N707DD grounded...

[David-aviator]

Yesterday I drained the fuel tanks, the reason being the only way the tank rivet paint blisters will ever get fixed it to quit flying for a while. I've intended to do this for 2-3 years and always put it off in favor of just one more flight...well there's no fuel in the airplane so I can't fly.

There are 3 ways to fix the paint blisters.

(!) Strip down and repaint
(2) Open aft tank baffle and reseal the inside joints, strip down and repaint
(3) Build new tanks

After consulting with professional pro seal consultant Rick Galati, it has been decided to build new tanks. The reason being, we do not know for sure why the blisters have occurred (although there are some strong theories) and starting from scratch has the best statistical chance of getting rid of the nuisance blisters. I figure it is about a 99% sure thing. I've had the tank kit for a couple years, why not use it?

So I am back in the builder mode, not flyer mode. So far things are moving along well, drilling, dimpling, clecoing, and making the parts not made by Vans. Good friend, Charlie Eubanks, cut the 5.25" holes in the inside ribs on his milling machine yesterday. They will be the most accurate cuts on the airplane this side of the spar attach holes.

The pro seal business will begin next week. (actually Flamemaster) I will use Rick's fay seal method and am confident the process will be not a big deal. The only major challenge not yet figured out is how to match drill the attach brackets to the existing holes in the wing spar skear web, but I know it has been done and we will come up with a way to do it.

[apkp777]

David,

One note from my experience. Don't wipe off wet sealant from rivet heads with MEK. Wait until it dries. It'll be a lot more work, but I suspect many fuel leaks are caused by people cleaning their tanks with MEK and thereby removing too much sealant.

Just my opinion.

[RV10Man]

I agree with Tony. At least let it cure for a while.

Marshall Alexander

[Hens07]

David. Keep us updated on how it goes! Good luck

[Bubblehead]

I think Vans has a procedure for match drilling a replacement tank. I almost bought an 8 that had struck a taxiway edge light and dented one tank. The owner contacted Vans and they helped him through it.

[David-aviator]

The assembly of the 2 new tanks was completed yesterday. I decided to let everything cure until Saturday before doing the balloon leak test.

Meanwhile a matter that has been most troublesome has been resolved.

While hemming and hawing on just how to match drill the new Z brackets to the shear web holes, it occurred to me the brackets on the old tanks could be used. They are drilled, have plate nuts installed and if I transfer them rib to rib as they were installed in the old tanks, they just might fit the existing holes.

This evening both tanks were trial fit the wings.

The old Z brackets on the new tanks are a perfect fit the wing shear web holes!! Thank you, Vans, for using incredibly accurate CNC tooling machines. The old tanks and wings were built 6 years before the new tank parts were made.

Now it is on to determining if the darn things will hold fuel. And then it is priming, painting and reinstalling.

(Another slightly bothersome question is this. Both tanks were cleaned of everything that could be sucked up with a vacuum before installing the aft baffle. However, in the course of riveting the attach brackets to the ribs ends and aft baffle after a fay seal with clecos, a #30 drill bit had to be run down through the holes to clean out partially cured sealant so the juiced up blind rivets could be inserted and set. Some junk from the clean out drilling could have fallen inside the tank.

I am wondering if running water through the tank to wash such junk out is a worthwhile effort? I've build composit tanks before that were clean and never saw junk in a filter but the need to clean out those 25 holes at each aft baffle makes me wonder if this should be done.)

[gasman]

Rinsing the tank out with water should not be an issue with the access plate removed. Rinse and slosh it good, then dry with a fan or a hair dryer.............

[David-aviator]

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The last 4 days were spent learning how to paint (again). I've painted before but it always takes a few tries to get it right.

Fortunately, the primer was easily sanded after 3 attempts to get the gun to work right. The gun is just an el-cheapo from Harbor Freight but it works well - if it is cleaned of all junk from the last use 2 years ago. It took some MEK to do that task.

The 4th prime job and final paint today went on like a charm with no sags and little dust. I moved outside and that really helped keep dust to a minimum.

The paint and primer are Dupont Nason Urethane 2K. It is a good auto paint and has held up well on the airplane.

Now it is let the paint cure for a few days and then install the tanks. Here's hoping they hold fuel. They did hold air for 6-8 hours with the balloon test. With any luck at all, the beast will be flying again next week end. I may even make it to OSH.

[David-aviator]

If I counted corrected, there are 144 #8 screws and 42 AN3 bolts attaching the fuel tanks to the wings. They all went in yesterday and I slept better last night than for a long time.

What was most troubling, even though a trial fit indicated the "Z" brackets would match the spar sheer web, was the possibility one or two would not. And then what? I did not have a plan B. The effort has established that if the old brackets can be salvaged and installed as removed from old rib to new rib, they will match the sheer web holes.

They did fit, very well. Of the 42 bolts going in through from the access holes, only one was a bit contrary. I could not see it but could tell the edge of spar web hole was off a tiny bit and while the bolt went through the hole, it would not catch the plate nut thread. I finally got a small electric screw driver in the area with a hex head 3/8" drill bit and took the edge off the hole and problem solved.

The tanks each had 5 gallons of fuel over night. The fuel supply, return and vent lines are hooked up and fuel is flowing from tank to FM200 and back to tank - leaks so far.

More work getting it back together today.

[Geico266]

Good fix Dave, thanks for the update. Not that it matters, but I think you made the right choice just to replace the tanks. Once the painting gods are upset it seems like it best to replace it, or live with it!

No LEAKS!