MD-11 Pylon Question
Posted by ZappaLlamaGamma@reddit | aviation | View on Reddit | 13 comments
Not sure if any other large aircraft have had the pylon fail from fatigue cracks as the NTSB noted but I was wondering about the design. Since there’s probably the most gyroscopic precession when the engine is rotated (at aircraft rotation), I’m wondering if that would’ve been made larger/thicker/stronger if it would’ve dealt with that twisting force a bit better. I’m not bringing AA191 as that is a different cause, but am only looking at UPS 2976. I’m also not talking about the bolts but the actual part of the pylon the bolt goes through when it is seated in the clevis. Anyway I’m curious if I’m just off in left field somewhere. I’m not an aeronautical engineer so I may be completely wrong here as I’m sure a lot more goes into this and I’m just speaking from a high level.
JusticeIsAProcess@reddit
I'm surprised there wasn't any redundancy where if one bearing fails, the rest of the structure or so will compensate for it, until the next heavy check spots the damage and fixes it.
upbeatelk2622@reddit
OP you're forgetting the issue with certain 747 pylons. China Airlines cargo plane crash in late 1991, El Al cargo plane crash in 1992 ten months later, and then a Northwest passenger 747 in 1994 landed with this (pictured). Are they related? Maybe, maybe not. I'm just pulling them out of my head first, which I have the right to without making claims, just saying oh that's curious...
So, part of flying will always be discovering new ways in which a well-devised mechanism can fail beyond your wildest imagination.
IM kinda sorta uneducated O, those cargo MD-11s are beyond the age they were intended to operate safely - chances are they can and will be safe, but we're now beyond normal age limit and into the margins afforded by manufacturing quality and precision. If your hobby includes anything with older machinery, you'll notice how there will be times when the Swiss Cheese doesn't go as planned and God throws you a curveball. Many cargo pilots will tell you the industry convention is cargo flights are allowed to be less safe because they don't involve compensation for passengers or negative publicity.
FZ_Milkshake@reddit
Gyroscopic precession is not a big factor compared to the thrust loads on the pylon, to my knowledge the thrust links are not even asymmetrical. Further it is important that the bolts can actually fail if engine vibration is too high, you'd rather have a damaged engine rip clean of than risk it tearing the wing apart.
ZappaLlamaGamma@reddit (OP)
When I saw the compressor stall from #2, my heart sank for those pilots. And yes, the thrust load on the pylon is much higher than gyroscopic precession so I’d anticipate that it’s overbuilt in the “thrust axis”. I know the final report will give more detail on #2. I’m not even going to speculate beyond what we all saw. But anyway just trying to understand the failure mode a bit better and what led out there and yes the pylon separating from the wing is what it’s designed to do - that I know. Ideally it won’t be at literally the worst time so that debris is easily pulled into #2 and the mount/wing/something is damaged enough to start a fire. I hope they find a solution to keep these birds flying safely a while longer.
rayfound@reddit
It's kind of wild point of consideration that if it had been the #3 engine that had initially separated, the gyroscopic effect would have pulled it away and likely avoided injection/apparent compressor stall on #2... and the event would potentially have been survivable.
A3bilbaNEO@reddit
Looking at the latest pictures though, there seems to be an increased twist on the outer wing compared to the inner, as if the front spar was broken.
FZ_Milkshake@reddit
I think that is just how the slats look, they extend further on the outboard side of the wing and with the engine out of the way you can see that difference as a sawtooth in the leading edge.
JaaacckONeill@reddit
You're right that it's not the biggest load factor, by itself. But, at rotation, that is when the mounts take on the most load, more than any other point in a typical flight.
If fatigue cracks cause a failure, it'll more often fail at the point where it receives the most stress. So it's actually not a coincidence at all that this happened at the worst possible moment.
(Not disagreeing, just adding my two cents)
Isssaman@reddit
Yes. But fatigue cracking is time related. My guess the fix will be one time inspection followed by routine NDI on a flight hour or cycle count.
Hammer466@reddit
Honestly, I think they set the time before a through check (i.e. all methods of crack detection) really high on the pylon. The designers may not have expected forward and aft flexing or didn't expect the amount of it. There is a lot of thrust and weight on those pieces and when you consider both forward and reverse thrust, just about any amount of flexing will lead to an eventual fatigue crack given enough flight cycles.
Isssaman@reddit
yes
ANITIX87@reddit
Are you asking, "Would a stronger pylon have been stronger?"
ZappaLlamaGamma@reddit (OP)
No. I tend to ramble on while forming my thoughts. Meaning more so could that have minimized the twisting forces. Guess my brain is ahead of guidance on the remedy here as I’m just wondering if that hole in the Swiss cheese actually helps or would it just push a failure elsewhere. I have no clue how much lateral or rotational force is put on that connection either.