I recall a lot of fingerpointing minutes after the crash by people blaming the presumably foreign maintenance crew.
Even now there is a lot of uncertainty around this crash, maintenance - or lack thereof - or even wrong maintenance could still be a factor. But given the location of the part asking for a 'visual inspection' is a pretty strange move, the part is all but inaccessible when it is in its normal position and even with an endoscope it would be pretty hard to determine whether or not the part had weakened. That's just not going to show up visually until it is way too late unless the part has been especially prepared to announce the presence of hairline cracks.
You'd have to disassemble a good chunk of the wing to gain access to the part based on the pictures I've seen of how it all holds together when assembled.
A former air accident investigator who works as an aviation safety consultant said "It's extraordinary that Boeing concluded that a failure of this part would not have safety consequences," and said the report was "disturbing"
Doesn't seem like gray to me. It seems a company who has a history of cutting corners and ignoring or downplaying safety problems did exactly that in this case too which resulted in the deaths of many people. UPS made an error here as well in trusting Boeing when they said it wasn't a safety issue and they should have installed the revised bearing assembly out of an abundance of caution, but I don't know much they would have known back in 2011 about the changes at Boeing that prioritized profit over safety following the merger with McDonnell Douglas
Because my naive conclusion after looking at the part in question is exactly the same "would not result in a safety of flight condition." if the bearing cracked at the point in question it is going nowhere, the bearing is still captive in its housing. hell it looks like it could have been designed as two pieces and it would work the same. the large bolt is what is holding the engine on.
The best I can come up with is that a split bearing causes increased wear on the mounting bracket and nobody noticed for a long time.
That's indeed a very naive conclusion. Once that bearing is gone the stress that it would normally allow to escape on account of rotation would be directly transferred to the metal around it and to the bolts holding the whole thing in place. Guess what broke first?
So if that bearing went that's not quite a smoking gun yet but it would definitely be a step closer to a root cause.
FWIW, the MD-11 was designed by McDonnell Douglas, and manufactured by McDonnell Douglas in 1991, before the Boeing merger. A McDonnell Douglas DC-10 failed in a similar way in Chicago in 1979, so it the issue may go way back.
AA Flight 191 in 1979, 273 dead. American Airlines invented their own engine removal procedure using a forklift and damaged a pylon and mounting bracket. The engine ripped off the wing on takeoff.
Some are forgetting how risk in technology works: No technology is designed or operated without flaws; that's an absurd approach and impossible to implement.
To reduce negative outcomes, we use risk management: assessing the likely lifetime cost of the flaw, and taking cost-effective measures to reduce the risk to an acceptable level. As a familiar example, redundant mass storage drives are much more cost-effective than high-reliability mass storage drives.
Insane that we can have places like the skunk works create the sr71 and operate on shoe string budgets but the largest passenger plane company in the world can’t accurately assess risk on planes far under the former planes Mach 3 record
Look up the hull loss numbers on the SR-71. More than a third of them were lost in incidents despite never making contact with the enemy.
It was also insanely expensive to operate: $300k/hour in 1990 dollars, and there aren’t reliable numbers on development costs with all of the black budgets.
I don't see that as a valid comparison. SR-71s could operate with a much higher level of risk than commercial passenger planes. IIRC, SR-71s leaked fuel on the ground, and their wings dragged on the ground without special attachments. Pilots needed special pressure suits, etc.
I also expect that they were much less complex than an aircraft that provides a comfortable, pressurized cabin; the high level of safety mentioned above; freight capacity; etc.
Also, despite Boeing's recent problems, I would guess that commerical passenger planes are far more safe than they were decades ago when the SR-71 was developed. Accidents were much more common despite many fewer flights, iirc.
12/32 SR71s were lost in the 33 years they were flying. 11/200 MD-11s have been hull-lost from 1988-2025. Not to mention that passenger/cargo planes will put on a lot more flight hours than the SR71s did in a given year.
the SR-71 leaking fuel on the ground was not a design flaw. it was designed to be operated at speed where things would expand to fill in. if they were filled in on the ground, they'd have no place to expand at speed/temps. the risk assessment was that it was better to leak fuel on the ground rather than blowing up at speed/temp
Every five years feels too infrequent. These are planes that are 30 years old and have done 100,000 hours of flying. Apparently UPS policy is to keep them around for about 35 years to maximize the ROI. But maybe once they hit a particular age they need to be inspected deeply every few months.
I am not an expert, however. Can metal fatigue be detected with such infrequent inspection?
On things like D check, the aircraft is essentially completely taken apart and inspected at that level typically taking 50,000 man hours and 6 month-1 year of time.
Thanks for this post. I’m blown away by that 50,000 hours figure.
The article mentions the cost and that Boeing underestimates it. When you divide the cost by the number of hours, it seems very reasonable. Parts and materials being included. I’m surprised any job that extensive isn’t even more expensive.
I recall a lot of fingerpointing minutes after the crash by people blaming the presumably foreign maintenance crew.
Even now there is a lot of uncertainty around this crash, maintenance - or lack thereof - or even wrong maintenance could still be a factor. But given the location of the part asking for a 'visual inspection' is a pretty strange move, the part is all but inaccessible when it is in its normal position and even with an endoscope it would be pretty hard to determine whether or not the part had weakened. That's just not going to show up visually until it is way too late unless the part has been especially prepared to announce the presence of hairline cracks.
You'd have to disassemble a good chunk of the wing to gain access to the part based on the pictures I've seen of how it all holds together when assembled.
The headline is missing an important bit.
Boeing knew of the flaw, and sent a letter to airlines about it. In 2011.
Well yes but Boeing also said it "would not result in a safety of flight condition."
There's a lot of gray going on here.
A former air accident investigator who works as an aviation safety consultant said "It's extraordinary that Boeing concluded that a failure of this part would not have safety consequences," and said the report was "disturbing"
Doesn't seem like gray to me. It seems a company who has a history of cutting corners and ignoring or downplaying safety problems did exactly that in this case too which resulted in the deaths of many people. UPS made an error here as well in trusting Boeing when they said it wasn't a safety issue and they should have installed the revised bearing assembly out of an abundance of caution, but I don't know much they would have known back in 2011 about the changes at Boeing that prioritized profit over safety following the merger with McDonnell Douglas
I am wondering what the exact fail mode here is.
Because my naive conclusion after looking at the part in question is exactly the same "would not result in a safety of flight condition." if the bearing cracked at the point in question it is going nowhere, the bearing is still captive in its housing. hell it looks like it could have been designed as two pieces and it would work the same. the large bolt is what is holding the engine on.
The best I can come up with is that a split bearing causes increased wear on the mounting bracket and nobody noticed for a long time.
Anyhow, here is the ntsb update in question https://www.ntsb.gov/investigations/Documents/DCA26MA024%20I...
That's indeed a very naive conclusion. Once that bearing is gone the stress that it would normally allow to escape on account of rotation would be directly transferred to the metal around it and to the bolts holding the whole thing in place. Guess what broke first?
So if that bearing went that's not quite a smoking gun yet but it would definitely be a step closer to a root cause.
The FAA has not determined that this flaw did lead to a safety of flight condition. Investigation is still ongoing.
Apparently they expected it to blow up on the ground, so technically the plane wasn't flying yet ...
Yeah saved boeing losing face and sales by requiring all the planes be grounded and fixed. Just eye it up every 5 years, if you want to.
FWIW, the MD-11 was designed by McDonnell Douglas, and manufactured by McDonnell Douglas in 1991, before the Boeing merger. A McDonnell Douglas DC-10 failed in a similar way in Chicago in 1979, so it the issue may go way back.
AA Flight 191 in 1979, 273 dead. American Airlines invented their own engine removal procedure using a forklift and damaged a pylon and mounting bracket. The engine ripped off the wing on takeoff.
Some are forgetting how risk in technology works: No technology is designed or operated without flaws; that's an absurd approach and impossible to implement.
To reduce negative outcomes, we use risk management: assessing the likely lifetime cost of the flaw, and taking cost-effective measures to reduce the risk to an acceptable level. As a familiar example, redundant mass storage drives are much more cost-effective than high-reliability mass storage drives.
They do mention that the DC10 (this plane's predecessor) was decommissioned for similar issues.
Insane that we can have places like the skunk works create the sr71 and operate on shoe string budgets but the largest passenger plane company in the world can’t accurately assess risk on planes far under the former planes Mach 3 record
Look up the hull loss numbers on the SR-71. More than a third of them were lost in incidents despite never making contact with the enemy.
It was also insanely expensive to operate: $300k/hour in 1990 dollars, and there aren’t reliable numbers on development costs with all of the black budgets.
I don't see that as a valid comparison. SR-71s could operate with a much higher level of risk than commercial passenger planes. IIRC, SR-71s leaked fuel on the ground, and their wings dragged on the ground without special attachments. Pilots needed special pressure suits, etc.
I also expect that they were much less complex than an aircraft that provides a comfortable, pressurized cabin; the high level of safety mentioned above; freight capacity; etc.
Also, despite Boeing's recent problems, I would guess that commerical passenger planes are far more safe than they were decades ago when the SR-71 was developed. Accidents were much more common despite many fewer flights, iirc.
12/32 SR71s were lost in the 33 years they were flying. 11/200 MD-11s have been hull-lost from 1988-2025. Not to mention that passenger/cargo planes will put on a lot more flight hours than the SR71s did in a given year.
the SR-71 leaking fuel on the ground was not a design flaw. it was designed to be operated at speed where things would expand to fill in. if they were filled in on the ground, they'd have no place to expand at speed/temps. the risk assessment was that it was better to leak fuel on the ground rather than blowing up at speed/temp
https://www.ntsb.gov/investigations/Documents/DCA26MA024%20I...
I’m guessing that manufacturers know of lots of flaws in the parts they make.
Hopefully they don't usually downplay the risks of dangerous known flaws in critical parts like Boeing seems to have done in this case.
Isn't it a mostly Boeing project that is going to go around the moon next month? I'm really afraid for that crew.
Why would you be afraid for them when the Orion capsule worked so flawlessly...oh right.
Every five years feels too infrequent. These are planes that are 30 years old and have done 100,000 hours of flying. Apparently UPS policy is to keep them around for about 35 years to maximize the ROI. But maybe once they hit a particular age they need to be inspected deeply every few months.
I am not an expert, however. Can metal fatigue be detected with such infrequent inspection?
Sounds like it is included as part of standardized airplane checks based on age of aircraft + hours flown.
[1]: https://en.wikipedia.org/wiki/Aircraft_maintenance_checks#AB...
On things like D check, the aircraft is essentially completely taken apart and inspected at that level typically taking 50,000 man hours and 6 month-1 year of time.
Thanks for this post. I’m blown away by that 50,000 hours figure.
The article mentions the cost and that Boeing underestimates it. When you divide the cost by the number of hours, it seems very reasonable. Parts and materials being included. I’m surprised any job that extensive isn’t even more expensive.