Comment by class3shock
12 hours ago
It depends. This aircraft was made near the beginning of the MD-11 production and if the original analysis for the fatigue life of this location was wrong, then you would expect to see that appear in older aircraft first. If that ends up being the case then it's not an inspection or maintenance issue, it's an engineering failure. Given aerospace accident history I would say that is less likely than some maintenance issue but we won't know for sure for a bit.
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.
> you’d think the engineers would overdo it there, if nothing else
No kidding, especially given the lack of redundancy in the design.