5 ms·

> 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 The b

by robocat 9mo ago

> 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 The bearing would have to sieze up and the bearing axle be locked to the race. There is some limit to rotational torque even with a siezed bearings. Metaphor: arthritic joints are not smooth, but they will rotate if given enough torque. From the images, it looks like the bearing had siezed. So presumably rotational vibration was transmitted to airframe and the vibration caused structural failure? I'm assuming it is not an issue of extreme rotational torque causing the issue (and given it is a bearing the design is for very little torque there!) IANAME (not a mech eng)

jacquesm 9mo ago
The forces on that mount are pretty extreme. Once the bearing seized it was really a matter of time before something gave and given the strength of the casing as well as the strength of the material and mount points it was a toss-up between the bolts and the casing. The previous evidence showed a clear order to the bolts breaking suggesting one bolt was heavier loaded than the remaining ones. The new evidence points to a much more extreme failure. As for your 'limit to rotational torque': seized bearings do not 'rotate if given enough torque' they will break right out of their casings and whatever those casings are surrounded by. The reason is that unlike your cartilage the bearings are orders of magnitude harder than the materials around them. For a bearing to seize indicates that the material has already deformed, you either catch it before the race goes or it will crack and after that all bets are quite literally off. I'm not aware of any design that would spec a bearing in a situation with such forces that would still happily work with that bearing replaced by a bushing welded to the shaft and the surrounding material even if it is statically in exactly the same position. What you describe is a worn bearing with an excess of play, not a seized one, which tends to exhibit roughly the same characteristics as a welded joint with dissimilar materials. Bearings are wear items, bearings that are worn or seized are something that should never ever happen in an aircraft, there is no way that this particular design would continue to function with sufficient margin if that bearing would fail. If not caught before it breaks the next flight is going to be a disaster. Take off in a fully loaded aircraft of this size puts extreme stress on the engine mounts. They are designed with all of their parts in working order, this is not a case of 'oh, we'll fix that the next time this craft is in for maintenance'. All parts of a plane that is certified as airworthy are supposed to be operating as originally specified. The default assumption is that it all looked good during the last inspection and that the time between the failure occurring and the plane going down was short. If it was not that would be highly unexpected. But again, until the final report is in that's speculative, and if anything the people at the NTSB are scary good at getting to root causes.
- potato3732842 9mo ago
  [flagged]
  - jacquesm 9mo ago
    I owned a machine shop, and I'm the founder of a mid sized CNC gear factory. I think I know my way around bearings, lubrication, press fits and other such bits & pieces. As for the rest of your comment: What a load of tripe. I'm doing the exact opposite of what you claim. I am just taking the bits of evidence already available and rejecting root causes that would require those bits of evidence to not exist, which is entirely valid, this still leaves a massive amount of uncertainty which I have underlined on more than one occasion. Your suggestion: > "A bearing that fails for whatever reason, welds it self, and then gets spun around in the bore by its shaft is nowhere near unheard of" is not compatible with what reputable operators of airliners would expect from their gear and if it happens as a rule people die and the NTSB gets involved, see TFA. This is not just any bearing and this is not your average bench top, industrial or vehicular application, this is an aircraft and a major load bearing component in that aircraft. > Unless you personally designed the mount of have insider knowledge of comparable ones you are speaking with degrees of certainty that are indicative of ignorance so massive it is functionally malice. I think that's worth a flag, especially coming from an anonymous potato. > The BS about how aircraft don't fly with worn bearings is just that, bullshit. Everything has service limits that allow degrees of wear. Now on some parts it might be zero or specific preload, but all that stuff is well defined. Yes, there is 'acceptable wear over the lifespan of a part' and then there is 'worn out'. Bearings in aircraft are replaced well before they are 'worn out'. Don't conflate design life wear with excessive wear to the point that a part can no longer function.
    - potato3732842 9mo ago
      >I owned a machine shop, and I'm the founder of a mid sized CNC gear factory. I think I know my way around bearings, lubrication, press fits and other such bits & pieces. Then you have no excuse for having such a nuance free opinion for you must know things are often not obvious at "first glance of pictures someone else took" which is what we're all doing here. >I'm doing the exact opposite of what you claim. I am just taking the bits of evidence already available and rejecting root causes that would require those bits of evidence to not exist, which is entirely valid, this still leaves a massive amount of uncertainty which I have underlined on more than one occasion. I disagree. You are acting like this is a cut and dry situation wherein the Boeing advice that this was not safety critical is just wrong on it's face. That assessment was made 15yr ago (perhaps by "old good boeing" engineers) and on a part already under a lot of scrutiny from the other MD11 that lost an engine. Sure they could be wrong, but I wouldn't bet on it so confidently. This bearing moves a few degrees. It's not like the engine is doing loops around the pylon. It's possible that for whatever reason the bearing stopped doing bearing things as well as it should. Now, this is a plane, everything is light, aluminum and made to flex to varying degrees. It's hard to say where exactly the movement was taking place in lieu of the bearing. Without specific knowledge it's hard to say how the failure happened. Maybe things got loose and failed from stress concentration. Maybe the movement happened in the wing assembly and the force+vibration of making that happen caused the engine mount to fail. You don't know. I don't know. Nobody in these comments know with a sufficiently low chance of being wrong to point the finger in any one direction. To act like "well of course when the bearing wore/failed/whatever it ripped its mount right in two because now the force was concentrated and the part it was concentrated on was sus to begin with" is to confidently oversimplify the situation. Engine pylons, landing gear, control surfaces, these are key systems, not the "built to within an inch of their life because they gotta be light" like a lot of other things on an airliner (though I admit the MD11 is a particularly questionable application of this heuristic) Big planes generally don't fall out of the sky because one party misleadingly labeled something in the service literature. I would be very surprised if there weren't also maintenance failing of some sort here.

by robocat 9mo ago

jacquesm 9mo ago

The forces on that mount are pretty extreme. Once the bearing seized it was really a matter of time before something gave and given the strength of the casing as well as the strength of the material and mount points it was a toss-up between the bolts and the casing. The previous evidence showed a clear order to the bolts breaking suggesting one bolt was heavier loaded than the remaining ones. The new evidence points to a much more extreme failure. As for your 'limit to rotational torque': seized bearings do not 'rotate if given enough torque' they will break right out of their casings and whatever those casings are surrounded by. The reason is that unlike your cartilage the bearings are orders of magnitude harder than the materials around them. For a bearing to seize indicates that the material has already deformed, you either catch it before the race goes or it will crack and after that all bets are quite literally off. I'm not aware of any design that would spec a bearing in a situation with such forces that would still happily work with that bearing replaced by a bushing welded to the shaft and the surrounding material even if it is statically in exactly the same position. What you describe is a worn bearing with an excess of play, not a seized one, which tends to exhibit roughly the same characteristics as a welded joint with dissimilar materials. Bearings are wear items, bearings that are worn or seized are something that should never ever happen in an aircraft, there is no way that this particular design would continue to function with sufficient margin if that bearing would fail. If not caught before it breaks the next flight is going to be a disaster. Take off in a fully loaded aircraft of this size puts extreme stress on the engine mounts. They are designed with all of their parts in working order, this is not a case of 'oh, we'll fix that the next time this craft is in for maintenance'. All parts of a plane that is certified as airworthy are supposed to be operating as originally specified. The default assumption is that it all looked good during the last inspection and that the time between the failure occurring and the plane going down was short. If it was not that would be highly unexpected. But again, until the final report is in that's speculative, and if anything the people at the NTSB are scary good at getting to root causes.

potato3732842 9mo ago
[flagged]
- jacquesm 9mo ago
  I owned a machine shop, and I'm the founder of a mid sized CNC gear factory. I think I know my way around bearings, lubrication, press fits and other such bits & pieces. As for the rest of your comment: What a load of tripe. I'm doing the exact opposite of what you claim. I am just taking the bits of evidence already available and rejecting root causes that would require those bits of evidence to not exist, which is entirely valid, this still leaves a massive amount of uncertainty which I have underlined on more than one occasion. Your suggestion: > "A bearing that fails for whatever reason, welds it self, and then gets spun around in the bore by its shaft is nowhere near unheard of" is not compatible with what reputable operators of airliners would expect from their gear and if it happens as a rule people die and the NTSB gets involved, see TFA. This is not just any bearing and this is not your average bench top, industrial or vehicular application, this is an aircraft and a major load bearing component in that aircraft. > Unless you personally designed the mount of have insider knowledge of comparable ones you are speaking with degrees of certainty that are indicative of ignorance so massive it is functionally malice. I think that's worth a flag, especially coming from an anonymous potato. > The BS about how aircraft don't fly with worn bearings is just that, bullshit. Everything has service limits that allow degrees of wear. Now on some parts it might be zero or specific preload, but all that stuff is well defined. Yes, there is 'acceptable wear over the lifespan of a part' and then there is 'worn out'. Bearings in aircraft are replaced well before they are 'worn out'. Don't conflate design life wear with excessive wear to the point that a part can no longer function.
  - potato3732842 9mo ago
    >I owned a machine shop, and I'm the founder of a mid sized CNC gear factory. I think I know my way around bearings, lubrication, press fits and other such bits & pieces. Then you have no excuse for having such a nuance free opinion for you must know things are often not obvious at "first glance of pictures someone else took" which is what we're all doing here. >I'm doing the exact opposite of what you claim. I am just taking the bits of evidence already available and rejecting root causes that would require those bits of evidence to not exist, which is entirely valid, this still leaves a massive amount of uncertainty which I have underlined on more than one occasion. I disagree. You are acting like this is a cut and dry situation wherein the Boeing advice that this was not safety critical is just wrong on it's face. That assessment was made 15yr ago (perhaps by "old good boeing" engineers) and on a part already under a lot of scrutiny from the other MD11 that lost an engine. Sure they could be wrong, but I wouldn't bet on it so confidently. This bearing moves a few degrees. It's not like the engine is doing loops around the pylon. It's possible that for whatever reason the bearing stopped doing bearing things as well as it should. Now, this is a plane, everything is light, aluminum and made to flex to varying degrees. It's hard to say where exactly the movement was taking place in lieu of the bearing. Without specific knowledge it's hard to say how the failure happened. Maybe things got loose and failed from stress concentration. Maybe the movement happened in the wing assembly and the force+vibration of making that happen caused the engine mount to fail. You don't know. I don't know. Nobody in these comments know with a sufficiently low chance of being wrong to point the finger in any one direction. To act like "well of course when the bearing wore/failed/whatever it ripped its mount right in two because now the force was concentrated and the part it was concentrated on was sus to begin with" is to confidently oversimplify the situation. Engine pylons, landing gear, control surfaces, these are key systems, not the "built to within an inch of their life because they gotta be light" like a lot of other things on an airliner (though I admit the MD11 is a particularly questionable application of this heuristic) Big planes generally don't fall out of the sky because one party misleadingly labeled something in the service literature. I would be very surprised if there weren't also maintenance failing of some sort here.