The timing and manner of the break make a lot more intuitive sense when you consider that the engine is essentially a massive gyroscope. As the plane starts to rotate, the spinning engine resists changes to the direction of its spin axis, putting load on the cowling. When the cowling and mount fail, that angular momentum helps fling the engine toward the fuselage.
https://en.wikipedia.org/wiki/El_Al_Flight_1862
Pictures of that accident are not for the faint of heart so think twice before clicking.
Planes can safely land with 0 engines, though this is obviously "not ideal." See: Gimli Glider.
(Mostly putting that out there for others.)
Admiral Cloudberg has a great article on AA 191 that covers exactly what happened: https://admiralcloudberg.medium.com/rain-of-fire-falling-the...
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...
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...
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.
There was a lubrication task in October, but according to tech comments that would just in greasing the zero fittings, no taking apart anything.
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.
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.
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.
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.
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.
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.
No kidding, especially given the lack of redundancy in the design.
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.
It seems like both are true, but doesn't necessarily prove WHY the mount failed.
* why it failed at rotation (the first/only sudden change of direction under full throttle) rather than as soon as it was mounted onto the plane, while taxiing, as soon as they throttled up, mid-flight, or on landing. This is important because at rotation is the worst possible time for this failure: no ability to abort take-off, no ability to land safety under no or severely limited power, little time to react at all, full fuel. Knowing these failures are likely to manifest then stresses the importance of avoiding them.
* why it failed in such a way that it damaged the rest of the plane.
Not so much what was wrong with the mounting in the first place, if that's what you're asking. Presumably it was designed to withstand the forces of this moment and clearly has done so many times before.
The report seems to suggest metal fatigue in the motor mount may be a possible culprit.
But yes, the report mentions stress factures where the aft pylon mount failed.
There are lots of candidates for a failing engine yeeting itself in any direction.
For the precise trajectory, certainly; for the general direction, not so much. Could you describe a combination of forces that would have thrown that engine to the left of the direction of travel? (We're talking about this accident, not any engine anywhere.)
Foreign object gets yeeted to the right. Internal component gets yeeted to the right. Engine exploded on its right side.
I think each of those is more likely than gyroscopics since the engine went to the left. Not left and up.
Whatever you're describing, it's not this accident. Over and out.
My broad comment is that gyroscopic precession having any critical role in this is incredibly far fetched. That said, I've never flown or worked on a turbofan so ¯\_(ツ)_/¯.
The final reports are always much more comprehensive.
> At the time of the accident, N259UP had accumulated a total time of about 92,992 hours and 21,043 cycles [..] A special detailed inspection (SDI) of the left pylon aft mount lugs would have been due at 29,200 cycles and of the left wing clevis support would have been due at 28,000 cycles
This isn't talking about replacement, only inspection; and it wasn't going to happen in the near future: 7k cycles at four flights/day means inspection is due in 5 years.
It wasn't doing four flights per day. As a long-distance cargo aircraft it was doing two flights per day, and I doubt it was flying every single day of the week.
So we are talking about at least 10 years before that inspection was due.
Or are some metals impervious?
> The fatigue limit or endurance limit is the stress level below which an infinite number of loading cycles can be applied to a material without causing fatigue failure.[1] Some metals such as ferrous alloys and titanium alloys have a distinct limit,[2] whereas others such as aluminium and copper do not and will eventually fail even from small stress amplitudes.