Comment by jacquesm | Hacker News Reader

Comment by jacquesm | Hacker News Reader

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Even if it was an inspection or maintenance issue (which given the kind of failure and available data looks increasingly doubtful, though it can not yet be ruled out) this part failed in a catastrophic way when it should have had ample engineering reserve over and beyond the load to which it was subjected. It just snapped clear in half those breaks are indicative of a material that has become brittle rather than that the part deformed first and then broke due to excess stress.

In other words, a slow motion video of a camera aimed at that part during the accident would have shown one of the four connections giving way due to fatigue cracks and then the other three got overstressed and let go as well, in the process damaging the housing of the spherical bearing.

The part at the bottom of page 9 is the key bit. Now I very much want to see the state of the mirror part on the other wing, that will show beyond a doubt whether it was maintenance or an over-estimation of the design life of that part.

It would also be interesting to have a couple of these pulled from the fleet and tested to destruction to determine how much reserve they still have compared to the originally engineered reserve.

According to the preliminary report, 3 of the 4 showed fatigue cracks, and the 4th overstressed. So yes, agree a random sample of these parts should be pulled from the fleet and tested - but something pretty crazy was happening here re: fatigue.

That it was so far from the maintenance schedule to be inspected AND that the fatigue cracks seem to have formed in areas that would be hard to visually inspect anyway points to either a engineering problem (especially bad, since the DC10 problem of a similar nature happened in roughly the same parts, albeit due to different abuse - you’d think the engineers would overdo it there, if nothing else), or some specific type of repeated abuse that particular pylon received, which is pointing more to a design problem.

Re-reading the 1979 report might be helpful here. This isn't my field, but it seems that the engine is attached "hard" to the pylon, then the pylon is attached via a bearing mount system to the wing frame. The bearings wear out, and hence have to be replaced (not sure how often, but they were doing it on the entire fleet prior to the 1979 crash). The 1979 investigators thought that the fatigue cracks were caused by removal of the entire pylon/engine assembly as one unit (because that put excess stress on the aft bearing, they suspected due to support being provided from below by a fork lift). After the 1979 accident engines had to be removed first, then pylon, supposedly removing that cause for mount cracking. Perhaps there was another cause.

Before the 1979 accident, engines also had to be removed first.

Airlines have to follow the approved maintenance manual procedures; that manual called for engine removal and installation from a pylon that was on the wing. American was improvising a maintenance procedure without the legal authority to do so, resulting in 191.

> you’d think the engineers would overdo it there, if nothing else

No kidding, especially given the lack of redundancy in the design.

There is nothing here to say it being a maintenance issue is doubtful. It could quite literally be a similar issue to Flight 191, we don't know yet.

It did have ample engineering reserve beyond the load it was subject to... before fatigue damage initiated a crack which then grew until there was no reserve left. The question is why the fatigue crack initiated prematurely? Maintenance damage? Analysis mistake? We don't know yet.

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