I was pretty sure we'd had a discussion thread on this one when it first happened back in October of 2022, but I couldn't find it. As I recall, I had speculated that the cause related to the odd geometry of the pivots at the ends of the struts.
As it turns out, the actual cause was much more prosaic. Basically, the upper end of the wing strut was anchored with a nut that was loaded in tension with almost the entire lifting load applied to the strut. Which, given prudent use of appropriate hardware, could have worked just fine.
However, the actual hardware was more than a little imprudent. The nut in question was an AN364-524, which is a thin nut intended for use only where the applied loads are primarily in shear. Not only that, but the bolt and nut were subject to rotation, which means that the nylock self-locking feature was the only thing keeping the nut on the bolt. Which, in the end, didn't seem to matter; in the case at hand the repeated application of axial tension simply stripped out the four or so threads in the 5/16" thin nut, releasing the wing from the wing strut attachment. All they found of the nut were its few threads wrapped around the bolt along with some strands of its nylon self-locking ring.
Report: https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/106079/pdf
Docket: https://data.ntsb.gov/Docket?ProjectID=106079
As it turns out, the actual cause was much more prosaic. Basically, the upper end of the wing strut was anchored with a nut that was loaded in tension with almost the entire lifting load applied to the strut. Which, given prudent use of appropriate hardware, could have worked just fine.
However, the actual hardware was more than a little imprudent. The nut in question was an AN364-524, which is a thin nut intended for use only where the applied loads are primarily in shear. Not only that, but the bolt and nut were subject to rotation, which means that the nylock self-locking feature was the only thing keeping the nut on the bolt. Which, in the end, didn't seem to matter; in the case at hand the repeated application of axial tension simply stripped out the four or so threads in the 5/16" thin nut, releasing the wing from the wing strut attachment. All they found of the nut were its few threads wrapped around the bolt along with some strands of its nylon self-locking ring.
Report: https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/106079/pdf
Docket: https://data.ntsb.gov/Docket?ProjectID=106079
