PPRuNe Forums - Bell 407 in-flight tail boom separation

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Jetstream67

7th December 2022 13:48

Quote:

Originally Posted by helispotter (Post 11343733)

wrench1: Isn't it possible the bolt would have plastically deformed (stretched) slightly in the process of the tail boom pulling the entire longeron attachment fitting from the longeron? If the bolt stretched in that way, then the remaining torque when tested would be less than what it had been prior to the failure simply because the bolt isn't pulling the nut up against the washers as much as it did beforehand. However in any case, NTSB doesn't seem to raise their torque readings as a direct issue*.

Yes very likely, but some measurable distortion of the stretched thread dimensions vs unstretched section of the same bolt should be evident if that was the case. I imagine they know this ;)

wrench1

7th December 2022 14:36

Quote:

Originally Posted by helispotter (Post 11343733)

Isn't it possible the bolt would have plastically deformed (stretched) slightly in the process of the tail boom pulling the entire longeron attachment fitting from the longeron?

Sure. Without knowing the failure sequence anything is possible with the bolts and ring. For example, why didn’t the upper RH bolt stretch as well? But if there was any bolt stretching I would think it would be the lower LH bolt with the upper bolt missing or broken. But it was over-torqued as well. Regardless as you mentioned it’s a moot point. I just base my observations on experience and other tailboom failures I’ve been around.

helispotter

9th December 2022 20:55

wrench1: Yes, good point about expecting other bolts, especially lower left to then also be subject to stretching if that was the case on the lower right. I just had tunnel vision about the single (seemingly) under-torqued bolt!

Jetstream67: Assessing bolt distortion via thread pitch measurements would certainly be a good check, but there is no reporting of this in the "Materials Laboratory Factual Report 22-071". They documented number of washers used and number of exposed threads (beyond nut end) on all three remaining bolts.

helofixer

9th December 2022 21:24

Bell issued ASB 407-22-128 today.Basically a one-time torque check of the four bolts and nuts, visual inspection of the surrounding structure, and then fill out a bunch of paperwork and email that to Bell but I'm sure an AD will be following, since ASB not mandatory unless you're Part 135 etc.

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