Originally explained on the blancolirio channel on YouTube -

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.

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PunchyHamster 7 hours ago | parent | next [-]

I think far simpler explanation is "the back part failed first and engine is making thrust so it just flipped over on now-hinge mounting

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loeg 22 minutes ago | parent | next [-]

That's why it flipped upwards, but not why it flipped towards the body of the plane / to the right.

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mrb 6 hours ago | parent | prev | next [-]

Yup. That's exactly what experts said of American Airlines flight 191 which was basically the same engine mount, same failure. Engine flipping over the wing.

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jsr0 3 hours ago | parent | next [-]

The failure of the pylon appears to be different. On AA 191, the pylon rear bulkhead cracked and came apart. In the case of UPS flight 2976, the pylon rear bulkhead looks to be in one piece, but the mounting lugs at the top of the rear bulkhead cracked.

Admiral Cloudberg has a great article on AA 191 that covers exactly what happened: https://admiralcloudberg.medium.com/rain-of-fire-falling-the...

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dreamcompiler 5 hours ago | parent | prev [-]

American 191's engine mount failed because of improper maintenance. It remains to be seen whether this failure had the same cause or if it was something else, such as metal fatigue.

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jacobgkau 5 hours ago | parent [-]

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.

	▲	dreamcompiler 4 hours ago \| parent \| next [-]
		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...
	▲	tremon 4 hours ago \| parent \| prev [-]
		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.

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inferiorhuman 4 hours ago | parent | prev [-]

Flipping backwards is what caused the engine to fly to the right and land to the right of the takeoff runway. The stills in the NTSB preliminary report clearly show the engine flying over the aircraft, to the right, and then heading straight down.

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cj 8 hours ago | parent | prev | next [-]

There might be some truth in that. But the report doesn't confirm that theory.

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rconti 8 hours ago | parent | next [-]

What theory? That the mount failed? Or the rotation of the engine in the photos going up and over the fuselage?

It seems like both are true, but doesn't necessarily prove WHY the mount failed.

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scottlamb 7 hours ago | parent | next [-]

Not an aviation expert at all, so take this with a grain of salt, but I think "the spinning engine resists changes to the direction of its spin axis" offers two important insights:

* 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.

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psunavy03 3 hours ago | parent [-]

> 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.

	▲	magicalhippo 2 hours ago \| parent [-]
		Not the motor mount but the pylon mount. The pylon was found attatched to the engine with both engine mounts attached. But yes, the report mentions stress factures where the aft pylon mount failed.

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cj 7 hours ago | parent | prev [-]

That the engine was flung into the fuselage due to gyroscopic forces.

	▲	inejge 6 hours ago \| parent [-]
		Well, some force flung it inboard and above the fuselage (gods, that CCTV stills sequence.) Knowing that the engine rotates CCW, there are not many candidates.

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philip1209 8 hours ago | parent | prev [-]

I'm presenting it "useful not true" - not an RCA.

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londons_explore 8 hours ago | parent | prev [-]

I assume such forces are calculated and added in when deciding hot thick to make those mounting brackets.

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loeg 7 hours ago | parent | next [-]

Yes, obviously; MD-11s aren't flinging engines off the wing every single takeoff. A 34 year old airframe may or may not actually match design strength, though.

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HumblyTossed 5 hours ago | parent | prev | next [-]

Yep. Now do 3 decades of metal fatigue.

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supportengineer 5 hours ago | parent [-]

Did I understand the report correctly that the part was scheduled to be replaced in the future after a certain number of hours, it just hadn't hit the threshold yet ?

	▲	tremon 4 hours ago \| parent [-]
		If you're referring to this quote (excerpted from the AVHerald article linked elsewhere in the thread), I don't think so: > 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.

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baggy_trough 8 hours ago | parent | prev | next [-]

Yes, but the point is that this moment of the takeoff is when a failure that's been waiting to happen is most likely both because of the thrust and the gyroscopic resistance.

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shtzvhdx 6 hours ago | parent | prev [-]

Aluminum has limited loading cycles

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dreamcompiler 5 hours ago | parent | next [-]

I'd be very surprised to read that the aft lug that cracked (and the bearing it contained) were made of aluminum. They were almost certainly steel or Inconel.

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CoastalCoder 2 hours ago | parent | prev [-]

Wouldn't that be true of all cast metal objects?

Or are some metals impervious?

	▲	harshreality 37 minutes ago \| parent [-]
		No; roughly, yes. In theory I suppose everything (even perfectly manufactured steel or titanium) fails even under light loads as # of cycles goes to infinity simply due to stochastic grain degradation over long periods of time from mechanical stress, radiation, etc. For practical engineering purposes, however, iron (steel) and titanium alloys have an endurance limit after which metal parts no longer degrade due purely to more cycles (on operationally-relevant timescales); aluminum and other metals, however, will. > 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. https://en.wikipedia.org/wiki/Fatigue_limit I don't know where metallurgists draw the line between metal fatigue purely do to cycle-weakening (e.g. of aluminum) and metal failure due to crack propagation (which can happen with any metal due to manufacturing impurities or damage after manufacture). The difference may be that fatigue cracks can be detected, and their propagation tracked, with less invasive, less involved methods (e.g. ultrasonic or dye).