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Old 11th Mar 2014, 13:57
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TheShadow
 
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On a Wing and a Prayer?

""The missing plane was involved in a crash in August, 2012, when it damaged the tail of a China Eastern Airlines plane at Shanghai Pudong Airport, according to unconfirmed reports.

....in the incident, the tip of the wing of the Malaysia Airlines Boeing 777 broke off..."


http://cdn.feeyo.com/pic/20120810/20...0951017177.jpg
China Eastern



One might assume that the wing(tip)(?) destroyed during the MH370 airframe's 2012 taxiing accident was repaired properly. However (see image) it did take one almighty clout when it struck the China Eastern acft's tail a few years back. That wing actually lost quite a few jagged feet of span. It was a classic case of a dominant force striking an immoveable object - and the failure was well inboard. A very pedestrian and mundane accident you might say. The fore-aft (i.e. chord-wise) propagation of the collision forces would have tended to concentrate, via the main- and sub-spar(s), at the wing-root - but the wing-root wouldn't necessarily be the weak focal point of any future turbulence (or metal fatigue) induced failure. I'll explain why below.

The wing out as far as the engine pylon mounting is quite beefy and rigid, because it has to contend with the inboard fuel tanks and the engine's weight and thrust - as well as minimizing the flexure caused by its inertia (you've seen how much the wing-mounted engines appear to move inflight relative to the fuselage, right?). From the engine's pylon-mount out to the wing-tip is less rigid, i.e. it's designed to allow a high aspect ratio wing to soak up turbulence-induced flexing. The mid-span of this pylon-to-tip distance was where the 777's RH wing was torn off. Not just damaged or dented, but TORN OFF. Did Boeing replace the entire RH wing? No it didn't. That starboard wing was "repaired". You'd have to wonder how the assessment of an "adequate" repair was done, given the extensive damage to a large section of outboard (i.e. missing) wing. Wingtip taxiing accidents tend to be dismissively "ground rash". This one may have been just a bit more than that. Wing repairs are several orders of magnitude more consequential than fuselage or tail repairs. "G", I wonder why? - you might ask. Obviously the considerations go far beyond the cosmetics of incidence, conformity and airflow-friendly re-skinning. Justifiably, the main concern would've been for ongoing operational structural integrity. This often leads to a very beefy repair scheme. No engineer would endorse or certify any repair less sturdy and reliable than afforded by the original structure..... and also because the question of "how much is enough?" is a tough one when considering a major wing repair scheme and the implications of undetected additional hidden damage. Replacement is often the better option. But that might not be what the insurance company is prepared to pay for, especially since there's a considerable history of simply replacing the wing-tip or its endplate winglet fairing after a bit of "ground rash". But this one was a little more than just a wing-tip. Try and convince an insurance assessor of that, in a circumstance that's so very familiar to insurance assessors and their bean-counters.

Catastrophic failure of a wing repair inflight is almost incredibly incomprehensible. But then again, the China Airlines 747's tail-scrape repair led to a high altitude instantaneous break-up at max cabin pressure differential a few years later - and consider as well the Boeing repair on the JAL 747 rear bulkhead that failed and led to a fatal loss of control? These events (and there have been similar others) should be convincing enough for an argument that a "sufficient" structural repair may not stand the test of time and the unforgiving operational environment. Consider also that a fuselage repair and a wing repair is chalk and cheese from a structural fatigue point of view. The wing-span is always "soaking up" an incredible continual imposition of very variable flight-loads, whilst the fuselage is merely taking a pressurization "hit" just twice per flight. So what are the chances and likelihood of the wing failing in the vicinity of the repair-patch? What could cause that and what would be the subsequent chain of events?

Consider that a wing is designed to have a natural harmonic and that this is achieved by a laterally harmonious gradation of each wing's structure from its wing-root to its wing-tip. Each side's original wing will have an almost identical natural harmonic (i.e. if you loaded up each wing-tip on the ground and simultaneously released those loads, each wing-tip's diminishing movements around the mean would be graphed as identical - as each wing's oscillation faded away cyclically to its static position). Would this be the case with a repaired wing? Not really, as the extra internal structure introduced by the repair's ironmongery would significantly change that sides' wing-flexure characteristics. Would Boeing engineers have compensated for this by ballasting (or beefing up?) the other wing? Not at all likely methinks.
Would a flight-crew detect any such dissimilar lateral flexure characteristic in the aircraft's gust responses? You have to take into account the "active controls" used in modern airliners for gust alleviation. Flight Control Computers compensate for turbulence-induced wing movements by minuscule aileron responses. It's designed to soak up and take the "bounce" out of turbulence and promote a more comfortable ride. If the wing on the collision side was slowly failing (i.e. structural fatigue damage propagating along micro-cracks in its repair doublers?), would the "active flight control" system disguise and (to a certain extent) alleviate or mitigate this? Possibly. Alternatively, could it exacerbate the cracking of a failing spar? Don't know .... but someone might. My suspicion is that "active flight controls" would promote crack growth in a weakened structure that was spider-webbing towards eventual failure. It would achieve a repetitive concentration of stress in its ongoing opposition to natural flexing.

If the taxiing collision occurred just two years ago, the MH370 aircraft may not have undergone a major servicing since its wing repair. Such servicings are predicated upon total flight-time and certainly that interval's not ever varied just because a major structural repair has been carried out. So anything going on inside that wing may have gone unnoticed in the long interim. It's unlikely that Boeing would have mandated any "how's it going?" non-routine inspection to see whether that repair was holding up OK - or to see whether there had been any further fatigue damage or developments (perhaps further inboard) that was beginning to manifest itself. As the manufacturer, Boeing would've been inclined to demonstrate "sight unseen" confidence in its repair work. Arrogant or not, the FAA wouldn't intervene. It's a Boeing supplicant.
When would such a culminating inflight failure be most likely? Possibly while the aircraft was still at its heaviest and on encountering clear air turbulence at or near top of climb (or whilst accelerating to cruise Mach). Would that be its most vulnerable point? If that repair gave way, (as most inadequate or improper wing repairs eventually do), what would be the sequence of events? Remember that up until the point of failure, the gust alleviation system would have been disguising (and even moderating?) any signs of imminent failure. In my opinion any such failure in turbulence would be in a DFDR identifiable two parts - firstly the progressive failure (over a few seconds) of primary structure (wing spars and internal bracing buckling as flight loads quickly transfer to inferior sub-structure) - and then the rapid deterioration of the scenario as the secondary structure failed under the increased loadings (the secondary structure being the wing-skin -as the skin does assume much of the inflight loading). As the wing folded, the aircraft would begin to roll to the right quite rapidly (at circa 180 - (increasing to about) 360 degrees per second - around its fore-aft axis). The pilots would be out of the equation at this point - as the aircraft spiralled rapidly down. However there are reports of a garbled transmission. This is likely to have been during the first phase of failure as the pilots became aware that something was happening. However they are unlikely to have discerned that the wing was slowly folding.... or rapidly losing its structural integrity.

What about ACARS reporting of these sudden developments? I'm wondering just what it could (or would) have reported to the company by way of exceedances or untoward abnormalities (??). Engines and systems would still be running normally, but the g forces in the spiral would've been quite high. In any case, would the ACARS report transmission succeed in a rapidly rolling and spiralling scenario? Or must its antenna be more or less static and upright in order to retain a synchronous lock with its associated satellite? Lastly, would the DFDR record of prior flights retain any record of differentially dissimilar flight control activity that may have indicated any deteriorating structural integrity in the RH wing? Probably not, as the compensating activity would've been via mutually synchronous aileron inputs - and not just the RH or LH spoilers. What would be the effect of landing arrivals impact be upon any propagating failure? Likely it would not tend to add to the deterioration of any cracking. It would be acting in the opposite sense.

If the pilots were disabled by the g forces and shock of a rapid roll into a spiral descent, the engines may have remained at high power and thus the aircraft's spiral would have tightened. The impact would have been at high speed under high positive g by an intact fuselage - and the damage would've been smithereening..... i.e. all fragments would likely have lost flotational dimensions - at least for anything visibly significant to aerial searchers.

Feel free to dismantle or disparage or to relate an alternative version of such an explanation. It is starting to look like a case of the simplest explanation being the most likely one. Falsetto passports don't necessarily promote accidents. Actual accidents always come complete with herrings rouge to some degree. Sometimes crashes just stem from the unexpected consequences of false economies. I'm reasonably confident that this will be the lesson learnt. They're always hard lessons and often they are quite revelational. Wing loadings are up. Think about that and respect that fact. It's an attritional process, post-repair degradation. Even when it's "just ground-rash", failure is never an option. How many other simplistic ground-rash repairs are out there awaiting their turn to insult Boeing or Airbus expertise? Think about it. Now repair the despair.

Accidents happen. But sometimes/oft-times, they are just a result.
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