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AF 447 Search to resume

Old 2nd Sep 2010, 12:53
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Why Aluminum Bar "36 g" broke -Part 1, Stress Factors

HN39 asked why I thought the material of part "arm 36g" mattered:
The NTSB report says about the equivalent part of the A300: "The support strut and its attachment fitting are made of an aluminum alloy."
In the context of Bearfoil's structural preferences, I would like to understand why the material is important. Whatever its material or method of manufacture, that part needs to be sized to support an ultimate load of 120,000 N, and a limit load of 80,000 N. The important evidence is that those design loads were exceeded.
Thanks for the information that it is an aluminum bar. I read the 587 report (after 447 Report #1) but either missed that, or did not attach enough significance to it at the time to remember the detail.

Metal structures are a bit better understood, having a longer history. That said, their design requirements tend to be writ in rather round numbers. For example, I could correctly say that a suddenly applied impact load would cause a stress in this bar twice that of the stress resulting from the same load gradually applied. That is, an 18 g load, applied as an impact, would fracture this bar.

But then there are 2 bars, and one probably bears on its bolt (these must be individual capscrews because of the divergent angle between the bars) before the other. There is in some applications a factor to reduce the effectiveness of bolt groups because of this; IIRC it is applied as a reduction in tabulated permissible bolt loading. I forget if it is a reduction to 90% or to 80%; let's say 80% in this example, to keep the math simple-- I want to illustrate the nature of the issue with somewhat real numbers, but not split hairs over any exact number. So now, a 15 g impact load would fracture one of the two bars.

Now the hole in the bar is a stress raiser. That factor can easily be two or three, although I don't think it is necessarily as high as 3-- we just can't see under the bolt head to judge (it may be an oversized washer-faced head) the hole size. This stress appears as a linear singularity rising to that value along the inside surface of the hole bore. Say the factor is 2. Then a 7.5 g impact load would raise the stress along this line to ultimate strength of the material, and start a crack.

But a fatigue crack might start at the limit load, given enough repetition. That is, fatigue cracking is a factor to inspect for if 5 g impact loads can result from rudder operation. I don't know at the moment; it is just a question to ask. There is a crack formed on each end of a diameter at right angles to the axis of the bar (but see below). If these cracks progress 1/3 the way into the aluminum on either side of the hole, a 5 g impact will break one of the bars. If they have only gotten half that far, a 10 g uncharacteristic (let's say) impact load will break one of the bars.

So you see that the 24 g limit load required by the US regulation is a sort of semantic thing. It is arrived at by multiplying some known impact loading determined in the distant (I assume distant) past to have caused trouble which was judged economically avoidable. It's because a certification test would not apply an impact load; the test setup would apply load at a steady rate over a specified number of minutes (say 2 to 5, as the technician has to take notes-- or did in my time).

So the limit load required is not an impact load; the part is new, so fatigue cracks are not present; metallic ductility may reduce the effect of stress concentration at the bolt hole to some extent; the requirement for one bar may be simply set at half the requirement for two-- remember regulations are a legal document, not an engineering lesson book.

Similarly the ultimate load factor, the additional "capacity" amounting to half the limit load, is intended for other intangibles-- things difficult to quantify, either at all, or at reasonable design or fabrication cost. These could be random defects in the metal, burring of finish on surfaces subject to fatigue, bolts not filling the bolt holes, bolts not concentric with the bores; uneven stress distribution over the bar section below the taper reduction in thickness; secondary (bending) stressing caused by corrosion binding the bolts in the bolt holes; and so forth. If you figure it, it is in the 24 g's; if not, it's in the 12 g's above that-- just a rule of thumb.

Not a very precise science, is it? And the more complex the object to be tested, the more costly the test; the fewer tests likely to be run; and the less likely the results will be repeatable from one test to the next. Well, the VS itself is an example of such a complex object.

This is just a general explanation. I'm sure that HN39 understands all this. I'd just say that I could not have made such a clear explanation had the link arm been fabricated from a composite material. I'd explain further, but it's a big job which I should not tackle late at night. Anyway, that's why I wanted to find out what the material was.

BTW, I did finish a further explanation late last night, and realized in doing it that I knew why the aluminum bar "36 g" had broken. It broke in lifting the rudder as the VS broke loose to somesault forward over the fuselage. The sequence is (a) VS breaks loose in carbon-composite over rear anchor set; (b) VS breaks loose at middle anchor set, pulling it out due to the brief greater strength of the carbon-composite under impact loading; (c) this frees the VS to rotate forward, accelerating; (d) the aluminum bar "36 g" breaks as it attemps to keep the rudder pintles properly aligned while the pintles lift the rudder in this sudden rotation that also starts as an impact. Far less than 36 g's of upward acceleration is required to break this bar. I have the detailed explanation written out but it is too long for this post (and the margin isn't big enough to hold it either). I'll post it latter.

OE
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Old 2nd Sep 2010, 14:53
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Old Engineer

Thank you for that analysis, it clears up, and frames, the discussion; it breaks the dam of understanding. I have been trying to crack it with a feather. I build, and the kind of language you use I have difficulty in framing.

The hinges are best described as "pivot" hinges, their mount is rotated 90 degrees from a butt hinge, where the leaves support a "pin" . Fracture at tip 36g is my working theory, what remains of the "strut" seems undisturbed, and as I have said, the bolt appears rather "unmoved" as well. Corrosion, a service life of high deck angle plants (no tail strike in the logs, is there?), and the final (hypothetical) High Altitude, high speed flutter may have sent the "tip" on the way. I still think that if the arm had been under (planned for) tension prior to failure, the Bolt would be displaced. If a capscrew, it has sufficient strength, if its face is "cupped" No one would attach a straight shouldered capscrew to a (round, cylindrical) seat. By this I mean a "saddle washer" may accomodate the joint, but the photo is not divulgent to that level. edit to say, that is not accurate, a washer would remain, and is not visible. That leaves a machined cup out of the arm itself, to ensure surface area and a tight marriage. This would attenuate the focal and linear stress on an unadorned joint. It would also mean even less cross section for the arm at its stressful monting. Perish the thought a flat Arm was merely bolted the "barrel". Impossible.

You mention the separation as two fold, involving two regimes of massive energy, divided by a stasis between. BEA makes much of the forward "flyaway" member, but has little to say about the prior, the vertical, necessary to yank the (whole) clevis from between two frames. They call it Vertical acceleration, no harm done, that is sufficient to capture a vague mental picture. For the VS to have remained uncorrupted at its roots (quite possible) the angle must be very close to strictly Vertical. The a/c had a forward component of velocity, and both together have stressed this assembly in extreme and unconsidered ways.

Without going down a trail to its pragmatic and unproductive end, one hasn't a chance of a weighted solution, arrived at by entering every dead end in the maze.

It is possible, even to be desired, to couch a mundane investigation in interesting prose. It is never (morally) allowed when the motive is ulterior, as in the Law, or in Economic gain. To sustain a popular interest in an investigation can be done. Likewise to discourage probing questions and out wait the public with a never arriving train of discovery can be done as well.

One has the resources, the others must have passion.

One can "comfort" the "afflicted", the other can afflict the comfortable

bear

edit I have re read Old Engineer a third time. Everything is in there to confidently examine all the disciplines. HazelNuts39, do you see how the stated numbers in certification can be very "soft"?

Last edited by bearfoil; 2nd Sep 2010 at 15:20.
 
Old 2nd Sep 2010, 16:50
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Old Engineer...

Thanks so much for that post (#2100). It's the clearest explanation I've seen of that particular matter (separation of the V/S).

Though I'm no engineer, I'd love to see your "long version" sometime. If it's too big for this forum please feel free to send it via PM.

Thanks again,
grizz
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Old 2nd Sep 2010, 17:13
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Originally Posted by bearfoil
HazelNuts39, do you see how the stated numbers in certification can be very "soft"?
bearfoil; I'm entirely happy with the conclusion of a long debate.
regards,
HN39
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Old 2nd Sep 2010, 17:16
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Hazel....

Ahhhhhh... music to our collective ears!

Thank you, thank you, thank you.

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Old 2nd Sep 2010, 19:13
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PBL
I am not encouraged ..... when I read of "Volumetric Area" in m^3 or "Force" given the dimensions of m.kg/sec (this is equally treated as a statement of energy where it actually has the dimensions of momentum)
Yes, and yes again, I should have been more careful with my words. Volume would have done nicely and likewise moment.

Old Engineer; Bearfoil; HazelNuts39 and others

Thanks to OE for a well thought-out description on the function of Arm 36g and the manner in which it failed. I look forward [along with I suspect others] to the notes associated with OE's summary of its failure in due course.

Cheers
mm43

Last edited by mm43; 3rd Sep 2010 at 00:04.
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Old 2nd Sep 2010, 19:15
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Yes! Thank you Old Engineer for your instruction and clarity about how the VS likely departed the aircraft. As with Grizzled, HN39, bearfoil, mm43, Finn47 and probably many others I, too, would love to see your detailed description. If as you say it's too large for a post on the thread, I could create a PDF file and post a link to it, if you would PM me the source.

The discourse here has sprung back to reason and reasonableness. Aaaah, relief. Only if BEA/France does NOT announce a Phase 4 search in the coming days will this thread become shrill again, if not bitter. My opinion: the CVR and FDR from AF447 are orders of magnitude more valuable to us than a few still-corked bottles of 1898 vintage Champagne lying in the Titanic's muddy debris field. Standing by...

GB
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Old 2nd Sep 2010, 23:58
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GB- I fear the thirst for information for all of us will go and remain unsatisfied if we expect the BEA to provide the libation we seek. Thus I humbly suggest that we will be seeking other sources for a likely explanation, probably one which in one way or another has already been offered. I hope that my previously stated suspicious will be proven incorrect, however, like you, and this is now September, standing by.
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Old 3rd Sep 2010, 07:00
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Comments--"Arm 36g": Implications for separation of VS

I appreciate the thanks for my study of the forces surrounding the failure of "arm 36g". Perhaps it is more a case of being glad that I was able to put together a written summary that was actually understandable. In any case the effort to do so really focused the mind.

At the time, I just concentrated on the one piece-- the aluminum arm and the immediate area around it. Its failure could be analysed without going into a lot of speculation. Well, the answer or endpoint of any analysis had to be that it broke at the bolt hole. And then a picture of the break turned up (meaning one clear enough to see almost the exact starting point of the crack). And after I'd helped the cause by having only one of the two bars break, someone commented that's exactly what had happened (while I was still checking the spelling).

By helping the cause, I mean finding the least amount of force that would explain why a part broke. Today I browsed around a bit in the qestion of how much force it would take to remove the VS. I looked at HN39's calculations and his sketch of the dimensions of the anchorage points. He is right-- the static pull to get it tumbling forward is 66 g's, perhaps even more.

That was a problem to him. It is a force that seems too high for the small damage seen to some debris. I have myself been reluctant to disagree with Bearfoil's instinct that there had to have been some damage at altitude, to the VS.

The only way that reasonably low forces (low to my mind, anyway) can remove the VS at water level is if there was prior damage to the aftmost anchorage area of the VS, making both clevises ineffective. Damage in this area is up in the carbon-composite. I think side loading from torque about the vertical axis of the fin would fit the bill here. Torque is caused by rudder action.

If that is the case, VS separation at water level could be accomplished by a forward deceleration of 16 g's, applied as an impact force to the forward four attachment points. That's kind of a nominal minimum; perhaps 16-20 is a better way to express that.

With this deceleration, the "arm 36" will have an upward lift of 10 g's required of it by the departing VS. Otherwise, if the VS attachments were undamaged at water contact, the force at "arm 36" would have been 40 g's upward. With this force, both of the pair of aluminum bars would have broken and we would be seeing a lot more damage in this area.

So the study of the aluminum bar fracture, after reflection on the total picture today, seems to compel the conclusion that the damage at the bottom rear of the VS, in the carbon-composite, had to have occurred at altitude, prior to water entry.

That is, in my previous sequence of events in the loss of the VS-- "a, b thru f"-- "a" occurred at altitude and due to side loading, likely torsional; and only "b thru F" occurred after water contact.

I appreciate the offer of help in placing some of my notes offline to save bandwidth. There is some background material on my work with fiber-composites that is more general than specific.

OE
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Old 3rd Sep 2010, 09:23
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old engineer, many thanks for your last 2 very informative postings, they now form for me and i am sure many others a very clear picture of just how the VS separated, and i am now in the camp with those that believe it did so on impact with the ocean. I wonder with your computational is it possible to calculate trajectory?
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Old 3rd Sep 2010, 14:24
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Old Engineer

I for one am immensely satisfied with your reasons for VS failure. I caution others not to rely too heavily on instinct, (though I may have!) It is invaluable if used appropriately. I found my inability to complete the picture frustrating, it is a bit embarrassing I took what may have appeared to be an "All or Nothing" position. I consider it less likely the assembly separated at altitude.

There is a caveat, naturally. Although the water impact is easier to "picture" as of now, there are two things. One, the Arm36 tip may have departed at altitude, if the Rudder was subject to emphatic Torsional Reversals, (perhaps even flutter). Two, to ennable the great load required to "flyaway" the VS, (diminished somewhat by your work),

Hazelnut's
figures and intuition remain, and I believe the sudden Horizontal cease of motion was ennabled by the sinking of the Nose, at the end of the "S" curve I propose is the profile of the CG from tail impact to cessation of motion (absent the final buoyancy sequence).

Let's not forget that the tip may not have been present on the a/c at Take-off. A night Walkaround would have spotted it, perhaps, then again...

bear
 
Old 3rd Sep 2010, 20:57
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Old Engineer wrote:-
So the study of the aluminum bar fracture, after reflection on the total picture today, seems to compel the conclusion that the damage at the bottom rear of the VS, in the carbon-composite, had to have occurred at altitude, prior to water entry.
... and

Bearfoil wrote:-
Although the water impact is easier to "picture" as of now, there are two things. One, the Arm36 tip may have departed at altitude, if the Rudder was subject to emphatic Torsional Reversals, (perhaps even flutter).
Which leads to:-
  1. How might have damage to the bottom area of the rudder occurred at altitude?
  2. Does the RTLU WNG at the beginning of the ACARS sequence have some relevance?
In the case of (1); vortex flutter due to an overspeed excursion could be possible? My eyes tend to tell me that the damage has both horizontal and vertical components??

In relation to (2); I am not sure that a RTLU WNG will normally be created on change from Normal to Alternate Law. Maybe someone with SIM experience of that situation can offer the answer. I have previously posited that the rudder must have been "booted" for the RTLU WNG to have been generated.

Old Engineer
's analysis of Arm 36g, its relationship with the rudder and the V/S, along with the impact separation sequence of the V/S has been extremely helpful. The new task (IMO) is to try and determine what if any airborne events were culpable along with the impact damage described by the BEA.

mm43
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Old 3rd Sep 2010, 21:53
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Old Engineer:

First of all: Really Great Work !!!

I have a careful question regarding:
I browsed around a bit in the qestion of how much force it would take to remove the VS. I looked at HN39's calculations and his sketch of the dimensions of the anchorage points. He is right-- the static pull to get it tumbling forward is 66 g's, perhaps even more.
Do you think the weakening of the mounting structure could have also happened by simply impacting the water with the tailcone first?
In other words: could it be possible that the supporting structure was damaged first by the impact and only then the VS departed ?

I'm alway a fan of simple explanations, although sometimes reality takes a more complex path...
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Old 3rd Sep 2010, 23:32
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OE, I still have in mind a severe tail strike situation.

I'm an EE not an ME so I could easily be all wet here. But I envision that the overall tail section design would be such that the pressure vessel would not be ripped open with a very severe tail strike. I figure it would break just aft of the cabin's end leaving the pressure vessel intact not spilling cargo. If that is the case, the stringers on the bottom break leading to a bending moment somewhere up among the clevis joints. The scenario I have in mind does not use acceleration to pull anything loose, it uses simple static stretch on the clevis joints with the front and rear of the VS as the pressure points and the clevis pins all going into varying degrees of tension. That would produce the damage to the nose of the VS that is very visible. I imagine it should have produced a characteristic damage to the rear of the VS as well.

Can that work into your analysis? And did the break on "36g" come with the force in the right direction to even entertain a scenario like the above?

(I am envisioning an attack angle closer to 10 to 20 degrees as the plan hits the water. But the wave effects might have been enough to do the trick even at 5 degrees.)
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Old 4th Sep 2010, 07:58
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In the answers offered to questions today (well, yesterday now), I just wanted to preface generally that I've formed a scenario of known failure events combined with the forces necessary to cause them. As I came across more clearer pictures, I had more events and had in part to modify the scenario.

My following answers or comments on the questions raised are based on making the my answer fit the scenario I put together as it now stands, slightly different from 2 days ago. If I had to change the scenario thinking again tomorrow, my answers now might wind up somewhat off the mark.

bia botal

By trajectory, I assume you must mean the angle of descent into the water just prior to contacting the water itself. I would have to say no, other than that the wings were approximately level. I had originally accepted per Report #2 that the rudder had pulled the arm downward in breaking it; only I thought the force was less. It was only when I saw that the arm(s) had pulled up on the rudder, in a later more clear picture, that I realized the break could not be the indicator of vertical deceleration on hitting the water. So any trajectory would fit, up to the point of being so steep as to cause 10 g's of vertical deceleration.

bearfoil

Yes, you are right that I am assuming that the break in "arm 36" is a fresh break. Which is to assume that the investigators did look for that, could confirm it, and would not have pointed to the break as evidence otherwise. But they didn't say that in so many words; instead they said their paragraph (I imply this from the preface) could now be put in final form. The things omitted from these reports make it awkward for the outsider.

mm43

1. It does not matter where this damage occurred.* It does matter that the damage to the fin at the rear attachments occurred at altitude.* It may matter that the rudder continued to work (the report said that it was stop-limited to 7.9 deg. IIRC) if the posited upset was such that the rudder was required to exit from it.

2. I suspect the WNG does have some significance. There has been some discussion. I had to deal with control systems and their reliability to some extent; all I can add is to say that fully testing a system with multiple states and inputs rapidly becomes very difficult. That is because each input or state increases a factorial parameter for tests. This factoral is equal to the number of machine states; plus the number of inputs, taken once for each of their individual states; plus who knows what for comparing 3 computers. Factorals increase very rapidly.

One could assume that WNG has occurred because the rudder lock has moved to not be at its most restrictive limit; and has done that at a time when the computers cannot know if speed and altitude permit that position. Obviously if fin damage at altitude is required,* it may be explained by excess rudder travel being available.

In your paragraph following these items: Flutter-- Normally one would like to have all the operating regime below the onset of flutter, analogous to the design of engine crankshafts in regard to torsional vibrations. But I have no idea what the margin is, or how it is affected by the variable configurations and attitudes of the airframe. Damage-- Agree; my eyes see lateral shift damage on one side, and tension damage on the other, both up in the carbon-composite area; and that the VS composite shell overall has not retained any obvious premanent distortion from the forces that caused this damage.

henra

I think that the scenario* I proposed is not compatible with more than 10 g's downward impact, because then the aluminum rod have broken before the VS broke loose, by the downward pull of the rudder, leading to a deformation of the standoffs different from that seen. I also am also uncomfortable imputing damage to this robust tailcone area from underneath by a slapping impact, seeing the minor damage to the recovered rack of flimsy cubicles.

JD-EE

Your suggestion of the lower longerons in the tailcone being broken by the pull of the VS fin on the attachment points, with resultant release of those points is interesting. There may be a substantial wedging action there. I don't see any conflict there with the senario I am using, which only requires that the pull from the fin pulls the attachment clevises loose.

My scenario* would only limit the attack angle such that the impact at the tail would not exceed 10 g's.

*Note for all comments: Means "scenario" as described in the second paragraph. That is, possible but not necessarily what actually happened. Omitting to qoute the question immediately above my comment or answer (to save bandwidth) is, on rereading, a little awkward in 2 or 3 spots. But the questions are right above in order on the same page & I am too tired to re-do the post.

OE
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Old 4th Sep 2010, 11:31
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Flutter-- Normally one would like to have all the operating regime below the onset of flutter, analogous to the design of engine crankshafts in regard to torsional vibrations.
A moot point maybe, but possibly a bad analogy in this context - engine crankshafts usually run through their torsional vibration speeds, sometimes work near their 2nd harmonic (I think!). Torsional dampers (metalastic bushed pulley wheels etc) control their worst effects on NVH and crank fatigue life.

The crank is simply made strong and stiff enough to have a sufficient fatigue life - one 4-cyl in-line engine I knew well had an easily recognised resonance at 5500 rpm, smoothing beautifully above that. Overdrives would in the early days help one avoid sitting at that rpm, their use often being ascribed solely to reducing fuel conusmption

Whereas as you say, flutter speds have to be a known margin above placarded safety speeds.

====

Thanks for your analysis OE... it might be worth reflecting that the VS could well have been the one major component that had no counter-balancing drag (hydraulic) to it's forward inertial forces under this hypothesis - sticking up high as it does?

Last edited by HarryMann; 4th Sep 2010 at 11:42.
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Old 4th Sep 2010, 12:48
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Originally Posted by bearfoil
Machaca; First, Sir, let me catch my breath. That pic is exquisite. (...) How is this wonder accepted by the Fuselage?
Some readers may not be aware that the A400M has a large rear-loading cargo door, and it is shown here:
A400M Cargo hold

Originally Posted by henra
... could it be possible that the supporting structure was damaged first by the impact and only then the VS departed ?
In my imagination the impact can be divided in two phases: vertical, then horizontal. The rear fuselage has just the perfect shape for minimizing resistance to forward motion, and hence impact forces are initially mostly vertical, pushing the light tailcone aft of frame 91 upwards against the rudder, crushing the rear fuselage structure while pushing the V/S upwards, causing the vertical acceleration that breaks arm 36g, and causing the fuselage break-up illustrated in mm43's #2067. Then large horizontal forces get into the act, as the water hits the blunt face of wing center section, engines, and wing, and rushes at high speed into rear freight hold and cabin open at the front.

BEA concludes from examination of the V/S that it separated in a forward motion relative to the fuselage. The frames were bent backwards, assembly 86-87 was slammed backwards against the V/S root rib, breaking the lateral load pick-ups in compression, and assembly 84-85 slammed backwards and broke its pick-ups in tension. Finally 86-87 was slammed forward against 84-85.

regards,
HN39

EDIT:: From BEA#2:
1.12.3.5.6 Examination of the fuselage parts (remains of the skin, frames and web frames)
The fuselage was sheared along the frames and centre and aft attachment lugs by loads applied bottom-upwards.

Last edited by HazelNuts39; 4th Sep 2010 at 21:47.
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Old 4th Sep 2010, 15:03
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Old engineer;

Thank you for your extensive and elucidating analysis. There are however a few details that I don't fully understand. For example:
The only way that reasonably low forces (low to my mind, anyway) can remove the VS at water level is if there was prior damage to the aftmost anchorage area of the VS, making both clevises ineffective. Damage in this area is up in the carbon-composite.
Could you perhaps elaborate on this? The mid and rear attachments were all recovered with the V/S and no damage to the carbon-composite parts was reported (*). The only damage to composite parts was in the back-up structure of the front attachments which separated from the V/S and presumably remained attached to the fuselage.

regards,
HN39
EDIT:: (*) Except the damage caused by the rearward rotation of the frame assemblies.

Last edited by HazelNuts39; 4th Sep 2010 at 16:20.
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Old 4th Sep 2010, 16:49
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, and hence impact forces are initially mostly vertical, pushing the light tailcone aft of frame 91 upwards against the rudder, crushing the rear fuselage structure while pushing the V/S upwards, causing the vertical acceleration that breaks arm 36g,
HazelNuts,
that's exactly the scenario that crossed my mind as well.

OldEngineer:
I was thinking that maybe the tip of the tailcone hit first, starting to cause damage to the cone especially when the HS starts to enter the water and the vertical drag in the water increasing enourmously, basically rupturing the entire tail structure.
In case of the middle and aft mounts it was the supporting structure that broke and not the clevisses. Maybe the aluminum structures surrounding the massive mounts had already been weakened by these forces.
That was my idea how the VS could have broken loose at a rather low decelleartion.
This sceanrio could also fit with the deformations of the frames in the tail (which seemed to show longitudinal compression in the upper parts) described by BEA, if I understood their description of the damage patterns correctly.
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Old 4th Sep 2010, 23:31
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HN39, keep in mind that once the fuselage cracks open OR the elevators hit the water drag goes up astronomically. That would tend to further rip things apart as well as bend the tail feathers upwards violently. Now that I think on that visualization I am not sure the tears in the clevis joints would match what is seen on the recovered VS.

I still think it is worth considering. It IS a way to impart a huge upwards force on the joints and perhaps on the rudder itself depending on its position at impact.
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