A Fix For Leaking Fuel Cells

by Ardis Almond

First, the background. My plane is N71AG. You might have seen it at Oshkosh in the show plane camping area in the last few years. It is white with maroon trim and the most noticeable differences is Val Bernhardt's drooped tips and oversize wheel fairings. It also has a modified Cherokee 140 instrument panel with Piper wheel controls.

When I built my wings in the mid 70's, a lot of people were experiencing problems with leaks. I had the "white" wing panels as compared to the ones with the "green" tint.  As I understand it, these are different types of fiberglass and the materials used for repair might be different. I overlaid the fuel tank area with very light dynel cloth and finished it with fiberglass resin and micro balloon slurry. After a couple of years I started getting bubbles all over the top of the tank area and pinhole leaks on the bottom.  The bubbles were apparently coming from vapor that was actually separating the paint from the fiberglass. I fought this for years. I patched small holes with pro-seal and repainted the fuel tank area several times.  I began to get delamination between the wings and the dynel. Now I had a problem of a small leak in one area that may travel several inches until it found another pinhole in the dynel.  This problem got worse and whole sections had to be redone.  I went through the procedure to cut holes in the top of the wing and try to fix them from the inside. Finally, I had a pretty sorry mess.

In the repairs detailed below, I used the following materials (all from Aircraft Spruce except the Pro Seal type product from Seal Pack Company, 2614 S. Hoover, Wichita, Kansas 67215, 316-943-9489) :

Fuel Tank Poly Resin* 6060-5-1G-----------1 gal.
Polyester Resin Catalyst-----------------------4 oz.
Surface Curing Agent ---------------------------4 oz.
Feather Fill #401-----------------------------------1 qt.
Glass Bubbles -------------------------------------1 lb.
Standard Fiberglass Cloth #7533 60"------6 yd.

CS3204** Class B-2 Two-Part Polysulfide based Aircraft Fuel Tank Sealant------------1 gal. kit

*This is an isophthalic resin used for making fuel tanks.  Iso resin is highly flexible. Has excellent adhesion to metal, wood, concrete, fiberglass and other "hard to adhere" surfaces. Compatible with most fuels except gasohol. Iso is a wax-free resin and must be over-coated with Type "A" Surfacing Resin to obtain a surface cure.

**I refer to this generically as pro seal. It is the equivalent of PRC 890 Proseal brand of sealer.

I knew I had to have a more permanent solution. I removed the wings and brought them back to my shop in the spring of '96. I removed the flaps and ailerons so that I had only the wing to work with. I peeled off the old dynel layer.  It came off very easily. I sanded, cleaned, and inspected the wing panels. I patched a couple of places where the panel joints were leaking. I used pro seal for that. I had also marked every pinhole spot that was leaking. I used the thick pro seal and forced it into the pores in those areas. I smoothed it right down to the surface. I again sanded the panels. Then I squeegeed the wing tank area with the standard pro seal (I originally thought I had used the paintable pro seal, but in checking my records, it was the paste type). While it was still wet, I wrapped the entire tank area with a very light fiberglass cloth. I overlapped the cloth at the trailing edge of the wing. Next, after the pro seal had set up for a couple of days, a very light coat of resin was applied to the cloth. It was wetted just enough to leave the cloth fairly rough. The panels had some areas that were "dished or slightly distorted.  I filled these areas with a very light weight micro balloon/ resin slurry and covered these (after set up and sanding) with cloth.  After this set up, I finished the rough cloth surface with a resin and micro balloon "composite type" finish.  I then repainted the entire wing. It looked nice.

The theory behind this is that the pro seal would seal the gas everywhere and would not delaminate as the resin had done. The cloth would add tensile strength to the whole wing and help keep the joints from trying to delaminate. I thought this was better than trying to cover the fuel tank area with metal because of the difference in expansion rates and different modulus of elasticity values. I used the Fuel Tank Poly Resin because I thought the strength was sufficient, it bonded well, it was flexible, and it would hold up better if fuel did get in contact with it. Believe it or not, the weight worked out to exactly the same as the wings when they were new.  I was very conscious of keeping the layers as thin as possible and the slurry as light as possible. When I first built the wings, I just had a very heavy layer with the dynel and heavy resin.

It worked great. My plane has been flying now for 6 years with the repaired wings. I have had only a couple of small bubbles show up on the upper surface and no stain.  I had one small leak appear at the spar inner panel interface and that was easily repaired with pro seal. The last problem was unrelated to the skin problem anyway.

If anyone wants to know more, I will be glad to help any way I can. 

See also: Ken Strite's fuel cell repair article.