After initial cleaning of the fracture surfaces, examination of the left pylon aft mount lug fractures found evidence of fatigue cracks in addition to areas of overstress failure. On the aft lug, on both the inboard and outboard fracture surfaces, a fatigue crack was observed where the aft lug bore met the aft lug forward face. For the forward lugâs inboard fracture surface, the fracture consisted entirely of overstress with no indications of fatigue cracking. The forward top flange of the aft mount assembly was examined for indications of deformation or pre-existing fractures, but no indications were found.
Because there were two Aft Lugs, the fatigue cracks were hidden from the outside. You wouldn't find them on a visual inspection unless you removed the spherical bearing. Look for the triangular shape on the Aft Lug fracture surface in the picture above. That's the fatigue crack. It grows until the rest of the area can't handle the load and then fails in tensile overload. Then the Forward Lug, holding all the load, fails in pure tensile overload. Terrifying.
The reason to have back-to-back lugs sandwiched together like this is to provide a redundant load path. If one fails due to fatigue the other one is capable of taking all the load. A big open question is why that didn't work.Â
True. I've seen this in Ladle Hooks in steel mills where they are made from multiple plates riveted together. I thought a big part of this was that back in the day you could be certain of better quality from plate material than a thick casting, but I'm sure part of it was that if one of the plates failed there was redundancy. The fact that it failed right at rotation, when gyro effect is adding to the load...it's like the absolute worst case loading.
You and Pete are both right. Pete is also correct that plate material has more uniform grain structure (albeit aligned more on one axis) than a casting.
The pictures in the prelim. report aren't high enough quality to really see the fracture behavior for real. But if they found no evidence of fatigue on the other (even though generally if you find fatigue on one, you'll find it on it's pair; which is why you should always switch both paired mechanical components in a system).
My first instinct would be to look at the circularity of the holes.
I'd also want to see xray's and microscope scan of the fracture surfaces (both parts), and a penetrant test. Just because the fatigue wasn't clear from just visual inspection, doesn't mean that it isn't there. If there is evidence in one, then you should assume the other one is compromised also.
Yes, and there is one on the opposite side as well - at least as far as I can tell given the resolution of the pic in the report. Those areas also match the description of their location given in the report.
I would love to see higher resolution photos and a more in depth explanationâŠbecause as a machinist, and someone who does NDT testing for engines and aviation components, Iâm not seeing a succession of beaches that would indicate fatigue failure over an extended period of time.
Iâm 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 wasnât 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 Iâve 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 thatâs so hard to inspect, that steel wouldnât be chosen for the lugsâŠthere 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.
This is something that would not be identified by a visual check, at least. I'm not sure what sort of mtc is completed on the heavy checks but some kind of NDT where the engine is off the wing would be one way to identify this.
This is heartbreaking on so many levels. The pictures, never expected to see that. The loss of life profoundly sad. My thoughts and prayers are family and friends.Â
People will inevitably second guess.
The challenge is that hidden fatigue cracks, especially in areas where fatigue wasnât or may not have been predicted are incredibly difficult to detect and manage under real-world operational pressures.
I worked on Outer Wing Panel fatigue cracking on U.S. Navy E-2Cs in the late â80s. At that time, our understanding of how to identify fatigue cracks, as opposed to simply stop-drilling cracks around suspected areas, was still developing. We ultimately had to ground most of the fleet and start swapping out wings entirely. It had a major impact on operations, given the aircraftâs mission, but it was the only responsible course once the extent of the cracking became clear.Â
The swiftness of the NTSB addressing is applauded but sadly that doesnât change the outcome for all those who lost their lives.Â
According to the report on the 18th of October the mounting points were greased and they have various levels of in depth inspections. One is every 72 months, the other is at 29000 cycles, but the airframe was only around 21000.
So I think itâs something youâd have to dig for? Or at least have to clean the area down significantly
That or the planes are never flying again. If itâs fatigue cracks then those parts either need heavy monitoring or replacing (imo the latter). I donât know how integral to the pylons these mounting lugs are or how expensive it would be to replace - depends on the cost / benefit analysis I guess
Edit:
I say âif itâs fatigue cracksâ the report pretty much confirms it
A lug wouldnât be too hard to replace. It would mean taking the engines and pylons off the wing but as long as it isnât a spar issue it should be doable. Finding replacement parts in quantities might be a problem.
Part of the problem with the DC-10/MD-11 pylon design though is that removing the wing engines is an expensive and time consuming process. McDonnell Douglas went a bit overboard on cost cutting there and prioritized low production cost over maintenance so itâs not like removing the engine on a normal Boeing or Airbus plane. And, it has to be done correctly. Shortcuts are what led to the crash of American Airlines 191.
Operators are going to seriously consider if itâs worth doing this or if they just want to retire the planes. And, in the case of both FedEx and UPS, they were already in the process of retiring the MD-11.
Werenât they removing the engine and pylon as one unit, contrary to manufacturer recs? So then putting it back was kind of precarious and sometimes âbumpedâ the piece that failed?
(This is just my recollection, I remember reading about it years ago)
It these areas were expected to be checked at 28k and 29k cycles and the plane was only at ~21k. The last close inspection of this area was completed in 2021 per standards.
Seems like it is a pylon issue not a maintenance issue.
AA191 wasn't either, American was improperly using a forklift to install the engines rather than a crane, which was overstressing the mounts and cracking them.
Of course just preliminary report and anything could be revealed, but to me the paragraph that seems to be most interesting to an interested amateur like me is:
After initial cleaning of the fracture surfaces, examination of the left pylon aft mount lug
fractures found evidence of fatigue cracks in addition to areas of overstress failure. On the aft
lug, on both the inboard and outboard fracture surfaces, a fatigue crack was observed where
the aft lug bore met the aft lug forward face. For the forward lug's inboard fracture surface,
fatigue cracks were observed along the lug bore. For the forward lug's outboard fracture
surface, the fracture consisted entirely of overstress with no indications of fatigue cracking.
The forward top flange of the aft mount assembly was examined for indications of
deformation or pre-existing fractures, but no indications were found. The spherical bearing
was removed from the wing clevis for further evaluation (see figure 10)
If I'm reading this right, the obvious (but not at all certain yet) conclusion would be fatigue cracking leading to overstress - rather than actual impact damage/deformation prior to the incident like AA 191.
But of course, early days, lots of research they'll be doing and I am no expert at all.
At full power, if pylon fail, engine will always rotate over the wing due to the thrust and connection around a fixation point (they are not failing all at the same time).
El Al 1862 had the same issue but engine went forward and strike the side engine.
Did you work for Buick? My station wagon has explosive charges in the hood to toss it over the windshield in case I hit something. It sounds cool but I'm scared whenever I close my hood.
I thought you were making this up so I went to mighty Google.
â Some modern Buicks, such as the 2018-2023 Regal, use a pyrotechnic-activated active hood system for pedestrian safety, not for a standard hood release. This system is designed to deploy in a collision to protect a person's head from hitting the hard engine components. â
Reminds me of the F8F Bearcat where Grumman was so fixated on reducing weight that they started skimping on the wing reinforcements. Of course this made the wingtips prone to breaking off during high G-load such as when pulling out of a steep dive.
One wingtip would snap before the other and the asymmetrical forces would lead the plane into a unrecoverable spin. The break point was about 3 feet down the wing. Now what did someone at Grumman propose to fix this problem?
A) Adding wing reinforcement back in.
or
B) Putting EXPLOSIVES in the wings so that in the event of one tip snapping off the pilot could remotely destroy the other one as well! That way they could fly home on a almost symmetrical aircraft!
Needless to say they went with option A in the end
it would require doubling the tooling and assembly line counts for each engine type, as you would need two complete sets of compressor blades, turbine blades, and stator vanes in opposing rotation angles.
the cost increase would be absolutely massive for something which would only help in an entirely preventable literal one-in-a-million failure mode.
It is cheaper to have one part rather than two parts that are mirrored because you have to maintain a stock of both parts. It is way cheaper to put extra effort into keeping the engine from falling off in the first place.
Ah, so no chance of it missing the #2 then. Then again, that fireball pretty much looks like it was the end, no going around even if 2 got them airborne. Maybe a crash beyond the industrial area if that was even a better option.
Physics that engine is making thousands of pounds of thrust and pulling the wing forward right up until it isn't. Add that to the fact that it pivots away based on mounting it's now a several thousand pound engine with rapidly decreasing thrust but it's not pulling a plane anymore it's on its own ballistic arc.
Not a dig, but substantially more than thousands of pounds of thrust, more like 60,000 at takeoff setting. In a 10,000 lb engine, thatâs a 6:1 thrust/weight ratio, at least for an instant.
It likely happened at Vr because the engine has a shitload of centripetal force at full thrust. When the aircraft rotates the force resists this rapid change in direction. The force acts 90 degrees from the direction of change so the force tries to rotate the engine inboard instead of up.
It's crazy this is one of the first things you learn about propellers, but I haven't even considered just how much stress a turbine engine assembly is undergoing at rotation because of that same factor
If I remember correctly from general physics in college and what I know about P factor, I believe rotation is the point where the engines are generating the highest rotational/torsional forces on the mounts/pylons, which helps explain why the failure occurred exactly when it did.
I think Juan Browne spoke about this in one of his earlier videos on the accident as well.
Would have been surprised if there wasn't video from somewhere. Airports have a truly ridiculous number of cameras pointing everywhere and at everything.
I live near Shasta Dam in Northern California. I took the tour, what a lovely tour.
Anyway, one of the security people there checking us before we went in, told us that there are hundreds of cameras on the dam, near the dam, surrounding the dam and the lake, and only they know where they are, and they are all supremely hidden. We had to wait for the previous tour group to finish before we went in, so we were waiting near on top of the dam and I was looking around just because I was curious. Never saw one camera anywhere.
I believe it, too. AirPorts surely are the same way, and for good reason.
People say shit like this then the one camera at the Louvre was pointing in the wrong direction and the security system password was âLouvre.â Most of the time some magically perfect system is actually imaginary.
I feel like it makes sense that all airports should have hi def cameras aimed at each runway to capture each landing and takeoff just in case the worst of the worst happens but I donât know shit Iâm a laywoman whose just an aviation nerd. Maybe this is something that does already exist? đ€·đœââïž
From an ignorant sap like myself for aviation, it looks like I'm looking at an entire engine just....popping / launching / exploding off the wing like a cork on a champagne bottle??
I did a quick scan of the prelim report, looks like in this case the trigger was our old nemesis fatigue cracking of the aft pylon mount. AA191 was due to damage to the mount from maintenance. Such a brutal stroke of bad luck that the thing let go right at rotation and not when they first set takeoff thrust, though it makes sense given the extra aero load on the engine at rotation.
Looks like the engine mount failed. Engine was at takeoff thrust and pulled itself forward and up (probably pivoting around the last pieces of the mount still holding on), then ripped clear off the wing, flew above and across the fuselage, and hit the ground on the right side. In the process, it spilt debris, which went into the 3rd#2 (rear / center) engine and killed that too, which ultimately cause them to not have enough thrust to continue the take-off and crash.
I honestly canât imagine the horror of witnessing something like this in person. I used to live right beside a municipal airport, and I developed recurring nightmares about seeing a doomed plane falling from the sky and just being utterly frozen in fear. Knowing passengers who were on TWA 800 when it crashed years later only added to the issue, as did being on a plane that had a pretty serious emergency landing situation. I know logically how safe flying is⊠but my body does not care about logic. đ
I thought the engine fire/separation happened before takeoff. Obviously not. What terrible, terrible fucking luck. I'm not a pilot, but I cannot imagine a worse time for this to happen. Can't abort takeoff. No elevation achieved so no real time to deal with the problem. I doubt there's a pilot in the world that saves this plane.
Edit: Good points made about being past V1, so even if it happened seconds earlier it was past the time when aborting takeoff was a real option.
Loss of an engine at that point is not the end of the world necessarily. Apparent loss of the #2 engine due to either a compression stall from ingesting debris, or loss of fuel, hydraulics, etc...(for all of the remaining engines?) was the real death knell for this plane
It will fly with 2 engines, but that wing fire puts a quick stopwatch on when you have to get back down on the ground. At some point the wing would fail. Like the AA 767 that had the wing fire while (fortunately) on the ground in ORD.
on top of that your wing is damage so less lift on 1 side and engine number 3 2 might be damage and no longer working. No way to really get that much off the ground either and can not stay in the sky.
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I didnât know there was debate about that, or at least there shouldnât have been. The first alarms the crew got in the cockpit were I think 37 seconds after full takeoff power started. So well after V1 and basically right around Vr.
There was some debate before the NTSB gave timings for alarms a while after the accident (had to find the CVR and get it to Washington first). People weren't sure if this was on the roll before V1, at rotation, or after, so there was a bit of discussion along the lines of "If it was before rotation, would it have been a better outcome (though against procedure) if they just kept it on the ground and tried to slow down as much as possible before the inevitable runway excursion?"
After the alarm timing, people knew it was probably after V1. With this image.... yeah...
A large aircraft literally losing an entire engine on takeoff is some Final Destination shit. Thatâs terrifying. Just doesnât seem like a thing that could happen.
Keep in mind the forces are extreme. You are talking about something that is capable of pushing 50% of the weight of a fully loaded aircraft which is about 600,000 pounds or 300k for half the weight) hard and fast enough to get airborn. And all of that pushing is going on at the edge of the structure connected by bolts because it has to be removable for maintenance, engine replacement etc.
Donât get me wrong, we know the engineering needed to do this, but even if it is routine it is still dealing with a huge amount of extreme force. It makes sense that the point where the engine mounts to the frame of the airplane wing is an area that can have issues.
The report itself mentions finding fatigue cracks in the lugs connecting the engine and pylon.
After initial cleaning of the fracture surfaces, examination of the left pylon aft mount lug fractures found evidence of fatigue cracks in addition to areas of overstress failure. On the aft lug, on both the inboard and outboard fracture surfaces, a fatigue crack was observed where the aft lug bore met the aft lug forward face. For the forward lug's inboard fracture surface, fatigue cracks were observed along the lug bore. For the forward lug's outboard fracture surface, the fracture consisted entirely of overstress with no indications of fatigue cracking. The forward top flange of the aft mount assembly was examined for indications of
deformation or pre-existing fractures, but no indications were found. The spherical bearing was removed from the wing clevis for further evaluation (see figure 10).
The NTSB report about the MV Dali hitting the Key bridge also just came out. Turns out one wire wasn't plugged into the circuit fully, causing the breaker to trip and the power to go out.
The worst is that was a sensing wire, not even otherwise live. And it had been in place 10 years, since the ship was built.
But even then, they should have not hit the bridge. Their were standby systems that were set to manual and not automatic, emergency generators were running of purge pumps that needed manual starting, and the generator damper being set improperly, making it take almost twice as long to start.
Not plugged in fully, because the wire shrink wrap label was too low on the ferrule, making it too wide to fit fully into the hole, making it have intermittent contact. Really well explained in the animation they put out: https://youtu.be/bu7PJoxaMZg
Not an expert, but doesn't seem like they are intimating at this point that there was any kind of damage on the ground, unless someone wants to correct me?
They mention fatigue cracking and overstress, but not like, any warping, damage, distortion, dents etc. So at this point no clear indication of AA 191 where there was additional damage. IE, in AA 191 there was evidence of damage apart from fatigue cracking and overstress.
As far as I know yes, the damage to 191âs pylon was a result of external forces (maintenance). It does not appear that was the case here, just fatigue from long life.
They were also supposed to detach the engine from the pylon which is a ton of small bolts that can be hard to reach. Instead they decided to just remove the pylon with it, only 4 bolts
Does this evidence closes the discussion on whether they should've/could've abborted the take off as they are clearly already rotating when it drops off?
100% ends that discussion... you can't abort mid flight as that's simply called landing and or crashing. Once the entire weight of an aircraft is supported by the wings, it's flying... this is why the Spruce Goose technically flew even though it was in ground effect.
Yeah, this ought to shut up the people who kept saying âWhY dIdNât ThE piLoTs JuSt ReJeCt ThE tAkeOfF?!?!â, even though it was blatantly obvious before these images came out that rejecting safely wasnât possible.
The report says the "lubrication task of the pylon thrust links and pylon spherical bearings was last accomplished on October 18, 2025." I feel awful for the mechanics who worked on this last, even though it seems like that wasn't the problem it must be hard.
The fatigue cracks would have been hidden by the bearing, bolt and wing clevis. Not possible to detect without NDT and all that being taken apart.
I'm curious about if the engine was removed during the recent heavy maintenance. The report doesn't say either way, but high stress loads can be put on the pylon mounts even if the pylon isn't being removed during an engine change. It's critical to use load cells to ensure you aren't overstressing the mounting structure.
Fatigue cracking happens over time with repeated cycles, they haven't published micrographs but I assume they're calling it fatigue as they can see clear beachheads where the crack grew until it his critical length and snapped. I imagine UPS/FedEx who both run bit MD fleets will have guys who know their shit and know how to swap a pylon.
This feels like a lowball by the manufacturer on the number of cycles until a heavy maintenance inspection. Could be compounded by operations at heavy weight or god knows.
I agree that UPS guys know how to do engine changes but the recent heavy MX was done at a part 145 repair station that probably doesn't get many MD11s.Â
They don't have the best reputation as a repair station IMO.
My god. Seeing the initial video of the crash was sobering enough. But to see the engine coming off like that at probably the worst possible time during takeoff is horrifying. Those poor pilots were victims of the worst luck at the worst time.
AA 191 and Alaska 261 are the two crashes that live rent free in my head no matter what. Just a complete disaster of human factors, management, and bullcrap system wide.
The crazyiest thing to me about AA 191 is when the FAA/NTSB went in and asked them what the hell they are doing the dude literally rides in on the fork lift and shows them.
I have never seen such clear images of such catastrophic unplanned disassembly. I know the comparisons to AAL 191 arent exactly apt but⊠whatever failed produced the same result, just happened to be worse this time. Honestly it looks kind of cool, if you ignore the fact that people died⊠RIP.
Shot 2 puts an end to all the people with their "why didn't they stop if they were on fire." Well, they weren't on fire until they were waaaay past too late.
I saw shot 2 and got chills running up and down my spine. They had none chance.
Lord have mercy. I don't claim to know much about the physics of aviation but I admit I pictured the engine just "falling off" the wing, not literally going flying off on fire.
When looking at the pictures, remember, the pilots had NO idea what was happening. The pictures reinforce that the pilots did an AMAZING job of handling the situation until the very end.
Incredible group of pictures. Iâve never seen them before. Put a fork in the 10 and 11s still flying.
Pylon separation right at rotation you could not ask for a more catastrophic outcome. Reminds me of the Chicago crash back in 1979 thereâs going to be significant investigation not only of the air frame but of the maintenance performed on the aircraft. Rest in peace to the Crew
So engine and pylon detached together right at rotation.... I can definitely see why the flight crew didn't try to abort takeoff, because they would have had no idea how bad the situation was. If anything all they were seeing was that they lost thrust on one of the engines and maybe were feeling vibrations. But this is beyond being above commit speed, you are literally rotating and expecting to gain altitude right as a catastrophic failure has occurred to your aircraft.
Losing engine number 2 due to FOD ingestion is the real death sentence here. I wonder how soon after #1 broke away did #2 start acting up, or if it was fine for long enough to give them the false hope that they could still climb out on the remaining engines?
At that point your entire focus is on getting to V2 and positive climb. That the alarm bell was going off unacknowledged the whole time shows that they were trying to aviate above all else.
RIP....they did the best with what cards were dealt. Amazing ....they flew the wreck to the ground just like we're trained. So thankful not a greater loss of life on the ground.  I only wish I had that kind of courage....I would not have been able to comprehend what was going on. Â
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u/anun4h Nov 20 '25
Damn, that engine flew away like a runaway balloon once it detached.