Monday, June 25, 2012

The Frikkin' Fuel Tank Fitting Fiasco

Van's Builder Assistance line opens at 10am Eastern time.

I started dialing at 9:57. I have no evidence to back up this assertion, but I believe first thing Monday is likely to be one of their busier times. I mean, how much airplane building goes on over an early summer weekend do you figure? Me, I figure it's a hella lot, and I wanted to be first in line.

At 10:01 I was on the horn with a very pleasant (and not at all condescending) Builder Assistant named Spencer. Keep in mind that 10am Monday morning to me is 7am Monday morning to a Van's employee (they're located in Oregon) and I was probably the first caller of the day for Spencer, so it took a little doing to try to paint a picture of my problem using only words. We all know that it takes a thousand words to stand in lieu of a picture, and both Spencer and I were pretty tired of the effort by word seventy-five.

"Hmmm," he said, "I might need some pictures."

This being my second rodeo, I already had an email window open and ready to send, the four most pertinent pictures already attached. All I needed was his email address. Even though we had only been on the phone for a couple of minutes, I got the feeling that there was no time to waste on his part, and what with me sitting there at my desk getting paid by an overly trusting employer, I wasn't keen on wasting a lot of time either while Spencer tried to sift my email out of the hundreds sitting in their shared inbox. He promised to retrieve the email and get back to me with an answer later.

Which he did:


After looking at your pictures and talking it over with one of the other guys we think your best option is to enlarge the hole in the tank attach bracket. If you take the nut plate off and enlarge the hole to accept the bolt you should be able to rotate the nut plate and drill new holes to attach it.


Concurrent with my help request from Van's, I had also posted as message on the peer-level Vans Air Force forum. I had also received a very helpful answer there:

I don't know if this is similar to your problem or not, but when recently doing the tank bracket modification on the Teen Flight RV-12 we had a bolt alignment problem.

Because of bend tolerances, the bolt hole in the bracket added to the left (inboard) side of the tank didn't align with the bolt hole in the lateral mounting plate on the tank. This made the bolt unable to be inserted when it came through the 1/4 inch thick center section C channel. It wasn't discovered until the tank was ready to be reinstalled.

We resolved it by removing the nut plate and then match drilling the #12 hole in the thinner bracket using the heavier mounting plate. This ovaled the hole in the bracket very slightly but none of the original design strength was lost because the hole in the mounting plate was undisturbed. A 10-32 floating nut plate was then installed in place of the original K1000, to allow for the slight offset in the nut plate position. The bolt went in easily at reassembly.

While this was very similar to my problem, I believe it was to a lesser degree. The suggested resolution was very similar, though, and had the additional suggestion of using a floating net plate. A floating nut plate is more forgiving of small misalignment issues because, as suggested by the name, there is actually a nut in it housed in a loose-fitting cage that allows it to, well.... float around a little bit.

Eh, I might as well save the other 960 words and show you a picture:

Naturally, I don't keep a collection of variously sized floating nut plates lying around, so I hoped that I wouldn't actually need one. After all, I would be match drilling the new hole and we all know how precise I am when it comes to match drilling....

That would come later, though. First I would have to remove the existing K1000-3 non-floating nut plate. This would be the third time drilling out one of the rivets in this particular nut plate. The first time was when I had to remove it for the Service Bulletin, and the second time was when I had to re-do the rivet after performing the Service Bulletin because I had accidentally left the flush head sitting proud when I squeezed it in. A flush head that doesn't sit flush pretty much defeats the design goal that drove the decision to use a flush head rivet in the first place, after all.  I had to fix that.

Why does it matter that the rivet had been drilled out twice already? Well, because there are only so many times that you can molest a rivet hole without ruining it.

With the nut plate removed, I could place the tank into the airplane and do the match drilling. Something was concerning me, though: would I be able to get a drill down into the center section?


I could have removed the wings to get those spars out of the way, but I would still have had the same problem, albeit to a lesser degree. It would still have been a little bit out of alignment, and just a little bit out of alignment is like being just a little bit pregnant.

I tried a different tack. I smeared some of the bright orange Torque Seal (which is really fingernail polish dolled up with a name that allows for a 500% price mark-up) on the end of the drill bit, then used that to mark the hole location on the tank.

I was reasonably sure that this would never, ever work, but I was wrong; the bolt fit in pretty well.

I screwed the nut plate in temporarily and marked its location with a Sharpie. All that was left to do was match drill the new rivet hole (after determining that only one new hole would be needed), but that presented a problem. There was no way to get a drill bit to it.

I had to put the nut plate on the other side and do the best I could at getting it in the correct position.

Here it is riveted in. Ugly, no? I suspect that my fuel tank installation is destined to be slightly more frangible than most.

Not just ugly, though. It also doesn't work. I got the nut plate alignment off by a minuscule amount, but it is just enough to make it very, very difficult to get the bolt in. Forcing a fit into a nut plate is a bad idea, generally, because if you ruin the nut plate it will be very difficult to replace. That's a somewhat obvious observation, I suppose, given that a nut plate is usually used for situations where the location is not accessible. This case is a little different in that I could choose to forego the use of a nut plate entirely and just use a lock nut, but what I'm going to do instead is order a floating nut plate, just like I should have done in the first place. That's okay, though, since I also need to replace the K1000-3 that I purloined from another part of the kit.

It's not like I have those lying around either, you know.

UPDATE: This appeared on the support forum after I shared my hypothesis that the center section must have been incorrectly drilled by Van's:

Your assumption is correct. The center section is drilled incorrectly. I measured mine and the dimension is the same as your tank bracket.

44.5 cm and the hole in the center section appears to be centered between the rivets.
So that's that. At least it wasn't something I did wrong, unless this is some form of Karmic retribution in which case, well, mea culpa.

The only tangible result of this validation is that I will forget the idea of ordering a new tank bracket to replace the one I've made such an unattractive mess of - it would require the same fix and it would more than likely end up being just as ugly.

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