[somat]

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...

[jacquesm]

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.

[ethbr1]

After watching the below video, it's the excess bearing play and thus no-longer-constrained force directions that would seem to be the issue.

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.

[jacquesm]

> 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.

[cbsks]

My company has a policy limiting the number of high level execs traveling on a plane at a time. I wonder if plane manufacturers have similar restrictions. It’d be an ironic to for them to simultaneously assert that their planes are safe for the general public, and also believe the risk is too high for a planeload of their execs to fly in one.

[tmp10423288442]

Controlled flight into terrain is a thing

[Someone]

> They'd have to show at least one plane with a bearing gone that still flies as intended.

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?

[ethbr1]

Some of it may be related to the 3-engine design, if Boeing had modeled that 2 engines still provided sufficient power in all scenarios.

But a takeoff does seem like the worst time to catastrophically lose 1/3 power, even without FOD intake by the central engine.

[foldr]

Certainly two engines would provide enough power in all scenarios, including take off. That’s a fundamental safety requirement that any airliner meets.

[i_am_jl]

Niki Lauda, eat your heart out

[LgWoodenBadger]

To see extreme examples of this, look at any wallowed-out/wallered-out through-bore in construction equipment (e.g. excavator buckets), particularly when a pin hasn't been greased, or is seized.

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.

[robocat]

> 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]

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.

[robocat]

> What you describe is a worn bearing with an excess of play, not a seized one

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 main mount is obviously not supposed to fail even if that bearing has broken.

[jacquesm]

Yes, you got it perfectly now.

[themafia]

Juan Browne (blancolirio) breaks this down:

https://www.youtube.com/watch?v=q5OQzpilyag

[ethbr1]

Deep link to the most relevant portion: https://www.youtube.com/watch?v=q5OQzpilyag&t=5m36s (spherical bearing cut-away diagram, actual bearing again, and failure mode explained)