This accident resembles AA191, a DC-10 that crashed in 1979. In both incidents, the left engine and its supporting pylon detached from the wing seconds after takeoff.
Investigators of AA191 discovered that critical pylon mount lugs suffered from metal fatigue.
The structural breaches forced the FAA to issue emergency grounding orders and mandate strict inspections.
That one is from cracks made during installing the engine from a forklift though. I wonder if it’s just this type of mount that is more susceptible to cracks there though.
Turns out tho THAT maintenance was to replace the pylon spherical bearings per Service Bulletin 54-48 of May 1975 which was a response to multiple outer ring cracks and failures which are strikingly similar to the recent failure.
The initial failure is the same, but what caused them to crash is completely different. With AAL191 it was the loss of hydraulic pressure which led to an un-commanded slat retraction on the left wing. With UPS it was FOD from engine 1 getting ingested by engine 2 that caused that engine to stall.
It's happened once before on an older generation of the plane (American 191) but that was due to the mounts getting banged up pretty bad by an incorrect maintenance procedure.... they had tried to hold the entire engine and pylon up with a forklift instead of with the proper tooling.
TIL that Dc-10 American 191 maintenance procedure was to replace the pylon spherical bearings resulted from a Safety Bulletin: 54-48 May 31, 1975!
That bulletin 54-48 was specifically due to a multiple instances of a strikingly similar cracking / failure of the aft pylon spherical bearings outer ring that occurred on the UPS 1011.
To add a bit more context I believe it wasn't just simply using the fork lift but there happened to be a shift change and when the shift change occurred the forks dropped a bit and no one noticed
I don't recall though if that resulted in a false torque or if it damaged the actual mount
It is the most dangerous time. Chernobyl even happened during a shift change; the test was meant for the more experienced day crew, but was put off for the less experienced night crew. Some of those poor men were barely adults.
So fatigue cracking in the bearing caused it to fail and initiate fatigue cracking in the clevis, which eventually failed and caused the other pylon attachment points to fail on the accident flight.
One thing they didn't mention is how long this took. Is this the kind of thing that can happen in a handful of flight hours, or was this a ticking time bomb that went through multiple maintenance cycles without being noticed?
From the investigative update and preliminary report linked under the video:
Boeing Service Letter MD-11-SL-54-104-A, dated February 7, 2011, informed operators of four previously reported bearing race failures (on three different airplanes) involving P/N S00399-1 spherical bearing assemblies. Specifically, each failure had initiated at the design recess groove on the interior surface of the bearing race.
The report goes on to say that the Service Letter stated inspection of the bearing be included in the general visual inspection, and detailed visual inspection of the pylon aft mount at 60-month intervals.
A new design of the part was recommended to be installed, in the event the bearing was found to be unserviceable, however using the old designed part to replace a failed part was not prohibited.
The last detailed inspection was October 28, 2021, and a 24-month/4,800 hour lubrication task of the bearing and pylon thrust links was done on October 18, 2025.
I'd be surprised if such fatigue was not visible in the month prior when the lubrication was completed. However, if they don't fully disassemble the bearing during that inspection, it don't doubt it could've gone unnoticed since the failed component was hidden within the clevis and bearing casing, and was in the corner of the grease channel.
It might be visible with some NDT that penetrates there but I doubt you would see it visually without a teardown as you said. I wonder if the service letter specified the inspection method or not.
I started wondering about effects of water ingress & ice and also bearing and bushing misalignments, given the huge torques both the pylon and the aft bearing itself experiences and shoulders.
Turns out historically in DC-10 a dry lubricant was first used before being changed to grease due to water ingress. And that minor alignment issues with either the bushing or lugs results in a large imbalance of force on one edge of the central groove of the outer ring. And that the forces involved “stack up” dramatically.
I did some gpt assisted looking and found a May 31, 1975 service bulletin “DC-10 Service Bulletin 54-48” that matches the UPS MD-11 failure almost point for point. The same fundamental failure mode (outer race fatigue fracture), consistent with the SB 54-48 mechanism class.
From the original SB 54-48 (1975), the key issue was:
Fatigue cracking of the wing/pylon aft bulkhead spherical bearing system
Cracks found after relatively low to moderate service hours (~1,500–8,000 hrs)
Initiation in the aft attach spherical bearing / bulkhead interface region
Core mechanism:
High cyclic load + constrained articulation → stress concentration in outer race / lug → fatigue crack initiation → propagation into pylon structure
Key structural reality (important):
The spherical bearing was part of the primary engine load path
It was not just a motion joint — it carried engine thrust reaction + bending loads
Mitigation at the time:
Increase bearing thickness / ductility
Oversize replacement options
Inspection / replacement campaigns
Removal/reinstall during engine change
From FAA/NTSB analysis of SB 54-48 work:
Maintenance procedures themselves could induce damage during installation/removal
Misalignment during pylon handling caused aft bulkhead cracking events during maintenance operations
So already in the DC-10 era, there were two overlapping failure drivers:
In-service fatigue cracking
Maintenance-induced geometric overload (secondary bending/torsion)
You first would identify the largest possible crack that could be missed during inspections and the smallest possible crack that could cause failure of the part. Then you calculate crack propagation speed between those two and however long it takes for this hypothetical crack to grow to dangerous levels, that is you minimum maintenance interval (divided by safety factor x). Either they missed a large crack during maintenance or miscalculated crack propagation or the size of the smallest dangerous crack.
Fantastic video, I now understand the root cause of the engine separation. Bravo to NTSB for explaining a complicated technical concept in words and pictures that everyone can understand.
What’s scary is that all wing mounted engines have some variation of this design.
Interesting though how this only has seemed to happen on the MD-11 and DC-10. Also, isn't that part supposed to be inspected in order to catch these fatigue cracks before total failure?
This is one of the reasons why the 777X is so delayed, the thrust links in the pylons kept cracking during it's test flights so they had to inspect them to find out why the behaviour was not matching the simulations and redesign them.
Putting the engines on pylons does allows you to have bigger engines, but obviously the further away the engines are from the centre of mass of the aircraft the stronger the mounts need to be.
The aft connetion as installed is statically undetermined. Therefore there is fatigue stress in the lugs. If they designed the aft connection 90 degrees turmed, with the lugs not pointing forward/aft, but in/outboard, the fatigue would probably not have occurred before D check or similar.
Im always amaze first how they get to design those parts with all those details I guess try and error, but how from a crash mess they can backtrack the steps and find out how and why happened
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So the lack of maintenance/ inspection and lubrication caused the bearing to fatigue and crack. What precautions are taken to prevent another accident like this one.
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SuperSaint77x@reddit
This accident resembles AA191, a DC-10 that crashed in 1979. In both incidents, the left engine and its supporting pylon detached from the wing seconds after takeoff.
Investigators of AA191 discovered that critical pylon mount lugs suffered from metal fatigue.
The structural breaches forced the FAA to issue emergency grounding orders and mandate strict inspections.
KnowledgeSafe3160@reddit
That one is from cracks made during installing the engine from a forklift though. I wonder if it’s just this type of mount that is more susceptible to cracks there though.
delicious-croissant@reddit
Turns out tho THAT maintenance was to replace the pylon spherical bearings per Service Bulletin 54-48 of May 1975 which was a response to multiple outer ring cracks and failures which are strikingly similar to the recent failure.
Flyinghud@reddit
The initial failure is the same, but what caused them to crash is completely different. With AAL191 it was the loss of hydraulic pressure which led to an un-commanded slat retraction on the left wing. With UPS it was FOD from engine 1 getting ingested by engine 2 that caused that engine to stall.
Hot-Cress7492@reddit
Just came here to say exactly the same thing.
Nostradamus_of_past@reddit
Are the MDs notorious for have an weak pylon - engine link?
greatlakesailors@reddit
It's happened once before on an older generation of the plane (American 191) but that was due to the mounts getting banged up pretty bad by an incorrect maintenance procedure.... they had tried to hold the entire engine and pylon up with a forklift instead of with the proper tooling.
delicious-croissant@reddit
TIL that Dc-10 American 191 maintenance procedure was to replace the pylon spherical bearings resulted from a Safety Bulletin: 54-48 May 31, 1975!
That bulletin 54-48 was specifically due to a multiple instances of a strikingly similar cracking / failure of the aft pylon spherical bearings outer ring that occurred on the UPS 1011.
FirstGT@reddit
To add a bit more context I believe it wasn't just simply using the fork lift but there happened to be a shift change and when the shift change occurred the forks dropped a bit and no one noticed
I don't recall though if that resulted in a false torque or if it damaged the actual mount
DasMo19@reddit
So much Shit happens during shifts changes in every industry. Even in medicine.
FirstGT@reddit
Note to self, schedule early morning surgeries
Azurehue22@reddit
It is the most dangerous time. Chernobyl even happened during a shift change; the test was meant for the more experienced day crew, but was put off for the less experienced night crew. Some of those poor men were barely adults.
EmotioneelKlootzak@reddit
So fatigue cracking in the bearing caused it to fail and initiate fatigue cracking in the clevis, which eventually failed and caused the other pylon attachment points to fail on the accident flight.
One thing they didn't mention is how long this took. Is this the kind of thing that can happen in a handful of flight hours, or was this a ticking time bomb that went through multiple maintenance cycles without being noticed?
HappyHHoovy@reddit
From the investigative update and preliminary report linked under the video:
The report goes on to say that the Service Letter stated inspection of the bearing be included in the general visual inspection, and detailed visual inspection of the pylon aft mount at 60-month intervals.
A new design of the part was recommended to be installed, in the event the bearing was found to be unserviceable, however using the old designed part to replace a failed part was not prohibited.
The last detailed inspection was October 28, 2021, and a 24-month/4,800 hour lubrication task of the bearing and pylon thrust links was done on October 18, 2025.
I'd be surprised if such fatigue was not visible in the month prior when the lubrication was completed. However, if they don't fully disassemble the bearing during that inspection, it don't doubt it could've gone unnoticed since the failed component was hidden within the clevis and bearing casing, and was in the corner of the grease channel.
Only_Razzmatazz_4498@reddit
It might be visible with some NDT that penetrates there but I doubt you would see it visually without a teardown as you said. I wonder if the service letter specified the inspection method or not.
Jazzlike_Climate4189@reddit
It would be very easy to detect with a magnetic particle inspection.
Only_Razzmatazz_4498@reddit
Ahh yeah I was thinking ultrasonic but that would probably work better.
delicious-croissant@reddit
I started wondering about effects of water ingress & ice and also bearing and bushing misalignments, given the huge torques both the pylon and the aft bearing itself experiences and shoulders.
Turns out historically in DC-10 a dry lubricant was first used before being changed to grease due to water ingress. And that minor alignment issues with either the bushing or lugs results in a large imbalance of force on one edge of the central groove of the outer ring. And that the forces involved “stack up” dramatically.
I did some gpt assisted looking and found a May 31, 1975 service bulletin “DC-10 Service Bulletin 54-48” that matches the UPS MD-11 failure almost point for point. The same fundamental failure mode (outer race fatigue fracture), consistent with the SB 54-48 mechanism class.
From the original SB 54-48 (1975), the key issue was: Fatigue cracking of the wing/pylon aft bulkhead spherical bearing system Cracks found after relatively low to moderate service hours (~1,500–8,000 hrs) Initiation in the aft attach spherical bearing / bulkhead interface region
Core mechanism: High cyclic load + constrained articulation → stress concentration in outer race / lug → fatigue crack initiation → propagation into pylon structure
Key structural reality (important): The spherical bearing was part of the primary engine load path It was not just a motion joint — it carried engine thrust reaction + bending loads
Mitigation at the time: Increase bearing thickness / ductility Oversize replacement options Inspection / replacement campaigns Removal/reinstall during engine change
From FAA/NTSB analysis of SB 54-48 work: Maintenance procedures themselves could induce damage during installation/removal Misalignment during pylon handling caused aft bulkhead cracking events during maintenance operations So already in the DC-10 era, there were two overlapping failure drivers: In-service fatigue cracking Maintenance-induced geometric overload (secondary bending/torsion)
Misalignment / constraint / wear ↓ Edge-loaded contact patch ↓ ↑ Hertz stress + ↑ frictional shear ↓ Outer race bending develops ↓ Groove root sees peak tensile stress ↓ Microcrack initiates at recess/groove ↓ Circumferential fatigue crack growth ↓ Load redistribution increases stress further ↓ Rapid crack acceleration ↓ Final overload fracture
FZ_Milkshake@reddit
You first would identify the largest possible crack that could be missed during inspections and the smallest possible crack that could cause failure of the part. Then you calculate crack propagation speed between those two and however long it takes for this hypothetical crack to grow to dangerous levels, that is you minimum maintenance interval (divided by safety factor x). Either they missed a large crack during maintenance or miscalculated crack propagation or the size of the smallest dangerous crack.
ecco311@reddit
Yeah I was wondering this too. At least once the bearing failed it was likely "relatively" quick or it would've been found during maintenance.
DullMind2023@reddit
Fantastic video, I now understand the root cause of the engine separation. Bravo to NTSB for explaining a complicated technical concept in words and pictures that everyone can understand.
What’s scary is that all wing mounted engines have some variation of this design.
CouchPotatoFamine@reddit
Interesting though how this only has seemed to happen on the MD-11 and DC-10. Also, isn't that part supposed to be inspected in order to catch these fatigue cracks before total failure?
spazturtle@reddit
This is one of the reasons why the 777X is so delayed, the thrust links in the pylons kept cracking during it's test flights so they had to inspect them to find out why the behaviour was not matching the simulations and redesign them.
Putting the engines on pylons does allows you to have bigger engines, but obviously the further away the engines are from the centre of mass of the aircraft the stronger the mounts need to be.
Blythyvxr@reddit
The biggest question I have is how a single point failure allowed for the aircraft to crash.
I know there's talk of engine 2 having issues, but FDR shows it didn't drop power below 98% N1, and even then only momentarily.
NorthernLions@reddit
Same question. Surprised that there isn’t redundancy built in for this failure mode.
MGreymanN@reddit
The the pertubations seen indicate turbulent airflow/stall. Engine was spinning fine but there was a huge loss in mass air flow.
aussiekd@reddit
The aft connetion as installed is statically undetermined. Therefore there is fatigue stress in the lugs. If they designed the aft connection 90 degrees turmed, with the lugs not pointing forward/aft, but in/outboard, the fatigue would probably not have occurred before D check or similar.
Nok1a_@reddit
Im always amaze first how they get to design those parts with all those details I guess try and error, but how from a crash mess they can backtrack the steps and find out how and why happened
Ecthelion-O-Fountain@reddit
Really good video
deleted_by_reddit@reddit
[removed]
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Any_Vacation8988@reddit
So the lack of maintenance/ inspection and lubrication caused the bearing to fatigue and crack. What precautions are taken to prevent another accident like this one.
803UPSer@reddit
There’s a new designed bearing without the groove and a significantly shorter inspection interval.
Dr__-__Beeper@reddit
Really bad video.
Dr__-__Beeper@reddit
Violently nauseating stupid video.
aviation-ModTeam@reddit
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If you post in this subreddit, you are expected to engage in the discussion. Do not post images, links, or videos just to karma farm or drive engagement. Questions with simple or easily-googled answers are not permitted.
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