Boeing new issue with 777X engine support
- Thread starter Wes Bucey
- Start date
The germ vs. immune system analogy is a little problematic here. In this scenario, there are generally 2 possible outcomes: Either the immune system smashes the attack (or, sometimes, has it contained indefinitely) - in which case the body/system prevails; or the germ wins and the host dies (or survives in a rather crippled state). It seldom ends in a systematic (or even localized) betterment of the host. That would necessitate a successful integration of the germ's genetic code with the host, in a beneficial and sustainable manner.
Maybe a vaccination is a better metaphor? There is sometimes an adverse reaction to the injected information, but if/once the host gets over it, it should gain some benefit from it. But then, sometimes the adverse reaction is so violent that the host dies. Or the host rejects it without gaining any improvement.
ChrisM
Quite Involved in Discussions
"The thrust link in question transfers thrust from the engine to the airframe.... there are two, one for redundancy in case one were to fail."
Perhaps I should have made it clearer.... if there are two links designed to carry the thrust in tandem, but the design is such that if one link fails, the other can safely carry the entire thrust load on its own, is that really "redundancy"? I suspect that there is a better engineering term for this but I cannot think of it at the moment (old age catching up with my memory, maybe??)
Also I wonder if one link fails, can the other carry the entire load on its own "for evermore" or is there a time-limited margin in which the broken link must be replaced (and maybe replace both links as a pair)?
Perhaps I should have made it clearer.... if there are two links designed to carry the thrust in tandem, but the design is such that if one link fails, the other can safely carry the entire thrust load on its own, is that really "redundancy"? I suspect that there is a better engineering term for this but I cannot think of it at the moment (old age catching up with my memory, maybe??)
Also I wonder if one link fails, can the other carry the entire load on its own "for evermore" or is there a time-limited margin in which the broken link must be replaced (and maybe replace both links as a pair)?
I would not call this redundancy. For it to be redundant, one would typically be a primary and the second a backup for the first. Both would have to be capable of handling the full load for some period of time before they were replaced. This seems more akin to a parallel arrangement as in resistors or capacitors wired in parallel. In neither case are they intended to handle the full load. There are probably engineering design reasons such as improving torsional strength or other reasons.
Hi Ronan, some good points here, a excerpt from your post best describes the scenario as shared by my professor for Global Leadership..."Or the host rejects it without gaining any improvement.", we said, as this is spot on. A eventual rejection of the "Change Agent"...the outsider.
I don't know what the true design intent was (does any of us?...), and I wasn't saying that in any case you have 2 of "something" in parallel it's a redundancy. Just going by the quoted text that I was commenting on - "there are two, one for redundancy in case one were to fail" (which, by the way, I have no way of determining the reliability of). This is almost the dictionary meaning of "redundancy". You have more than one, in case it fails; then the other(s) kick in. Again, this may or may not have been the case with the engine link - I don't know.
Resistors in parallel can be redundant or not - it's a matter of sizing and the actual use cases.
Wes Bucey
Prophet of Profit
The part did not FAIL, the report said "crack," which to me says, "who knows WHEN or IF it will actually fail and cease to function?" We have no idea yet how Boeing will conduct its root cause investigation nor its conclusion on future corrective and/or preventive action. I expect it may not be defective part, but require a redesign of the entire engine support system.
What do you folks think should be the process Boeing should follow in the root cause investigation? Is my suspicion that Boeing's past history of Bandaid solutions a valid concern?
So the idea that a cracked part is not a failure or at least a defect is as erroneous as it is dangerous. Our words matter. The parts are not intended to crack. This is exactly like the challenger: the clevis and tang were not intended to separate (as evidence by soot on the o-rings); the fan blade on the plane that was flight UA 232 that disintegrated and tore thru the one point in the plane where all 3 hydraulic lines came together wasn’t supposed to be cracked. A crack presents a weaker system. A crack will propagate because the part isn’t going to get stronger and the stresses aren’t going to get weaker. This is a classic stress-strength interaction. Either the part was too weak or the stress was too great. Since stress is not a very controllable condition the path to a solution most likely lies in the strength of the part. This is a fairly straight forward causal mechanism search. How did the part crack; how do you prevent cracks in the future…I’ve lead coached and trained literally thousands of these in my career in automotive aerospace and medical devices…What Boing will do is something I cannot predict.
I certainly agree that a cracked part is a failed part, regardless.
Contrary to common belief, cracks are usually not the result of insufficient strength for an ordinary applied load. If that was the case, it would have been a ductile failure (deformed part). Rather, they are typically the result of a material flaw or irregularity at the microscopic level, and can be promoted by environmental conditions. The main problem with cracks is that their behaviour is unpredictable (despite our best science and research) and they will typically propagte sooner or later, until they reach a critical length, then quickly lead to macroscopic failure.
The more exotic the materials involved, the less those mechanisms are understood.
When engineers are not confident in their understanding of failure mechanisms, they will typically apply heavy safety factors and redundancy. The problem is that when you operate on the envelope or outside it (e.g. spacecraft), there isn't too much room for safety factors and redundancies.
Contrary to common belief, cracks are usually not the result of insufficient strength for an ordinary applied load. If that was the case, it would have been a ductile failure (deformed part). Rather, they are typically the result of a material flaw or irregularity at the microscopic level, and can be promoted by environmental conditions. The main problem with cracks is that their behaviour is unpredictable (despite our best science and research) and they will typically propagte sooner or later, until they reach a critical length, then quickly lead to macroscopic failure.
The more exotic the materials involved, the less those mechanisms are understood.
When engineers are not confident in their understanding of failure mechanisms, they will typically apply heavy safety factors and redundancy. The problem is that when you operate on the envelope or outside it (e.g. spacecraft), there isn't too much room for safety factors and redundancies.
Irregularities are ‘typically’ the initiation point of a crack where the local stress is greater than the local strength.
I once had an (aircraft engine) set of 3 parts where two parts were held together by a (interference fit) clip. The clips were cracking. There were only 4 possible causal paths: the clip was too small, the clip was too weak, part A was too big or part B was too big. ( or a stack up of all of these). The applied force was ruled out almost immediately as the clips were cracking regarding of the device and there were clips right next to the cracking clip assemblies that never cracked. Part A was found to always be too big when there was a cracked clip and small when there was no cracked clip. The solution was to ensure that part A was always smaller (near the center of the spec). Of course there was more work to be done to understand how A could be too big…it was a bit of a comedy of errors. In the case of UA 232, the crack was present at manufacture (GE actually found it and returned it the manufacturer where the part got ‘lost’ and resent without rework or repair to GE who missed it the second time around. The crack then propagated thru 5 subsequent routine inspections until its fatal fracture.
I once had an (aircraft engine) set of 3 parts where two parts were held together by a (interference fit) clip. The clips were cracking. There were only 4 possible causal paths: the clip was too small, the clip was too weak, part A was too big or part B was too big. ( or a stack up of all of these). The applied force was ruled out almost immediately as the clips were cracking regarding of the device and there were clips right next to the cracking clip assemblies that never cracked. Part A was found to always be too big when there was a cracked clip and small when there was no cracked clip. The solution was to ensure that part A was always smaller (near the center of the spec). Of course there was more work to be done to understand how A could be too big…it was a bit of a comedy of errors. In the case of UA 232, the crack was present at manufacture (GE actually found it and returned it the manufacturer where the part got ‘lost’ and resent without rework or repair to GE who missed it the second time around. The crack then propagated thru 5 subsequent routine inspections until its fatal fracture.
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