Icelandair 757 Gear collapse at BIKF
Can't blame Icelandair for doing that... the interim report is pretty clear! It looks like you would need to fabricate a new nut to the spec in the CMM. That's something that is hard to miss... so perhaps the nut was fabricated but the wrong one fitted? Or fabricated to the incorrect specs...? All speculation for now, we'll have to await the final report.
The repair involved recutting the threads on the swivel for a second time (having been done originally in 2008), which of course then required a correspondingly (further) undersized nut to be used during reassembly. It appears that instead of a 1/8" undersize nut being used, a 1/16" undersized one was fitted similar (or possibly even the same one) as that which had been removed.
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Hopefully the two major components in this saga have a paper trail that will permit scrutiny and perhaps come up with some useful detail, but what interests me at this point is the final link or two in the chain.
To this end I had an in depth discussion with a colleague last night as to just at what point in our respective careers we'd have been able to determine simply by 'feel' that there was something awry with the fit of a nut+bolt, and was it something we were taught, or did the knowledge somehow just occur one day? Neither of us could say, we both started messing with mechanical things around 8-10 years of age and went through later training in our respective fields of expertise, but we didn't recall being specifically taught about what 'felt' right, yet we'd be pretty sure we could both identify an issue such as this, either by the nut simply being too loose on the shaft thread, and/or by the rotation of the torque wrench as the nut was tightened. Although we didn't reach a definitive conclusion it might be that we'd be able to do this by the age of say 18-20.
Thus this incident raised questions in our mind around what training is given on fundamentals such as tightening nuts (do apprentices even get the opportunity to overtighten, to fit 'wrong' nuts and to see/feel what that's like?), and just how one might ensure a system that is able to deal with possible issues such as occurred here. To some extent it would be useful to know if the nut was fitted by, say, a first year apprentice and then 'checked' by a senior person (I say 'checked' not to impugn that senior person, but simply because what can you really check once it's in place?), or whether it was fitted by a journey[wo]man.
Knowing the answer to this might allow a focus on certain aspects of training, environment, or even competency assessment, but in any event it behoves us all to think well around how we might better approach such a simple task as fitting a nut, or how we might properly train a new person coming on board. I hope it's clear that I'm looking here at the system, not the individual, although I don't believe we can totally divorce the vagaries of the individual from the mix (at the end of the day if someone, after a lot of training, just doesn't 'get' it then they just cannot adequately perform that task - and should not be doing so).
Quite possibly we'll find this issue has come about as another example of the holes in a cheese lining up; the supplier supplied mismatched components, the fitter fitted them and didn't notice something was awry, the inspector didn't see anything awry, the pilot (on their walkround) didn't see that the nut had moved or 'loosened' after a few cycles, but how do we go about making a cheese with less holes? Or is there another lateral way of better seeing the individual holes before they line up to result in a calamity?
There will, almost certainly, never be a perfect answer, but nothing should stop us from striving for it. It'd be interesting to hear what people think about how a seemingly simple task such as this might be better addressed, or if it's even possible?
FP.
Agreed, which is why I asked a question about this a couple of posts above
I used to race formula cars and had a business in Seattle maintaining and renting same. I can't imagine starting a nut and not feeling something was not right. Either it fit or it didn't and that would send a loud signal and ask for corrective action. I guess this is simple experience, but it sure seems basic..... especially when you apply the toque wrench.
How could the wrong size nut be threaded on? It wouldn't even start......
Quite possibly we'll find this issue has come about as another example of the holes in a cheese lining up; the supplier supplied mismatched components, the fitter fitted them and didn't notice something was awry, the inspector didn't see anything awry, the pilot (on their walkround) didn't see that the nut had moved or 'loosened' after a few cycles
The flats on the nut threads look pretty wide to me, and the minor diameter is around .020 above the spec for a 1/16" undersize nut. That makes me think that some material might have sheared off the thread tips uniformly, and the fit on assembly might have been a little tighter than the post-accident measurements indicate.
