Fuel tank cover issue - Factory Q/B kit

[DBeaulieu]

Okay...trying to get my wings ready for paint. Getting some of the last little steps done. The fuel tank bay was final drilled for nutplates holding the tank cover last Fall and I left fuel tank cover panel at #40 pilot holes, supposedly already match drilled. This week, after opening up the tank cover panel #40 holes to #27 I find that there is no longer a good match of the cover plate upsized holes to the tank bay nutplate holes. No dimpling or edge breaks on the cover plate at this point. Not happy with the misalignment. I'm certain I have the correct fuel tank cover for the associated wing. The misalignment is almost half a hole along one side of the cover panel. and perfect along two adjacent sides. Seems like a slight twist has manifested in that area.

Sorry so lengthy, but I am perplexed and slightly annoyed over the situation. I don't look forward to trying to match drill a new cover to the 96 existing final size holes around the tank bay. Open to suggestion on how to proceed with the matter.

Thanks for any thoughts and considering this issue!

Dave (from Maine)

[Steve W]

Hi Dave. I have done 4 sets of wings and up to the last set, didn’t have any issue with cover hole alignment. The last set, model 5, some of the holes were off a considerable amount on one wing only to the point of where I would like to have made another cover. It was the row along the root side of the bay. The wings came back to me for completion along with the rest of the kit. I’ll reassess after I look at the alignment again. I slightly opened up some of the bad ones. I will have to see what diameter the numbered drill people are suggesting to use for the hole size and see if I’m close. I don’t countersink the screws and normally leave the holes at 9/64ths which makes a tight assembly. Probably a couple of hours or more to duplicate another cover. The holes along the three good sides are easy to duplicate. You would need a strap duplicator for the bad side. And I’m not a fan of those either!

[DBeaulieu]

Steve W Steve, I do not plan on countersinking or dimpling for the cover. Pan head screws only. Bob was okay with that as long as I used aircraft grade hardware

[Dpearson]

Awww what a headache! IIRC the fuel tank cover is a structural part as it carries some of the load of the monocoque design. So getting the screws and screw holes right matters... IMHO it sounds like making a new cover is probably required.

Are the nut plates already installed in the wing? If so, you could use my "set screw + blue tape" trick in the attached photos, which I had to use when re-making an inspection cover that suffered from a 2006 factory goof (the nut plates were offset vs. the screw holes, and re-making the cover with holes to match the wandering nut plates seemed to be the best solution):
1. Install set screws at nut plate locations.
2. Put blue tape on back side of cover.
3. Tape cover in place, with blue tape pushed up against set screws.
4. Tap around the screw locations with a rubber mallet, which causes the set screws to leave a mark in the blue tape.
5. Drill the cover at the marks.
I think I probably did two corners (diagonally opposite) first, and then used screws to hold the cover in place for the rest of the process.

image.png
image.png

If you haven't installed the nut plates yet, a strap duplicator as Steve W mentioned seems like the best approach. I'd do corners first, then work my way around using lots of brass clecoes as you go.

Finally (and I know may be too late for you, sorry), I've attached my tank cover nutplate instructions I wrote for myself some time ago. In a nutshell, I updrilled #40 to #27 through the cover and wing structure in one pass, with the cover clecoed + screwed in place. Then removed the cover and did the rest of the nutplate install stuff.

[DBeaulieu]

Dpearson "I updrilled #40 to #27 through the cover and wing structure in one pass, with the cover clecoed + screwed in place. Then removed the cover and did the rest of the nutplate install stuff" This is the way it should be done IMHO !! Would be a non-issue then.

[Bcone1381]

Another idea if you decide to remanufacture the cover is Blind hole pins, or blind hole spotters. Both Aircraft tool supply and The Yard Store carry them. I also have some bild hole spotters that thread into the nutplate.

https://www.yardstore.com/sheet-metal-hand-tools/hole-finders/blind-hole-spotters

[Steve W]

Yea, I like using those too but are they available in #6? I’ve used larger sizes but not the 6.

[DBeaulieu]

@Bcone That was my first thought when I realized/accepted that I may have to make a new cover.

[noema]

Transferring hole patterns with a clear sheet of thin, transparent plastic (e.g. acetate sheets) works well.

You can also increase the accuracy in situation like this with the following procedure. Drill small holes (#40) first, mark direction of inaccuracies. Then make smaller adjustments with an end mill (oval out the hole by the same amount in the opposite direction). Drill final size.

[Steve W]

Just checked the wings and it's the right wing model 5 ser #15 so an early wing in their production. Just in case it’s the same wing affected. I realize yours is a Patrol but they should be assembled in the same jig. Tank cover is also .040. I noticed that when we unbuttoned another set of M5 wings a few weeks ago. Those were .032 and a much newer kit.

[DBeaulieu]

Thank you all very much for the ideas and support. Once I have some Aluminum material to work with I have some methods to go forward with. Going up to the hangar today to check the other wing fuel tank cover (which I have not upsized from #40 yet) I plan on getting two diagonal corners started and clecoed then work my way to opposing corners with incrementally larger drill bit sizes once I'm certain I have good hole alignment. Will be a PITA but better than fabricating a new cover

Dave (from Maine...where the black flies are thriving!)

[TLyon]

I just did this a couple weeks ago used Dpearson’s technique of clecoing the cover in place through the #40 holes then upsizing the cover and underlining structure to #27 all together. Worked well assuming the #40 holes all match to start with.

[DBeaulieu]

Agree that is an absolutely great way to do it!

[Bcone1381]

Another idea.

Use your existing tank cover as a template. Lots of the holes are fine. Some are not.

An example you might consider....what I am about to write might be a bad idea that might ignite a better idea. Drill all the offending holes oversize with a step drill....way oversize. Then fabricate on your bench away from the tank a small 2" x 2" " coupon from scrap aluminum. Drill a single hole #27 in the center of the coupon to mate with each screw location in the wing.

Then install your old tank cover (what is now a new drilling template) and put a coupon over each unsat hole.... adjust the coupon position so its right....exactly right and fix the coupon in place with painters tape, This your drilling template is ready to use. You might test the template by fabricating a cover from a couple pieces of posterboard married with spray adhesive before you drill out four aluminum.

If nut plates were installed before the template was made the precision of the hole location using a machine screw might improve.

[DBeaulieu]

Bcone1381 Thanks for considering the issue Brooks.

I was in the hangar this morning with a couple of very experienced A&P/IA types with diverse backgrounds on certified aircraft. Problem situation is such that 3 sides of the cover and tank bay are in good alignment, and allow for fasteners though a slight bit of cover wrinkle. The remaining side (aft tank spar) has good alignment on about 2/3 of the fasteners with about 8 or so that were at about a 1/3 to 1/2 half hole off. A satisfactory solution to them (which I have started to implement) is this: (1) I upsized all tank cover holes to #21 from #27 and for the misaligned remaining nutplate holes (already drilled to #27, but without nutplates) I'm going to open up to about #12 and install floating nutplates. The lateral freedom of the floating nutplates allow me to pickup the screws thru the #21 holes in the fuel tank cover to those particular floating nutplates. The #21 holes allow me to achieve a wrinkle free panel and still have a good tight fit of the fasteners thru the holes with no shifting of the cover.

The decision to go this route seems reasonable to us, and likely better congruence of holes that would have to be match drilled to existing structure if fabricating a new panel from stock. I also do not plan on dimpling or countersinking for these particular fasteners associated with the fuel bay. When I discussed that with Bob (not countersinking) he was okay with that as long as aircraft grade quality fasteners were used.

The ideal procedure for avoiding this type of problem is just doing the initial and final match drilling together with all pieces...live and learn!

Comments welcome!

Dave (from Maine)

[Bcone1381]

I imagine its a relief to have a path forward! Ive been there many times. (And am there right now...hehe) Great to hear that you talked it over with Bob.