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Comment by jacobgkau | Hacker News Reader

A failure due to metal fatigue would still be a failure to properly maintain the aircraft, right? I know by "improper maintenance," you're referring to actual improper things being done during maintenance, and not simply a lack of maintenance. But I'm reading things like "the next check would've occurred at X miles," and, well... it seems like the schedule for that might need to be adjusted, since this happened.

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Yes, when I said "improper" I meant the American 191 maintenance crew took shortcuts. The manual basically said "When removing the engine, first remove the engine from the pylon, then remove the pylon from the wing. When reattaching, do those things in reverse order." But the crew (more likely their management) wanted to save time so they just removed the pylon while the engine was still attached to it. They used a forklift to reattach the engine/pylon assembly and its lack of precision damaged the wing.[0]

Fatigue cracking would be a maintenance issue too but that's more like passive negligence while the 191 situation was actively disregarding the manual to cut corners. The crew chief of the 191 maintenance incident died by suicide before he could testify.

[0] https://en.wikipedia.org/wiki/American_Airlines_Flight_191#E...

> The crew chief of the 191 maintenance incident died by suicide before he could testify.

To be clear, a crew chief (Earl Russell Marshall) did. But he wasn't directly involved in maintenance of the specific DC-10 that crashed. Or at least, I haven't found a source saying he was, and some sources say he wasn't. https://www.upi.com/Archives/1981/03/26/The-wife-of-an-airli...

If the (FAA-approved) maintenance schedule says "the next check should occur at X miles" and X miles hasn't happened yet, then it's not going to be classified as improper maintenance -- it's going to be classified as an incomplete/faulty manual.

Now, of course, if that maintenance schedule was not FAA-approved or if the check was not performed at X miles, that's going to be classified as improper maintenance.

A more likely metric for this particular inspection would be hours or cycles, in other words starts and landings, not 'miles'.

According to various comments the plane was nowhere near the cycling for a special detailed inspection of the aft pylon mount lugs: SDI is at 29200 cycles and the plane had 21043.

There was a lubrication task in October, but according to tech comments that would just in greasing the zero fittings, no taking apart anything.

Those pictures of that torn up part are pretty hefty, that's a clean break, no stretching as far as I can see it just tore the material in half, you can see the grain. There does not seem to be any torsion either so most likely that was the first part to go, if the problem had been in the engine then I would expect this part to be mangled, not pulled apart. What stress damage there is occurred shortly after that first break. A valid question would be whether or not that crack was there before take-off or not.

I'm very curious what the metallurgic analysis of the mirror part on the other wing will come up with, especially whether there are any signs of stress fractures in there. If there are that will have substantial consequences for the rest of the still flying MD11's, about 50 or so are still in service.

The preliminary report mentions fatigue cracks on both sides of the aft lug, and one side of the forward lug, with the other showing no trace of fatigue, only overstress.

From this it seems like the aft lug was way fucked, and the forward lug was hanging on for dear life, until it could not.

Yes, that's exactly how I read it. The aft lug was the first to go, the forward lug shows signs of stress so it held on longer.

I don't think they're going to be skimpy on the metallurgy report so looking forward to the analysis of the mirror parts on the other wing. Those will tell without a doubt whether it was maintenance related or age related fatigue. Right now I would bet on the latter but the former could also still be a factor, for instance, that bearing might not have had enough lubricant.

Zerk fittings, not zero (stupid autocorrect), the grease fittings.

Grease nipples. They had a perfectly fine name already, which autocarrot was perfectly happy to spell.

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.

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.

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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