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In-flight breakup Yellowhead Helicopters Ltd. Bell Textron Inc. 212 (helicopter), C-GYHQ 7

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hpaircraft

hpaircraft

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Mar 23, 2018
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Not necessarily an engineering failure (except maybe process engineering), but one that should be of interest to any aero or mechanical engineer.


Long story short: Bell Helicopter subcontractor should have made blade retention pins from H11 tool steel hardened to Rockwell C 53, actually made them from 316 stainless of Rockwell C 8.

How one of these pins actually made it into a flying helicopter, and how all the opportunities to prevent it were missed, is fascinating.
 
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Technically that's all correct. But NC machines have sensors to measure tool load and will compensate.
And the high-school dropout that sweeps the chips out of the NC machine doesn't know, or care, what they look like.
I bet the hardness tester could have been used on the B scale, and god knows what they thought of HRB 72, or why that odd result didn't make someone in the know check the helper's result. Yet another mistake with no checks and balances.
 
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It's sort of similar to the calibration of the angle of attack sensor with the max.

 
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I can see the FMECA for it in my mind, along with a mitigation of "Manufacturing process controls ensure that parts meet design requirements".

Except nobody flowed that mitigation down to the contract manufacturer or the incoming inspection department.
 
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I'd like to have seen some analysis of how exactly a lack of training can get you the expected result out of the hardness test, rather than the actual value or some more random number.
 
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pellucidar said:
I'd like to have seen some analysis of how exactly a lack of training can get you the expected result out of the hardness test, rather than the actual value or some more random number.
Click to expand...

In my mind 'training' would include teaching an employee how critical it is that every part is tested, and that parts that yield results that don't make sense need to be segregated and investigated.

When I read that part of the report, I assumed this was a possible case of an untrained employee going "7?!? that doesn't make any sense at all. Can't be right. I'll just write down 50.5 like all the other ones were and move on".
 
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Would there not be a witness mark on the pin if it were hardness tested? That witness mark could indicate what the hardness reading was at the time of test. It seems like the pin was either not tested or the operator pencil whipped the number.
 
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The hardness test isn't performed on a finished part - I suspect it's removed as part of the process of finish grinding and coating. You wouldn't want to leave a Rockwell C test mark on the surface of a highly stressed part anyway - it'd be a perfect stress riser.
 
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I was thinking they did it on the end. The lathe turning prior to heat treat should leave the part very close to the finish diameter. I wouldn't expect more than. 0.010" to be removed during surface grinding. How deep is a Rockwell indentation? It's been a long time since I've had access to such a device.
 
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Aviation parts have a testing area defined sometimes etched depending what they are.

Well landing gear gear legs and rims do. Which are the ones I see daily.

Various yokes and other things have evidence of regular tests.
 
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These have a washer bonded to them on each end, and they are plated - both of which happen after hardness testing.

Agree that maybe the mark isn't completely removed, but it's very likely obscured.
 
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Yes, but proper QC (and training) costs $$. This was likely a “low cost supplier”. Sigh.
 
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Huh - never heard about this accident but this is incredibly close to home for me. Very familiar with Chip Lake (the lake to the west of the accident site).
 
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