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CONCORDE ACCIDENT - PART 2

Rumours & News Reporting Points that may affect our jobs or lives as professional pilots. Also, items that may be of interest to professional pilots.

CONCORDE ACCIDENT - PART 2

Old 1st Sep 2001, 23:05
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I am awestruck by the quality of this thread. It's what makes PPRuNe a unique aviation resource.
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Old 2nd Sep 2001, 02:39
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Covenant

Check this link for information about Low Aspect Ratio Wings at High Angles of Attack

There are some graphs that quite clearly explains what happens to delta wings at high AOA.

At high angles of attack, several phenomena usually distinct from the cruise flow appear. Usually part of the wing begins to stall (separation occurs and the lift over that section is reduced).

but,

When the sweep is very large, or aspect ratio low, this approach does not work. Separation tends to occur near the leading edge of the wing, but unlike in the low sweep situation, the separated region is not large and does not reduce the lift.

--

When the vortex burst occurs on the wing (as opposed to downstream of the wing) the lift drops substantially.

but,

For many SST designs, however, the maximum CL may be predicted by assuming that the vortex does not burst at the maximum permissible angle of attack.
All of the above is backed up by a lot of formulas that doesn't make sense, though, to "normal" human beings

you wrote:
Without a whole load of power at your disposal, increasing angle of attack is an inefficient trade-off to gain a little short-term lift for a lot of airspeed - much more so than with a conventional wing. Sooner or later gravity will inexorably take over and the aircraft will spin out of the sky.

If that's not stalling, then I don't know what is!!
I think most here would define stall as the state of flight where the critical angle of attack has been exceeded (i.e. the wing stops producing lift). Drag is another matter and an excess of drag is not the same as a stall, although the consequences may be the same eventually.

-------

But is any of this, or how the Captain flew the aircraft relevant? Isn't it more relevant why they ended up in the messy situation that they did?

[ 01 September 2001: Message edited by: cosmo kramer ]
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Old 2nd Sep 2001, 17:06
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Cosmo

Could you join me on Tech Log – Votex bursting?

Regards
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Old 2nd Sep 2001, 18:45
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John Farley

I just want to thank you for a beautifully simple and professional analysis of what Capt Marty had to deal with. I am "just" a private pilot but would have been more if I wasn't so short-sighted. I have loved Concorde since she first flew and anything that will keep this wonderful bird flying safely and help mere mortals (regular non-Concorde/non Delta-wing pilots) understand the issue is very welcome. From what others have said, and I have read, of the career of Capt Marty, I suspect he would be delighted that (a) we appreciate his predicament and (b) we are debating what happened in Paris so that it need never happen again.

I have to say your analysis of the incredibly fine balance he would have had to maintain in extraordinarily stressful circumstances just shows what a great pilot he was - in extremisrelying on his skills, experience and instinct while knowing, probably, that he and his aircraft would not survive the incident.



P.S. any word on when Concorde will make her first flight to/from LHR?
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Old 3rd Sep 2001, 10:42
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Although the Vulcan was a 'conventional' delta rather than a 'slender' delta, it too had unusual handling charcteristics at low IAS - or more correctly at high AoA. These included significant adverse yaw and ultimately roll reversal with use of elevon in the rolling plane requiring cautious use of rudder to control yaw/roll and rapid IAS decay in level flight unless a lower pitch attitude was selected and a descent established. We didn't take the ac to anything close to the limits during our OCU on type, but I certainly remember how unnatural the low IAS/level flight regime was.

Even in the cruise the high drag at low IAS could catch the unwary! We didn't have any low speed buffet boundary limits as I recall, so when day over the south of France at FL410 when the nav plotter asked for a large speed reduction to cope with the unusual high level wind and timing requirements to make good a low level entry time, the captain decelerated to around 190 KIAS; when we tried to accelerate again it wouldn't do so even with the 4 Olympi bellowing as loud as they could. He had seemingly discovered Vzrc4 for that particular flight level and thrust available and it was only with the agreement of ATC allowing us to descend 4000 ft that we managed to accelerate out of the flight regime.

[ 03 September 2001: Message edited by: BEagle ]
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Old 3rd Sep 2001, 22:25
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Covenant

In the way of these things I have acquired some manufacturer’s data relevant to flying Concorde slowly that covers some of the points you made here in your 1 Sep 13.01 & 14.35 posts.

I’ll put it over on the Tech Log Vortex bursting thread.

Regards
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Old 4th Sep 2001, 08:39
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Having read some of the posts here and in the Tech Log, from John Farley in particular, I think I should withdraw any speculation on the appropriateness of Capt. Marty's actions that I made in my second post of 1st September. It's quite clear to me now that, all other factors being equal, he was making the very best of a very bad job.

I should have just left my first post to stand as it was. That'll teach me to stick to the facts!

Cosmo Kramer
Thanks for the link to the low aspect ratio wings at high alpha treatise. I don't think anything in there contradicts what I said in my first post, although the graph does show the non-linear vortex lift continuing for higher alphas than on my graph - probably because it is describing the theoretical total lift and doesn't allow for vortex bursting which, as the author points out, is not easily predicted using theorectical models.

On the tech log, WOK noted that concorde departs at slightly less than 25 degrees alpha in the sims which I suspect is more due to other stability factors than vortex bursting. I also tend to agree with John Farley that, even with all four engines producing max thrust, concorde would not be able to maintain IAS at such high alpha, which effectively puts vortex bursting well outside the normal (or even abnormal) operating envelope of the aircraft.

Nevertheless, at the point of departure from controlled flight, the aircraft, for whatever reason, experienced a dramatic loss of lift, which is essentially what people mean when they talk about stalling. That this was ultimately due to high alpha, high drag, low power or any combination of the three is not really important. One or both of the wings did, in fact, stop producing lift - which is why the aircraft went down and not up!

As you say, although discussion of this point is interesting, it has little to do with what we should be concentrating on with regard to the causes of the accident. My aim was really to clear up any misunderstanding that may have arisen about the nature of slender delta wings and their performance at high alpha. I think between us all, we have managed to do that fairly comprehensively!
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Old 4th Sep 2001, 11:36
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Covenant
Nevertheless, at the point of departure from controlled flight, the aircraft, for whatever reason, experienced a dramatic loss of lift, which is essentially what people mean when they talk about stalling. That this was ultimately due to high alpha, high drag, low power or any combination of the three is not really important.
You are right that isn't important as far as the result goes.

But speaking from a strict definition point of view, I would say that a stall is caused by an exceedance of the angle of attack. Is a wing that is not producing lift stalled? If so is a parked aircraft stalled?

Perhaps, not to clutter up the topic, further posts on this issue should be in Vortex bursting or another new thread in Tech log?
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Old 4th Sep 2001, 14:01
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I hesitate to point out to Brit PPRuNers that a Concorde TV documentary is due on Thursday night (BBC 2?) at about 2100. Although I've not seen it, and have had nothing to do with it, I fear that it may generate some anti-journo/anti-media feeling.

But don't blame ME.

PS: What an epic thread! Many thanks to JF and covenant for educating me!
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Old 4th Sep 2001, 16:02
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BEagle

Very interested in your comments about Vulcan handling at high A of A. When the lovely bird was flying, the airshow routine seemd to comprise a steep climb, sustained 'till very low airspeed, then a 'wingover', to the right and a dive to recover speed. Given the odd handling at high alpha, that must have been 'interesting', especially for the guys in the back with no bang seats.

Cheers

SSD


PS

Just like to add my thanks to John Farley for his superb contribution to this most interesting thread.
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Old 4th Sep 2001, 21:52
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Re weights

Based on that day's met data (a twelve kt headwind, a low QNH (1008), the higher than normal temperature and the usable length of the runway), the dispatcher calculated the maximum weight as 177,930 kg. I'm unclear as to whether this took account of the non-availability of thrust reverser 2, which led to a reduction of 2.5% in the maximum weight permitted.

However, flight preparation showed a takeoff weight of 184,800 kg with one hundred passengers (but not all of the baggage) checked in, and the despatcher then started getting anxious suggesting a route with an optional tech/refuel stop or unloading baggage to follow on another flight - it was at this stage that the crew took over flight planning (in order to stretch or bend the rules?).

The max structural weight is 185.075 tonnes.

From Captain P's reading of the report it has been said that they took off at 187.2 tonnes, though in the latest report it is suggested that the accident investigators recalculated the figures, and came to the conclusion that actual TO weight was an estimated 184,802 kg.

So was the aircraft marginally below its absolute structural limit (as suggested by N1 TOGA) by the time it reached its take off position, or two tonnes overweight (cf max structural), or ten tonnes over the max weight originally calculated by the despatcher. And all of those overweight figures are without the 8 kt tailwind.

Ten tonnes. TEN tonnes? Can some-one confirm that I've got that right?
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Old 4th Sep 2001, 22:24
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After re-reading the BEA reports, I have just noticed a rather critical erratum in the preliminary report which was corrected on page 7 of the first interim report (15/12/2000).

Rather significantly to my earlier post, it says that, with reference to the control column position graph, positive values indicate nose down input rather than nose up, which was how it was shown on the key in the preliminary report.

What this means is that contrary to what I had previously believed, the pilot was constantly using nose down input to the control column from about 12 seconds after rotation. This is especially significant at around time 97667, where it appears the AoA begins to climb out of control. I had originally thought that the pilot responded by pulling the stick back through some 4 degrees (which seemed to make little sense, I admit).

It appears to me now that Marty was indeed trying to hold the AoA to around 12-13 degrees for the bulk of the flight against a tendency to go nose-up, rather than trying to hold the nose of the aircraft up to maintain height, as I had originally thought.

What a difference a little "+" sign makes!

[Edited for the usual typos]

[ 04 September 2001: Message edited by: Covenant ]
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Old 5th Sep 2001, 00:16
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Jacko - I'm pretty sure that the flight despatcher's figure allowed for the known thrust reverser unserviceability and it doesn't have a material bearing on the accident.

The "investigators' recalculation" doesn't make clear whether that was a RTOW or ATOW estimate; it was way short of the ATOW figure known to the crew and that figure was known by them to be in excess of the max permitted certificated structural TOW.

Nowhere in the report do I read a clear and accurate assessment of RTOW based upon the known 8kt tailwind, the actual OAT and QNH at the time. WHY NOT?? Informed comment in this thread puts the figure at around 177T...

Elsewhere, John Farley comments upon Vzrc2 both with the landing gear up and with the landing gear down. Also, we learn that any decrease in thrust at low speed will lead to a rapid loss of speed and an increase in Vzrc. So why would anyone ever consider shutting down any engine still producing thrust with the landing gear still down??

[ 04 September 2001: Message edited by: BEagle ]
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Old 5th Sep 2001, 02:08
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BEags: If it's 177 tonnes for that day's OAT and QNH, but with a 12 kt headwind, how much lower for an 8 kt tailwind? What ballpark difference would we expect?

Incidentally, for those who doubt it, the Structural weight limit for take off is 185,070 kg and for taxying is 186,880 kg.

The accident report suggests that the aircraft taxied out at 186,757 - 87,251 kg, and took off at 185,757 - 186,251 kg, though these are guesstimates, based on what the two Air France Concorde chaps producing the report for the magistrate regard as unduly optimistic estimates of baggage and pax weights.

Covenant: With regard to the trace of control column inputs, the idea that all the +s represent stick forward seems bizarre - do the traces show that they pushed forward to rotate (it would be a neat trick)?

Also, the AoA was held to a reasonable level (without much stick input at all), so the subject of stick inputs is hardly relevant - 2 either way hardly represents the Captain pushing forward to keep the nose down (nor pulling back to keep it up) terribly hard. Right up until the final turn, where the speed started to decay (from 208 kts down to 181, and later much lower), the AoA went up and the aircraft's rate of climb began to increase markedly, the Captain was flying it with great delicacy, nursing the aircraft into a gentle climb straight ahead. The lack of 'fighting to keep the AoA down' may also be indicated by the fact that the pilots' had wound in significant nose up trim however, and did nothing to select more nose-down trim.

Also, while the co-pilot kept shouting for airspeed, the captain kept it pegged at 199-211 kts (precision flying, under the circumstances), and the aircraft gently but steadily climbed to 182 ft as it crossed the motorway, and to 300 ft as it began the final turn.

Would you guys have done that, or would you have stayed at 100 ft and tried to get closer to the Vzrc speed as you belted for Le Bourget?

gear up:
two engines 262 kts, three engines 193 kts
gear down:
two engines 300 kts, three engines 205 kts

VMCA:
three engines 132 kts
two engines 157 kts

Did they begin turning for Le Bourget too late (forcing them into turning more tightly, with more angle of bank, than was perhaps wise)? Would you have begun turning earlier (even before crossing the motorway, perhaps turning to intercept the Le Bourget extended centreline, rather than flying a mile beyond it?) would the aircraft have departed in a gentler, faster but much lower turn?

I hear what you've all said about Delta wings stalling, but looking at the altitude/airspeed/AoA traces you can see that the aircraft effectively stalled and spun once airspeed decayed/alpha increased beyond a certain critical point.

If I'm being over-simplistic or showing my PPL limitations, please educate me!
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Old 5th Sep 2001, 04:37
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Jackonicko

The original trace in the preliminary report shows control column negative input as the pilot rotated and for the next approximately 12 seconds where it became positive and remained so for the duration of the flight. The key says "+ Nose Up". The first interim report corrects this to "+ Nose Down".

Of course, if I'd been thinking properly, I'd have realised the error since, as you say, it would be a pretty neat trick to rotate with nose down input on the control column!

I don't think I was implying that he was "fighting" to keep the nose down, more that there was a moderate amount of nose down input, contrary to what I had originally thought which led me to wonder if he was trying to maintain altitude at the expense of airspeed.

I see more clearly now , and think he was more likely, as you suggest, to have been doing some delicate speed control by nudging the nose forward under what power he had available.

To comment on your last few paragraphs, I've been trying to visualise the flight path from the CDR traces, and ended up actually modelling the aircraft for the last 30 seconds of the flight, in a 3D program I use, to try and get my head round it.

Having done that, I would venture to suggest that the real critical problem was not attitude or angle of attack, but roll. As you say, neither of the former parameters was all that excessive for most of the flight, and by the time AoA did become excessive, the roll to the left was around or above 90 degrees. At that point, lift production is purely academic. I don't think the roll angle got this bad because the pilot tried to turn too hard or too late.

I suspect it would have had more to do with control surface problems, specifically elevon failure on the left wing. Since the pilot was demanding slight nose-down during the fire, if one of the left elevons became frozen in place, it's not hard to see how that would cause an unstable situation leading to uncontrollable anticlockwise roll. It may even be that it was this extreme angle of bank which caused the #1 engine to ingest greater quantities of the burning fuel (leaking from inboard of the engines) and finally give up the ghost.

Whatever causes and effects or the actual sequence of events, by this time, the plane was uncontrollable with no more options left to the pilot. Furthermore, with this event about to happen, I don't think there was anything more he could have done at any point during the flight.

In many ways, this also makes all the other questions about TOW, CoG, tailwind, missing spacer bar, and anything else that didn't directly contribute to the fuel fire, purely academic. If your control surfaces lock up, you're in big trouble regardless of your height, speed, or anything else for that matter.

In my mind, it's back to the question of how and why the fuel leak and subsequent fire started; all other questions being interesting but not significant.

[ 05 September 2001: Message edited by: Covenant ]
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Old 5th Sep 2001, 10:03
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Jacko - the 177T RTOW figure was, I believe, an unofficial figure based upon the actual 090/08 wind passed to the crew with their take-off clearance - not the pre-flight calculated value using the ATIS conditions at that time.

We introduced 'late change' data for the Vulcan in 1978 specifically to allow for last minute wind/temp/pressure changes influencing the V speeds - and it's something we teach our VC10 students during their simulator training. It is absolutely fundamental on heavy transport category aircraft to recompute RTOW if the weather values are different to the planned values on departure; equally it is never acceptable to ignore RTOW or max permitted take-off weight limits. I would be very, very unhappy at ever flying in an aircraft whose crews seemingly ignore such limits......
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Old 5th Sep 2001, 13:05
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Covenant & BEagle,

Many thanks.

But the AoA was kept as 'tight' as the airspeed until the turn (beginning at point 7 on the photo), where roll rate also increased dramatically.

At point 8 (181 Kts, angle of bank going from 38 to 93, AoA up to 19.5) it looks as if he'd lost it - and to my uneducated eye, had over-banked and tried to 'hold' the nose up. At that point, the left wing had (sorry) seemingly 'stalled'. Four seconds later the speed was down to 136 kts, and AoA was 25.15, while the roll angle was beyond 90 (spin or incipient spin?).

1) Why climb to (and indeed through 200 ft) without coming close to VZRC2?

2) How much more airspeed might they have got if they'd flown level, rather than climbing at about 180 ft per minute?

3) Why not turn gently towards Le Bourget much earlier (the direction the aircraft wants to go, and to keep it straight you're using right rudder)?

I believe that the dispatcher's originally calculated maximum weight was the RTOW for that aircraft on that day in those conditions (177,930 kg - very close to our 177 tonnes) and the fact that the actual TOW was so much higher was what made him start kicking up a fuss. Is it interesting or sinister that the accident report does nothing to explain why his RTOW figure was so much lower than the one they came up with as being 'appropriate', or is this the difference we'd expect for the different RWs - the dispatcher planned for 27, they planned for 26?

From the report:

"Based on data on the wind (a twelve kt headwind), the QNH (low, 1008 hPa), the temperature (higher than the norm) and the usable length of the runway, the dispatcher calculated the maximum weight as 177,930 kg. However, flight preparation showed a takeoff weight of 184,800 kg with the one hundred passengers checked in.

At about 09 h 30, the dispatcher informed the duty officer of the weight problem, without however specifying the QFU used for the calculation. The duty officer first thought of using another aircraft, then tried to resolve the technical problem with the reverser and finally thought of loading the baggage onto another flight.

On his side, the dispatcher studied two hypotheses for routes (one direct and one with an optional technical stop) and loading so that the flight could take place in terms of its weight.

A little before 10 h 00, the crew called the dispatcher who informed them of the problem. The crew informed him that they had asked for the replacement of the failed pneumatic motor on reverser 2, asked him to file a direct ATC flight plan and told him that they were going to take over the flight preparation themselves."
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Old 5th Sep 2001, 15:56
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Jacko, perhaps prompted by V1TOGA's posting I can only say that as a pilot and having lost both parents through an airliner crash I, and I think many others, applaud your efforts to obtain a clearer understanding into whatever facts can be reasonably ascertained in the sad ending of that Concord flight.
It can only be hoped that the information thus gleaned will be of use to others. In the meantime I follow your findings and the informed discussions with great interest and hope like us all to see Concord airborne again.
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Old 5th Sep 2001, 21:13
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Jackonicko
I hear what you've all said about Delta wings stalling, but looking at the altitude/airspeed/AoA traces you can see that the aircraft effectively stalled and spun once airspeed decayed/alpha increased beyond a certain critical point.
How can you see that? The reason I keep digging in this stall thing (which by now we should agree that Concorde doesn't) is because it somewhat implies pilot error. You have no facts to support vortex bursting, which as far as I can read from the Vortex bursting thread would require a hypothically high AoA, and the aircraft would probably become unstablile before reaching that AoA. All you know is that for unknow reasons the aircraft rolled over and crashed (speculation as to why may be e.g. fire damage to the control surfaces). This does not necessarily imply that the wings were not producing lift. However, as Covenant mentioned it is academic to discuss lift if it doesn't act in the opposite direction of gravity (i.e when roll is greather than 90 deg).

At point 8 (181 Kts, angle of bank going from 38 to 93, AoA up to 19.5) it looks as if he'd lost it - and to my uneducated eye, had over-banked and tried to 'hold' the nose up.
See above, and atleast support speculation with some facts. Otherwise one might just aswell say that aliens shot it down with a laserbeam

At that point, the left wing had (sorry) seemingly 'stalled'.
If talking about a conventional wing (that stalls ), the right wing would stall first in a left turn because it has the highest AoA.

3) Why not turn gently towards Le Bourget much earlier (the direction the aircraft wants to go, and to keep it straight you're using right rudder)?
Are you suggesting that they should have let it sideslip towards Le Bourget (keeping the wings level and let the nose drift)? That is not the most aerodynamic efficient way of flying.

After 150+ post on this topic, I for one, feel more and more confident that the accident was indeed caused by the tireburst and subsequent fire. If it was the fire damage that caused the crash, overweight, engine shut down, etc. would not have been factors that directly contributed to the crash.
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Old 5th Sep 2001, 22:15
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Cosmo,

Sorry to upset you with my inexact terminology.

JF wrote: "The increase in drag that you refer to (when a delta flies slowly at higher angles of attack than ordinary wings can reach without stalling) is the dominating characteristic of such flight. Indeed it leads to the notion of the zero rate of climb speed (Vzrc)..... If you slow down to this speed you (by definition) need full throttle just to hold that speed in level flight. One knot (or more) slower and you are in big trouble. You must lower the nose so as to reduce lift and the associated induced drag, which means you give away height in order to pick up speed. Just like the stall recovery case for conventional types. When I left that scene the boffins were seeing this Vzrc as the direct equivalent of Vs for all certification purposes. It is not a stall but it has the same effect as one and margins (1.3 or whatever) would need to be provided to keep pilots away from it just like the stall."

This is why I used the word 'effectively' when I described the process as stalling. Perhaps I should have written:
"looking at the altitude/airspeed/AoA traces you can see that the aircraft departed and spun (there is no doubt that it's final manoeuvre was a spin) once airspeed decayed/and induced drag due to excessive alpha increased beyond a certain critical point."

To suggest that the cause of the departure was control damage (even the BEA does not claim that the controls were not functioning at the time of impact) is as speculative as my suggestion that the pilot mishandled the aircraft (so perhaps you too should support speculation with some facts, or shall we both agree that aliens shot it down with a laserbeam?).

(And to be fair, I don't think my 'speculation' was so very far fetched: "At point 8 (181 Kts, angle of bank going from 38 to 93, AoA up to 19.5) it looks as if he'd lost it - and to my uneducated eye, had over-banked and tried to 'hold' the nose up." does it?)

With regard to wings stalling in a turn, isn't the out-of-turn wing travelling faster than the inside wing, with faster relative airflow? In an erect left handed turn which wing stalls first?

To clarify, I'm not suggesting that the pilot should have sideslipped all the way to Le Bourget (though in those circumstances, keeping the ball centred might not have been a prime concern). Do we even know that the aircraft wasn't perhaps already sideslipping, with the constant starboard rudder pressure?

May I challenge you on two questions, since you express yourself so confident that you 'know' what happened?

1) How confident are you that the combination of excessive weight and undercarriage problem didn't exacerbate or even cause the tyre blowout, or, if not that, that it didn't cause the ignition of the fuel?

2) How confident are you that shutting down the No.2 had no effect on the outcome?

3) Can you explain how the outcome would have been the same had Marty traded altitude for airspeed, by not climbing above 100 ft, and had he made a faster, gentler earlier turn towards Le Bourget? Or is it OK for you to speculate, but no-one else.

I don't know the answers, I don't have a firm idea of what caused the tragedy, but I do believe that there may have been multiple factors at work. Why do we need to over-simplify it without evidence that it was simple?
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