Gotcha? Wing rib to spar bolt, extra washer required

Hi guys, maybe this is just me being dumb and missing a step but somehow I can't accept that I was the only one who fell into this hole. I did search the forums and online, but I couldn't find any similar gotcha's that anyone talked about. I made a writeup on our build blog about this yesterday, but thought I would also post it here.

In Section 14, page 6 for the Wing Ribs, there is ONE of the 24 bolts (per wing) that requires two washers stacked on each other instead of the typical single washer. The plans only call this out with a small "2X" mentioned in the parts listing, but it is neither depicted in the graphic; nor is it highlighted, flagged, or mentioned anywhere else in the plans. In my opinion, this is an extremely easy detail for a builder to miss. They call for two washers here in order to keep the nut from reaching the end of the threaded portion of the AN3-20A bolt to be used. I missed this callout when assembling the wing ribs, and the tolerances here are so close that using just one washer will still barely snug up on the two parts (as the nut reaches the end of the bolt threads) just as you reach 25 in-lbs torque on the bolt.

You could probably use a shorter bolt instead (like an AN3-17A) with one washer, but I think the reason Van's did it this way is so that the shank of each bolt protrudes all the way through the spar and rib flange, and the threads for each bolt begin past the rib flange. This probably helps with the shear strength characteristics of the fastener.

I doubt I would have ever caught this mistake, and I'm pretty confident it would have never been caught, had my Pitts builder friends Gary and Justin not stopped over to set a few rivets for their project and talk shop for a little while. We got into a discussion about bolt lengths, and I went over to demonstrate the way the spar and ribs had been assembled on our wings, when Gary just so happened to grab the ONE bolt (remember, out of 24) that was incorrectly done and was able to make it twist with his fingers. You can imagine my horror!

It took all three of us a good 20 minutes staring at this page of the plans, and checking our work to realize the problem. After quickly remedying the issue and dabbing an emotionally comforting extra-thick blob of torque seal on the bolt, I showed the plans to another friend of mine who specializes in engineering/installation drawings and diagrams for retrofitting the big jets (Boeing/Airbus) with electrical upgrades. He was appalled by the lack of a flag or some other highlight to point out the discrepancy for that one bolt versus the others, and recommended I contact Van's to suggest they highlight this step a little more clearly. That made me feel a little bit better, and I might just do that... but for now I wanted to at least document this finding.

The good news here is that I don't believe this one bolt being out of tolerance would have caused anywhere near a 'real' safety hazard for the airplane itself. With the skins riveted into place, and 23 other bolts and 81 more rivets holding the ribs to each spar, it's not as though the whole wing structure was compromised by this one bolt not being fully snug against the parts. That being said, it wasn't fastened as well as Van's engineers wanted it to be, and that is indeed not a good thing. Care needs to be taken to ensure you don't miss these kinds of details hidden in the plans, and plain and simple we failed with that here. I'd be willing to bet, though, that there's more than one RV-10 flying around out there with a missing washer in this location.

Believe it or not, I had this problem in a certified (182) airplane. Maintenance manual did not show washer, and none was there (but it was needed). Parts manual did show washer.
Bottom line is every bolt needs to be checked that it will not bottom out on the threads. Easy to grab the wrong bolt, thin washer, etc. On my QB wing someone put in too long bolts, compensated with a stack of washers. That's no good either.
As you said, the objective is to keep the threads out of the hole but still have enough for the nut to not bottom out. With two washers called for, I'd put one under the bolt head and see if the shank was going all the way thru. If so, I'd leave the washer under the bolt head and put just one under the nut.
The tolerances on these are sometimes pretty tight, which is why there are thin washers.

I agree that it's easy to miss. That said, you can't bottom a nut on a bolt without too many threads showing. That should always be your touchstone, plans or no; sometimes manufacturing tolerances conspire against you and an extra washer or at least a thin one is required. Sometimes it goes the other way - there are not enough threads showing so you have to substitute a thinner washer. As with rivet callouts, the plans are not the last word. I think you also make a good case for (gently) attempting to spin the nut (with no wrench on the bolt) after torquing a bolt in tension to be sure it truly is in tension. (Shear applications should spin freely.)