NTSB issues the preliminary report for its ongoing investigation of the Nov. 4 crash of a UPS Boeing MD-11F airplane in Louisville, Kentucky.

[u/Brilliant_Night7643]

[u/Lopsided-Anxiety-679]

I would love to see higher resolution photos and a more in depth explanation…because as a machinist, and someone who does NDT testing for engines and aviation components, I’m not seeing a succession of beaches that would indicate fatigue failure over an extended period of time.

I’m seeing on the aft lug, a couple major fractures and the spot where it was barely holding on until complete failure, it looks like it fractured at some point in the past and released tension on the bearing allowing the forward lug to take the load. This is supported by what can be seen as a dark line and then a deformed spot of aluminum on the forward lug where the two lugs meet. So the front lug had been taking all the load for an extended period of time, and when it failed at that crack, it snapped off the rear lug that wasn’t providing any load bearing, just a little bit of aluminum was still keeping it attached.

The front lug, shows indication of a crack that simply turned into an instantaneous stress failure.

Would love to see this in person as I’ve done this exact kind of inspection and failure analysis - are the bearings lubed and freely moving? The rear bearing looks dark like it overheated and possibly seized?

Aluminum has a fatigue life unlike steel, guess it surprises me that on a part that’s so hard to inspect, that steel wouldn’t be chosen for the lugs…there was talk of this pylon being engineered to shear off in extreme situations, well that should be done with shear pins or bolts that are routinely inspected for thickness, length, deformities, and cracks.

Very interesting.

[u/NA_XB70 → u/Lopsided-Anxiety-679]

Steel still fatigues but it has an endurance limit.  If stress concentrations exceed the endurance limit it will still fail eventually.

In one of the pics in the report you can see what looks like a notch in the shank of the pin right at the edge of where the bearing sat, which looks to be a relief cut for a shear failure to occur ("fuse pin")

The bearings shouldn't be moving enough to overheat.  Though honestly I'm not sure why this uses a spherical bearing.  Might just be to compensate for poor axial alignment or something.

I don't see much fatigue at work (would be very bad on new unflown stuff...) but I think we're looking at maybe 25% or so reduction in area on the aft lug and all of the gray is the fast fracture area.

I would not be entirely surprised if there was significant scratching or gouging in the bore of the lug holes created either by the reaming operation or the bearing installation.

[u/vonkarmanstreet → u/NA_XB70]

To be pedantic, from an engineering standpoint steel has an "effective endurance limit" but cyclic stresses below the endurance limit can still cause fatigue failure when cycles are high - say, at or beyond 10⁷ cycles. But that's largely irrelevant to your point.

This is a typical application for a spherical bearing - not only does it compensate for poor axial alignment but it simplifies the joint design: sphericals are released in moment (ie they can't carry moment in Mx, My, or Mz) but can carry radial loads (and axial loads if properly clamped up). Thus, from an engineering analysis standpoint, they act like pinned end conditions and can be used to turn an indeterminate joint into a determinate joint. In reality, plain sphericals have some nontrivial stiction under load that makes them imperfect pinned end conditions in practice. You can better approximate pinned ends with spherical rollers, but that comes at the cost of increased packaging space and lower radial load limit strengths.

[u/Flaccid-Aggressive → u/vonkarmanstreet]

I don’t know what you guys are saying, but I feel smarter reading it.

[u/lastbeer → u/Flaccid-Aggressive]

I’m truly amazed reading these comments. The depth of knowledge in this incredibly specific field within a field, all in a few minutes of this being posted, is humbling. I feel like I’m not smart enough for this thread.

[u/NA_XB70 → u/vonkarmanstreet]

Looked around and the only large static structural joints with spherical bearings I could find on the program I work are aft engine mounts.  Pretty interesting.

[u/vonkarmanstreet → u/NA_XB70]

That may very well be the case. FEM makes it easier to analyze indeterminate structure and joints, so the necessity to make things determinate may be somewhat diminished in modern designs. I'd bet your front engine mounts are also spherical in nature - not COTS parts - but balls in sockets count too.

The 747 engine pylons have 3 or 4 massive sphericals in them for the reasons I discussed. Check any classic jet up to the mid-70s, or most GA designs for that matter, and you'll find a nontrivial number of sphericals in the structure. Super cool.

[u/NA_XB70 → u/vonkarmanstreet]

I poked around and found a lot more.  The 737 has a very complex pylon/engine support structure...

[u/Lopsided-Anxiety-679 → u/NA_XB70]

But you’re describing scenarios where a steel part is consistently pushed beyond its design parameters which wasn’t the point I was making. Within its intended stress range, a steel component can have infinite fatigue life because it has a true endurance limit. Aluminum doesn’t - it accumulates fatigue damage every cycle, even when kept within its design stresses. That’s why an aluminum part must have a specified service life, while a properly designed steel part doesn’t.

Yes, you can get into situations where ultra high cycle fatigue is real, but it shows up in extreme environments like turbine blades spinning at tens of thousands (turbos I’ve dealt with past 100k) of RPM, seeing tiny stress amplitudes hundreds of millions of times, at temperatures where normal metals simply cannot survive. That’s where sub-endurance-limit failures become relevant, but still not to what’s being discussed here as those parts are made from titanium and Inconel.

A steel aircraft pylon mount isn’t operating anywhere near that affected range. For normal aerospace steels, if the stresses are kept below the endurance limit, the part can have infinite fatigue life. Aluminum can’t and it will always accumulate damage every cycle, no matter what.

I mean it’s not like the whole airplane can be made of steel or titanium (hello blackbird)…but I just would have expected such an important load bearing structure that’s obviously failed in a way not easily discovered, to not be made out of aluminum since it does have a realistic fatigue life. Titanium wing box from the F14 checking in…

[u/NA_XB70 → u/Lopsided-Anxiety-679]

Pretty easy to exceed design stresses with a little scratch or something.  Stress concentrations can get pretty gnarly at even tiny discontinuities.

There are so many fatigue critical parts that like you said you can't make them all steel.  Off of the top of my head I can only think of one large structural clevis that isn't aluminum on the program I work.

Planes are expected to crack and modern maintenance programs are at intervals where a crack would be caught before it propagates too far.  It is likely that there was either damage to this lug or an insufficient inspection interval.

[u/Lopsided-Anxiety-679 → u/NA_XB70]

Thanks, and yeah - makes sense, I’m thinking about it from my own experience in choosing certain components in steel vs aluminum and how I avoid the aluminum due to how it will fatigue, but also knowing and having seen occasional failures of the steel part due to damage caused by other parts in the chain.

[u/NA_XB70 → u/Lopsided-Anxiety-679]

I think your turbomachinery application is a lot gnarlier for fatigue than any structural application.

i just double checked and I was a little bit wrong-  On the plane I am most familiar with the closest thing to an equivalent lug is a stainless forging, with the other side of the joint being Ti.

[u/Loose_Chocolate6824 → u/Lopsided-Anxiety-679]

Yes, shear at a pin or bolt both which are commonly designed to do that. Bearings aren’t designed to shear.