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Andrews Web Site
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Date: 7-2-2020
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Number of Hours: 6.00
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Manual Reference:
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Brief Description: Spar Fitting Issues
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While drilling the final holes in the last set of fittings, I noticed that the set-up pins in the large 17/32 holes as supplied by Mustang Aero were loose. When I inspected them further, I noticed that they were actually even bigger than the 35/64 holes that were supposed to be drilled next while in assembly with the outer wing spars. I also noticed that the holes as drilled were deeply scored. I went ahead and reamed a couple of the fittings to see how big the holes needed to get before the scores were cleaned up and the size came out to be 0.562. That's too big for the type of fit needed. Yes, I could order oversized bolts, but I don't want to start with that. I have two options.
1) Buy new fittings from Mustang Aero WITHOUT holes this time, or
2) Make new fittings and find a heat treat facility that will do the job.
I tried to get in touch with Mustang Aero, but my phone calls and emails were not returned. After a week with no contact, I decided to build a completely new set of fittings. Since I was building them from scratch, I added an additional 0.050 all around just to have some margin (I am not a huge fan of the Mustang wing joint in terms of margin for build errors). Once completed, I contacted some heat treat shops to see what that will cost to bring the 4130N up to 125ksi. The answer came back that the minimum charge is about $440 with inspection (minimum lot size problem). Around the time I got that quote, Mustang Aero got back in touch with me and said they could ship blank fittings fully heat treated with certificates immediately. The price for the Mustang Aero fittings is slightly less, so I told him to ship them, even though I have work in on the others. I also advised Mustang Aero about the hole issues, but that certainly did not get me a discount on the replacement...ha ha .
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Original fittings with large hole issues, and new blank fittings yet to be heat treated
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