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.
Actual question: would an airline have the engineering competence to second-guess an airplane manufacturer's engineering guidance? They operate airplanes but don't build them, and I'd assume they'd out of necessity need to trust the manufacturer's judgement.
Airlines have every reason to be skeptical of their supplier even if they do not have the engineering competence to second guess them. They could for instance look through their past communications with the manufacturer and see for themselves which advisories they agree with because for instance they are obviously not safety critical, this would then allow them hire specialists to evaluate the remainder for a second opinion.
For almost any act, we rely on other people. That doesn't absolve us of our personal responsibility.
But I'm just some guy with no incentive to endanger human life if I think it will save money so what do I know
Frankly I put it squarely on Boeing.
No, UPS bought the plane from Thai Airways International.
> Boeing built them
No, McDonnell Douglas built the plane in question; Boeing hadn't merged with MD at the time this aircraft was manufactured.
The other elements are probably true, but this was not a Boeing aircraft.
Why did you need to tell me about the wire then?
The answer is an attempt to transfer the liability to me. The liability for a thing they think could happen, but didn't tell me about.
And the worst thing is I don't think even after mcas things have substantially improved there. I've seen more spin and damage control than actual safety focus. They could have launched a huge company program and management reorganization to really turn this mindset around.
I think the biggest issue that Boeing is too big to fail. They'll never fall because the government needs them for all their warplanes.
This is how you get mentally and morally weak bean counters running companies instead of engineers with a conscience. It’s an engineering company and yet it’s run like a bank that just so happens to have an engineering branch.
$5mln? $100mln? Old school, $50?
In the US we've done a pretty poor job of doing that and it's resulted in countless lives lost and every living person and animal on earth being poisoned. It's long past time our government and its legal system took their responsibility to public safety more seriously.
I’m sorry, but this phrase has worn out its welcome.
In aviation, there is little room for error. It’s also the case that resources and time are limited. So there are multiple constraints.
We both agree that Boeing is the big problem. I’d also say its a problem of the FAA and the aviation industry.
But UPS? Why would they be taking action “out of an abundance of caution”?
The worst you can say for UPS is they could have sought a second opinion out of “an abundance of caution”, and recommendations of next actions and how.
Keep in mind UPS core competency isn’t aerospace and aeronautical engineering.
Would they even be able to assess the risk of changing said bearings en masse?
The actual lesson here is that most of the advisories and self-certifying from Boeing over the past 30 years need to be reconsidered; most likely redone, by independent third parties and also an FAA with a mandate to be fully independent.
Seems like a perfectly fine phrase to me.
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...
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 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)
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.
>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.
Have you ever dicked around with servicing stuff in an application that has a lot of bearing, plain or roller element? Because it sure doesn't sound like you have with the way you're talking. Words like "sized" and "deformed" are far less binary that your spherical cow assessment implies they are. 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. For bearings that are there to allow for misalignment/flex and only move a few degrees often times they simply flex the parent assembly if seized. I can't say what would be a reasonable failure mode for this assembly though because I didn't design it nor similar ones.
Unless you personally designed the mount of have insider knowledge of you are speaking with degrees of certainty that are indicative of ignorance so massive it is functionally malice. The lying bullshit 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.
This is a highly nuanced technical topic. None of us have the full facts at our disposal. I think the degree to which you are up and down these comments peddling a very specific chain of causality and putting down anyone who even has the most narrow or nuanced suggestion that it might not be exactly as you suggest is indicative of a moral failing.
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.
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.
Well, those good old Boeing engineers and their management have misled the world more than once and no longer deserve the benefit of the doubt. That advisory is black-and-white, there is no arguing with what it says or does not say, you can read it for yourself. If your conclusion is the same as Boeing's then that's fine, you can have a different opinion. My conclusion is that if a load bearing component has these kind of potential issues that you need to act with an abundance of caution because of the price in case you get it wrong.
Yes, that bearing only moves a few degrees. But this is not about how much it moves, this is about what happens when it can not move and given the forces involved the outcome of that is fairly predictable, in spite of your previous statements. There is absolutely no way in which if that bearing is seized or otherwise constrained that this is safe.
> I would be very surprised if there weren't also maintenance failing of some sort here.
I explicitly left the door open for that. But regardless, this bearing should have never failed.
There are a couple of HN members whose pension depends on Boeing stock so I can see how this might ruffle some feathers but this is not a company that has behaved in a morally responsible way when it came to issues such as these om the past and you are effectively already blaming the maintenance people with your 'I would be very surprised if there weren't also maintenance failing of some sort here.'.
That is jumping to conclusions.
I would not be surprised if it were the case, but I also would not be surprised if it wasn't the case. That's the degree to which Boeing has squandered its erstwhile stellar reputation.
But, since you feel comfortable attacking my reputation from behind your shield of anonymity I suggest you flesh out your profile and Bio and tell us a bit about yourself and why you feel so emotionally involved in this.
You and potato followed that wrong path and unfortunately didn't correct me or yourselves. I tried to correct myself later (see my sibling comment), but I wouldn't be surprised if I've made another huge cockup with the facts.
> Yes, that bearing only moves a few degrees
It certainly does not move a few degrees (except maybe after a crash).
Thinking back to my one undergrad mechanics paper, I think the design purpose was to make torsions equal zero, so that the mechanical analysis would be tractable.
The torsions should still be extremely low because nothing can rotate (except maybe a tiny amount due to deformations).
If we can't get the engineering facts straight, then our opinions on engineering management are likely to be even more pointless and flawed.
You seem to have gone off the rails as much as potato.
The report seems to be implying that the broken bearing is the cause of the accident. The bearing was still in place after the accident so presumably the bolt didn't shear. If the engine has full acceleration then the engine is pitching upwards against the wing and the force on that bearing is upwards?
I think the report implied the fire occurred at the same time as the structural failure.
What might be the chain of events from a broken bearing to a fire?
Yeah. Worn or seized bearings are relevant to rotation, but on second thoughts, rotation isn't the issue here.
Rereading the PDF, I can see that I entirely misunderstood the function of the bearing and how it failed, and I suspect I've mislead you. The two lugs mislead me! I would guess they make the lug as two parts for redundancy (if the lug was a single part then it's failure would be bad). My previous comment was wildly incorrect about rotation, but now I think rotation is not the issue.
The casing split in half all the way around the circumference at the weakest point (where the recess is), splitting into two pieces, a forward half and a rearward half. The half forward of the split moves forward and the half rearward of the split moves rearward. That is what they inspect for every sixty months to see if the bearing casing has broken.
An unbroken casing is normally prevented from moving forward or backwards by the ball (how the hell do they make the bearing like that?!).
It appears that the unbroken casing itself is designed for the outside to be able to slide forwards and backwards within the lugs (very little movement?).
The primary force this bearing is preventing is pitching of the engine relative to the wing (vertical force). And secondarily to prevent yawing of the engine relative to the wing (horizontal force). Rotation (roll of the engine relative to the wing) has to be prevented by the main mount and the engine surely can't twist much therefore I suspect rotational forces at that bearing are rather irrelevant.
As the engine thrusts and stops thrusting, the thrust changes create pitching forces on the engine, and there would be vertical movement at the broken bearing - a clunk!?
The main mount would flex a little more due to the extra pitch movement; and I guess we'll have to wait and see whether the bearing failure is relevant to the crash. It appears to be a smoking gun, but could be a red herring?
The bolts are obviously not supposed to fail even if that bearing has broken.
With a proper tolerance bearing in place, the force is constrained so that other parts are only stressed in directions they're well suited to handle (because the bearing takes the load).
Once the bearing develops excess tolerance, you've got a bucking engine that (to your point) is directly loading other parts in unexpected ways/directions, eventually causing failure.
The fact that Boeing supposedly modeled this and came up with non-safety critical in the event of bearing breakage... curious how that will turn out.
They'd have to show at least one plane with a bearing gone that still flies as intended. I suggest we break one on purpose, put the full complement of Boeing execs on that plane to prove its safety given the alternative of retracting that statement.
That depends on the meaning of “safety of flight”. I don’t know what it means in aviation, but do not rule out that there is significant room between “flies as intended” and “result in a safety of flight condition”.
For example, if an engine were to complete drop off the plane, would that necessarily result in a safety of flight condition, or does “the plane will be able to continue take off and land again” mean safety of flight isn’t affected?
But a takeoff does seem like the worst time to catastrophically lose 1/3 power, even without FOD intake by the central engine.
This same scenario combined with the amount of vibration and stresses caused by the engine, should scream "this is a catastrophe waiting to happen" for any engineer.
The NTSB investigation found that for this crash, not only did the bearing race crack, but also that the bearing lugs, which hold the bearing in place, were fractured. I don't have access to the original text of the letter Boeing sent out, but based on the NTSB report, it sounds like only the issue with the bearing race was previously identified. The two may very well be related, but that doesn't mean that the lug fractures are an expected result of the race failure - perhaps some contributing factor made the lugs more susceptible than predicted. It also remains possible that the bearing damage is a red herring; the aircraft was nearing the end of its service life and had known structural issues in other parts of the pylon. The fact is that for more than a decade after the bearing race issue was reported, it didn't result in a safety of flight condition.
The insinuation that Boeing was deliberately trying to hide or downplay a known issue is simply unwarranted. It would be irresponsible for the NSTB not to mention a known issue that could have potentially been relevant, it's not evidence the issue was improperly handled.
There's a lot of gray going on here.