height).
Why on earth they were so close to Alpha Max AND Low Height AND Idle Thrust is beyond me.

[DozyWannabe]

The baulked landing procedure (Select TOGA power and rotate to 10 degs pitch) allows you to fly away from any attempted landing - even after touch down.

Indeed. A bit of background - The very first Software Engineering module lecture I attended in my first year at Uni had my late Prof. showing us the infamous video of the accident - he then took a show of hands to see how many of us thought it was an issue with the computer/software implementation - quite a few went up. He then proceeded to explain why it wasn't what so many of us believed, and provided an overview of how the systems were designed/implemented - finishing up by pointing out that it only takes a small mistake to cause a disaster, and just how much work and effort it takes to keep those mistakes to a minimum.

I've said it before, but I should reiterate that said Prof. was originally a dyed-in-the-wool sceptic when it came to digital FBW in civil aviation, and was well-known on the old Risks list for holding people's feet to the fire!

Some pilots seem to forget that the only thing that makes an aircraft climb is thrust (when you have no more kinetic energy to swap for height).
Why on earth they were so close to Alpha Max AND Low Height AND Idle Thrust is beyond me.

At least some of the answers are found in the lesser-known "Other factors" section of the report:

(Emphasis mine. Please excuse ropey Google translation - it was the best I could do in the time!)

3.2.2 . other factors

- Flight preparation was insufficient, particularly because of the brevity of the record provided late and information on the air show ;
- The distribution of tasks scheduled for overflight the crew was incomplete and has not been complied with;
- The party atmosphere for both passengers and spectators chip transmit the captain ;
-*A 320 has new features that could induce in the mind of the commander overconfidence
- These new characteristics of the aircraft were sometimes attacked and the captain wanted to defend ;
- neither pilot had experience such pre- flight presentations ;
- The late identification of the land has led some precipitation events and the realization of the descent and the entire passage above the track with full engine reduced and without stabilization of flight parameters ;
- Neither pilot had experience overshoot from flight idle in similar conditions.

In short - the airline and crew forgot the old "5 Ps" maxim*, and thus fell victim to the consequences.

* - i.e "Proper Planning Prevents P*ss-poor Performance".

** - @CONF iture - could you please point me to the page/section where you're getting the 2.5deg value from? Thanks.

[toffeez]

3.2.2 . other factors:

The pilot was known to his Air France colleagues as "Rambo".

[vilas]

r r r
"Why on earth they were so close to Alpha Max AND Low Height AND Idle Thrust is beyond me".

One guess is that the crew wanted to keep the aircraft in view for a longer time by taking advantage of high AOA protection to slow it down to minimum speed and maintain it/AOA by adding thrust. The computers adjust alpha max to stay below Vs1g stall, may be that slight adjustment was enough to cause a descent from 100ft to 50ft. When pilot decided to go round from twin evils of idle thrust and max angle of attack the aircraft performance was less than desired in the situation. One thing is certain unplanned manuevres at lower altitude is asking for trouble. Perpignan for example.

[rudderrudderrat]

vilas
The computers adjust alpha max to stay below Vs1g stall, may be that slight adjustment was enough to cause a descent from 100ft to 50ft.
From the link to the accident report I posted yesterday:
"The first officer informed the captain that the aircraft was reaching 100 feet at 14:45:14. The descent continued to 50 feet 8 seconds later and further to 30-35 feet. Go-around power was added at 14:45:35. The A.320 continued and touched trees at the end of the runway at 14:45:40 with a 14 deg. pitch attitude and engine speed being 83% N1. The plane sank slowly into the forest and a fire broke out."

I would suggest the aircraft sank below 100 feet because they didn't apply any power for 21 seconds, and hence they deliberately flew below 100 feet.

Villas, please explain why you think it was the computers that were not able to keep the aircraft airborne for over 26 seconds with idle power from 100ft at a speed close to the stall.

Hi DW, @CONF iture - could you please point me to the page/section where you're getting the 2.5deg value from?
May be some confusion with 2.5%?
See FCB 17
"TOLERANCE OF FAC COMPUTED SPEEDS
Due to the data accuracy used to compute the characteristic speeds, and specifically the AOA accuracy, the precision of the computation is specified to be within ​2.5 %.
During acceptance flight, the tolerances are as following:

Clean aircraft
Green Dot ±​5 kt
VLS ±​4 kt
Vαprot ±​5 kt
Vαmax ±​5 kt

Conf full
VLS ±​3 kt
Vαprot ±​5 kt
Vαmax ±​5 kt"

[jcjeant]

vilas
One thing is certain unplanned manuevres at lower altitude is asking for trouble. Perpignan for example. In the case of Perpignan this was a planned manoeuvre but at a wrong altitude
Methink if they had made the planned manoeuvre (it was a test) at a higher altitude the result was to be the same .. splash in water .... they were engaged in a deadly yoyo game

[captainsmiffy]

For DozyWannabes benefit.....Prior Planning Prevents P*ss Poor Performance is actually not the '5 Ps' as advertised but, actually 6!! Hopefully the performance will be better next time with a little prior planning.....?!!!

[vilas]

r r r
I do not have access to English version of the report. From what I read in the posts,I consider the following. The pilot wanted to fly as slow as possible by flying at Valpha Max by holding the stick back. Alpha floor gets activated between Valpha prot and Valpha Max and if it was instantly disconnected before speed increase beyond Valpha prot then it will be
deactivated till the speed has increased beyond Alpha prot before dropping below again. He did not seem aware of this that is why he was surprised by the lack of alpha floor response and moved thrust levers manually. Now he was at alpha Max and idle thrust at 100ft.Considering the response time of the engine and for the speed to recover from back side of the curve descent from 100ft and ground contact seems possible.

[rudderrudderrat]

villas

The French report, page 35:
12h 46' 26'' CDB "Bon Je vais bien la debrayer l'auto-manette." (Google translate = "Good I'm fine to disengage the auto-throttle"). Radio Altimeter shows 40 / 50 / 40 feet.

The Captain is aware the auto thrust is disconnected, yet he doesn't apply TOGA thrust for another 10 seconds with a plane full of passengers. Incredible!
He did not seem aware of this that is why he was surprised by the lack of alpha floor response and moved thrust levers manually.

I think you are right. It is the best explanation so far.

[CONF iture]

The oft-repeated claim that "the aircraft thought it was landing and wouldn't let the pilot pull up" was a complete fabrication of the press*.
As was also the claim that : "had it been a 737 no one would have survived but thanks to the Airbus protections, almost all survived"
* - Given the usual short shrift accredited to journalists among pilots, it's surprising how long that particular rumour has lasted and how many were willing to believe it.
Not more surprising than most of you who have read or not the report are still pretending that the airplane was at alpha max.

1.16.1.2. Lois de pilotage de l'Airbus A320
Cette loi de pilotage assure en particulier une protection automatique empêchant l'avion d'atteindre une incidence supérieure à 17.5 degrés, pour conserver une marge suffisante par rapport au décrochage, même si le pilote maintient sa demande au plein cabré.

[tdracer]

OK, I'm going to duck for cover after I post this, but....


There were reports that the DFDR and CVR didn't jibe - that there was a 3 second discrepancy. The implication was that the DFDR had been 'altered' to 'remove' a 3 second delay in the engines responding to the GA command http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/puppy_dog_eyes.gif. Basically, that there was a cover-up of some shortcoming in the aircraft and blame it on the pilot.


Based on an 'Air Disasters' program about the crash, the pilot is still claiming he was framed, pointing at the 3 second gap. http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/confused.gif


So, knowing relatively little about this accident (and most of what I do know is based on the press and TV reports), is there anything to the alleged gap in the data?http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/worry.gif

[jcjeant]

One dedicated site for this accident (in french ..from a journalist who has followed the case since the first minute ... because he was a passenger on that plane)
Crashdehabsheim.net (http://www.crashdehabsheim.net/index.htm)
So it is an analysis among others, and which are often contradictory :)

[Chris Scott]

Quote from Conf_iture:
Originally Posted by BEA [see its Habsheim final report of 1990/04/24]
1.16.1.2. Lois de pilotage de l'Airbus A320
Cette loi de pilotage assure en particulier une protection automatique empêchant l'avion d'atteindre une incidence supérieure à 17.5 degrés, pour conserver une marge suffisante par rapport au décrochage, même si le pilote maintient sa demande au plein cabré.

In case it helps anyone, this is my translation into received English:
This flight law provides a special automatic protection preventing the aeroplane from reaching an incidence [angle of attack] greater than 17.5 degrees, to conserve a sufficient margin with respect to the stall, even if the pilot maintains a full climb [pitch-up] demand.

Note that this BEA description does not specify that an alpha of 17.5 deg will be achieved if the pilot maintains full back-stick.

An earlier part of 1.16.1.2 sheds light on the question of whether any engagement of Landing Mode might have at any stage affected the ability of the pilot to achieve Alpha Max. Many of you will prefer to read the original in French, but for those who don't and to save space I'll simply offer this translation:

During all flight-phases prior to the moment the machine reaches a height of 50 feet, measured by the radio altimeter, the flight law is the usual one, called C*.... [there follows a description of the latter] ...
During the phase of descent between 50 and 30 feet, the flight law is modified progressively to take into account (*), instead of the term of load factor [normal G] , a term in pitch (difference between the pitch recorded passing 50 feet, called the reference pitch, and the instantaneous actual pitch);
when passing 30 feet, a supplementary order intended to simulate an effect of conventional landing is introduced to complement [modify] the previous pitch law: this order progressively reduces the the reference pitch (established at 50 feet) to -2 degrees in 8 seconds, creating a dive [pitch-down] moment that the pilot must counteract ("derotation").
At all times, if the angle of attack reaches 14.5 degrees, the flight law is modified and the term of load factor or the term of pitch (whether modified or not by the derotation order) is replaced by a term of incidence [Alpha Prot] (difference between the current angle of attack and 14.5 degrees). This flight law provides a special automatic protection preventing the aeroplane from reaching an angle of attack greater than 17.5 degrees, to conserve a sufficient margin with respect to the stall, even if the pilot maintains a full climb [pitch-up] demand.
This latter flight law, by the name of Alpha Prot (angle-of-attack protection), is a priority mode once the angle of attack reaches 14.5 deg. It does not constitute a degraded mode and cannot be de-activated by the crew.
[The section continues with a description of Alpha Floor.]

(*) This transition is progressive: the change of law is effected in one second.

The reference to Alpha Prot being a priority mode suggests that it would take priority over all others, including Landing mode. The report states that the modes/laws of engagement were specifically recorded by the DFDR as follows, where the time base is in seconds relative to the time of "impact", notated as "t" (see also my previous post (http://www.pprune.org/tech-log/528034-habsheim-2.html#post8202617)). These are listed as follows:
Before [and including?] t - 22, Normal C*;
between t - 21 and t - 20, in transition to landing mode;
between t - 20 and t - 18, progressive return to C*;
between t - 18 and t - 04, landing mode (a reference pitch of +6 deg having been measured passing 50 ft);
at t - 4, switch to Alpha Prot, the angle of attack having exceeded 14.5 degrees; this law maintained thereafter.

Confit and others,
Please let me know soonest of any anomalies in my translations...

[balsa model]

...pretending that the airplane was at alpha max
Please excuse (or ignore) my intrusion into this discussion - I'm the least qualified to comment here, but I do have a question:
Would the extra couple of degrees of AoA, that are presumed to be held back by the s/w (is that your point, CONF iture?), have changed anything for the better in the outcome?
I understand that the lift coef curve is mostly flat near stall as it is. In addition, IF the extra increment in lift extended the flight long enough for the engines to spool up, wouldn't the go-around thrust-pitch couple just push the airplane into a stall? The crash would then be tail-first, perhaps.

[awblain]

If the angle of attack had been greater, and the slowing with the same power had thus been more alarming, then perhaps the power would have been added earlier, and the trees would have been avoided. The trees would also have been further away in time, allowing the engines to spool up more before they turned into chippers.

Crashing on the airfield might also have been more benign than crashing in the forest. There'd probably have been less chance of smashing open all the fuel tanks, and it would have been easier for whatever rescuers were available to get to the wreckage.

As it was, it was close. Perhaps with three fewer feet of treetops in the engines… it might have flown away. The aircraft wasn't dragged to a dead stop - it was moving through the forest for five seconds before it stopped and burned.

[BOAC]

balsa - we did this topic to death on a previous Habsheim thread, but the view is that had the a/c been a 'traditional' non-fbw type then the trees might well have been cleared with the extra degree or two of 'alpha', but conversely if the crew had over-rotated in their anxiety they could well have stalled and possibly dropped a wing which would almost certainly have caused far greater loss of life. The jury, as they say, remains out.

[Chris Scott]

Hi balsa model and awblain,

Reading your posts, a couple of points spring to mind.

(1) Pitch rose through +10 deg at about t -13. No particular significance in +10, but my guess is that, assuming the pilot's eye height was adjusted (using the seat-height adjustment) to the recommended level, the trees would not have been visible through the windshields after that. Also, not many pilots are used to the perspective at that kind of attitude - except on T/O. At t -7 pitch was about +14. Unfortunately I don't have an A320 AWOPS guide to hand, which might help with the relevant geometry.

At about t -8 the co-pilot warned the captain of some pylons ahead. They were very much further away than the trees, so one can assume that either the co-pilot could not see the trees, or that the steep deck-angle caused him to think that the a/c was higher than the treetops.

(2) Some readers may be surprised to learn that at t = 0 (defined as impact) the stick was still not fully back. IIRC, full deflection is 20 degrees. The last recorded stick position was -17 deg (17 deg nose-up command), rising rapidly from -5 deg at about t -3. What happened after that is not recorded, because the power supplies to both the DFDR and CVR were simultaneously cut (by severing of shared conduit).

Between t -9 and t -4, the stick command was between -6 deg and -7 deg, but this was relaxed slightly at t -4, falling to -5 deg.

ALPHA PROTECTION

As mentioned in the BEA report (see previous posts), the flight-control law changes from Normal Law to Alpha Protection (in effect a sub-law of Normal Law) if the angle of attack rises to an arbitrary figure known as alpha-prot. Alpha Prot "law" tries to maintain that angle of attack if the stick is neutral. A higher angle of attack, known as alpha-max, can be achieved with sustained, full back-stick. Alpha-max provides a small margin from the stall (the latter usually defined as alpha-CL-max), and cannot be exceeded. Intermediate angles of attack are commanded acording to the amount of back-stick applied by the pilot.

The BEA report states that, in their configuration (Flaps 3 and L/G):
alpha-prot was 14.5 deg;
alpha-max was 17.5 deg.

At t = 0 ("impact") both pitch and alpha were about +15. The latter represents the apparent discrepancy of about 2.5 deg that Conf_iture has called into question. As stated above, alpha-max can only be achieved by sustained, full back-stick.

[awblain]

The trees being out of sight under the nose is interesting, which might have contributed to a lack of urgency - although the approach of the line of the trees should have been perceptible in peripheral vision, even if they were below the sill directly ahead. From the video, the whole of the flyby appears to have been done at a substantial deck angle, so any visual impression of height throughout the whole pass would have had to have been from the side. It's interesting that the first officer was able to see the pylons 8-s from impact though - distant 30-m-high pylons (which look like they're 3km away on Google Earth) - and nearby treetops are not likely to be at very different elevation angles from 8-s (~350m?) away from the trees. Looking forward, it would be trees all the way from the base of the pylons to the sill, and with the ground whizzing by at 40 feet or so to the side and the tree line approaching, it seems to be no place for a confidence-building visual illusion.

Isn't it all navel-gazing anyway? - the plan for the show presumably didn't have any tree surgery in it, and the arrival at the treetops was presumably at the end of a substantial deviation from the intended plan.

Prompt cutting of the power to the recorders implies a substantial impact - rather more than from twigs hitting the rear fuselage - perhaps a few extra feet wouldn't have made any difference. With more angle of attack, the rear might have been thumped into that conduit-slicer even harder (3 degrees being about a meter at the tail from the center of mass), helping to pitch back down and further into trouble.

Would you agree that the prime human factor is that no-one would ever have dreamed of trying this in a non-FBW airliner? Perhaps the same sort of overconfidence factor that has been claimed to account for the accident statistics when ABS systems were first fitted as options to cars, and the serious accident rate was higher in the otherwise identical cars with the ABS than those without.

[RoyHudd]

I fly Airbus types, inc. 320, and I used to fly out of Habsheim in a previous job. (Not A320's) It is a small airfield surrounded by forestation on 3 sides.

The CM1 was taking an enormous risk in flying too low and slow in an aircraft he clearly did not understand well. (Parallels with the Costa Concordia accident exist in Human Factors terms)

5 dead are on this plonker's conscience. Forget nationality, or background. On the day, he showed off, screwed up, and killed people.

[CONF iture]

Note that this BEA description does not specify that an alpha of 17.5 deg will be achieved if the pilot maintains full back-stick.
The BEA does not elaborate at all but applying full back stick is what it takes to go to alpha max.
Some readers may be surprised to learn that at t = 0 (defined as impact) the stick was still not fully back. IIRC, full deflection is 20 degrees. The last recorded stick position was -17 deg (17 deg nose-up command)
Full deflection in roll is 20 deg but 16 in pitch. The stick was fully back.

[balsa model]

So to get to alpha max, one has to apply sustained full back stick. I'm guessing that this means that as the full back stick is applied, the alpha limitter progressively increases its limit value from alpha prot to alpha max.
Out of curiosity, how much delay would this entail?

Also, I recall something about Airbus fbw going to "direct law" at expected landing radar altitudes, and that this mode gives the pilot direct stick-to-pitch control. Could that have been the pilot's motivation for going so low? To have a better feel over the remaining pitch authority?

[CONF iture]

Would the extra couple of degrees of AoA, that are presumed to be held back by the s/w (is that you point, CONF iture?), have changed anything for the better in the outcome?
To be clear, I'm questioning why BEA+Airbus kept absolutely silent on the fact that the airplane had no intention to deliver anything more than alpha prot despite the pilot request.
But to answer your question, my opinion is that it would have made the difference considering that according to the report the engines have been registered at 91% N1.
IF the extra increment in lift extended the flight long enough for the engines to spool up, wouldn't the go-around thrust-pitch couple just push the airplane into a stall? The crash would then be tail-first, perhaps.
No, alpha max is part of the protection, and so whatever the thrust setting.

[vilas]

Dozy, Chris r r r
I think unnecessary lengthy technical discussion is going on when there are some simple aviation facts staring in the face. I do not know what was planned but you can see what was executed. The pilot decided to slow the aircraft to minimum speed taking advantage of the AOA protection, fly level and go around. In any aircraft primary instrument of flight path change is pitch. The thrust maintains speed. The pitch up caused by thrust is secondary effect. Incidentally it is dampened in A320 by FBW and you need to pitch up. Not doing so was a possible cause of another crash. When pilot was late in setting the precise thrust to fly level at 100ft the aircraft sank. He could not pull back because he was at MAX or near MAX AOA so unable to change the flight path. Thrust was the only alternative but being at idle for a while it takes about 4 seconds to get out of idle and when you are sinking from 40ft, 4seconds must have seemed like a life time. So he panicked and thought ATHR was not behaving and moved the thrust levers manually. Alpha floor should have come between alpha prot and alpha max. Now the question is how and when exactly was it disconnected? There are three possibilities.
1. It was disconnected permanently before slowing down started. This does not seem probable as the pilot expected thrust activation.
2. It was disconnected as it got activated between alpha prot and alpha Max. In this case alpha floor will remain dormant till the speed increases beyond Valpha prot.
It is possible pilot did not know this considering the technology was new and that is why he was surprised by lack of response.
3. He may have been holding on to the disconnect button to prevent alpha floor till speed came to alpha max and inadvertently exceeded 15 seconds because he was preoccupied visually and disabled it.
In any case he needed quick response to gain those critical extra 50ft which was not possible because of the max angle of attack.
Why should a line pilot who only has access to FCOMS indulge in manoeuvres which even the test pilot did not, in radically different aircraft? I am unable to comprehend that.

[Clandestino]

ATHR disabling (by holding disconnect buttons for more than I-forgot-the-exact-number seconds) was planned but not executed. Alpha floor didn't activate because it's inhibited below 100 ft per design, in order not to spoil the landing.

Why should a line pilot who only has access to FCOMS indulge in manoeuvres which even the test pilot did not, in radically different aircraft?

Showing off. Careless planing combined with sloppy execution. Chances are they never meant to fly that low, which does not imply it is acceptable to make high alpha pass with planeload of passengers at 200 instead of 20 ft.

Also, I recall something about Airbus fbw going to "direct law" at expected landing radar altitudes, and that this mode gives the pilot direct stick-to-pitch control.Some widebodies stop autotrim, narrowbodies add automatic pitch-down command to facilitate the flare but going to direct law happens only with gear extension when aeroplane is already in alternate law and that was definitively not factor here. Aeroplane was totally sound just before controlled flight into trees.

[rudderrudderrat]

vilas
Alpha floor should have come between alpha prot and alpha max. Now the question is how and when exactly was it disconnected?
We don't need to discuss that - because as Clandestino points out, "Alpha Floor is available, when the flight controls are in Normal Law, from lift off to 100 ft RA at landing." (FCTM)

Alpha Floor was never going to activate whilst they were below 100 RA.

[vilas]

r r r
If the speed went below alpha prot only when they went below 100ft then offcourse alpha floor was disabled. Did the pilot know that? The Airbus design is such that it is possible to fly hundreds of normal routine operations without much knowledge. I consider that as a positive but the pilots have to understand that if they try out something beyond normal they could be asking for trouble because that requires much deeper study of the design and its implications which is beyond the scope of FCOMs and line pilots. Protections on civil transport aircraft are not built to routinely fly the aircraft on the brink but to save the situation when forced into it by abnormal factors. At Perpignan they planned it then abandoned it and last moment executed it at low level and were not able to handle the consequences.

[rudderrudderrat]

vilas
If the speed went below alpha prot only when they went below 100ft
From page 40 of the BEA report, at time 1245
100 RA Speed 151 kts.
48 RA Speed 141 kts.
40 RA Speed 137 kts.
30 RA Speed 116 kts.

It appears to me that the aircraft was deliberately flown below 50 RA with the speed allowed to wash off, apparently waiting for Alpha Floor to "save the day".
then offcourse alpha floor was disabled. Did the pilot know that? Probably he did not know that, as the FCTM was published later, and I still can't find a reference in FCOM.
the design and its implications which is beyond the scope of FCOMs and line pilots. I agree.

[Chris Scott]

vilas,

As you know, this thread was started from the transfer of a few posts on the AF447 Thread No. 11. (Not my idea!)

If you look again at post #1, you may be reminded that - in an earlier post on the AF 447 thread - I had opined that the crew had been relying on Alpha-Floor to initiate the G/A (i.e., when the AoA reached +15), but that they had allowed the a/c to sink below the Alpha-Floor inhibition height without realising the consequences. That assumption on my part was naive, and on reflection would have made it very difficult for the captain to go around at a position (in relation to the crowd) of his choosing. Alpha-floor (+15 deg) is only half a degree above alpha-prot.

As CONF_iture pointed out, however, the captain had specifically briefed that he would be inhibiting Alpha-Floor, in order to continue pitching up to alpha-max. His plan was to maintain alpha-max (therefore necessitating full back-stick) in level flight (he said at 100ft), using manual thrust to maintain height. (Note that, at a stedy AoA, thrust controls VS, not speed - rather more like a light piston-engined a/c on the approach.)

The two key defficiencies in the execution of the briefed game-plan seem to have been:

(1) Rushed, straight-in approach, arriving over the airfield boundary (inbound) with too much energy to allow a reduction in speed to the planned Valpha-max during the transit of an unfamiliarly-small airfield. This may have tempted the PF to delay the go-around while he waited for the IAS to decay to an acceptable (in his terms) value.

(2) Descent below 100R. From the time they were approaching 100 ft on the approach until about t -3, the PF consistently did not pull hard enough on the stick to maintain or recover to the briefed height. This apparent reluctance to pull harder is all the harder to explain, because the PF was looking for the highest-possible deck angle to show the crowd.

Quote from CONF_iture:
Originally Posted by Chris Scott:
"Note that this BEA description does not specify that an alpha of 17.5 deg will be achieved if the pilot maintains full back-stick."
The BEA does not elaborate at all but applying full back stick is what it takes to go to alpha max.

Yes, we are in accord on that one. :ok: However, in the earlier post you've quoted, I was just trying to point out that the BEA seems to have carefully emphasised that alpha-max (in this case 17.5 deg) will not be exceeded, without stating that it will necessarily be precisely achieved.

Quote from CONF_iture:
"Full deflection in roll is 20 deg but 16 in pitch."

Thanks, I haven't found that figure yet. The DFDR trace seems to shows a rapid rise to about -17, at which point the trace ends without becoming steady.

Quote from CONF_iture:
"The stick was fully back."

As I've already said:
"Between t -9 and t -4, the stick command was between -6 deg and -7 deg, but this was relaxed slightly at t -4, falling to -5 deg.
"The last recorded stick position was -17 deg (17 deg nose-up command), rising rapidly from -5 deg at about t -3."

The back-stick reached about 17 deg (presumably maximum), but not for a measurable period prior to impact. That's the reason for my observation in (2), above. Why he did not pull harder from t -26 to t -3 is inexplicable, if only because it was delaying the desired attainment of alpha-max. The modest back-stick up to t -4 would, as you know, never achieve alpha-max.

[vilas]

r r r
I am unable to paste the report in google for translation. Below is what you wanted.
DSC-27-20-10-20 P 4/6
In addition, below 200 ft, the angle of attack protection is also deactivated, when:
‐ Sidestick deflection is less than half nose-up, and
‐ Actual α is less than αprot –2 °
Note: 1. At takeoff α prot is equal to α MAX for 5 s.
2. αfloor is activated through the A/THR system, when:
‐ αis greater than αfloor (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 ft RA before landing.

[Chris Scott]

Quotes

From rudderrudderrat:
"It appears to me that the aircraft was deliberately flown below 50 RA with the speed allowed to wash off, apparently waiting for Alpha Floor to 'save the day'."

From vilas:
"If the speed went below alpha prot only when they went below 100ft then of course alpha floor was disabled. Did the pilot know that?"

Response from rudderrudderrat (my emphasis):
"Probably he did not know that, as the FCTM was published later, and I still can't find a reference in FCOM."

Firstly, the evidence strongly suggests that he was not relying on Alpha-Floor; contrary to what I originally thought.

Secondly, I think he should and would have known of its inhibition below 100R.

Regrettably, I have mislaid the Airbus FCOM volumes dished out to us BCAL (British Caledonian) pilots on arrival at Blagnac for our conversion course on 1/1/1988. Prior to that, we had perused the BCAL Ops Manuals (our own FCOM) dated August 1987. After finishing our ground course and simulator (we were just a week behind the first Air France course), BCAL issued a wide-ranging amendment, dated 17FEB1988, which included considerable changes to the Flight Controls chapter in the Technical Manual. The changes were partly necessitated by changes in FBW logic (Dozy please note), and partly an improvement in the explanations and diagrams.

At both 25AUG1987 and 17FEB1988, the BCAL chapter on Flying Controls describing the Alpha protections aparently fails to mention that Alpha Floor is inhibited below 100R.

However, the BCAL chapter on Power Plant - which includes a description of A/THR, and which remained unchanged from 25AUG1987 in the 17FEB1988 amendments - includes the following:

A/THR can be engaged:
- Manually, by.....
- Automatically, when the pilot initiates a T/O or G/A....
or - if there is an alpha-floor detection after lift-off and down to 100 ft (R.A.) on landing.
In the particular case of alpha-floor detection, the MAX T.O. thrust is automatically selected whatever the position of the thrust lever.

[DozyWannabe]

I do not have access to English version of the report.

That's because there isn't one - as I've said before - all I've had to work with over the years are extracts - but I've managed to OCR and translate some of the lesser-known aspects of the report recently.

Not more surprising than most of you who have read or not the report are still pretending that the airplane was at alpha max.

To the best of my knowledge, no-one's claimed that, and we've already confirmed that the FCTM does not state that pulling back in High AoA Protection mode will definitely give Alpha Max - just the closest to that value that the systems can give with the flight parameters at the time. If a pilot assumes that it will give them Alpha Max precisely, then they are in error.

OK, I'm going to duck for cover after I post this, but....

There were reports that the DFDR and CVR didn't jibe - that there was a 3 second discrepancy.

Nope - that was based on a misinterpretation of the data by the retired investigator the lawyers used to give a second opinion. Ray Davis was an excellent investigator, but he had no prior experience with DFDRs.

Posted a while back:
DFDR matches CVR. If you are referring to Ray Davis's independent reading of the FDR, he was not experienced in dealing with the new digital models, and got it wrong.

From the document posted by Franzl months ago : http://www.crashdehabsheim.net/Rapport%20Airbus.pdf

Mr. Davis was apparently not aware of the convention (which is apparently unique to France) that requires that the transcripts of forward accelerations are shown with a negative sign. [He is] therefore claiming that in the last seconds the negative acceleration shown in the transcript demonstrates that the aircraft was decelerating and therefore one or both engines were not providing sufficient thrust.

The flight recorders did not stop instantaneously at Habsheim. In the final report produced by the Commission of Inquiry it clearly states that -after the first impact with the trees, the CVR continued to operate for around 1.5 seconds and then stopped. The DFDR continued to operate
for around one second [after impact] then gave incoherent data for around two seconds". The exact cause as to why the recorders stopped almost simultaneously before the aircraft finally came to rest could not be determined. The most probable cause is that the power supply cables of the two recorders broke.


Additionally, as the (admittely poorly-)translated "Other Factors" section I posted earlier confirms, the official BEA report does not find that the factors leading to the crash lay with the flight crew alone - the sequence began with poor preparation and briefing materials on the part of the airline. The BEA's report does not explicitly apportion responsibility - because like other agencies that are nominally independent, yet still civil service (like the UK AAIB, ATSB etc.), their remit does not extend far enough to allow them to do so. Of course, the legal teams were counting on the press they were briefing to be unaware of that distinction.

Because the crew's lawyers were arranged by the union, it could be argued that they tried to paint a picture of a cover-up on AI's behalf, because to draw attention to the report's actual content - i.e. implicit but robust criticism of AF - would be biting the hand that fed the union's members.

For their part, AF's defence during the criminal and civil proceedings involved falling back on an old document that specified the airshow "hard floor" (i.e. minimum altitude) to be 600ft. This raises the question of how, if that rule was still in effect, they were able to sign off the Habsheim display in the first place - and it also demonstrates that it was in fact AF who were primarily responsible for the effort to have the crew "thrown under the bus".

As for "how did they end up so low and slow?", as I understand it the chain of events was set in motion during the (rushed) approach. It wasn't that the PIC was waiting for an "acceptable" (as in sufficiently breathtaking for the spectators) altitude/speed during the flypast, it was because they were so high and hot at the start of the final that they chopped off too much power.

[CONF iture]

So to get to alpha max, one has to apply sustained full back stick. I'm guessing that this means that as the full back stick is applied, the alpha limitter progressively increases its limit value from alpha prot to alpha max.
Out of curiosity, how much delay would this entail?
Fast process - The protected aircraft takes advantage of the electronics to get the maximum performance in the minimum time and outperform the non protected aircraft.

Thanks, I haven't found that figure yet.
I have some unofficial references for now, but I will try to locate and post some official ones ... Wilco
The back-stick reached about 17 deg (presumably maximum), but not for a measurable period prior to impact. That's the reason for my observation in (2), above. Why he did not pull harder from t -26 to t -3 is inexplicable, if only because it was delaying the desired attainment of alpha-max. The modest back-stick up to t -4 would, as you know, never achieve alpha-max.
I think it was all about the show ... Even if the planned altitude was for 100 ft AGL and the pilot pretended he thought he was maintaining just that, I would not be immensely surprised if his real intention was to go lower ... (?)
Pulling harder early in the sequence would have made the aircraft that was too fast climb far too early to his liking.
Late in the sequence, the pilot, now fully aware of the approaching trees, but well aware that thrust was not coming as he expected it, delayed as long as possible the pull up request.
Regrettably, I have mislaid the Airbus FCOM volumes dished out to us BCAL (British Caledonian) pilots on arrival at Blagnac for our conversion course on 1/1/1988.
I certainly would not blame you for courageously making space ...

[DozyWannabe]

Fast process - The protected aircraft takes advantage of the electronics to get the maximum performance in the minimum time and outperform the non protected aircraft.

Real Time process to be more precise. Actual (as opposed to theoretical) Alpha Prot, Floor and Max boundaries are calculated constantly in real time. The values are functions - not hard-coded constants.

I have some unofficial references for now, but I will try to locate and post some official ones ...

I'd be as interested in the unofficial values (and where they came from) as I am in the official ones.

I would not be immensely surprised if his real intention was to go lower

I'd be very surprised - as the Airbus test pilots who came up with the scenario were very specific about not going under 100ft RA to maintain a reasonable safety margin.

Pulling harder early in the sequence would have made the aircraft that was too fast climb far too early to his liking.

With Alpha Floor disabled, the aircraft would maintain current altitude at calculated max AoA up to theoretical Alpha Max - it wouldn't climb, if the documentation is accurate.

Late in the sequence, the pilot, now fully aware of the approaching trees, but well aware that thrust was not coming as he expected it, delayed as long as possible the pull up request.

Can you confirm in the FDR trace?

(I'm still waiting for the source that gives you 2.5deg, by the way...)

Additionally, if the PIC was expecting thrust sooner than it was provided, then he was clearly ignorant of the properties of high-bypass turbofans. Either that or ignorant of the consequences (namely allowing the engines to spool down) of the thrust settings he ordered to make the approach.

[HazelNuts39]

With Alpha Floor disabled, the aircraft would maintain current altitude at calculated max AoA up to theoretical Alpha Max - it wouldn't climb, if the documentation is accurate.It will only do that at constant airspeed, i.e thrust equal to drag. With thrust less than drag the aircraft decelerates and AoA has to increase progressively to maintain lift equal to weight, i.e. the sidestick has to move progressively backwards. When alpha-max is reached and the aircraft is still decelerating it will descend.

[DozyWannabe]

It will only do that at constant airspeed, i.e thrust equal to drag. With thrust less than drag the aircraft decelerates and AoA has to increase progressively to maintain lift equal to weight, i.e. the sidestick has to move progressively backwards. When alpha-max is reached and the aircraft is still decelerating it will descend.

Correct - but in this case the aircraft had finished decelerating before it crossed the grass strip threshold. If the aircraft's forward momentum was still subject to cutting power then it would continue to decelerate, but it did not - it maintained the airspeed determined by the approach settings.

[RetiredF4]

When the accident happened just in front of my homebase, we fast jet pilots discussed the possible traps of this failed display long hours. That short time after the accident we had no official information available except the location and environment of the nearby accident site. We could look at the crash site when departing visual from our base.

Any pilot of my outfit was familiar with low flying down to 100 feet and up to 540 KIAS in Labrador, and we all agreed that flying an airliner at 100 feet or below over a small airfield at minimum speed was suicidal planing.

We discussed, why the crew even went considerably below that planned altitude and came up with the suggestion, that it might not have been the intention of the crew, but happened due to the perspective of the small little airfield. Flying a visual approach to a small little runway / airfield like Habsheim when being used to bigger airfields and buildings like Basel just minutes before gives the visual illusion of being at a higher altitude above ground than actual. Which raises the question, why the information by the Radar Altimeter was neglected. When you expect in advance that the thing will give altitude warnings you have no intention to obey to (as the intention was to do a visual pass and not one based on inside cockpit information) the information is filtered to zero as non essential background noise by the brain.

This flyover should never have happened, we would never have gotten the permission to execute such a flyover with our jets. In my view all the speculation about possible technical aspects is noise to cover up the fact, that the crew f** ed up badly and didn't register the descent below 100 feet. Only altitude information comes from the Radar altimeter, Captain and FO did neither acknowledge nor react to those warnings.

[DozyWannabe]

We discussed, why the crew even went considerably below that planned altitude and came up with the suggestion, that it might not have been the intention of the crew, but happened due to the perspective of the small little airfield.

Not even that. As far as I can tell from the report, AF signed off on the profile provided that the flypast was over the paved strip (which had no obstructions in the run-off area) and the crew were comfortable with the profile. The briefing materials provided were inadequate - specifically the photocopies of the airfield charts provided to the crew effectively deleted the graphics indicating the trees.

In the event, the crew spotted the airfield late, and noticed that the spectators were lined up on the grass strip, not the paved strip on which they had been briefed. At this point the crew would have been well within their rights to abort the flypast altogether, and press on to the sightseeing trip over the Pyrenees which made up the latter part of their flightplan. Instead, the PIC elected to perform an unauthorised approach maneouvre to match the grass strip, even though they were considerably high and fast to do so.

[HazelNuts39]

it maintained the airspeed determined by the approach settings.I'm not sure what you mean by that. Are you referring to a point where the airspeed bottomed, changing from decelerating to accelerating while the engines continued to spool up from idle to TOGA?

[DozyWannabe]

I'm not sure what you mean by that. Are you referring to a point where the airspeed bottomed, changing from decelerating to accelerating while the engines continued to spool up from idle to TOGA?

I'm referring to the constantly changing variable of airspeed throughout. The airspeed did not increase enough to have a significant impact on calculated maximum alpha until a point where they were just about to make contact with the trees. The engines spooled down well before the aircraft crossed the threshold. The aircraft maintained airspeed based on that initial value, and maintained pitch attitude equivalent to maximum AoA available. The airspeed didn't increase substantially until after the engines spooled up again.

[RetiredF4]

Dozy
In the event, the crew spotted the airfield late, and noticed that the spectators were lined up on the grass strip, not the paved strip on which they had been briefed. At this point the crew would have been well within their rights to abort the flypast altogether, and press on to the sightseeing trip over the Pyrenees which made up the latter part of their flightplan. Instead, the PIC elected to perform an unauthorised approach maneouvre to match the grass strip, even though they were considerably high and fast to do so.

Wherever you got that information from, it does not look right. The heading difference between the flown grass strip track and the paved strip is 035 degrees, and would have required a total different track from beginning and flight over the houses of Habsheim, which was not permitted for airshow traffic. They would have flown a different track to approach the airfield some miles out with the track of 020 degrees for the paved strip. But the track was flown along the east side of the highway Basel-Straßbourg which is about heading 345 like the grass strip and which is over thin populated areas and offers good visual clues, which makes sense.

The failure was not maintaining the intended altitude of 100 feet AGL. I could go over and measure those trees today after they continued to grow another 25 years, but i doubt that i could find a tree today with a hight of 100 feet.

You may check my points by the maps published under
Habsheim cartes crash (http://www.crashdehabsheim.net/cartes.htm)

Edit: You might refer to the article in flight international edition 17.april 1990. I think the mentioned obstacle clearance calculation by AirFrance for the paved strip was just that, a calcukation without reference to the intended approach path.

1990 | 1071 | Flight Archive (http://www.flightglobal.com/pdfarchive/view/1990/1990%20-%201071.html)

[DozyWannabe]

Wherever you got that information from, it does not look right. The heading difference between the flown grass strip track and the paved strip is 035 degrees, and would have required a total different track from beginning and flight over the houses of Habsheim, which was not permitted for airshow traffic. They would have flown a different track to approach the airfield some miles out with the track of 020 degrees for the paved strip. But the track was flown along the east side of the highway Basel-Straßbourg which is about heading 345 like the grass strip and which is over thin populated areas and offers good visual clues, which makes sense.


Check the BEA report page 19, which confirms what I said. Rather that turn around and make a second and exact approach to the grass strip, the PIC elects to "fudge" an approach from their current position. The reasons behind that decision we will never know - but that decision was nevertheless made and executed.

[HazelNuts39]

I'm referring to the constantly changing variable of airspeed throughout. (...) The aircraft maintained airspeed based on that initial value ????
The airspeed did not increase enough to have a significant impact on calculated maximum alphaHow does airspeed affect the 'calculated maximum alpha', isn't alpha-max equal to 17.5 degrees?
The aircraft maintained airspeed based on that initial value, and maintained pitch attitude equivalent to maximum AoA available.I don't understand how an airplane can maintain a constant pitch attitude in essentially level flight while it decelerates from 140 kts to 112 kts, and the whole point of the discussion is that the airplane never reached alpha-max.

[DozyWannabe]

How does airspeed affect the 'calculated maximum alpha', isn't alpha-max equal to 17.5 degrees?

Not sure - but AoA is a function of airspeed and pitch attitude, no?

I don't understand how an airplane can maintain a constant pitch attitude in essentially level flight while it decelerates from 140 kts to 112 kts,

To the best of my recollection, the pitch attitude was not constant.

[RetiredF4]

Dozy,
could you point to the exact paragraph on page 19?
my french is better than Spain, which i don't speak at all, but not suitable for pinpointing the fact you cite.

[HazelNuts39]

Dozy,

I'm sorry, but we seem to be having some semantic misunderstandings, mainly about 'maintaining' and 'maximum'. In my vocabulary maximum available alpha is alpha-max and according to the accident report that is 17.5 degrees. The AoA that provides a lift equal to the weight is a function of airspeed. In level flight without wind that AoA is equal to the pitch attitude, controlled through sidestick inputs. Yes, the pitch attitude increased during the level deceleration.

The certificated 1g stall speed is obtained at alpha-max, with the sidestick held fully back during 3 seconds, after decelerating at 1 kt/second. But that is in free flight, out of ground effect, i.e. above 100 ft RA.

[DozyWannabe]

Dozy,
could you point to the exact paragraph on page 19?

Section 2.3 on p.19 covers the crew's decision and attempt to define an approach path that would return them to the position they were intending to hit from the outset, offset from the original (incorrect) co-ordinates.

In my vocabulary maximum available alpha is alpha-max...

Indeed - but Airbus documentation explicitly states that pulling up in High AoA Protection mode does not guarantee a precise application of theoretical Alpha Max.

[HazelNuts39]

Dozy,

IMHO there is nothing 'theoretical' about alpha-max. It is a somewhat arbitrary value chosen by the airplane manufacturer and programmed into the flight control system, just as alpha-prot and alpha-floor. It varies with configuration and (on some airplanes) with Mach number (when M>0.3 typically). And, as I said, it must be obtainable in free air by pulling the sidestick to the aft stop and holding it there until there is no further increase of AoA. I haven't seen any Airbus material 'explicitly' contradicting that.

[DozyWannabe]

Dozy,

IMHO there is nothing 'theoretical' about alpha-max. It is a somewhat arbitrary value chosen by the airplane manufacturer and programmed into the flight control system, just as alpha-prot and alpha-floor. It varies with configuration and (on some airplanes) with Mach number (when M>0.3 typically).

Agreed - the function that determines the value is programmed into the flight control software as you describe.

And, as I said, it must be obtainable by pulling the sidestick to the aft stop and holding it there.

No - according to the documentation, Alpha Max (as illustrated by the graph in the doc) *may* be achieved by pulling the sidestick full aft, but the documentation clearly states that the theoretical alpha max is not guaranteed to be reached.

[Chris Scott]

Quote from RetiredF4 (my emphasis):
"Any pilot of my outfit was familiar with low flying down to 100 feet and up to 540 KIAS in Labrador, and we all agreed that flying an airliner at 100 feet or below over a small airfield at minimum speed was suicidal planing.
We discussed, why the crew even went considerably below that planned altitude and came up with the suggestion, that it might not have been the intention of the crew, but happened due to the perspective of the small little airfield. Flying a visual approach to a small little runway / airfield like Habsheim when being used to bigger airfields and buildings like Basel just minutes before gives the visual illusion of being at a higher altitude above ground than actual."

Absolutely. And, on the approach, the distance-to-go would seem greater, once they had "gone visual". (They would initially have been using the FMS with a suitable waypoint constructed and inserted.)

You also make a very relevant point about the routing along the autoroute apparently being impracticable for the stated plan to fly along the paved Rwy 02. That would have involved an awkwardly-tight right turn.

Quote:
the crew... [...] didn't register the descent below 100 feet. Only altitude information comes from the Radar altimeter, Captain and FO did neither acknowledge nor react to those warnings.

...which would be very unusual for an A320 crew, even in those early days. The synthesised voice is clear and loud, calling 100, 50, and 30 ft (etc.). I notice that the BEA report recommended that the calls be included in the pilots' headphones, as well as the cockpit speakers. But I don't remember ever having a problem with our a/c at that time, and we always wore our earphones. Personally, in the L/H seat, I always wore my left earphone and boom mic, but usually kept the right earpiece just behind my right ear to allow conversation without necessarily using the intercom. (We did not get noise-reducing headphones until about a year later.)

HN39,
Bon courage... ;)

[EMIT]

What will have been very unusual for the crew will have been the over the nose visibility during the low speed, low altitude pass - the pitch attitude was so high that the trees may well have been hidden just behind the nose - cutoff angle over the nose, if I recall correctly, was 15 degrees down.
Believe it or not, even in an F-16 you have to fly a level, low speed pass on the old fashioned dial altimeter head down, because, sitting at 25 degrees AoA, the outside horizon is 10 degrees below your nose and the Flight Path Marker is way below the bottom of the HUD.
The Habsheim AF crew, in a way, just didn't know what they were really doing.

By the way, the scars in the trees can still be seen when you are on approach to BSL RW 15, an accident takes a long time to heal, in many aspects.

[vilas]

According to me planning this fly past at 100ft was not a problem but doing so at Valpha Max was an accident waiting to happen. Nobody can fly at dead 100ft at minimumm speed with hardly any elevator authority left for such precision altitude keeping. Such precision with purely thrust management is impossible considering also that the pilot had to look more outside.

[CONF iture]

The back-stick reached about 17 deg (presumably maximum), but not for a measurable period prior to impact. That's the reason for my observation in (2), above. Why he did not pull harder from t -26 to t -3 is inexplicable, if only because it was delaying the desired attainment of alpha-max. The modest back-stick up to t -4 would, as you know, never achieve alpha-max.
The pilot version is that he was following the altimeter set on the QFE of Habsheim and was indicating 100 ft all along. He also said he has never heard the RA announcements. That can be the reason he did not pull harder earlier during the initial phase of the low pass.

Also, I was having a look at the AF447 data (http://www.pprune.org/6626046-post1756.html), and the max deflection in pitch for the sidestick was at 16 deg.

[CONF iture]

In my view all the speculation about possible technical aspects is noise to cover up the fact, that the crew f** ed up badly and didn't register the descent below 100 feet.
I think we all agree on the crew responsibility in that event, but the technical aspect is still of interest.

[CONF iture]

I'm referring to the constantly changing variable of airspeed throughout.
?
The airspeed did not increase enough to have a significant impact on calculated maximum alpha
?

The more you write on alpha max, the more you look confused.

[roulishollandais]

At this point the crew would have been well within their rights to abort the flypast altogether, and press on to the sightseeing trip over the Pyrenees which made up the latter part of their flightplan.
Pyrenees were a little far from Mulhouse-Bâle Airport. The possible trip was over Swiss Alps not far away.

[HazelNuts39]

Dozy, please read again in your post #26 your quote from the second (A320 series) reference, then look at the graph immediately below the quoted text, in particular the legend which says "ɑMax Full aftstick - VɑMax". Then read the reply by Tester78 in post #27.

The FCOM says that VɑMax is another name for Vs1g.

[Chris Scott]

Quote from EMIT:
"What will have been very unusual for the crew will have been the over the nose visibility during the low speed, low altitude pass - the pitch attitude was so high that the trees may well have been hidden just behind the nose - cutoff angle over the nose, if I recall correctly, was 15 degrees down."

Yes, as I commented in an earlier post (http://www.pprune.org/tech-log/528034-habsheim-3.html#post8208343):
"Pitch rose through +10 deg at about t -13. No particular significance in +10, but my guess is that, assuming the pilot's eye height was adjusted (using the seat-height adjustment) to the recommended level, the trees would not have been visible through the windshields after that. Also, not many pilots are used to the perspective at that kind of attitude - except on T/O. At t -7 pitch was about +14. Unfortunately I don't have an A320 AWOPS guide to hand, which might help with the relevant geometry."

I added:
At about t -8 the co-pilot warned the captain of some pylons ahead. They were very much further away than the trees, so one can assume that either the co-pilot could not see the trees, or that the steep deck-angle caused him to think that the a/c was higher than the treetops.

EMIT is right. At that deck-angle, the PF would inevitably have been flying on instruments. The A320 RA readings are presented (like most EIS cockpits) in purely digital form - at the bottom of the PFD.The trend of the readings is therefore more difficult to interpret than would be the case with an analogue (dial) presentation. The synthetic voice normally calls the transits of 100, 50, 30, 20, and 10 on a landing approach. If the RA reading hesitates (due premature flare, or passing over a gully), the value can be repeated. The former happened, in effect, during this fly-past, which is why the CVR records the "Thirty" announcement several times.

Quote from CONF_iture (my emphasis):
The pilot version is that he was following the altimeter set on the QFE of Habsheim and was indicating 100 ft all along. He also said he has never heard the RA announcements. That can be the reason he did not pull harder earlier during the initial phase of the low pass.

Thanks for that. The DFDR trace shows the a/c consistently lower, eventually below 30 ft QFE. No pilot expects a baro reading to be that accurate, which is the main reason that, AFAIK, radio-altimetry is mandatory for any AWOPS procedure with a decision height below Category 1 minima (~ 200ft).

The geometry of the pilots' ability to see the trees would benefit from the diagrams in an AWOPS guide. However, there is another aspect of geometry which is interesting: the difference in the pilots' eye-height above the ground, and the RA reading.

The four rad-alt TRx antennae on the A320 were mounted on the bottom of the rear fuselage, just forward of the fuselage tapering area. Examination of the a/c profile diagram shows that, at a pitch angle of +13.5 degrees, the pilot eye height above flat ground would be about 25 ft higher than the RA reading. At the final pitch angle of about +15, the tail-cone and tapering part of the rear fuselage would have been the first to touch any horizontal series of treetops. At that stage, the engine nacelles would have been between 5 and 10 feet clear. (However, we cannot assume the treetops were all the same height.)

So the pilot's eye would have been over 25 ft above the tree-top that the a/c first struck...

Also, FWIW, the static ports that sense the static pressure for the ADCs that feed the altimeters are not far below the pilots.

[HazelNuts39]

In AoA protection mode the sidestick commands an angle of attack. Attainment of the commanded angle of attack will not be instantaneous. A nose-up movement of the sidestick will request a change of the elevator position to pitch the airplane nose-up until the actual AoA is equal to the commanded value. That will take a certain time, depending on the magnitude of the change, the airspeed, the gains in the control loop and the pitch inertia of the airplane. If the airplane is decelerating or accelerating, the phugoid damping comes into play. The accident report on an A320 hard landing at Bilbao in 2001 describes it thus:

AoA protection (high angle of attack protection) ELAC, L80

(...) in these high angle of attack situations, with a tendency toward phugoid movement, studied by longitudinal dynamic stability, in which the aeroplane oscillates between two kinetic and potential energy levels, the EFCS behaves as a damper of the oscillations, commanding appropriate variations of angle of attack in a way that, when the aircraft is slowing down, makes it pitch downward and vice versa.

[CONF iture]

The DFDR trace shows the a/c consistently lower, eventually below 30 ft QFE.
I don't see any QFE altimeter reading in the DFDR traces, do you ?

A nose-up movement of the sidestick will request a change of the elevator position to pitch the airplane nose-up until the actual AoA is equal to the commanded value.
Where is that change to expect 17.5 deg of AoA ?

[HazelNuts39]

Where is that change to expect 17.5 deg of AoA ? Attenuated by phugoid damping?

[Chris Scott]

Quote from me:
"The DFDR trace shows the a/c consistently lower, eventually below 30 ft QFE."

Quote from CONF_iture:
"I don't see any QFE altimeter reading in the DFDR traces, do you ?"

Sorry, Confit, that was sloppy of me. I should have acknowledged that the "trace" I was looking at is in the report that Airbus themselves did in 1995, not the BEA report.

However, Annexe VII of the BEA Rapport Finale, "Tome 1", represents one of the lists of second-by-second flight parameters decoded from the DFDR. Inconveniently, on my PDF copy the print is reminiscent of the blurred, bottom carbon-copy of a document created on a cheap portable typewriter. (How DID we ever manage in those days?) ;)

Column 2 is a second-by-second timeline.
Columns 3 - 5 are barometric readings, which I'll now try to explain in fundamental terms for our non-aviator readers.
Column 3 seems to be the flight level, sampled every 4 secs, and rounded down. (So "+018" means FL 018.)
Column 4 is either the QNE (i.e., the pressure altitude, based on the standard sea-level pressure of 1013.2 hPa) or the altitude above sea-level, based on the current QNH of 1012. (More likely the QNE.) I am ignoring that parameter.

Column 5 is almost certainly the actual reading on the captain's altimeter, the sub-scale of which would have been set to the QFE of 984 hPa.
(For the uninitiated, that would theoretically read between zero and -15 ft for an a/c parked on the airfield with a perfect altimeter. The lower figure of -15 ft would apply if the precise pressure at the airfield had been 984.5 hPA, which ATC would have broadcast as 984 hPa.) So it is the best reading of height above the airfield obtainable by barometric means. (See my previous post.)

At time 332.0 secs, the value (as far as I can see) is 0030.
At time 333.0 secs, the value (as far as I can see) is 0024 or 0026.
At time 334.0 secs, the value (as far as I can see) is 0024 or 0026.
At time 335.0 secs, the value (as far as I can see) is 0?48 or 0248.
(The latter may be the defined point of "impact".)
At time 336.0 secs, the value (as far as I can see) is 0000, and remains so until the end of the recording at 339.0 secs.

[DozyWannabe]

Dozy, please read again ... in particular the legend which says "ɑMax Full aftstick - VɑMax". Then read the reply by Tester78 in post #27.

The FCOM says that VɑMax is another name for Vs1g.

Understood (and I did read it the first time :}), however that's not what I was getting at. The FCTMs use very equivocal language when referring to the procedure of pulling back in High AoA Protection mode - e.g. "Alpha Max *may* be achieved" and "*Approximately* correspond[ing]".

Given Airbus's notorious pedantry when it comes to language, this choice of words seems to indicate that achieving the precise alpha value shown on the graph as "ɑMax Full aftstick - VɑMax" will not necessarily be the case if suitable conditions aren't met. Therefore CONF iture's argument that AF296's failure to achieve absolute Alpha Max is tantamount to a technical problem seems to be negated by the language used in the FCTM.

[HazelNuts39]

... achieving the precise alpha value shown on the graph as "ɑMax Full aftstick - VɑMax" will not necessarily be the case if suitable conditions aren't met. What are the 'suitable conditions' that must be met in order that the airplane can support its own weight at VɑMax/Vs1g?

[AlphaZuluRomeo]

The BEA report states that, in their configuration (Flaps 3 and L/G):
alpha-prot was 14.5 deg;
alpha-max was 17.5 deg.

At t = 0 ("impact") both pitch and alpha were about +15. The latter represents the apparent discrepancy of about 2.5 deg that Conf_iture has called into question. As stated above, alpha-max can only be achieved by sustained, full back-stick.

Indeed, I think Conf_iture is comparing the "17.5° AoA = AlphaMax" from § 1.16.1.2 and the column labelled "INC" (short for Incidence = AoA) in the FDR annex (page 37 of the french original PDF report). We see on that chart that in the end of the flight (~12h45'33"), the AoA reaches and maintains 15°.
17.5 - 15 = 2.5. There we are.

The BEA does not elaborate at all but applying full back stick is what it takes to go to alpha max.

Full deflection in roll is 20 deg but 16 in pitch. The stick was fully back.

Thanks for the value (16=max deflection), was looking for it. :)

Two remarks, regarding your point:

1/ I think the last column of values (in the FDR annex) are the decimals for the AoA. If I'm correct, then the last AoA value registerd is 15.4°, not 15°. The aircraft would have been then 2.1° short of AlphaMax, not 2.5.
[edit] This remark prooved to be null and void, as pointed by Conf below. Disregard, sorry.

2/ Between timestamps 1245.328 and 1245.335, the registered AoA varies between 2 extreme values of 13° and 15°. This variation led me to believe that, either:

something prevented the aircraft to reach AlphaMax despite the crew demand (some are advocating this, but no you, unless I'm mistaken),
or calculated AlphaMax at the time was lower than theorical max AlphaMax of 17.5° (DW... why?),
or that the crew did not maintain full aft back stick. You said above that the crew did. Ahem... really?
Do we have reliable sources on that topic? In fact, yes: the very same report. See: Why AF A320 didn’t reach AlphaMax @ Habsheim (https://drive.google.com/file/d/0B8OO8d13J2aCQy1POTQ0bm5kQTg/edit?usp=sharing).


My conclusion: indeed, as Conf_iture said, the aircraft did not reach AlphaMax. Because the pilot didn't ask for it (unless, perhaps, at the very last second, i.e. too late).

Your explanation as to why is there:
Late in the sequence, the pilot, now fully aware of the approaching trees, but well aware that thrust was not coming as he expected it, delayed as long as possible the pull up request.
That doesn't sound logical if the pilot had decided to rely on the AoA protection, because as you said yourself, to get AlphaMax you have to pull the stick full aft and maintain it there.

[HazelNuts39]

AZR,

Thanks, your post together with Chris Scott's was very helpful for reading the DFDR print-out. The graph below shows some parameters for the last 5 seconds of the accident flight.

My timeline differs 1 second from Chris, because the report puts t=0 at 12:45:39 and states that the airplane entered alpha protection mode at 12:45:34 (t-5) at 122 kIAS and alpha=13 degrees.

AlphaCmd in the graph represents the sidestick position PROFON_CAP, assuming a linear relation between AlphaProt=14.5 for SS=0 and AlphaMax=17.5 for SS=-16.

http://i.imgur.com/xDSJu8l.gif

P.S.
There is something that bothers me in the DFDR print, and that is that all parameter values are shown for the same timestamp in seconds, suggesting that all values are sampled simultaneously at the same instant. As I understand the recording format of current DFDR's, the parameters are sampled sequentially, i.e. each parameter sampling time is offset a fraction of a second from the time recorded in the first word of each 1-second (sub)frame of data, depending on the position of the parameter 'word' within the 'frame'. (See for example the DFDR read-out recently released by the NTSB for the Asiana crash). IOW, the timing of a parameter recorded near the end of a frame can differ nearly one second from that of a parameter recorded early in the frame.

Is that different for this recorder?

[Chris Scott]

Hi HN39,

That graph is going to be a very useful tool, but I'm wondering if your time base might be one second adrift at the moment? I agree that the defined moment of "impact" is 12:45:39, but I'm still trying to find your quoted reference that Alpha Prot started at 12:45:34 (i.e., t -5), at an AoA of only +13.

Quote from the BEA (please excuse lack of accents):
"A t - 4 s, commutation sur la loi de pilotage en incidence, la valeur de 14.5 deg ayant ete atteinte, cette loi etant ensuite conservee."

Also, the DFDR print-out shows a corrupted reading ("D39"?) of longitudinal stick position ("commande profonde" or, in this case, alpha-cmd) at what I interpret as time 12:45;39. The two preceding values are U17 [sic] at 12:45:38 (t - 1) and U16 at 12:45:37 (t - 2)

Does the above make sense?


PS
Just noted your PS. I'm not sure, but I've been working on the assumption that the first value in a time frame is the exact second. So if there are 4 values in one second of a time frame, say 316.0 (s), the first would be at 316.0, the second at 316.25, the third at 316.5, and the fourth at 316.75. So, as you say, the values are sequential.
Likewise, with the hours and minutes, I've been assuming that the first line of "1245" refers to a time of 12:45:00. What I don't yet understand is why the associated value (column 2 on the same line) reads "296.0 (s)", but am hoping it doesn't matter!

[CONF iture]

I should have acknowledged that the "trace" I was looking at is in the report that Airbus themselves did in 1995, not the BEA report.
By any chance, would you have a link for that Airbus report ?
Inconveniently, on my PDF copy the print is reminiscent of the blurred, bottom carbon-copy of a document created on a cheap portable typewriter.
It's not your PDF copy, it is how the BEA conveniently (?) published its report ...

Attenuated by phugoid damping?
At such level it is not attenuation but plain restriction.

We see on that chart that in the end of the flight (~12h45'33"), the AoA reaches and maintains 15°
There is no flight ending around 12h45'33" ... but apparently more confusion to come with the timings ...
I think the last column of values (in the FDR annex) are the decimals for the AoA. If I'm correct, then the last AoA value registerd is 15.4°, not 15°. The aircraft would have been then 2.1° short of AlphaMax, not 2.5
I don't know what that column is, where the same sequence 1234 is reproduced, but it is not for the decimals.
Bottom line, the System had no intention to deliver those extra degrees.
That doesn't sound logical if the pilot had decided to rely on the AoA protection, because as you said yourself, to get AlphaMax you have to pull the stick full aft and maintain it there.
It is very logical for a guy of his experience to not pull too early when thrust is not felt yet ... but that's another part of the topic ...

[HazelNuts39]

Hi Chris,

About the time base: On page 10, RH column, halfway down the page:
- les manettes de poussée des moteurs ...
A 12 h 45 mn 34 s la vitesse est de 122 kt et l'incidence 13 (deg). You are correct (I was wrong) that entry into alpha-prot mode occurred at t-4, one second later (paragraph 1.16.1.2 refers).

Re your comment on my P.S.: The DFDR recorded 200 parameters, including 141 discretes (yes/no) - para.1.11.4. We don't know if the sequence on the print is the same as the recording sequence, but the first word in the recorded frame normally contains the date/time of that frame. I read the second column headed TGEN (s) e.g. 296.0 as a frame counter starting from some arbitrary datum.

[CONF iture]

In the recording of the Af A340 'level bust' event is is evident that the phugoid is indeed heavily damped.
Makes sense too at FL350, but less at SL when the engines are supposedly approaching 90% N1 and Performance is requested NOW.

The FCOM says that VɑMax is another name for Vs1g
I understand Vs1g = 1.06 Vs but I'm not sure about VɑMax = Vs1g ?
Would you have the reference ?
What I get from the FCOM is VɑCLMax = Vs1g

[Owain Glyndwr]

I had told JT that I didn't want to participate in this thread, but I must admit that the debate has been to a much higher standard than I feared it might be.

At last I see what Confiture has been getting at, although I am not convinced that the difference matters much.

So far as 'phugoid damping' is concerned, the discussion has centred on the response in the last three or four seconds. The phugoid period at 118 kts is around 27 seconds - there is no way that phugoid motion is going to affect the issue.

The point made about timing deduced from the DFDR record is well made; since any one of the parameters can be up to a second adrift relative to another and the whole debate relates to a period of four or five seconds it is, IMHO, almost pointless to attempt to deduce much from that data.

In any case, the data available does not give enough detail to assess the motion, and in particular the effect of elevator lift on the motion. It has been said that the AI laws attempt to drive the aircraft so as to maximise performance. The usual way of improving pitch response would be to overdrive the elevator to improve the pitch acceleration and then to back it off to avoid overswing.
But when you apply a load of 'up' elevator the first thing that happens is that the aircraft starts to accelerate downwards because of the negative lift. It is only after the pitch/AoA has built up enough to overcome this negative lift that the aircraft starts to accelerate upwards. The net result is that there is a slight delay after application of elevator before the CG gets above the original height. The delay isn't much - from memory it would be around 0.5 to 1 second on something like an A320, which is unimportant in normal operations but if you are only one second away from disaster it is very important. And of course the back end will lag the CG in terms of height development!

At the very end of the BEA report there is a graph of an AI flight test to repeat the Habsheim event (done at a safe height of course). That graph, with pretty much the same stick input as the actual event, shows that once alpha-prot was triggered the AOA followed the stick position over the first five seconds at the end of which time the AOA was about 16 deg. It then took another 10 seconds with the stick held fully back until the AOA got to 17.5 deg. This is entirely consistent with the AI report mentioned by Chris Scott which says:

The maximum achievable angle of attack is frequently called "Alpha Max". To maintain the AOA near this value the crew must maintain full nose up input to the sidestick which requires the pilot to sustain a high nose-up force.
It also explains why the aircraft did not develop 17.5 deg AOA during the accident sequence - it was simply not given enough time to do so before contact with the trees.

That is one reason why I am not convinced that failure to achieve 17.5 deg AoA is important - the important thing IMO was the timing. The other reason is also related to timing; increasing AoA is not going to give an increase in flight path angle unless accompanied by enough thrust. Increasing AoA to 17.5 deg would indeed produce more lift but it would also increase the drag significantly and unless thrust were increased the aircraft would either decelerate further towards stall or sink. But if he had increased thrust earlier there would not have been a problem .......

[rudderrudderrat]

Hi Owain Glyndwr,
increasing AoA is not going to give an increase in flight path angle unless accompanied by enough thrust.
I don't quite understand that statement.

If we consider the conversion of kinetic energy to potential energy from say 112 kts (achieved) to 107 kts (a guess of flying possibly 5 kts slower).
1/2 * m * Vsquared = m * g * h.
re-arranging, Delta h = {(189 ft per sec)squared - (180 ft per sec)squared} / 2 * 32.
By my maths, if the aircraft had swapped another 5 kts of airspeed for a gain in height they could have climbed 52 feet. Even then, they would still be below 100 radio.

[HazelNuts39]

I understand Vs1g = 1.06 Vs but I'm not sure about VɑMax = Vs1g ?
Would you have the reference ?
What I get from the FCOM is VɑCLMax = Vs1g

Apologies - I quoted incorrectly from memory from the A330 FCOM which actually says in 3.04.10: "The FCOM uses VS for VS1g".

However, the explanation preceding that statement makes it quite clear IMHO that VɑMax is identical to VS1g:

Because the A330 has a low-speed protection feature (alpha limit) that the flight crew cannot override, the airworthiness authorities have reconsidered the definition of stall speed for this aircraft.
All the operating speeds must be referenced to a speed that can be demonstrated by flight test. This speed is designated VS1g.Certification flight tests to determine the reference stall speed are conducted with all airplane systems functioning as designed, i.e. in normal law. VɑMax is the minimum speed that can be demonstrated by flight test in normal law for A320 and A330.

[Owain Glyndwr]

OK - I should have written sustained flight path angle.If they had zoomed to 85 feet they would then have the thrust they started with and the drag associated with the higher AOA - and as you get near to stalling AoA the drag goes up rapidly.

[vilas]

In a non precision approach MDA all authorities insist on +50ft. because when you execute a GA you are expected to loose that much and You are at Vapp at that point which is at least say 30 kts higher than alpha max. Then if you lost 100ft from alpha max is it surprising? this fly past was doomed even at 100ft because you will have no elevator left to effect flight path change. Some height loss had to be accepted.

[HazelNuts39]

So far as 'phugoid damping' is concerned, the discussion has centred on the response in the last three or four seconds. The phugoid period at 118 kts is around 27 seconds - there is no way that phugoid motion is going to affect the issue.No way? While I agree that the phugoid damping would not be sufficient to explain the apparently 'attenuated' response to the sidestick command, I'm not convinced that it could not have contributed to it.

As described in the Bilbao report " the EFCS behaves as a damper of the oscillations, commanding appropriate variations of angle of attack in a way that, when the aircraft is slowing down, makes it pitch downward and vice versa". In the last 5 seconds of the Habsheim flight, the airplane was decelerating at about 2 kt/sec. In the A340 airprox incident there was about 1 degree of alpha-prot bias between accelerating and decelerating parts of the trajectory for 2 kt/sec difference between acceleration and deceleration.

[Owain Glyndwr]

Originally posted by HN39
No way? While I agree that the phugoid damping would not be sufficient to explain the apparently 'attenuated' response to the sidestick command, I'm not convinced that it could not have contributed to it.

I'm not saying that phugoid damping won't affect long term response when under alpha-prot, only that in the first 4 or 5 seconds of a 27 sec period motion the effect will be very small and that the motion will be dominated by the short period mode.

[Chris Scott]

Quote from vilas:
"In a non precision approach MDA all authorities insist on +50ft. because when you execute a GA you are expected to lose that much..."

You are not comparing apples with apples. Your instrument-approach analogy addresses a NPA of the modern variety, in which the descent fom the FAF (final approach fix), at the final approach speed, may be predicated on a steady approach angle in excess of 3 degrees to an MDA, at which point the G/A is initiated immediately (unless the descent can be continued visually). During the G/A, the a/c must not descend below the OCH (obstacle clearance height), so the chosen MDA may vary from type to type. The fly-past at Habsheim was not planned or briefed in that fashion.

Many years ago, the concept of MDA did not exist (as such). Even on precision approaaches like ILSs, the approach was flown to a CH ("critical height") or OCH, at which the a/c normally flew level until passing the threshold of the runway (usually assessed by stopwatch) unless the descent could be continued visually. Approaching the critical height, the pilot gently flared the a/c to fly level at it (but never below it - going below the CH/OCH on instruments was a fail-point on an Instrument-Rating test). In order to have a chance of "getting in" to the runway in marginal conditions of cloud ceiling and/or visibility, a large jet would have to fly the instrument descent from the FAF at a steeper angle than the average slope from FAF to threshold; in order to have manoeuvering space to level off, see the runway, and resume the descent. (In these days of reliable ground-speed information and - since 1983 on the A310 - the FPA "bird" - the concept of leveling off at the MDA/OCH/CH is no longer necessary on big jets.)

Quote:
"...and You are at Vapp at that point which is at least say 30 kts higher than alpha max. Then if you lost 100ft from alpha max is it surprising? this fly past was doomed even at 100ft because you will have no elevator left to effect flight path change. Some height loss had to be accepted."

No, that idea needs to be put to bed immediately. The PF would have planned to initiate a flare prior to reaching 100ft QFE or 100R. He was visual, and not constrained to descend steeply prior to levelling off. His descent below 100 ft was prolonged for nearly half a minute, and was therefore either deliberate, or due to sloppy flying.

If the PF was finding the prolonged application of back-stick physically difficult, he should have gone-around earlier. (Even if that was not the problem, he should have gone-around earlier...) The briefed fly-past at 100 ft, however that height was going to be assessed/defined, was a risky manoeuvre that led to an accident. As the whole game plan was voluntary, as well as plainly and irresponsibly risky, any excuses for its shoddy execution are incompatible with the essential concepts of command and responsibility. (End of sermon!)

Now, back to the minutiae of flight mechanics...

[AlphaZuluRomeo]

I don't know what that column is, where the same sequence 1234 is reproduced, but it is not for the decimals.
:ugh: What an idiot I'm, for not having seen this repetition of sequence. Of course you're right on that. Editing my previous post now. Thanks.:ok:

[HazelNuts39]

Hi Owain,

I find it hard to believe that the EFCS waits until it has diagnosed a phugoid motion of the airplane. As I understand the description in the Bilbao report, the EFCS in alpha-protect mode applies a bias to the commanded alpha as soon as it detects a deceleration or acceleration of the airplane, which by the way is good for stall protection as well as for phugoid damping.

[Owain Glyndwr]

Hi Hazelnuts

You may be right in that, but I'd be willing to bet that the acceleration information is passed through a low pass filter before use in that way, if only to eliminate nuisance signals due to noise. This is going to slug the action I think.

[Chris Scott]

Re your interest in the AI report of 1995, I only have a PDF that someone kindly sent me. If you PM me with an e-mail address, I will send it to you.

I see that you and AZR have been using the last (data) column of Tome 1 for "INC" (AoA) data.

There are other "INCIDENCE" (AoA) data in Tome 6. On the line for each second are recorded two values, each to one place of decimals; unlike the single column you are using. It is unclear to me at the moment whether the 2 values are from different sources (e.g., FAC1 and FAC2), or if they represent 2 samples per second from a single source. I favour the former explanation, because next to them are twin values of what appear to be sideslip, which is another parameter handled by the FACs.

Using UTC 12:45:39** as the "t" reference, here are the decoded AoA values, from Tomes 1 and 6 respectively, for the last 20 seconds of flight:

t -19 +08 U08.3 U07.2
t -18 +08 U07.6 U08.2
t -17 +09 U09.3 U09.3
t -16 +09 U09.0 U08.9
t -15 +08 U08.4 U07.9
t -14 +09 U08.9 U10.6
t -13 +11 U11.3 U10.8
t -12 +12 U11.6 U11.5
t -11 +11 U10.9 U10.1
t -10 +10 U09.7 U11.4
t -09 +12 U12.1 U13.1
t -08 +14 U13.5 U13.9
t -07 +14 U13.9 U13.1
t -06 +13 U13.1 U12.7
t -05 +14 U14.4 U14.4
t -04 +15 U15.3 U14.4
t -03 +14 U14.3 U13.8
t -02 +14 U13.8 U15.6
t -01 +15 U15.1 U14.9
t -00 +15 U15.3 D73.8
t +01 -00 D00.2 D00.2

** (i.e., elapsed time 335.0 seconds in the second column)

Can anyone inform us of the precise sources of the above AoAs, and if the Tome 6 pairs of data are sequential, or from two different sources simultaneously?

HN39,
I agree that the sequential times in seconds in column 2 are elapsed times from an arbitrary point, which may be the selection of TOGA thrust at the start of the T/O run. (The report states that the a/c was airborne at "12:41", and that the R/H turn was commenced during the second which followed.) :sad:
Perhaps the BEA could supply us with pages 1 - 6 of the DFDR print-outs... ;)

Quote from Owain Glyndwr:
I must admit that the debate has been to a much higher standard than I feared it might be.

Praise indeed... :cool:

[HazelNuts39]

Hi Chris,

thanks for the good work. I support your thought that the two columns in Tome 6 probably are from two sources. The values in the first column rounded to the nearest degree correspond to the value in Tome 1.

It is now also apparent what the 1234 in the last column represent. Apparently 4 subframes form one full frame. That permits the number of parameters to be increased, by using a single location in a subframe for multiple alternating parameters, e.g. A and B each sampled every other second, or A, B, C and D once every 4 seconds.

[AlphaZuluRomeo]

Hi Owain Glyndwr,
increasing AoA is not going to give an increase in flight path angle unless accompanied by enough thrust.
I don't quite understand that statement.

Hi,

As I'm not sure many of the readers of this thread can read French, allow me to quote and (try to) translate some relevant sentences from the BEA report:

(end of § 1.16.1.4.2) :
Les autres exploitations effectuées montrent par ailleurs que si la commande de profondeur avait été amenée plus rapidement ou plus tôt en arrière lors du vol de l'accident, une incidence supérieure à 15° aurait pu être obtenue avant l'impact sur les arbres. Elles montrent également que sans une remise de gaz plus précoce, une telle manœuvre, qui aurait conduit à augmenter la trainée instantanée de l'avion, n'aurait pas permis d'éviter l'impact avec les arbres.

Other [data-analysis] operations performed also show that if the sidestick had been brought faster or earlier back during the accident flight, an AoA greater than 15° would have been obtained before the impact on the trees. They also show that without an earlier go around [understand: advancing the throttle levers earlier], such a maneuver, which led to increase the instantaneous drag of the aircraft would not have enabled to avoid impact with the trees.


Also noteworthy:

§ 1.16.1.3 (wrongly labelled 1.16.1.13 in the BEA report/PDF) "Expertises effectuées"
Le fonctionnement normal des commandes de vol n'a jamais été mis en doute par l'équipage de l'A320 accidenté. La commission d'enquête a cependant jugé nécessaire de vérifier ce point de façon approfondie.

The proper functioning of the flight controls has never been questioned by the crew of the accidented A320. The Board of Inquiry, however, found it necessary to check this thoroughly.


§ 1.16.2 "Fonctionnement des moteurs"
Contrairement au cas des commandes de vol, le fonctionnement des moteurs a été mis en cause par l'équipage immédiatement après l'accident : il a en effet déclaré qu'après la remise de gaz, la poussée des moteurs ne s'était pas réalisée.
Dès les premières exploitations des enregistreurs (C.V.R. et D.F.D.R.), il a été toutefois établi que les moteurs avaient répondu à la commande de remise de gaz effectuée entre 5 et 5,5 secondes avant l'impact sur les arbres

In contrast to the flight controls case, engine performance has been questioned by the crew immediately after the accident; the crew indeed declared that after advancing throttle levers, engine thrust had not been delivered.
However, it was established from the early readings of the recorders (CVR and DFDR), that the engines had responded to the request of go around thrust which was performed between 5 and 5.5 seconds before the impact on the trees

Hope this helps.

[Chris Scott]

Thanks AZR,

I think some of us are enjoying deliberately delaying looking at the BEA's conclusions ! ;)

The crew's perception that the two CFM 56-5-A1s were slow to spool up probably indicates more about the effects on their metabolic-rate of the urgency of the situation they suddenly recognised than it does about the performance of the engines.

Why would the two engines, controlled independently by their respective FADECs, accelerate slower than normal; but in perfect unison?

[Owain Glyndwr]

Yes, thanks AZR, that was very helpful.

I would translate it slightly more positively, but essentially those BEA remarks are consistent with what I was trying to say.

Other calculations show that if the sidestick had been brought back faster or earlier during the accident flight, an AoA greater than 15° would have been obtained before the impact on the trees. They also show that without an earlier thrust increase such a manoeuvre, which would have led to an increase in the instantaneous drag of the aircraft would not have permitted (the aircraft) to avoid impact with the trees.

[HazelNuts39]

The plane refused to deliver alpha max, it kept 2.5 deg short of it.

http://i.imgur.com/ahkxsdT.gif

[Owain Glyndwr]

The quality of the traces in the published BEA report is so poor that I imagine many people would like to see something better. This is an extract from that AI report cited by Chris Scott

http://i1081.photobucket.com/albums/j351/OwainGlyndwr/scan0247_zps2f727183.jpg

[Owain Glyndwr]

and this is a somewhat lower quality record of the Airbus flight test replication of Habsheim.
http://i1081.photobucket.com/albums/j351/OwainGlyndwr/scan0250_zps7e3c62ea.jpg

The crosses are, I think, Habsheim actuals. The AOA trace shows a similar restricted AOA gain to Habsheim over the first few seconds, but goes on to achieve Alphamax a few seconds later.

[Chris Scott]

Good evening HN39,

That's a super graph! I was going to ask you which sources you chose for the Alpha1 and Alpha2 curves, and I see that you have now averaged the two values into one curve.

However, although it doesn't affect the overall picture radically, I remain unhappy with the apparent one-second discrepancy between the time bases you and I have been using. (See also my posts of Dec18/0021z (http://www.pprune.org/tech-log/528034-habsheim-6.html#post8212886) and Dec18/2008z (http://www.pprune.org/tech-log/528034-habsheim-7.html#post8214459).)

In the text accompanying your first graph (http://www.pprune.org/tech-log/528034-habsheim-6.html#post8212705), you quoted the BEA as follows:
"...the report puts t=0 at 12:45:39 and states that the airplane entered alpha protection mode at 12:45:34 (t-5) at 122 kIAS and alpha=13 degrees."

Why would Alpha-Prot have engaged at an AoA of +13, i.e., 1.5 degrees early? As previously mentioned, the only reference I've found so far is as follows (1.16.1.2):
"A t - 4 s, commutation sur la loi de pilotage en incidence, la valeur de 14.5 deg ayant ete atteinte, cette loi etant ensuite conservee."
[At t - 4 s, switching into the angle-of-attack law, the value of 14.5 deg having been attained, this law being maintained thereafter.]

It also defines "t" as "GMT" 12:45:39; as you have stated. My interpretation of the DFDR print-out is that 12:45:39 is paired with an elapsed time in seconds of 335.0 - not the "334" which you state.

Referring to the first of the many lines annotated "1245", which must represent 12:45:00, the elapsed time is 296.0. So 12:45:30 would be 326.0, and 12:45:39 would be 335.0. That is the last line on each DFDR "Tome", page 8. (BTW, the elapsed times continue at the top of Page 9, but - curiously - the associated GMT times are erroneous.)

So, using your time base, where "t" (12:45:39) is 334.0, t -5 is 329.0, at which the AoA on Tome 1 is indeed +13 (and the Tome 6 values U13.1 and U12.7).

If I am right, however, the time-vs-AoA figures are as follows:
t -06 12:45:33 (329.0) +13 U13.1 U12.7 (Landing mode of Normal Law)
t -05 12:45:34 (330.0) +14 U14.4 U14.4 (Landing mode of Normal Law)
t -04 12:45:35 (331.0) +15 U15.3 U14.4 (Alpha Prot)
t -03 12:45:36 (332.0) +14 U14.3 U13.8 (Alpha Prot)
t -02 12:45:37 (333.0) +14 U13.8 U15.6 (Alpha Prot)
t -01 12:45:38 (334.0) +15 U15.1 U14.9 (Alpha Prot)
t -00 12:45:39 (335.0) +15 U15.3 D73.8 (Alpha Prot)
t +01 12:45:40 (336.0) -00 D00.2 D00.2

I suggest that the numbers annotated on your time axis need to be moved one space to the right. Also, the first two values of "AlphaCmd" need to be deleted, because Alpha Prot did not start until t -4.

[HazelNuts39]

Hi Chris,

In reply to your post#107 I already admitted in post #109 that I was in error for the time of entry into alpha-protection mode, which is not t-5 but is t-4. I'm on the move right now but will remove the AlphaCmd point at t-5 as soon as I have retrieved my laptop from my luggage.

At 12:45:34 airspeed is122 kIAS and Alpha is 13 degrees is recorded at 329.0s
Five seconds later at 12:45:39 t=0 so that must be at 329.0s + 5 = 334.0s

[AlphaZuluRomeo]

Hi Owain Glyndwr,
Thx for your comment (the same goes to you, Chris Scott) :)

Your translation is better than mine, I agree :ok:

[Chris Scott]

Hi HN39,

Sorry to catch you "on the hoof"! When you have time to have a closer look, our one-second discrepancy in timings is identified in your answer.

Quote from your response (my emphasis):
"At 12:45:34 airspeed is122 kIAS and Alpha is 13 degrees is recorded at 329.0s"

My interpretation is that 329.0s = 12:45:33 (see explanation offered in my previous post). But I have little experience in DFDR readings.

Quote:
"Five seconds later at 12:45:39 t=0 so that must be at 329.0s + 5 = 334.0s"

According to my interpretation, that should read "six seconds later". As you can see in my previous post, I have the AoA datum of +13 at t -6.

PS
I now understand what you meant about all the parameters in a one-second time frame not necessarily being recorded at the same instant, but possibly sequentially during the whole second. Do you or anyone else have any news on that?

[awblain]

You're right. They could have exchanged another 5kt (2.5m/s) of airspeed, from 110 to 105 (~55m/s). for height h (in m) ~ v.Delta-v/g ~ 55(2.5)/10 ~ 14m.

The key question is… did they have that 5kt of airspeed to exchange (FBW doesn't change the physics), while avoiding a stall.

There was also an interesting point about the motion of the aircraft when the elevator changes position - there is indeed an immediate tail-down pitch acceleration, about the center of mass, plus a linear downward acceleration, due to an increase in negative lift from the horizontal stabilizer... the change to the attitude then gives an upward acceleration from the larger, increasing wing lift, traded for forward speed. These changes in response to an instant movement of the elevator would extend over ~ a second.

[HazelNuts39]

Hi Chris,

My timeline is based on the information given in the report for 12:45:34 and 12:45:39. That would put frame no. 296 at 12:45:01.

While I agree that one would expect the first 1245 frame at 12:45:00, that doesn't fit with the two points mentioned in the report. Maybe BEA's GMT's are one second off, but who cares? What matters are the seconds to some reference time in the recording. The report on page 10 also puts the impact with the trees between 12:45:39 and 12:45:40. The data clearly show a discontinuity between frames 334 and 335, so that fits also. Another fit is 114 kIAS at 'ts'.

[jcjeant]

Hi,

If not yet read .. you can read this Airbus report
"The A320 Habcheim Accident
An Airbus Industrie response to allegations made in television programme and other medias"
Rapport Airbus.pdf - Petit Fichier (http://www.petit-fichier.fr/2013/12/18/rapport-airbus)

[Chris Scott]

Owain Glyndwr's translation of a BEA conclusion:
"Other calculations show that if the elevator had been brought back faster or earlier during the accident flight, an AoA greater than 15° would have been obtained before the impact on the trees. They also show that without an earlier thrust increase such a manoeuvre, which would have led to an increase in the instantaneous drag of the aircraft would not have permitted (the aircraft) to avoid impact with the trees."

I notice that OG has so far not specifically disputed rudderrudderrat's calculation that a trade of kinetic energy by zoom climb from an airspeed of 112 kt to 107 kt (assuming a steady wind) would provide an altitude gain of 52 ft. (I assume that the 112 kt was based on the IAS recorded at DFDR time-frame of 333.0 secs, which is only 1 or 2 seconds before the defined point of impact.)

awblain has also taken up the cudgels (my emphasis):
"You're right. They could have exchanged another 5kt (2.5m/s) of airspeed, from 110 to 105 (~55m/s). for height h (in m) ~ v.Delta-v/g ~ 55(2.5)/10 ~ 14m.
"The key question is… did they have that 5kt of airspeed to exchange (FBW doesn't change the physics), while avoiding a stall. "

The short answer is in the negative. The GW was about 59 tonnes, with "Flaps" (config) 3 and L/G down. For an A320-100 (no winglets), the VS1G is a CAS of 114 kt. Given that VS1G is said to be defined on the A320 as the steady airspeed at the alpha-max in this confiuration of 17.5 deg (not the CL-MAX), provided the Nz (normal acceleration) is 1G, that begs the question of why the AoA coincident with 112 kt on the Habsheim fly-past was only +14.

My suggestion is that the a/c was in a slight bunt-manoeuvre. The Nz at that stage is around 0.93G, and the pitch has been reducing. Although the IAS had fallen from 116 in the previous second, the ground-speed remained the same (112 kt). That suggests a mini-tailwind shear in that second. The following second shows an IAS of 114 at an AoA of +15 with 1.00G. However, it appears that either the a/c was overperforming slightly, or the ZFW (zero fuel weight) may have been lower than calculated on the loadsheet.

I think it might be more realistic, therefore, to postulate a zoom climb initiated at time frame 329.0 sec; 5 or 6 secs before impact. That is the point at which TOGA thrust had been commanded, at IAS 122. Therefore, the PF was already anxious to expedite his go-around from that point. If rudderrudderrat's energy calculation is good, that should provide at least as much height-gain as he calciulated for his.

As for the zoom climb per-se, I would be very interested to hear the views of Owain Glyndwr, John Farley, and others as to whether it is in any way analogous to the ski-jump concept used operationally, 6 years earlier, to increase the take-off performance of the Hawker/BAC Harrier to permit - in effect - overweight take-offs in STO mode.

I'm wondering if the gain in altitude achieved at the expense of IAS, followed by a semi-ballistic trajectory - maintaining alpha-max at an Nz below 1G - might have provided two advantages, compared with maintaining height/altitude into the treetops.
(1) The initial impact would be avoided (and the engines were rapidly spooling up).
(2) The semi-ballistic segment would delay the stall, providing even more time for the engines to add energy to the total-energy equation. In the case of the Harrier, the engine is already at its TOGA thrust (and, admittedly, with thrust-vectoring). The A320 at Habsheim was light, of course, with very great surplus performance at TOGA, and less than two seconds short of achieving TOGA thrust at the time of impact.

[Owain Glyndwr]

@Chris

I notice that OG has so far not specifically disputed rudderrudderrat's calculation that a trade of kinetic energy by zoom climb from an airspeed of 112 kt to 107 kt (assuming a steady wind) would provide an altitude gain of 52 ft.Partly because as an energy calculation it is unexceptional, partly because of proccupation with another task in conjunction with a mutual friend and partly because I didn't want to get too involved in a debate about what is actually a complex issue.
The missing parameter is, once again, time. A zoom climb being essentially a pull up manoeuvre one has to consider the 'g' available to execute such a pull up. One might expect the lift curve to be nonlinear up near alphamax, so that is one complication. Ignoring any nonlinearity but allowing for the zero lift AOA, we might expect an available 'g' of about 1.1 at 14.5 deg AOA falling to 1.00 at 17.5 deg.

That is going to mean that the latter part of any zoom climb is going to be prolonged, and in practice indistinguishable from an (unsustainable) steady climb. If a full zoom is going to take a long time then if the intent were to clear the trees by another 50ft or so the zoom would have to have been initiated well back down the runway. In fact of course any zoom is only needed to give just enough clearance to avoid impact so that only the first part of any pull up is relevant and once again timing of such a manoeuvre is critical.

For me, that issue is covered by the BEA report statement I quoted in earlier posts.

Other calculations show that if the sidestick had been brought back faster or earlier during the accident flight, an AoA greater than 15° would have been obtained before the impact on the trees. They also show that without an earlier thrust increase such a manoeuvre, which would have led to an increase in the instantaneous drag of the aircraft would not have permitted (the aircraft) to avoid impact with the trees. PS I agree with your comments on 'stall' getting in the way.
PPS I'll let John Farley make any comments on ski-jumping.

[DozyWannabe]

Given that VS1G is said to be defined on the A320 as the steady airspeed at the alpha-max in this confiuration of 17.5 deg (not the CL-MAX), provided the Nz (normal acceleration) is 1G, that begs the question of why the AoA coincident with 112 kt on the Habsheim fly-past was only +14. [/SIZE]

*Dons the Helmet Of Speculation*

If I were to make an educated guess, I'd say it was because the CAS trend indicated a steady loss of airspeed, and the logic based its pitch limit on the assumption that the trend would continue - which it did right up until t-1.

[C_Star]

Hi,

Another thing regarding the zoom climb - the calculation presented assumes that all of the kinetic energy from the speed loss would be converted into height.

I think there would be some loss due to increased drag due to higer AoA required to pull g's and to maintain lower speed afterwards. Any idea how that could affect the height gained?

[awblain]

Drag would increase. But only marginally.
They were descending and slowing gently with the existing settings, so that rate of slowing from drag would have increased, in addition to the slowing from energy exchange.

OG is right. There was probably not time in any case. Being gentle, they'd need a while to make the exchange.

Free falling a distance h=14m takes a time t = sqrt(2h/g)=1.7s. Could they have pulled up to 2g to take this time? If so, they'd surely have enjoyed an accelerated stall, which in this case wouldn't have lead to them doing a Bud Holland/Elmendorf C17, but rather an unavailability of sufficient lift to execute the commanded climb.

If they didn't realize they needed more power, it's unlikely that they'd realize they needed to exchange speed for height either.

Climbing to over tree-top height would also have reduced their ground effect… perhaps leading to a stall? Would flopping onto the trees have been less destructive than settling into them? Almost all the passengers were unshaken enough to climb out. If it had dropped from 30m, that might not have been the case.

A skijump is very different - that points you upwards using the reaction force with the ground through the wheels. Zeus, for whatever reason, forgot to grab under the A320 and push to give them such an advantage.

[rudderrudderrat]

Hi C_Star,
Any idea how that could affect the height gained?
My calculation was for 100% efficient energy swap, which is reasonably close to what an aircraft could do on the front side of the drag curve when thrust> drag with a speed reduction.

However this flight was well below min drag speed on the curve, so any speed reduction results in more drag - hence less time to impact.

The flight was doomed when they attempted Alpha Max with idle power without having either an infinitely long runway ahead of them, or sufficient height to allow for 6 seconds of engine spool up time.
Pretty basic stuff really.

[Chris Scott]

Quote from C_Star:
"I think there would be some loss due to increased drag due to higer AoA required to pull g's and to maintain lower speed afterwards. Any idea how that could affect the height gained?"

Welcome! That may be the nub of the issue. OG implies it above:
"A zoom climb being essentially a pull up manoeuvre one has to consider the 'g' available to execute such a pull up. One might expect the lift curve to be nonlinear up near alphamax, so that is one complication. Ignoring any nonlinearity but allowing for the zero lift AOA, we might expect an available 'g' of about 1.1 at 14.5 deg AOA falling to 1.00 at 17.5 deg."

That's just one of the reasons I'm uneasy about my ski-jump analogy (but thought I'd air it anyway, particularly as it's now the wintersports season). ;) In the Harrier ski-jump, the vertical acceleration results from the reaction between the vehicle and what amounts to terra-firma.
The A320 has to generate it by an increase in AoA (and therefore drag),

(I'm going to be simplistic and empirical here. The Harrier ski-jump looks like a gentle, continuous curve, rather than the level-change ramps at a multi-storey car park, but I'm going to describe the latter in the hope that the principle would remain roughly the same.)

Once the rotation of the A320 has been completed and the climb established, the extra lift associated with the increasing AoA can be used to enable a gain of altitude. Well, not much, because it has lost airspeed in the rotation... The airspeed decay-rate for a given thrust (still negligible) has increased, due to the climb angle. As r-r-rat points out, the a/c is seriously on the wrong side of its drag curve...

The Harrier is supported by the ramp throughout, and does not need any lift from its wing. That must reduce the drag considerably.

Once the two a/c have left their respective "ski-jumps" (in the case of the A320, when it reaches alpha-max), they become semi-ballistic (please pardon that expression, but I can't think of a better one). The A320 follows a quasi-parabolic (:}) trajectory at alpha-max. The Harrier presumably has to rotate to a suitable AoA, perhaps not far short of the stall, and vector its thrust upwards?

The A320 will return rapidly towards its start-altitude, and the resulting VS will have to be arrested before the treetops are reached, The Harrier, on carrier ops, has the luxury of having the extra height of the carrier deck to play with before it hits the water...

[CONF iture]

Other calculations show that if the elevator had been brought back faster or earlier during the accident flight, an AoA greater than 15° would have been obtained before the impact on the trees.
Totally misleading translation here, commande de profondeur stands for sidestick not elevator.
Question is why the elevators did the opposite of the sidestick displacement ?
That is the question that all of you should try to find an answer for ... but don't worry both BEA + Airbus have taken great care to avoid the question in the first place ...

[Owain Glyndwr]

Totally misleading???

Only in your mind I think

Ok, I have changed it in both posts.

[awblain]

The skijump ramp isn't a flat plane with a sharp change of gradient at the bottom, as that would probably smash the nose wheel as it hit it.

The role of the ramp is simply to rotate the aircraft quickly to the best compromise of attitude and flight path to climb away from the ship. Both are set by the inclination at the top - the profile is just chosen to minimize the stress on the landing gear. The forward speed of the ship then determines the angle of attack off the top of ramp.

This has no relationship with the pre-tree 5 seconds at Habsheim.

[Saint-Ex]

The dreadful Box Film programme on the accident found an accident investigator who also asked why the up elevator command resulted in a pitch down movement. They also asked the opinion of a PPL expert, also ignorant of the 320`s fby philosophy. I did the translation of the initial report and it appears the elevator acted exactly as it was programmed to. To their great credit, BA had the aircraft back in service shortly afterwards and they had a highly respected and very critical flight safety department.

[rogerg]

BA had the aircraft back in service shortly
BA?

[HazelNuts39]

From the NTSB investigation of the A320 ditchng in the Hudson river on 15 January 2009:

NTSB Accident Number DCA09MAS026, Docket Item 86
Aircraft Performance 13 - Factual Report of Group Chairman
url=http://dms.ntsb.gov/pubdms/search/document.cfm?docID=322563&docketID=47230&mkey

Reference 8 documents an Airbus simulation of the last 300 ft of the flight, and indicates that the airplane was performing as designed as was in α-protection mode from 150 ft to touchdown. Per Reference 6 (quoted above), in α-protection mode, “the angle of attack is proportional to side stick deflection. That is, in the αprot range, from αprot to αmax the side stick commands α directly” while keeping α < αmax. However, in α-protection mode, the flight control system incorporates a phugoid-damping feedback term in addition to side stick commands when computing the commanded elevator position (which in turn determines the pitch angle response). As described by Airbus,

… the aircraft was in angle-of attack (AoA) protection from about 150 ft RA.
When in AoA protection law, stick command is AoA objective. Stick neutral commands alpha-prot and
full back stick commands alpha-max.
However, AoA protection shall take care of the A/C trajectory and, thus, looks after phugoid damping
as well as AoA control: there are feedbacks within the AoA protection law aiming at damping the
phugoid mode (low frequency mode). The feedbacks are CAS and pitch attitude variations. Without
these feedbacks, an aircraft upset from its stabilized flight point up to constant high AoA would enter
a phugoid (which is, by definition, a constant AoA oscillation) without possibility to stabilize the
trajectory. As a consequence, commanded AoA is modulated as a function of speed and attitude
variations: for instance, if A/C speed is decreasing and/or pitch attitude is increasing, pilot's
commanded AoA is lowered in order to avoid such a situation to degrade.
On the last 10 sec of the "Hudson" event, it is confirmed that pitch attitude is increasing and CAS
decreasing. Then, the phugoid damping terms are non nul and are acting in the sense to decrease
the finally commanded AoA vs. the stick command, in order to prevent the aircraft from increasing the
phugoid features.

Based on this explanation, it appears that on the accident flight, the nose-up side stick commands from 15:30:36 to 15:30:43 were offset somewhat by the phugoid-damping feedback term, thereby limiting the pitch angle and α increase below 150 ft radio altitude.


E. Conclusions
(…) A phugoid damping feedback term in the flight control laws, that is active in α−protection mode, attenuated the airplane’s nose-up pitch response to progressively larger aft side stick inputs made below 100 ft radio altitude.

[DozyWannabe]

Question is why the elevators did the opposite of the sidestick displacement ?
That is the question that all of you should try to find an answer for ... but don't worry both BEA + Airbus have taken great care to avoid the question in the first place ...

Now who's being misleading? The answer to that question is summarised right there in the report - the behaviour of the aircraft was consistent with High AoA Protection mode. I suspect that the problem is not that you don't know the answer so much as you don't *like* the answer.

I must admit that despite initial reservations, creating this thread has given me an opportunity to understand the situation in much more detail, and everything new I've seen seems to back up the BEA's summary.

Correct me if I'm wrong, but you seem to be operating on the assumption that pulling up when in High AoA Protection will command the flight control systems to achieve and hold Alpha Max (as it is displayed on the graph) almost immediately based on a snapshot of the aircraft's orientation and configuration at that precise point. If so, this assumption appears to be in error.

For starters, the FCTM material earlier in the thread seems to flatly contradict this assumption - the wording states either that Alpha Max "may" be achieved, or doesn't refer to Alpha Max at all ("a maximum AoA" isn't the same thing). What they state is that the system will maintain a setting which will provide maximum lift based on the current status of the aircraft.

As OG correctly points out, in order for a digital system to work reliably in real-time there needs to be a degree of "filtering" of the data. I'd be very surprised if the systems did not, in addition to this filtering, observe trends over time (as they would have to when, for example, checking that the A/THR disconnect switches were held down for a certain period of time).

Presuming that the FCTMs are correct, what pulling back on the sidestick will do when in High AoA Protection mode is command the systems to configure the flight surfaces to provide maximum lift based on the data over a certain period of time. Airbus spent the best part of a decade, with their experimental FBW A300 and the first A320s off the line, collecting and refining the data used to define the aircraft's behaviour.

As I was saying when I first donned my Speculation Helmet, the systems would have been fed with data indicating that the aircraft was slowing down. Because the laws of physics demand that there must be a delay between the movement of a flight surface and the aircraft responding to that movement, as well as other uncontrollable external factors such as wind speed and direction, it follows that the pitch attitude commanded in a situation where the aircraft is slowing down will need to be somewhat shy of what it is at that precise moment - otherwise it risks encroaching on approach to stall, which would defeat the whole purpose of the protection.

Therefore, as pulling back in that mode cannot be assumed to give Alpha Max (or, at least, not immediately), what that sidestick command actually "tells" the flight control system is "give me the best AoA you can". So, going back to the beginning, the elevators were briefly commanded nose-down because the systems were trying to maintain the optimum (not necessarily maximum) AoA, and that was what was required to maintain it. And if I were to hazard a guess, given the period in which the elevator position was noted, the reason it was required would have been to counter the pitch-up moment from the engines as they spooled up and started producing significant thrust.

[roulishollandais]

Because the laws of physics demand that there must be a delay between the movement of a flight surface and the aircraft responding to that movement, as well as other uncontrollable external factors such as wind speed and direction, it follows that the pitch attitude commanded in a situation where the aircraft is slowing down will need to be somewhat shy of what it is at that precise moment - otherwise it risks encroaching on approach to stall, which would defeat the whole purpose of the protection.

Therefore, as pulling back in that mode cannot be assumed to give Alpha Max, what that sidestick command actually "tells" the flight control system is "give me the best AoA you can". So, going back to the beginning, the elevators were briefly commanded nose-down because the systems were trying to maintain the optimum (not necessarily maximum) AoA, and that was what was required to maintain it.Laws of physic don't request much time : As freefaller we are used to modify the position, to feel on the body the aerodynamic forces and moments created, and resume the movement in less than 1/20 second. Not the tens of seconds of the phugoïd low period. An aerobatic's plane needs a longer time than /20 second but is reacting very fast. What needs time on your plane is the result of cheap and low sampling (perhaps to avoid high frequency transient response?) or a bad model and algorithm specialy near the stall.

And what is optimized? Cost as usual?

It is often question of "time needed" but no figures are given. The expert Max Venet said during the Habsheim trial - answering to the President of the Court - "we don't know very well how long the aircraft needed to be less than 30 feet RA over the trees (before the runway threeshold, not after the end of the runway..) to get in flare - I listened and
understood that - what is sure is that he did not find in the hard- and software documentation and modification history the answer to that question.

I wonder too to see now the ref of 150 FT RA in the landing algorithm -Hudson- (and phugoid damping) which is much more than the most oftened read 100 FT RA or 50 or 30 FT : How could the crew and the Court know really how the system works?

[rudderrudderrat]

Hi roulishollandais,
How could the crew and the Court know really how the system works?They didn't know exactly how it works, because FCOM is not that detailed. However, the system did exactly as it says on the tin - by preventing a stall.
I don't know how close to the stall you, as a passenger on that flight, would want to be flown at below 50' radio, but please tell us - how much closer to the stall would you still have been happy?

[HazelNuts39]

For starters, the FCTM material earlier in the thread seems to flatly contradict this assumption - the wording states either that Alpha Max "may" be achieved, or doesn't refer to Alpha Max at all ("a maximum AoA" isn't the same thing).

Perhaps we are are dealing with semantics again, but I think Airbus offers a more accurate description in the above reference:

Stick neutral commands alpha-prot and full back stick commands alpha-max.
However,(...) there are feedbacks within the AoA protection law aiming at damping the phugoid mode (low frequency mode). The feedbacks are CAS and pitch attitude variations. (...) if A/C speed is decreasing and/or pitch attitude is increasing, pilot's commanded AoA is lowered (...).

[roulishollandais]

I don't know how close to the stall you, as a passenger on that flight, would want to be flown at below 50' radio, but please tell us - how much closer to the stall would you still have been happy?If it is only protection against stall 150 FT RA is already a little low:mad:

[Owain Glyndwr]

Hi Hazlenuts


However,(...) there are feedbacks within the AoA protection law aiming at damping the phugoid mode (low frequency mode). The feedbacks are CAS and pitch attitude variations. (...) if A/C speed is decreasing and/or pitch attitude is increasing, pilot's commanded AoA is lowered (...).

OK, I can go along with that. I was bitching that the actual phugoid mode would not have much effect on the short term motion, but I can see that the phase advance damping terms in the control laws might well do so.

[Chris Scott]

roulishollandais

Correct me if I'm wrong, but I think you may be misinterpreting two sentences of the NTSB report into the Hudson Bay ditching:
"… the aircraft was in angle-of attack (AoA) protection from about 150 ft RA."
AND
"...Based on this explanation, it appears that on the accident flight, the nose-up side stick commands from 15:30:36 to 15:30:43 were offset somewhat by the phugoid-damping feedback term, thereby limiting the pitch angle and α increase below 150 ft radio altitude."

So, on the A320's ditching approach, Alpha-Prot Law was activated at 150R, because the AoA happened to reach alpha-prot at that height. It remained engaged thereafter because it takes priority over Normal Law, including the landing mode (which normally commences at 50R, and which at 30R starts to require an increasing amount of back-stick if the pilot wants the pitch-attitude to be maintained).

You state in your post, above:
I wonder too to see now the ref of 150 FT RA in the landing algorithm -Hudson- (and phugoid damping) which is much more than the most oftened read 100 FT RA or 50 or 30 FT : How could the crew and the Court know really how the system works?

So, on the Hudson approach, 150R was an arbitrary height at which Alpha-Prot Law had to take over from C* (Normal Law). In the extract quoted by HN39, the report does not state that the feedbacks used in Alpha-Prot Law - aimed at damping the pitch-phugoid tendency - change at any height. As for 100R, that is simply the height below which Alpha-Floor is inhibited.

However, it may be worth reminding ourselves that the Hudson accident was 20 years after Habsheim.

HN39,

FWIW, I don't regard any of your discussion about phugoid damping as semantic!

[Chris Scott]

Quotes from Saint-Ex:

"To their great credit, BA had the aircraft back in service shortly afterwards..."

Just in case that causes any reader to infer that the Habsheim accident a/c was operated by, or owned by, BA: it was neither.

I think we did ground our A320s for about 24 hours. The AF accident took place on a Sunday afternoon...

"...they [BA] had a highly respected and very critical flight safety department."

Yes, although I don't think there were any A320-qualified people specifically on it at that stage. We had only been operating our a/c down at Gatwick for a couple of months, and all of us aircrew and our fleet management were ex-BCAL.

"I did the translation of the initial report..."

Which report was that?

[DozyWannabe]

Is he talking about the A320 that almost landed on Crawley High St?

[EDIT : As discussion later in the thread details, this comment came as a result of reading the Andrew Weir book "The Tombstone Imperative", the first edition of which mentioned this incident in the book, and prominently used the phrase "almost landed on Crawley High Street" on the back cover. In order to verify this, I tracked down a later edition of the book, in which both references were deleted. If this error has caused any upset, I apologise.]

[Saint-Ex]

Hi Chris. I was on an A320 fllght the night before the Air France crash. I didn`t think it necessary to mention the aircraft was not one of BA`s as it was so widely reported.
As far as Flight Safety is concerned you may remember the first BA pilot to be allocated to the Airbus actually headed the Flight Safety department.
I was given the first French report very soon after the crash and it comprised the full CVR recording (unbelieveable) and, if I remember correctly, an analysis of the removeable FDR.

[CONF iture]

For an A320-100 (no winglets), the VS1G is a CAS of 114 kt. Given that VS1G is said to be defined on the A320 as the steady airspeed at the alpha-max in this confiuration of 17.5 deg (not the CL-MAX), provided the Nz (normal acceleration) is 1G, that begs the question of why the AoA coincident with 112 kt on the Habsheim fly-past was only +14.
Maybe something does not add up here ...
The comment made by HN39 has its merit but if Vs1g is stalling speed at 1g how V alpha max would be Vs1g ?


I don't know how close to the stall you, as a passenger on that flight, would want to be flown at below 50' radio, but please tell us - how much closer to the stall would you still have been happy?
Very unhappy to be intentionally flown that way, but considering how badly the situation was engaged ... very happy if the system commanded alpha max as requested by the pilot to give a chance to avoid the crash.

[CONF iture]

However, in α-protection mode, the flight control system incorporates a phugoid-damping feedback term in addition to side stick commands when computing the commanded elevator position.
It was the duty of the BEA, 20 years earlier, to detail such characteristic.
NTSB has done what BEA failed to do.

Reference 8 documents an Airbus simulation of the last 300 ft of the flight, and indicates that the airplane was performing as designed
When the design prevents Sully to obtain a better touchdown, and Asseline, as wrong as he was in the first place, to avoid the trees, the design must be firstly detailed, and secondly can be questioned.

[rudderrudderrat]

Hi CONF iture,
the design must be firstly detailed, and secondly can be questioned.
It is and has been since 2001 after several heavy landing incidents. See:
Accident of an Iberian Airbus A320 in Bilbao (http://www.iasa.com.au/folders/Safety_Issues/others/Bilbao.html)

You may be protected against stalling, but flying too slowly limits your pitch authority beyond what you might expect in a conventional aircraft.

Edit. I've just noticed your previous comment #160 very happy if the system commanded alpha max as requested by the pilot to give a chance to avoid the crash.
I agree it may have helped in that particular case (deliberately flown), but the design is to help avoid stalls during windshear GAs etc. Would you prefer the extra couple of degrees of AoA with an increased probability of overshoot beyond stall + then possible wing drop, or the present design?

[roulishollandais]

@Confiture : Thanks to complete my answer to Rudderruddererrat mentioning alpha max.

It was the duty of the BEA, 20 years earlier, to detail such characteristic. NTSB has done what BEA failed to do.Agreed. The Court of Colmar was not able to discover that by themselves, too new technic, unaware "experts" and counter-"experts" of these new technic. Perhaps it could be a legal grief to ask revision of the trial, with that new information.:*

[Chris Scott]

Quotes from Saint-Ex

"Hi Chris. I was on an A320 fllght the night before the Air France crash. I didn`t think it necessary to mention the aircraft was not one of BA`s as it was so widely reported."
Yes, but many of our readers were too young to be taking an interest in such matters 25 years ago. You're not giving much away, but I presume your flight was as a passenger on a BA a/c? If so, I'd like to apologise for any delay ;) - we were having a lot of teething probs, both with the a/c and with the systems at the newly-opened Gatwick North Terminal. For example, we were the first short-haul fleet in BA (or, for that matter, the first of any airline operating at Gatwick and elsewhere) to use containerised baggage. I don't know if AF were doing it, although it seems probable, and wonder about Air Inter.

"As far as Flight Safety is concerned you may remember the first BA pilot to be allocated to the Airbus actually headed the Flight Safety department."
You're nearly "right" (pun intended). The first proper BA pilot checked out on the A320 (AYK, the rest of us were all ex-BCAL) was ColinW, who joined us as an Assistant Flight-Manager in the summer. he then masterminded the transfer of the fleet to Heathrow in the October/November, and did a superb job of promoting the A320 in BA. Much later, when BA rationalisation abolished assistant managers, he joined Flight Safety, and eventually headed it.
In 1988, head of Flight Safety was JimP, who wasn't checked-out on the A320 until a year or two later.

[Chris Scott]

Quote from Dozy Wannabe:
"Is he talking about the A320 that almost landed on Crawley High St?"

Perhaps you could supply some details?

[AlphaZuluRomeo]

It was the duty of the BEA, 20 years earlier, to detail such characteristic.
NTSB has done what BEA failed to do.

When the design prevents Sully to obtain a better touchdown, and Asseline, as wrong as he was in the first place, to avoid the trees, the design must be firstly detailed, and secondly can be questioned.
I agree that it would have been nice to have (earlier) a more thorough description of the system, for our armchair analysis.
But...
Is it prooved that a higher AoA would have permitted to clear the trees? After reading the current thread, I'm under the impression it's not sure, far from it... Plus as awblain said:
If they didn't realize they needed more power, it's unlikely that they'd realize they needed to exchange speed for height either.
With this in mind, why do you point a finger at the BEA? I'll quote its website: The safety investigation, whose sole objective is to prevent future accidents and incidents, includes the gathering and analysis of information, the drawing of conclusions, including the determination of cause(s) and/or contributing factors and, when appropriate, the making of safety recommendations (European Regulation 996/2010 article 2 part 14).
In conclusion, I share your regret but do not feel entitled to accuse the BEA.

[CONF iture]

You may be protected against stalling, but flying too slowly limits your pitch authority beyond what you might expect in a conventional aircraft.
But for the conventional aircraft, without touching the trim, pitch authority limit is naturally known as the full command deflection has necessarily implied full elevator deflection.
On the contrary, what happens here is a total disconnection between the elevator movement and the sidestick displacement even if beyond alpha prot the sidestick should directly command the alpha ... That leaves the pilot out of the loop ... how really are deflected my elevators ?

I agree it may have helped in that particular case (deliberately flown), but the design is to help avoid stalls during windshear GAs etc. Would you prefer the extra couple of degrees of AoA with an increased probability of overshoot beyond stall + then possible wing drop, or the present design?
Help avoiding stall but without compromising the performance. Windshear at 50 feet on take off is all about avoiding ground contact where full back stick may be necessary. For such exercise I have seen the speed well into the red for an obvious temporary overshoot of alpha max ... Which brings another question : Is real thrust output also part of the feedbacks within the AoA protection law aiming at damping the phugoid mode ?

[Saint-Ex]

Hi Chris, I thought Colin was with us on one of the first Toulouse courses, but you are right JP was head of Flight Safety when the Airbus was introduced into BA.

[AlphaZuluRomeo]

On the contrary, what happens here is a total disconnection between the elevator movement and the sidestick displacement even if beyond alpha prot the sidestick should directly command the alpha ... That leaves the pilot out of the loop ... how really are deflected my elevators ?
This disconnection is at the core of the FBW / C* law, so it's not some specific degraded thing. It sure is different from a conventional aircraft. It has ups and downs. There is a nice F/CTL page that tells you how really are deflected your elevators if you need it. But it's obviously not meant to be used in a rush, as it's not as "easy" or immediate to interpret as is the stick placement in conventional aircraft.
If the only way to keep the pilot in the loop is, in your eyes, strictly proportionnal stick and surfaces displacements, then I understand you don't like FBW aircraft. I suppose you weighted pro and cons?

[rudderrudderrat]

Hi AlphaZuluRomeo,
If the only way to keep the pilot in the loop is, in your eyes, strictly proportional stick and surfaces displacements, then I understand you don't like FBW aircraft.
I think the B777 FBW system would satisfy CONF iture. Page 3. Design Philosophy. http://www.davi.ws/avionics/TheAvionicsHandbook_Cap_11.pdf

"What should be noted, however, is that none of these features limit the action of the pilot. The 777 design utilizes envelope protection in all of its functionality rather than envelope limiting. Envelope protection deters pilot inputs from exceeding certain predefined limits but does not prohibit it. Envelope limiting prevents the pilot from commanding the airplane beyond set limits. For example, the 777 bank angle protection feature will significantly increase the wheel force a pilot encounters when attempting to roll the airplane past a predefined bank angle. This acts as a prompt to the pilot that the airplane is approaching the bank angle limit. However, if deemed necessary, the pilot may override this protection by exerting a greater force on the wheel than is being exerted by the backdrive actuator. The intent is to inform the pilot that the command being given would put the airplane outside of its normal operating envelope, but the ability to do so is not precluded. This concept is central to the design philosophy of the 777 Primary Flight Control System."

I guess the same is true in pitch control.

[DozyWannabe]

In conclusion, I share your regret but do not feel entitled to accuse the BEA.

Agreed. The BEA stated - correctly - that the behaviour was consistent with High AoA Protection mode. Given that the behaviour had little - if anything - to do with the factors leading to the accident, it is unsurprising that the report does not go into more detail.

Aside from the fact that CONF's assertion that, in the Hudson incident, the protections "prevented a perfect touchdown" is a fiction to which he appears to be the only subscriber - the NTSB were freer to look into aircraft behaviour in their report as there were far fewer preceding factors (barring the birdstrike itself) to write up. In true half-informed fashion, some of the press wrote up the behaviour as "assisting" the landing on water, but since when have any of you ever taken the press seriously? An attitude which is entirely understandable given the frequent mistakes and mischaracterisations they make.

RRR - we don't know exactly what the B777 systems would have done in a similar scenario, but based on the understanding I have, the servos driving the yokes would have significantly increased resistance to the pilot's inputs the closer the aircraft came to maximum permissible alpha. This in turn would have increased the physical effort required to pull up. The behaviour may be more akin to a conventional setup in some respects, but the systems would still have in effect been "fighting" the pilot's inputs.

"Crawley High St." refers to an incident related in a dead-tree book I owned, the first edition of 'The Tombstone Imperative' by Andrew Weir. It apparently happened to a G- registered A320 in July 1988 and was tangentially related to the Bangalore and Strasbourg accidents (i.e. autoflight-related). Given the lack of online references I can find I can only think it must have come from a CHIRP or similar reporting regime.

[CONF iture]

Is it prooved that a higher AoA would have permitted to clear the trees? After reading the current thread, I'm under the impression it's not sure, far from it...
To prove anything is not the point, but cooperative elevators was obviously a necessary element to expect the better.

With this in mind, why do you point a finger at the BEA? I'll quote its website: ...
NTSB has probably a similar mission ... still, they did what the BEA failed to do.

[AlphaZuluRomeo]

Hi rudderrudderrat,
I think the B777 FBW system would satisfy CONF iture.
Thanks, I'm aware of the different philosphies when it come to protections/limits. I know that a number of pilots feel like they would rather have the ultimate control, and hence don't like the A because it's like "the damn thing is better than I'm" (sometimes).
But that's not the point, here: in an A320 as in a 777, command (stick/yoke) and elevator are disconnected, and their relative displacement not (always) proportionnal. That's any FBW with anything else than Direct Law. E.g. C* law.

Hi CONF iture,
To prove anything is not the point, but cooperative elevators was obviously a necessary element to expect the better.
So now, the elevators were un-cooperative? Feels like the great return of the killer plane (and/or computer).
Now we're at rudderrudderrat'point. I dare say I fail to see how the aircraft should have saved the day, while not (hard) imposing hard limits to the crew...
With "cooperative" elevators as you put it, and stick full back (human reaction just before impacting the trees), stall is on the way = worse end-result.

NTSB has probably a similar mission ... still, they did what the BEA failed to do.
How many years later?
Once again, I share the regret. The NTSB did? Yes. We're happy it did? Yes. We agree on that. I just feel it's not fair to write that the BEA "should have". :)

[CONF iture]

With "cooperative" elevators as you put it, and stick full back (human reaction just before impacting the trees), stall is on the way = worse end-result.
Why should it stall when alpha max is still 2.5 deg away ... ?

[Chris Scott]

Quote from Dozy Wannabe:
" 'Crawley High St.' refers to an incident related in a dead-tree book I owned, the first edition of 'The Tombstone Imperative' by Andrew Weir. It apparently happened to a G- registered A320 in July 1988 and was tangentially related to the Bangalore and Strasbourg accidents (i.e. autoflight-related). Given the lack of online references I can find I can only think it must have come from a CHIRP or similar reporting regime."

In those days NDB approaches were still available at LGW, using the Locator-NDBs west and east of the airfield (on the extended centrelines of Rwys 08R/26L respectively, about 4nm from their thresholds). With TRK-FPA available on the A320 FD (and with a better PFD display than we had enjoyed on the FPA-equipped A310), we were using it - sometimes with the AP engaged - to provide a form of track and glide-slope guidance (the latter as an alternative to the cumbersome step-down technique, which was still standard in most airlines).

IIRC, there was a known incident in which the crew mis-selected a VS (vertical speed) instead of an intended FPA (flight-path angle). With the original ergonomics of the A320 AFS, the format of the shared VS/FPA display of the resulting selected VS on the FCU would not necessarily have alerted a crew to the mistake. The latter would soon become evident, however, by an excessive pitch-down, and rising VS.

Until corrected, such a mis-selection could quickly take the a/c below the normal approach profile. It would not, however, affect the tracking in azimuth. Crawley High Street is a couple of miles due south of Gatwick - nowhere near the approach paths for either runway.

If such an incident was considered hazardous, an MOR would have been submitted by the crew and/or ATC (hopefully both), which should remain on record. If not, but one or both pilots felt it needed to be made known, an anonymous 'Chirp' report might have been filed for the information of other UK-licensed crews. Unless and until we can find one or the other, your scenario remains mysterious.

[DozyWannabe]

Why should it stall when alpha max is still 2.5 deg away ... ?

Well, for one thing because the engines will be trying to pitch the plane nose-up as they begin to provide power. Without the flight control system counteracting this tendency, even neutral elevator could allow the aircraft to overshoot 17.5 degrees in very short order.

What the computers do is not especially clever, the advantage they have is that they can detect and counteract unwanted deltas (changes) in attitude much faster than a human. Again, nothing in the documentation states that full back-stick will command alpha max instantaneously - indeed experience shows that it can take up to around 20-30 seconds to stabilise enough to reach 17.5 degrees. Thus the BEA did not "fail" to do anything - only if Airbus's documentation claimed instant alpha max would the aircraft's behaviour have needed more thorough examination.

@Chris Scott - thanks for the input. I suspect "Crawley High St." was a bit of journalistic licence on the part of the author. I mentioned it only because Saint-Ex mentioned BA, and to the best of my knowledge that was the only BA A320 to have any difficulty around that time.

@rudderrudderrat - the other thing important to remember about the High Alpha modes is that they are designed to allow for a limited amount of roll authority to be retained even at alpha max. What the flight control systems are doing is assimilating all the external forces acting on the aircraft along with the control inputs and responding to control demands while maintaining a stable flight regime. In this case you have the increase in thrust from the podded engines causing an increase in pitch attitude, as well as a somewhat belated full back-stick demand. The Bilbao case you link seems to highlight what us engineers call an unknown "edge case" (i.e. a scenario that was unaccounted for in the original design, and sometimes results in non-optimal behaviour). However it also demonstrates that if Airbus are presented with evidence of such an occurrence, they can and will turn around a fix and will announce it publicly even before the final report is written.

EDIT : Looking at the report on the Iberia Bilbao incident:

http://www.smartcockpit.com/download.php?path=docs/&file=A320-Gear_Collapsed_After_Hard_Landing.pdf

It seems that the problematic aggressive phugoid damping was initiated by a tailwind gradient which manifested itself as an excessive reduction in airspeed (a delta of around -10kts over one second). The EFCS logic incorrectly considered this to be a result of phugoid motion and applied nose-down elevator to arrest the pitch-up demand.

As there was no such tailwind gradient at Habsheim, and the airspeed deltas were much less extreme, I reckon it unlikely that phugoid damping applied here.

[jcjeant]

The behaviour may be more akin to a conventional setup in some respects, but the systems would still have in effect been "fighting" the pilot's inputs.I do not like the word "fighting"
In a conventional system (direct control cable and without or with counterweight) there will have the same effect that is to say an application force to the controls which will increase with deflection requested ... it does not comes to fighting .. but simply to the laws of physics and that is exactly what is reproduced in the 777 system
When you pilot a Tiger Moth .. the system is no fighting your inputs .. it's simply physical laws in action

[DozyWannabe]

In normal situations yes, but the forces applied to counter the inputs when in an envelope protection mode on the B777 will be noticeably greater than they would be if it were simply emulating aerodynamic resistance. In that mode it's not trying to mimic old-fashioned controls, it's trying to tell the pilot they are in danger of departing the flight envelope by significantly increasing flight control resistance. It's a modern, digitally-controlled descendant of the old "stick-push" systems, but it acts pre-emptively. A Tiger Moth won't do that.

[roulishollandais]

Again, nothing in the documentation states that full back-stick will command alphamax instantaneously - indeed experience shows that it can take up to around 20-30 seconds to stabilise enough to reach 17.5degrees. Thus the BEA did not "fail" to do anything - only if Airbus's documentation claimed instant alpha max would the aircraft'sbehaviourhaveneededmore thorough examination.That is concierge's work in cow-boy land building unmanned toys.

As a computer guy who had responsibility of methods choices in our research center, I would NEVER have accepted to register such "systems"(?) without the complete boolean tables showing how passing from EVERY mode to another is managed, and due description of ALL the transient regimes ASWELL theoric and from experience.

I wonder why the BEA and other safety agencies did not see or/and point that lack of computer science method.

Many lifes are deliberatly sacrified by unresponsible engineers (probably able and taught to perform much more, that fact does the things still worse) playing only stats of opportunity to be paid by criminal blind airlines and unaware persons on board and their family.

[misd-agin]

CONF iture - "When the design prevents Sully to obtain a better touchdown,"




http://www.ntsb.gov/doclib/reports/2010/aar1003.pdf


pdf pages 65, 73, 106, 107 of 216. Lack of airspeed, and not the airplane, was the primary problem.

[CONF iture]

Well, for one thing because the engines will be trying to pitch the plane nose-up as they begin to provide power.
Perfect then, let them bring the AoA to 17.5 deg.
Thrust + alpha max = PERF ... as advertised by Airbus themselves, just what we look for after all.

[CONF iture]

pdf pages 65, 73, 106, 107 of 216. Lack of airspeed, and not the airplane, was the primary problem.
We can start all over here ... but a good deal of it has already be done there even if the title does not necessarily reflect it :
http://www.pprune.org/rumours-news/508424-american-airlines-flight-742-flight-control-system-problems.html

[AlphaZuluRomeo]

Hi,

So, does anyone has an answer to CONF iture? I don't.

The question, as I understand it, is: why the A320 FCS didn't immediately reach for AlphaMax (17.5° AoA) in the specific AoA protection mode?

My guess would be that the software keeps a margin to avoid being in a position where the aircraft will lack down-pitch authority if needed (to avoid busting the strict 17.5° limit, that would lead to risking stall).

If correct, is the apparent 2.5° margin (in Habsheim's scenario) consistent? Isn't it too much?

Phenomenons that could explain the margin (non-exhaustive list, feel free to complete):
- Phugoïd and/or any sort of longitudinal aerodynamics-led unstability?
- Diminishing IAS?
- Provision for engine-led pitch up (throttling up)?
- ... ?

[CONF iture]

The question, as I understand it, is: why the A320 FCS didn't immediately reach for AlphaMax (17.5° AoA) in the specific AoA protection mode?
My questions would be more like :
1-Why the FCS limited the AoA to alpha prot when the request was for alpha max ?
Or why the elevators did the opposite of the sidestick displacement ?
2-Why BEA + Airbus sat on the question ?

[roulishollandais]

HabscheimIs it any reason to use that nickname ? It may be surprising, but the true name of that alsacian french airfield and village is Habsheim and must be said habs - heim ;).

[DozyWannabe]

As a computer guy who had responsibility of methods choices in our research center, I would NEVER have accepted to register such "systems"(?) without the complete boolean tables showing how passing from EVERY mode to another is managed, and due description of ALL the transient regimes ASWELL theoric and from experience.

I wonder why the BEA and other safety agencies did not see or/and point that lack of computer science method.

Fair warning - now you're very much on my territory, and I can tell you categorically that what you have posted above is absolute nonsense.

Computer Science methodologies were used for every aspect of the initial specification, but I can assure you that the engineers went even further - because pure CS processes as they stood at the end of the 1970s would not be sufficient to provide enough assurance in a system requiring this level of safety and reliability. It was therefore easily the biggest and most thorough application of Software Engineering and Reliability principles in Europe at the time, if not the world. And where existing practices weren't quite up to scratch, they extended those practices and invented new ones. Multiple implementations, regression testing, test-driven development and automatic code-generation along with manual review processes were all used, and in some cases pioneered, with this programme.

Have a look at my late Prof's visit (made in 1993):

Report on visit to Airbus Industrie - 28-29th Jan. 1993 (http://www.kls2.com/cgi-bin/arcfetch?db=sci.aeronautics.airliners&id=%[email protected]%3E)

Extract:

The EFCS life cycle involves requirements capture resulting in an equipment specification, including hardware, software, and functional specifications. The pilot is very definitely "in the loop'' for requirements capture, which is an iterative process using rapid prototyping and flight tests. Emphasis is placed on validation of functional requirements, which is clearly distinguished from verification.

Test pilots and pilot engineers were involved in every step of the development and testing process, and no software spec was signed off without their OK - so your claim of "unresponsible [sic] engineers" working in an ivory tower is not just incorrect, but bordering on libellous.



I think that then, as now, there is a significant gap in understanding the significant differences in approach to safety-critical real-time software compared with the better-known imperative programming methods that your research department was using. For example, Truth Tables are a practical proving method when using formal methods to define a single-threaded function - but in real-time systems made up of a network of finite automata, the algebra is too complex to render in such a simple form. Hence while truth tables may be used to prove low-level binary functions, a set of more thorough and more engineering-orientated testing strategies must be used.

The fundamental difference between what most people understand regarding how computer programming works and the nature of real-time systems is that in the latter there is no such thing as a "main" loop from which procedures and functions are fired - instead you have a network of finite automata modules that are constantly assessing each other.

The idea that somewhere in the code there's a function along the lines of:

IF STICK_Y == STICK_Y_MAX_UP THEN PITCH_UP(MAX_ALPHA)

is a fallacy. Instead, you'd have tens of these finite automata modules cross-checking each other, and based on the constantly changing values in real-time, they'd be configuring the flight surfaces to provide optimum response to the command inputs while taking into account what the aircraft is doing.

For example, it would do no good to set pitch attitude to match an AoA of 17.5 degrees based on the current airspeed if, by the time that pitch attitude was reached, the aircraft was flying a few knots slower.



In fact, just as mechanical technology pioneered in the aviation industry tends to make its way into more commonplace things (e.g. antilock brakes on cars), the software and engineering processes pioneered during the development of civil FBW have been filtering into more general software development over the last decade or so - particularly continuous integration, regression testing and test-driven development.

My questions would be more like :
1-Why the FCS limited the AoA to alpha prot when the request was for alpha max ?

Simple, it didn't. What the EFCS provided was the maximum alpha it could based on the aircraft's orientation and status - which was changing constantly. The deltas in airspeed indicated that the aircraft was *slowing down* right up until the last second, so I'd suppose the pitch angle was calculated to match the airspeed based on that deceleration trend, given the inherent delay in actuating the pitch angle to match 17.5 degrees of AoA in a few seconds' time.

If the aircraft's airspeed was consistent and stable - or indeed if the aircraft was accelerating - at the time the full backstick command was given, then the EFCS would have been able to attain 17.5 degrees from that stable value much sooner.

Here's a properly-flown Alpha Max demonstration in a Cathay A330 for comparison (approx. 0:18 onwards, though it's useful to see from the beginning to establish speed comparison from take-off):

Cathay Pacific - Airbus A330 Crazy Alpha Max Low Speed - YouTube

As you can see, when the approach is flown correctly, speed is stable and approach power is maintained (i.e. the engines have not spooled down), then the systems will quite happily provide and maintain 17.5 degrees for as long as desired. All the tests flown regarding Habsheim matched the aircraft's trajectory (i.e. slowing down with late application of power), hence the significant delay in achieving Alpha Max.

Or why the elevators did the opposite of the sidestick displacement ?

Already answered. See my posts #150 and #176.

Sidestick commands rate in Normal Law, even in High AoA Protection mode. Aircraft was at maximum alpha based on the projected airspeed with the current rate of deceleration. Like a human pilot, the EFCS uses deltas and trends to stay ahead of the aircraft. Until the engines were producing enough thrust to bring the airspeed up (or if the nose was lowered to build up a little more speed sooner), then the EFCS would be basing its calculations on the rate the aircraft was decelerating.

The elevators were probably commanded nose-down to arrest the pitch-up tendency from the engines, and until the deceleration trend was itself arrested by increased thrust, that was absolutely the correct action for the scenario - indeed, the Airbus test flight which re-enacted the scenario demonstrated this, with optimum alpha max achieved once the airspeed was stable and increasing.


2-Why BEA + Airbus sat on the question ?

Again, they didn't. The data linked in earlier posts that was released by Airbus (was it in 1995?) was originally performed as part of the investigation by the BEA. The re-enaction of the flightpath at Toulouse (presumably at the expense of either the French civil service or Airbus) was part of that original investigation, and Capt. Bechet (head of the investigation) was personally involved. I don't think you could reasonably say that was "sitting on the question".

[roulishollandais]

DozyWanabbe,
Thank you for that worthful report. I have to agree that it does not comply to an ivory tower.
But accept that such work quality, such teams have been in work in AS & others (the same companies) building the fabulous Ariane501 launch and crash for 8 billions FF : only one bit was wrong (the famous carry) coming from only one unuseful variable BH imported from the former ArianeIV on the ground. That story showed that reliability must get much better in softwares. Jacques-Louis LIONS had been the head of the CNES (maîtred'oeuvre) but could not see the hidden bug from his office, and discivered it only when he was the President of the enquiry Board.
I know my position is harsh, strong, strict, etc. I will not change a comma.
The fact that I am an (retired) airline pilot too is an obligation to save lifes of my colleagues, not probability of live.
Thank you to allow that open discussion.
HAPPY NEW YEAR,
No reliability number is assigned to software, nor to any other design aspect.)

It was interesting to note, however, that nobody to whom I spoke believed that ``reliability = 1'' for any part of any system, although they are not aware of any method of demonstrating such high reliability figures for software. In fact, I was given a copy of a paper [6] which argued the impossibility of so doing, using arguments similar to those employed by Littlewood and Strigini[7], with which paper everyone was also familiar.

One interesting aside was that the 10^-9 figure is justified within the JAA by an argument that I do not believe is used by the FAA. It depends on actual statistics, which indicate that the probability of an aircraft crash is about 10^-7 per flying hour. Given that there may be around 100 critical systems on any aircraft (on the A320 there are 68) the additional factor of 10^-2 is applied to allow for the fact that *any one* of them may fail catastrophically. This justification is contained in one of the JAR interpretive documents.

[DozyWannabe]

But accept that such work quality, such teams have been in work in AS & others (the same companies) building the fabulous Ariane501 launch and crash...

Same parent group, but categorically *not* the same company, let alone same team(s). Don't let the common nationality of some of the people involved fool you into thinking there's a common problem.

For one thing, I think that all bit-depth conversions performed in the Airbus EFCS software come from generated code (i.e. thoroughly tested repeatedly and regressionally). There are no "low-level" code changes of that nature allowed using the Airbus process.

I know my position is harsh, strong, strict, etc. I will not change a comma.

It's not that your position is any of those things - what bothers me is that your (vehemently stated) assumptions were incorrect in this case.

The fact that I am an (retired) airline pilot too is an obligation to save lifes of my colleagues, not probability of live.

All well and good, but let's be clear - the number of line incidents involving both Airbus and Boeing FBW types that can be put down to a syntactic or logical (code-level) error in the computer code is zero - I repeat, zero.

The few issues that have transpired have always been down to assumptions made at the design level (and signed off by pilot engineers) which turned out to be either incorrect or incomplete, as was the case before digital flight control systems existed.

[HazelNuts39]

... the number of line incidents involving both Airbus and Boeing FBW types that can be put down to a syntactic or logical (code-level) error in the computer code is zero - I repeat, zero.

For one not familiar with COMPUTERGEEKSPEAK, it would seem that that statement is open to question.

From the ATSB Final Report on the inflight upset accident in flight QF72:

5.2.3 Reasons for the design limitation
Non-awareness of the failure scenario
(...)
The A330/A340 FCPC algorithm for processing AOA data was redesigned after a problem was found with the initial algorithm during flight testing that was conducted before the aircraft type was certified. The redesign unintentionally introduced the design limitation in the algorithm, and the fault-tolerant features of the system were not able to fully mitigate the problem. The design limitation was not identified during the redesign activities. Although the SSA identified the relevant failure condition (incorrect, high AOA data leading to a pitch-down command), it did not identify the scenario that led to this condition on the 7 October 2008 flight.

[DozyWannabe]

For one not familiar with COMPUTERGEEKSPEAK, it would seem that that statement is open to question.

Indeed, which is the reason I book-ended that section in my post - I'll do my best to elaborate.

At code level, a syntactic error is one in which the rules of the programming language itself are violated, thus causing the program to stop running. When a program is compiled into object and machine language, these errors are usually flagged, halting compilation and resulting in no working output. A code level logical error is one in which the syntax is correct, but the code's behaviour deviates from the specification and design.

The issue you raise from the ATSB report on QF32 [EDIT : corrected from QF72 - my bad] falls into the category of spec/design-level errors, as I said:

...assumptions made at the design level ... which turned out to be either incorrect or incomplete...

in which the code implements the specification and/or design correctly, but the spec or design itself (signed off by the aero engineers and pilot engineers - not just the software engineers) was at fault.

This is a very important distinction to make from my perspective - because strictly speaking this is not a "computer" or "software" error at all, but an engineering problem. Not that the media will hesitate to report it as a "computer error", which, needless to say, grinds my gears in much the same way as yours when you read a sentence like "the wing flaps which help the aircraft land".

[CONF iture]

I don't think you could reasonably say that was "sitting on the question".
Of course I can, as the only reply from Bechet was :
"That's the normal functioning of the aircraft, and the normal functioning of the aircraft is not detailed in the report."
You can write as lengthy posts as you like and proceed with your suppositions ... you simply don't have access to the lines of coding. Airbus has, but how embarrassing is it to admit that a direct command of the elevators was more adapted to save the day than a complex inefficient design ...

The elevators were probably commanded nose-down to arrest the pitch-up tendency from the engines
Which would be a total waste of the available thrust.
Thrust + alpha max = PERF ... as advertised by Airbus themselves.
What don't you understand here Dozy ... ?

[rudderrudderrat]

Hi CONF iture,
Thrust + alpha max = PERF ... as advertised by Airbus themselves. I agree.
However Alpha Max is approached asymptotically and with full back stick will require some time before it is achieved.
Are you really suggesting that despite the Alpha floor being disabled by deliberately flying below 100 radio + engines back at idle + a very late application of TOGA TL selection, that the prime reason for this crash was because Alpha Max was not achieved "instantaneously"?

What don't you understand here CONF ....?

[DozyWannabe]

Of course I can, as the only reply from Bechet was :
"That's the normal functioning of the aircraft, and the normal functioning of the aircraft is not detailed in the report."

Did he say that at a press conference or at the legal proceedings? There's no point going into a lengthy, technical explanation of the systems as described above if the audience has no technical expertise and will not understand it!

but how embarrassing is it to admit that a direct command of the elevators was more adapted to save the day than a complex inefficient design ...

So, as rudderrudderrat suggests, you *are* actually arguing that an aircraft that is near maximum alpha for the current speed *and decelerating* will not stall if the elevators are subsequently deflected to the maximum? Rather you than me...

And as for your accusations of "complex, inefficient design", you couldn't be more wrong. For one thing, use of obsolete hardware meant that code efficiency was watched *very* closely and proven engineering principles were used to keep complexity down. The systems would be aware of deltas not just in airspeed and pitch, but also thrust setting and status. There simply wasn't enough airspeed to allow for more positive pitch and not enough thrust to power out of it until the aircraft was at the boundary of the forest - which was far too late. As people have pointed out before, Airbus invited pilots to pit their reflexes against the EFCS in simulated scenarios in the early days of the A320's life and to the best of my knowledge the protected aircraft outperformed even the most skilled pilot every time. Lest some pilots' hackles be raised at that assertion, this was not down to any particular genius on the part of the computers or software guys, but mainly by years and many thousands of flying hours put in by test pilots as performance data was gathered and the systems behaviour was refined.

It's not about embarrassment, it's about a bunch of disgruntled French pilots who can't let go of a two and a half-decade old issue no matter how many times they are reasonably proved wrong. If Airbus were truly trying to avoid embarrassment and deflect criticism, then why did they publicly acknowledge and correct a systems design issue - well ahead of the respective report -when one showed up after Bilbao?

Finally, I may not have direct access to the "lines of coding [sic]" - but I was taught by a guy who saw the end-to-end process first hand and could explain it thoroughly.

I should also make clear again that his visit in 1993 was not a jolly, organised by a grateful Airbus Industrie for a supporter, it was quite the opposite. Prof. Mellor was a well-known sceptic of civil digital FBW, and something of a constant thorn in their side - not just in specialist arenas such as the RISKS list, but also a prominent contributor to the highly-regarded "Black Box" TV series. While his report above concedes that he was pleasantly surprised by what he found, he retained a very cautious attitude several years later - when I first showed up in his lecture halls in 1997.

[CONF iture]

However Alpha Max is approached asymptotically and with full back stick will require some time before it is achieved.
It certainly won't efficiently as the elevators are commanded nose-down as a start.
And if the elevators are commanded nose-down to arrest the pitch-up tendency from the engines, as submitted by dozy, it is even a poorer concept.
Remember, we are still 2.5 deg short of alpha max.

Are you really suggesting that despite the Alpha floor being disabled by deliberately flying below 100 radio + engines back at idle + a very late application of TOGA TL selection, that the prime reason for this crash was because Alpha Max was not achieved "instantaneously"?
Instantaneous ?
It does not have to be, what matters is the intention, the intention to efficiently deliver.
The prime reason for the crash ?
Why would it be when the pilots put themselves in such a undesirable situation, but a contributory factor for the crash it has to be.

[vilas]

Conf iture
The aircraft was in high AOA protection mode which has priority over everything else. If the pilot was applying back stick why would the aircraft apply down elevators to prevent thrust pitch up?

[RetiredF4]

vilas @Conf iture
The aircraft was in high AOA protection mode which has priority over everything else. If the pilot was applying back stick why would the aircraft apply down elevators to prevent thrust pitch up?


I think it is time to review the principle and function modes of FBW C*.

[rudderrudderrat]

Hi CONF iture,
It certainly won't efficiently as the elevators are commanded nose-down as a start....
Instantaneous ?
It does not have to be, what matters is the intention, the intention to efficiently deliver.

Please have a look at Airbus A320 Fly By Wire Demo - YouTube (http://www.youtube.com/watch?v=9fqy8uPzW90&feature=youtu.be) from time 12:00. (link posted by alonso1986 in another thread)
There is an AoA gauge fitted and visible to the bottom left of the PFD.

During the demonstrated pull up manoeuvre, the AoA increases rapidly to 15 degs during the +ve delta g. During the steady 1 g climb, as the speed washes off, the AoA increases towards 15 degs again steadily over about 10 seconds. The AoA never exceeds its design limit (apparently 15 degs mentioned around time 10:00).

That is the sort of manoeuvre the system was designed for, and performs it very well.

[DozyWannabe]

I think it is time to review the principle and function modes of FBW C*.

Why?

The systems were doing as designed, and as refined by thousands of hours of flight testing by the best test pilots from across Europe. Not to mention that the system has been operating safely worldwide with a safety record that matches or exceeds older/more "conventional" types for over two and a half decades!

I've long been resigned to the fact that CONF iture will never let go of the idea that the aircraft coulda-woulda-shoulda cleared those trees if the nasty computers just let Asseline have those elevators up, but that opinion simply doesn't hold water against the known facts - the aircraft was decelerating, TOGA thrust from spooled-down high-bypass turbofans takes some time in coming, when it does come the aircraft will tend to pitch up (if corrective action is not applied) and above all that aircraft must attain stability before achieving full alpha max.

Based on the fact that the fudged approach was conducted in such a way that at least one step of the procedure was forgotten, that the altitude, thrust and speed stability were shot to pieces and the pilot actions once the danger was realised were poorly-co-ordinated and reactive, I have a tough time believing that the PF was in a suitable state of mind to take into account the thrust/pitch couple, the insufficient airspeed and the proximity to stall to "finesse" the controls in a superior manner to the EFCS.

Maybe the aircraft would not have immediately stalled if the elevators were actuated nose-up, but the resulting lack of stability if the aircraft exceeded alpha max or Vs1g would certainly have put the aircraft at a greater risk of no longer flying. I'd put money on the aircraft stalling with the combined pitch-up tendency of the thrust increase plus nose-up elevator in that scenario.

Perhaps a better way of looking at the way those systems work is not in terms of an implacable hunk of technology, but a system set up with the combined experience of top-notch test pilots to give the person in command of the aircraft a steadying hand if and when necessary.

[HazelNuts39]

@RRR,

thanks for the link to the A320 demo.

BTW the demo pilot was Gordon Corps, Airbus' chief (test) pilot at the time.

[DozyWannabe]

BTW the demo pilot was Gordon Corps, Airbus' chief (test) pilot at the time.

Something of an unsung hero, to say the very least. I'll say it again - I doubt very much that we'd still be having these misunderstandings and arguments had he not died so tragically and prematurely. I've had the pleasure of asking those who knew him about the way he worked, and they all agreed that on top of his professionalism and meticulous nature, he had an almost unique ability to explain and demonstrate things to both pilot and engineering audiences in a way that inspired confidence and respect from all. They also said that while his loss was tragic, his insistence on personally heading the safety team that went to the Himalayas to get to the bottom of the A300 accident was entirely in character. Did I say he's a hero of mine yet? :ok:

Worth clarifying that the second demonstration (12:00 or thereabouts) in the video is of a higher-altitude escape maneouvre and not a low-level flypast. My supposition is that the aircraft stabilises at around 15 degrees of AoA rather than the theoretical max of 17.5 degrees because there is a bank/turn component to the input, which was not the case at Habsheim.

Interestingly, in the first demonstration (approx. 9:50), Capt. Corps states that 15 degrees is the maximum permitted alpha in the landing configuration. Was AF296 in this configuration?

[RetiredF4]

@ Dozy
you misunderstood my post, which is probably my fault.
I'm with you in most of your posts in this thread here. My remark was pointed at those, who are either unable or unwilling to understand, that only in direct law the elevator deflection corresponds to the stick deflection, and that it is an basic principle of FBW C* law of AB (at least that's how i see it) that the pitch command of the stick represents an g command blended by pitch rate when applicable.
Therefore it is normal, that the pitch rate change of the increasing thrust enters the equation as just that, a pitch change. If the pilot commanded pitch change corrected by the protections is less than the actual pitch change by thrust, then the elevators will command a pitch reduction despite the stick position.

By the way, a pilot in the loop in a conventional aircraft in such a pitch up situation due to increasing engine thrust would react how (without available protections)? By controlling the pitchup by use of proper stick input, which might be stick forward!
Who wants to end like the B737 at Kazan?

[DozyWannabe]

@ Dozy
you misunderstood my post, which is probably my fault. I'm with you in most of your posts in this thread here.

Much appreciated - I know we've crossed swords in the past, so maybe I was also a little quick to rush to judgement.


Therefore it is normal, that the pitch rate change of the increasing thrust enters the equation as just that, a pitch change. If the commanded pitch change corrected by the protections is less than the actual pitch change by thrust, then the elevators will command a pitch reduction despite the stick position.

Understood. However in the High AoA protection scenario, back-stick commands maximum or best possible alpha, based on the aircraft's previous, current and projected (via deltas/trends) state. If the current pitch attitude already corresponds with maximum/optimum AoA based on the current/projected state, then the command is to *maintain*, rather than change, that optimum attitude. So, as you say, if the EFCS detects even the slightest pitch-up tendency - e.g. from the gradual thrust increase - then it must counteract that, and because Alpha Floor was unavailable, the only aspect it could control to counter it was pitch.

By the way, a pilot in the loop in such a pitch up situation due to increasing engine thrust would react how (without available protections)? By controlling the pitchup by use of proper stick input, which might be stick forward!

I get your point, though I would say that there's no guarantee that a pilot *would* do that, no matter how "in the loop" or otherwise. It's worth remembering that the A320, while considered a very nimble airliner, is still nevertheless an airliner. The whole point of using digital technology in civil flight control systems is that they can, and do, detect and correct minute changes (as in fractions of a degree/knot etc.) in aircraft orientation more quickly and accurately than a human can. This is not because they're capable of outperforming humans in general, but because unlike a human pilot that is all they are designed to do.

In a pre-FBW fighter or a specialist aerobatic aircraft, a pilot will be trained and given experience in recognising responses and making those split-second
decisions/inputs - and because those aircraft are designed to be lightweight and nimble above all else, momentum is less of a problem. Even smaller short/medium-haul airliners like the A320 are still big and unwieldy aircraft by comparison - and this is the reason high AoA protections were a very important thing to demonstrate in the early days, being a scenario no previous type could pull off safely. I'd be prepared to bet that if you asked any one of Yeager, Armstrong, Cunningham or Beamont et al. to get in a conventional jetliner and hold 15 degrees of AoA precisely while flying slowly (as in near stall speed) at 1000ft, let alone 100ft, their refusals would have been both emphatic and unprintable.

The flight envelope protections afforded by modern FBW systems have been consistently characterised by the doubters as being primarily a technological vote-of-no-confidence in piloting skills, and this has been a very difficult idea to dispel amongst pilots. Again at the risk of repeating myself, the overarching premise is nothing of the sort - the main impetus is in allowing pilots to have confidence in making positive maneouvres without having to worry about the aircraft's ability to stay in the air.

[Chris Scott]

Dozy,
To answer your recent question: yes, the (Habsheim) a/c was in a normal landing configuration, i.e., L/G down and "Flaps 3" (Slats 22 deg, Flaps 20 deg). BTW, the other normal landing config is "Flaps Full" (Slats 27 deg, Flaps 35).

Quote from RetiredF4:
"By the way, a pilot in the loop in a conventional aircraft in such a pitch up situation due to increasing engine thrust would react how (without available protections)? By controlling the pitchup by use of proper stick input, which might be stick forward!
"Who wants to end like the B737 at Kazan?"

Quite so, and - IIRC - there's a precedent on the A310. I imagine most twin-engined airliners with underslung engines have comparable characteristics in the go-around; partly because they are essentially overpowered when both donks are running, and it is normal practice to use TOGA thrust when going-around from a low height.

On the A310, which for this discussion we can regard as being in Direct Law, the amount of down-elevator to counter the pitch-couple effect of TOGA is cosiderable, even from Vref. This is highlighted best if the AP is off, when the PF has to exert an almost unsustainable forward force on the control column, manually trimming the THS forward from the landing setting as fast as possible to relieve the elevator load. (When in use, the AP and auto-trim obviously have the same task.) The down-elevator requirement would be much greater from Valpha-max. (BTW, the A310 also has an Alpha-Floor mode in A/THR, so the latter must have been demonstrated for certification, presumably at an aft CG.)

FWIW, I don't remember ever doing a G/A on an A320 in Direct Law (certainly not in anger, because in 14 years I never lost Normal Law). However, a glance at the relative profiles of the A320 and A310 suggest that the A320 elevators may enjoy a more effective pitch-moment than the A310's. (The A319 might be more comparable to the A310 in that respect, and the A320 more like the A300-600.)

[DozyWannabe]

Dozy,
To answer your recent question: yes, the a/c was in a normal landing configuration, i.e., L/G down and "Flaps 3" (Slats 22 deg, Flaps 20 deg). BTW, the other normal landing config is "Flaps Full" (Slats 27 deg, Flaps 35).

In which case (and I'm looking for guidance here), does this not render the question of why the aircraft did not attain an AoA of 17.5 degrees somewhat moot?

[HazelNuts39]

He actually says 'in this configuration'. Do we know which configuration that is?

[Chris Scott]

Dozy,

As you know, I'm not the best one to answer that, but I've no reason to doubt the BEA figure of Alpha-max = 17.5 deg, which is specific to Flaps 3.


AFAIK, the AoA is measured in relation to a specific part of the wing chord, ignoring the varying positions (angles) of the slats and flaps.

[DozyWannabe]

He actually says 'in this configuration'. Do we know which configuration that is?

He says "..which is the landing configuration" at about 09:40.

As you know, I'm not the best one to answer that, but I've no reason to doubt the BEA figure of Alpha-max = 17.5 deg, which is specific to Flaps 3.

Thanks for providing that info. What has tickled my grey cells is the "gear down" distinction. While not directly related to the calculation, would the increased drag from the gear require a reduction in maximum AoA in the protected mode to compensate?

[Chris Scott]

Hi HN39,
Re the flap settings. Regardless of briefings and configuration commands from PF to PNF, the DFDR shows that the fly-past was flown at Flaps/Config 3.

BEA Rapport Finale, Annexe VII, Tome 2
Published DFDR data start at 1244Z, Time mark 240.0 sec (this may be 240 secs from selection of TOGA at the start of T/O).
At 240.0 sec, Slats18/Flaps10 (= "Flaps 1+F", maintained from T/O).
At 263.0 sec, Slats22/Flaps15 (= "Flaps 2").
At 279.0 sec, Slats22/Flaps20 (= "Flaps 3").
At impact (334.0 or 335.0), Slats22/Flaps20 ("Flaps 3").

Dozy,
Re possible effect of L/G on alpha-max: again, I think the BEA would have done their homework. L/G increses drag, which offsets thrust, but I don't think the aerodynamics are changed significantly. For example, alpha-floor remains the same at 15.0 deg in that config, according to my contemporary FCOM, which unfortunately gives no equivalent figures for alpha-max. Placarded VLS (on the ASI) does not change, IIRC, with gear position, whereas it does with speedbrake extension.

PS
HN 39, I now see your exchanges with Dozy refer to the video of the Gordon Corps demo flight...

[DozyWannabe]

Dozy,
Re possible effect of L/G on alpha-max: again, I think the BEA would have done their homework.

I'd expect so too - however at the same time I must take into account that there was in all likelihood no single person more knowledgeable about the A320 systems specifics at the time than Gordon Corps - and from what I've heard about the man I doubt very much that he'd have allowed that demonstration video to be released with a verbal mistake on his part.

To be somewhat brutal, the only person I've known to make a big deal about the 17.5 degree figure is CONF iture on these forums. The BEA alluded obliquely to the figure but did not question the systems' behavioural integrity, however at the same time neither the SNPL nor Capt. Asseline's legal team (who had access to the report and those figures) questioned it during the court case. If this apparent 2.5 degree discrepancy was the 'smoking gun' CONF claims it is then why was it not brought up at the time?

The way I see it there are two main possible conclusions - either Capt. Corps was mistaken when narrating the video, or the BEA included the 17.5 degree figure in their report as background information only, not intended for comparison with the aircraft's actual AoA during the sequence.

[Chris Scott]

Sorry Dozy, nothing I previously posted this evening refered to the Gordon Corps video, which I had not watched. I hadn't realised you and HN39 were refering to it in your exchanges about the landing config.

Have now watched the video, and cannot shed any light on which of the two standard landing configs Gordon was using, or why he implied that 15 degrees was about the maximum angle of attack for it. If he was using Flaps Full, I think alpha-max MIGHT be slightly less than for Flaps 3, but that guess is only based on the fact that my FCOM says the Flaps-Full alpha-floor is 14.5 deg - 0.5 deg less than at Flaps 3. That said, the small, electro-mechanical AoA indicator seems to exceed 15 deg occasionally in the video.

[rudderrudderrat]

Hi Chris,
the small, electro-mechanical AoA indicator seems to exceed 15 deg occasionally in the video.
I agree. It exceeds 15 momentarilly when he rolls on some bank.
During the steady turn, the AoA gauge then returns to 15 and when he levels the wings again, the AoA shows less than 15. All that is done with full back stick.

In level flight, the system seems to be limiting the AoA shy of Alpha Max - so that some bank can be applied with the same initial pitch attitude without exceeding Alpha Max.

From my FCOM:
• αfloor is activated through the A/THR system, when
α is greater than αfloor (9.5 ° in configuration 0; 15 ° in configuration 1, 2; 14 ° in configuration 3; 13 ° in configuration FULL)

FCOM FCB5 Avoiding Tail strikes; “Assuming an 8 kt speed decrease during flare, and a -1 ° flight path angle at touchdown, the pitch attitude will increase by approximately 4.5 °”

So during the flare, the flight path is changed by +2° (-3° to -1°) for a loss of 8kt. Therefore the angle of attack increases by 2.5° (4.5°-2°) for about 8 kts reduction in speed.
http://www.blackholes.org.uk/PP/AoA.png
From the diagram, αfloor is about 8 kts from αMax, therefore in CONF 3, αMax would seem to be about 14° + 2.5° = 16.5°

[Jesse Pinkman]

A FLOOR doesn't work below 100'RA.

[Chris Scott]

Quote from rudderrudderrat:
"From my FCOM:
• αfloor is activated through the A/THR system, when
α is greater than αfloor (9.5 ° in configuration 0; 15 ° in configuration 1, 2; 14 ° in configuration 3; 13 ° in configuration FULL)"

I guess that must be a FCOM from a later era than June 1988? Here is the equivalent paragraph from the BCAL Tech Manual, dated 17FEB1988, around the time of A320-100 type-certification.

[I]Alpha-floor is activated when:
- alpha > alpha-floor (9.5 deg in conf 0; 15 deg in conf 1, 2, 3; 14.5 deg in conf FULL, or
- sidestick deflection > 14 deg nose-up and pitch-attitude > 25 deg or in angle-of-attack protection [sic...].

[The ambiguous mix of "and" and "or" in the final, un-punctuated sentence is as printed.]

A slightly earlier, late-1987 edition of the Airbus FCOM Flight Controls chapter (1.09.10 P8, REV 03, SEQ 001) omits any specific alpha figures; instead providing the following figures of the relationship between the alphas at the protection stages and the alpha at Vs.

All configurations at low speed:
alpha-prot >= alpha (1.13 Vs)
alpha-floor = alpha (1.1 Vs)
alpha-max = alpha (1.06 Vs)

Clean configuration at high speed:
alpha-prot = alpha (buffet)
alpha-max = Czmax
When alpha protection is active nose-up auto-pitch-trim is inhibited.

One might assume in the above that Vs is a speed associated with the alpha at CL -max, but it does not state whether that Vs is at 1.0G or a lower figure. And whether it is the same value used in the "Stalling Speeds" graph (FCOM 3.01.20 P5) is unclear.

Therefore, I think any attempt, using a combination of the above sources, to infer specific values of alpha for the three different protection levels might be ill-founded. Also, the two versions may originate either side of the FBW certification process.

Hi Jesse,
"A FLOOR doesn't work below 100'RA."

Yes, we have discussed that previously, although my 1988 manuals only seemed to mention it in the Power Plant section (A/THR automatic-engagement criteria). The Habsheim crew had planned to disable Alpha-Floor to ensure it did not intervene at their briefed display height of 100ft.

[jcjeant]

For a non-professional it is fun to read all these comments about official manuals for the same type of aircraft that change whenever the manufacturer decides and when those are used by professionals to analyze a sequence of a particular flight they come in of conlusions that contain the words .. ill founded ... ambiguous ... unclear ... probable ... etc ...
So I guess the results when everything is explained in a court where everyone tries to blame the other party :rolleyes:

[Chris Scott]

Quote from jcjeant:
"For a non-professional it is fun to read all these comments about official manuals for the same type of aircraft that change whenever the manufacturer decides and when those are used by professionals to analyze a sequence of a particular flight they come in of conlusions that contain the words .. ill founded ... ambiguous ... unclear ... probable ... etc ..."

Well, that's not entirely true. Haven't noticed any (ex) professional recently using the word "probable" here... ;)

[Chris Scott]

Quote from rudderrudderrat:
FCOM FCB5 Avoiding Tail strikes; “Assuming an 8 kt speed decrease during flare, and a -1 ° flight path angle at touchdown, the pitch attitude will increase by approximately 4.5 °”
"So during the flare, the flight path is changed by +2° (-3° to -1°) for a loss of 8kt. Therefore the angle of attack increases by 2.5° (4.5°-2°) for about 8 kts reduction in speed.
"From the diagram, αfloor is about 8 kts from αMax, therefore in CONF 3, αMax would seem to be about 14° + 2.5° = 16.5°

I take it you are assuming Nz is 1G before and after the flare? I'm wondering about the possible change of ground effect on the 1G-AoAs between (say) 30'R and 5'R.

[CONF iture]

Please have a look at Airbus A320 Fly By Wire Demo
What a Great video - Never seen it before.
The presence of the AoA indicator is perfect for the demo, only a simultaneous view on the sidestick and the flight controls page could be better.
The AoA never exceeds its design limit (apparently 15 degs mentioned around time 10:00)
Alpha max is at 15 deg in landing CONF FULL but 17.5 CONF 3
During the demonstrated pull up manoeuvre, the AoA increases rapidly to 15 degs during the +ve delta g.
Absolutely no hesitation to go rapidly to alpha max.
Why no such intention in Habsheim ?

[Chris Scott]

Quote from CONF_iture:
Alpha max is at 15 deg in landing CONF FULL but 17.5 CONF 3

That seems a big difference between the two configurations, particularly as my contemporary FCOM states that the alpha-floor figures are 14.5 deg and 15 deg respectively (see my post above).

[HazelNuts39]

A slightly earlier, late-1987 edition of the Airbus FCOM Flight Controls chapter (1.09.10 P8, REV 03, SEQ 001) omits any specific alpha figures; instead providing the following figures of the relationship between the alphas at the protection stages and the alpha at Vs.

All configurations at low speed:
alpha-prot >= alpha (1.13 Vs)
alpha-floor = alpha (1.1 Vs)
alpha-max = alpha (1.06 Vs)
I wonder, does your late-1987 (i.e. pre-cert) edition define V2min in relation to Vs (in the A330 FCOM it's in 3.04.10 - Characteristic speeds)?

[rudderrudderrat]

Hi CONF iture,
Absolutely no hesitation to go rapidly to alpha max.
Why no such intention in Habsheim ?
Maybe the system can rapidly achieve aMax during +ve delta g, because it is able to avoid exceeding aMax by simply reducing the rate of rotation.
Whereas at Habsheim, there was only the last 2.5° (if 17.5° is correct) of rotation available, and would only approach the limit gradually.

[DozyWannabe]

Maybe the system can rapidly achieve aMax during +ve delta g, because it is able to avoid exceeding aMax by simply reducing the rate of rotation.
Whereas at Habsheim, there was only the last 2.5° (if 17.5° is correct) of rotation available, and would only approach the limit gradually.

Plus on the Airbus video demo he was at low speed, but keeping the thrust on (i.e. the engines were not spooled down - unlike Habsheim). Also the demo airspeed was stable and not decelerating prior to full back-stick.

Alpha max is at 15 deg in landing CONF FULL but 17.5 CONF 3

Do you have a reference handy for that, or is it a guess?

Being a programmer, I am naturally averse to numbers without knowing what they are and where they're from (aka "magic" numbers).

[Chris Scott]

Dozy,
That's an interesting hypothesis, i.e., the EFCS might deliberately keep a margin to alpha-max in idle thrust. I wonder.
AFAIK, however, we haven't established what config/flap GC was using for that demo, and in any case I currently disagree with Confit's figure of 15 deg for alpha-max with config FULL (see my post).

HN39,
That's a good call, although 3.04.10 was re Flight Instruments, and not included in my issue. However, 3.04.01 "Operating Speeds Definition" may provide the best explanation of how the peculiarities of certification for the A320 in Normal Law - which (as you know) was unique in being incapable of meeting the contemporary norms of stall testing - were addressed, and a compromise reached to avoid it being unfairly disadvantaged, field-performance-wise, relative to types like the B737.

Airbus FCOM 3.04.01, REV01 SEQ001 (still current at SEP 1987, prior to formal type-certification) (my emphasis):
"For a conventional a/c the stall speed which is used for reference is VSmin based on a load factor lower than 1G, which therefore gives a stalling speed less than that obtained at 1G. All operating speeds are expressed in relation with this speed (for example Vref = 1.3 VSmin). Since the A320 icorporates a low speed protection feature (alpha limit) which cannot be overridden by the flight crew, the airworthiness authorities have reconsidered their position regarding the stall speed definition.
"All the operational speeds have to be referenced to a speed which can be demonstrated by flight test. This speed is VS1g for A320 and VSmin for previous a/c types. As a result the Authorities have agreed to allow A320 to have the following factors:
V2 = 1.2 x 0.94 = 1.13 VS1g
VREF = 1.3 x 0.94 = 1.23 VS1g
"It can be seen that these speeds are identical to those that would have been achieved, had the a/c been conventionally certificated to the 94% rule. Compared to a conventional a/c the A320 has exactly the same manoeuvre margin at its reference speeds.
"In the FCOM VS1g will be designed [designated] VS. "

* VS = "Reference stalling speed (equal to VS1g)."

[HazelNuts39]

... the A320 in Normal Law - which (as you know) was unique in being incapable of meeting the contemporary norms of stall testing ...I wouldn't know about that - under the 'contemporary norms' Vsmin would have been equal to Valpha-max.
All the operational speeds have to be referenced to a speed which can be demonstrated by flight test. This speed is VS1g for A320 ...If "alpha-max = alpha (1.06 Vs)" and Vs=Vs1g, how was Vs1g demonstrated in flight test?

Also, if "alpha-prot >= alpha (1.13 Vs)" and "V2 = 1.2 x 0.94 = 1.13 VS1g", then V2>=V(alpha-prot). So V2 can be equal to V(alpha-prot)?

[rudderrudderrat]

Hi HazelNuts39,
So V2 can be equal to V(alpha-prot)?
Well spotted!

FCOM, Aircraft Systems, DSC-27-20-10, Protections:
"At Take off aProt is equal to aMAX for 5s."
By which time, the SRS will have commanded about V2+10.
During OEI, aFloor is disabled.

@Chris Scott
Hi Chris,

Thanks for those figures in post #213 because if we assume the Cl is proportional to AoA (straight line graph between afloor and aMax), then we can guesstimate aMax.

Lift = 1/2 rho * A*Cl*Vsquared.
Since the lift remains constant at 1g, and A is constant,
Cl(amax) * V(amax)squared = Cl(afloor) * V(afloor) squared.
Rearranging Cl(max) = Cl(afloor) * V(afloor)squared / V(amax)squared.
If we assume Cl is proportional to AoA then,

AoA(max) in conf3 = 15 * (1.1)squared/(1.06)squared = 16.15°.
Using the same assumptions, astall = 18.15°
Since the accuracy of the AoA probes is 2.5% (see post #46), then the error could be +/- 0.45°.
18.15 - 0.45 = 17.7

I wonder if that where the 17.5° came from?

[HazelNuts39]

"At Take off aProt is equal to aMAX for 5s."
By which time, the SRS will have commanded about V2+10.With one engine inoperative?
If we assume Cl is proportional to AoA then, ...Is that a valid assumption? In the landing configuration AoA for Cl=0 is almost certainly some significant negative value. Also Cl-alpha is non-linear near Clmax.

[rudderrudderrat]

Hi HazelNuts39,
With one engine inoperative?
OEI - SRS commands V2, but aFloor is disabled so there is no problem.
Also Cl-alpha is non-linear near Clmax.
Correct.
That is why I've only considered "straight line graph between afloor and aMax" (see post #211 for the picture).

[Chris Scott]

Quotes from HN39:
If "alpha-max = alpha (1.06 Vs)" and Vs=Vs1g, how was Vs1g demonstrated in flight test?

It could not have been (in Normal Law), which is inconsistent with the text I quoted. The plot thickens... One possible explanation is that the "Operating Speeds Definition" comes from REV01, whereas the factors you mention are from REV03 in a different volume. All three volumes were issued to us at Blagnac in early January 1988, but we were warned that amendments were already pending.

Also, if "alpha-prot >= alpha (1.13 Vs)" and "V2 = 1.2 x 0.94 = 1.13 VS1g", then V2>=V(alpha-prot). So V2 can be equal to V(alpha-prot)?

Presumably not!

[rudderrudderrat]

Hi Chris,

Vs1g was demonstrated in flight test in ALT LAW.
It was demonstrated at 1g, whereas in a conventional aircraft, there is a tendency for the nose to drop and the demonstrated value is around 94% of 1g (allegedly).

So V2 can be equal to V(alpha-prot) - apparently it can be.

[CONF iture]

Maybe the system can rapidly achieve aMax during +ve delta g, because it is able to avoid exceeding aMax by simply reducing the rate of rotation.
Whereas at Habsheim, there was only the last 2.5° (if 17.5° is correct) of rotation available, and would only approach the limit gradually.
If the system is fast enough to efficiently control the elevators to rapidly command achieve or control alpha max in one case I don't see how it would not in another case.
Also, an excursion over alpha max is not an issue, as demonstrated in the video.

[Owain Glyndwr]

rrr

OK, but in Conf 3 the zero lift AOA Would be -6.5 deg

In earlier stall demonstrations Vsmin was taken as the minimum speed measured in the stall and stall recovery - this usually corresponded to about 0.94g

[HazelNuts39]

Vs1g was demonstrated in flight test in ALT LAW.Vs1g=reference stall speed is the current norm for conventional aircraft. In ALT LAW the A320 can be stalled as a conventional aircraft, so why would it need a different treatment of stall speed? The explanation given in the FCOM makes no sense if Vs1g was demonstrated in ALT LAW.

[rudderrudderrat]

Hi HazelNuts39,
why would it need a different treatment of stall speed?
A conventional aircraft stall speed is demonstrated by trimming the stab only as far as Vref, then decelerating at 1 kt/sec whilst applying back elevator until the nose drop. It was found that this always produced slightly less than 1g (94% ish) at the measured stall speed value.
In Alt Law, the Airbus continues to trim and level flight was achieved right up until the speed where you will lose control - all apparently done at 1g.

See page 36:http://www.skybrary.aero/bookshelf/books/2263.pdf

"Airworthiness Authorities have agreed that a factor of 0.94 represents the relationship between VS1g for aircraft of the A320 family and VSmin for conventional aircraft types."

[DozyWannabe]

Dozy,
That's an interesting hypothesis, i.e., the EFCS might deliberately keep a margin to alpha-max in idle thrust. I wonder.

Not so much that it keeps a margin for idle thrust (though it may do so) as it takes into account whether the aircraft is accelerating/decelerating and by how much, then uses the trends to stay ahead of the aircraft.

I tried to illustrate this earlier:

For example, it would do no good to set pitch attitude to match an AoA of 17.5 degrees based on the current airspeed if, by the time that pitch attitude was reached, the aircraft was flying a few knots slower.

Capt. Corps alludes to the idea that the overriding ethos behind the EFCS in this particular mode is retaining stability and keeping the aircraft flying.

[EDIT : In fact, if we go back to Chris Scott's earlier post:

In case it helps anyone, this is my translation into received English:
This flight law provides a special automatic protection preventing the aeroplane from reaching an incidence [angle of attack] greater than 17.5 degrees, to conserve a sufficient margin with respect to the stall, even if the pilot maintains a full climb [pitch-up] demand.

Note that this BEA description does not specify that an alpha of 17.5 deg will be achieved if the pilot maintains full back-stick.

What if the hypothesis that 17.5 degrees should be consistently held with full back-stick in High AoA mode is incorrect?

Is it not possible that 17.5 degrees AoA represents the absolute "do-not-exceed" maximum alpha from a normal limit of around 15 degrees AoA? If Chris's translation is correct, the wording of the BEA report is certainly consistent with that scenario. This interpretation would also be consistent with Capt. Corps' narration on the video.

]

[CONF iture]

or is the max AOA available strictly a function of actual slat/flap position being detected by the FMGEC regardless of the MCDU CONF prompt selection?
I would go for that one - I did not find anything in the FCOM to state otherwise.
As you mentioned, CONF 3 selection on the MCDU will modify VLS and VAPP, but it is also a signal for the GPWS in its FLAP MODE operation.

Alpha max at 17.5 deg CONF 3 as per BEA
Alpha max at 15 deg in landing configuration as per video
CONF FULL is the usual landing configuration as per FCOM.

For example, it would do no good to set pitch attitude to match an AoA of 17.5 degrees based on the current airspeed if, by the time that pitch attitude was reached, the aircraft was flying a few knots slower.
To lose 1kt/sec is very soft and normal and certainly nothing to restrain the AoA at 15 deg.

[DozyWannabe]

I would go for that one - I did not find anything in the FCOM to state otherwise.

And nothing in the FCTM states that 17.5 degrees AoA is guaranteed in such a scenario, but you still seem to take it as gospel.

To lose 1kt/sec is very soft and normal and certainly nothing to restrain the AoA at 15 deg.

The deceleration delta was nothing like stable at -1kt/sec throughout the sequence - try not to think about it as "restraining" AoA as anticipating where it will be if the aircraft continues to slow (which it would do until the engines have spooled back up).

[CONF iture]

The deceleration delta was nothing like stable at -1kt/sec throughout the sequence
How much was the deceleration in the last 10 sec as the airplane was level ?
try not to think about it as "restraining" AoA as anticipating where it will be if the aircraft continues to slow (which it would do until the engines have spooled back up).
When the elevators movement oppose the AoA increase, it is a clear restriction.
But nice theory of your own making, too bad it is not detailed either in the documentation or the BEA report.
What if the hypothesis that 17.5 degrees should be consistently held with full back-stick in High AoA mode is incorrect?
What if the Airbus documentation is wrong ?
Is it not possible that 17.5 degrees AoA represents the absolute "do-not-exceed" maximum alpha from a normal limit of around 15 degrees AoA? If Chris's translation is correct, the wording of the BEA report is certainly consistent with that scenario. This interpretation would also be consistent with Capt. Corps' narration on the video.
17.5 deg of AoA for CONF 3 is the target to maximize the performance, that's why Airbus chose to call it Alpha Max.
The video demonstrates how rapidly Alpha Max (15 deg for CONF FULL) is obtained.
But again ... nice theory of your own making.

[rudderrudderrat]

Hi CONF iture,
The video demonstrates how rapidly Alpha Max (15 deg for CONF FULL) is obtained.
The video also shows (at time 10:50 ish) an AoA of 16° after rolling on about 15° of bank. I would guess from that demo that Alpha Max is >15°, but the system remains on the conservative side of Alpha Max to allow for rapid changes in bank.

[CONF iture]

The video also shows (at time 10:50 ish) an AoA of 16° after rolling on about 15° of bank.
I can even read 17 deg for a very temporary excursion over alpha max.
I would guess from that demo that Alpha Max is >15°, but the system remains on the conservative side of Alpha Max to allow for rapid changes in bank.
Then it would be publicized as such ... Any reference ?

[rudderrudderrat]

Hi CONF iture,
Any reference ?
It simply does this rather well. No promise of achieving aMax - only of not exceeding it.
FCOM 27-20-10 Protections.
"HIGH ANGLE OF ATTACK PROTECTION

Under normal law, when the angle-of-attack becomes greater than αprot, the system switches elevator control from normal mode to a protection mode, in which the angle-of-attack is proportional to sidestick deflection. That is, in the αprot range, from α prot to αMAX, the sidestick commands α directly. However, the angle-of-attack will not exceed αMAX, even if the pilot gently pulls the sidestick all the way back. If the pilot releases the sidestick, the angle-of-attack returns to αprot and stays there."

[HazelNuts39]

No promise of achieving aMax - only of not exceeding it.You and I must be reading this differently:
the angle-of-attack is proportional to sidestick deflection. That is, in the αprot range, from α prot to αMAX, the sidestick commands α directly.

[CONF iture]

However, the angle-of-attack will not exceed αMAX, even if the pilot gently pulls the sidestick all the way back.
And we all got the demonstration by one flight test pilot from Airbus that :

the airplane will rapidly reach alpha max
temporary excursions over alpha max may happen, and so without stalling or crashing

[HazelNuts39]

the airplane will rapidly reach alpha maxYes, with a pilot who knows his airplane inside and out and how to best demonstrate its capabilities.

[CONF iture]

Yes, with a pilot who knows his airplane inside and out and how to best demonstrate its capabilities.
I would like to think I know that much ... but when I pull the sidestick following a GPWS warning, my simulator is also rapidly to alpha max, and over for temporary excursions.

[HazelNuts39]

when I pull the sidestick following a GPWS warning, my simulator is also rapidly to alpha maxWould you be decelerating with increasing pitch attitude when you pull the sidestick?

In the investigation of the Hudson ditching, the NTSB accepted Airbus' explanation of the phugoid damping. In the Habsheim accident the airplane was decelerating more rapidly and the pitch attitude increasing more rapidly than in the Hudson ditching. The effectivity of the phugoid damping was convincingly demonstrated in the A340 level-bust incident.

[rudderrudderrat]

Hi HazelNuts39,
You and I must be reading this differently:
The behaviour of Alpha Max protection demonstrated in the video around time 10:50 shows an AoA of between 16° (me) to 17° (CONF iture) which is very close to the theoretical Alpha Max attempted to be demonstrated at Habsheim.

The complex system behaviour is described in FCOM in just one short paragraph, so I'm not surprised we interpret what it is trying to say differently.

[roulishollandais]

The complex system behaviour is described in FCOM in just one short paragraph, so I'm not surprised we interpret what it is trying to say differently. "KISS" Airbus V2.0:} other exemple Airbus doesn't stall

[CONF iture]

The complex system behaviour is described in FCOM in just one short paragraph, so I'm not surprised we interpret what it is trying to say differently.
The EXTREMELY complex and EVER EVOLVING system behavior is much better described in Official Reports ... as long as they are not from the BEA.

[roulishollandais]

as long as they are not from the BEAthe men of the BEA comment what they are able to understand.

[CONF iture]

I'd rather say they take their directives from la maison mère in TLS ...

[CONF iture]

Would you be decelerating with increasing pitch attitude when you pull the sidestick?
Any scenario is possible.
Whatever the weight, configuration, and circumstances ... Airbus has one single procedure to be applied : PULL UP TOGA
Did any of them refused to deliver alpha max ? Not that I can remember.
Did any of them was under severe deceleration or pitch attitude increase ? To be honest I could not tell but I would be glad to experiment and report if only I had free access to a simulator.

In the investigation of the Hudson ditching, the NTSB accepted Airbus' explanation of the phugoid damping. In the Habsheim accident the airplane was decelerating more rapidly and the pitch attitude increasing more rapidly than in the Hudson ditching. The effectivity of the phugoid damping was convincingly demonstrated in the A340 level-bust incident.
Phugoid damping in Habsheim ... ?
Why not ?
But where has been the BEA on it ?
Why the NTSB or the investigators from Spain can detail how the elevators can refuse the pilot's orders, but the BEA is mute ... ?

What strikes me is how in Habsheim, New-York or Bilbao, the pitch authority has been compromised by features built into the system, whenever the aerodynamic had still more to deliver.