Warbird Information Exchange

Who is the expert out here about BT-13A issues with Fuel leaks and rivets?

Need some guidance on some issues.


thanks!

Paul,

Fuel leaks in the BT-13 are a mess. Our BT (N56360) still leaks after about 800 man hours of stripping and resealing. I hate to discourage anyone, but it seems as though it is just something that cannot be fixed. If anyone out there has had a contrary experience, I'd love to hear about it.

Regards,

Mike Hogan
Nationa Capitol Squadron
CAF

thanks for the information Mike.

can i pass along your contact information to a third party who contacted me about fixing the leaks?

Hi you have to coat every rivet and seam with P.R.C. compound.There are galleys drilled through out the fuel tank area were they use to pump zink chromate paste through a grease gun to seal up the leaks.If the flaps are lowered at too high of air speed it flexes the center section enough to start major leakage.If the a/c is disassembled build a fixture so you can rotate the center section and pour a dilluted P.R.C. compount in the tanks , rotate both directions and drain excess.If its together remove all tank insp. covers, scuff all seams and start brushing P.R.C. on every seam and shop head of each rivet.If done right they should not leak.Good luck! Mike

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IF YOU CAN FIND IT WE CAN FIX IT

I own a BT 13 project and love the airplane. I am not an A/P IA but I have done a lot of looking and talking with people who have worked on these airplanes and here is how my experiences have gone.


The wings leak, period. They leaked when new, and they leak worse when older. Inexperienced pilots and high time doesn't help. There are several which are under re-construction right now with complete disassembly of the center section, but they haven't gone back together yet so who knows if they will leak or not leak. If I was betting, I'd say they are going to leak.

First thing to do is classify the leaks. Minor weeping of fuel, or blue residue around joints shouldn't be a big deal (although the FAA might have a thing to say about it). Leaks which produce visible trails or drips from the structure are signifigant and should be investigated. This is paraphrase, but WWII journals said the same thing pretty much.

The major issue is the rear spar, which is where the flap hinges are attached to, and is also the aft bulkhead of the tank. When you take apart original center sections, you find all kinds of dum-dum and extra coatings of tank coating put there by the original mechanics to try and keep the tanks from leaking. I think this ultimately attracts water into the dead space between the dum-dum and rear spar and lower skin, leading to corrosion. It doesn't take much separation on those joints to have a minor or even major leak.

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The above figure is what the rear spar looks like attached to the lower skin. There is one rivet line which closes the space between the skin (the dashed line) and the rear spar web (_/ in the figure)

I would say that no amount of filler is going to make up for for a bad rivet line, especially in a frequently flying aircraft. It is difficult to replace, but there is a lot of space there for hidden corrosion and my guess, from examining lots of wing center sections, is that you need to de-rivet the whole line, clean it up, and re-rivet it. Of course, you need to build a jig and hold the spars in alignment and deal with whatever you find along the way.

I don't think there is any problem with structural integrity in the wing center sections. If you see how they are overbuilt, you could lose half of the wing structure and the BT would still hold 6 times its weight in the air. But I do think (in the long term interest of the BT's preservation, and as a true BT 13 lover) that many of them which have signifigant leaks need to come apart. I am not alone in this conclusion and that is why there are a few projects out there with wing center sections coming apart and getting totally re-done. Of course, these guys are the laughingstock of many experts. But what they are finding totally supports their decisions to do a restoration this way. And of course, the REAL problem is doing a $25K repair to an airplane that is worth less than $100K, because that is a conservative estimate on what taking apart the center section and putting it back together would cost.

I'd be curious to hear what you find.

BT series leaks are really not that bad to fix if you want to take the time and do it correctly. To do it right, you are basically going to pull the bottom skins off one at a time and clean and reseal each sidel. I helped a friend do his and it takes a decent pair of sheetmetal guys about two weeks of hard work to each tank. Spread it out working on it during nights and weekends and it will jump the time quite a bit. On the one I was involved in, complete new lower skins were installed at the same time, so it ran up the manhours fairly quickly.

Once you remove the skin, clean and strip the tank to bare metal. Reprime with a fuel compatable primer.

Cap seal every visable rivet and joint with the approriate PRC sealant. Word to the wise is use the tube based semi-kits and use air driven sealant guns to apply. Allow to cure and inspect for any missed areas.

When ready to reinstall or install the new skin, this will be a wet layup...ie every place that will have contact with the skin will get a 1/16-1/8" layer of the same sealant just prior to installing the skin. Cleco the skin up with clecos in every other hole so that you get a good seal and flow of the sealant. Clean up all the wet ooze that you can and form nice fillets everywhere you can. Rivet it all up now using wet installed rivets.

Once you are done with all the riveting and bolting, go back and cap seal everything on the bottom skin from the inside, including all of the inspection plate nutplates. When you install the plates, use new gasket material or you will have leaks there. Fillet seal all the joints that are exterior that were not filleted when the skin was off.

Give the wing at least 96 hours (based on the sealants used) to cure. Then plug all the holes and do an air pressure check for leaks. We pressurised the wing to about 2psig and watched a 0-5 psig gauge for leakage. Once we were assured that there was no leakage other than the fuel cap, we then rotated the wing to the normal flight position and filled with water and pressure checked again.

Now repeat for the other cell and you will have no more leaks again...

If you have small leaks you can apply a vacuum (just enough to start to deform the skins) and brush on sloshing compound in the leak areas. This will suck it into the gaps.

I have had some experience with fuel leaks in a Mooney. A key is to scrape out all the old sealer or the PRC goo won't stick everywhere, don'tknow if this helps with BTs. It sounds like it could drive one to drink. First the BTs then the DTs. Bill G.

Bill: It's a lot like the Mooneys, except that the factory method at the time to seal every thing was to pack it and encapsulate it with zinc chromate paste. Over the years, the paste has slowly been washed out, thinned or just plain dried up and broken away from the structure.

The fix can be done through the inspection ports but it takes a considerably higher number of manhours to get it all cleaned. Also, you still run a big risk of missing an area and not being able to get it resealed. I've got a coworker that did a Mooney just last year and swore that if he ever had to do one again that he would pull the wing apart first. I think he ended up spending something like 300 manhours before he go the Mooney to quit leaking.