Roger Rucker Fuel Tank Repair

From Bondline

[Update: As of today (1/5/10) it has been one year since the repair described below was completed and it would seem that the test or proof of such a repair is time. No wing tank leaks have been observed throughout this period, so it appears that the repair was successful.]

Bondline contributor and Cheetah owner Roger Rucker shares the story of repairing the fuel tank leak on his Cheetah.

In November of 2008, while the plane was at a maintenance shop it developed a leak at the outboard spar on the left wing. Since this shop had considerable experience repairing these leaks, I approved the repair, but on the trip home from the maintenance shop I found that the leak was still present. When the wing strap was removed, it was clear that fuel was flowing from the spar area. Returning the plane to that maintenance shop was not feasible. Eventually a couple of options arose but an A&P friend of mine agreed to give the repair a try after researching the repair on Bondline (and Bluejay's website) and a series of questions sent to Bob Steward. Note: All of my work was performed under the direction and supervision of an A&P.

This repair is not for the timid or for someone not willing to put considerable time, effort and patience into the job plus the inevitable scraping or damage to the hands, arms and knuckles with loss of blood from these injuries. My A&P and I estimate that between the two of us we put approximately 50 hours into scraping, cleaning and sealing the tank from start to finish.

My 1978 AA5A has a different fuel flange that protrudes further into the tank than Bluejay's Tiger, which makes it a little more difficult to work around this area to remove old sealant from the top of the spar/wing/rib area. We disturbed the sealant on the spar side of this flange and at the end we cleaned/resealed it. The sealant round the vent tube that runs through the rib forward of the spar should also be inspected, as scraping around the spar may possibly disturb this area. This is probably a good time to inspect the fuel scupper drain hose for security or possible replacement and any nicks you may have put in the hose during the scraping activity. The doubler inside the tank on the bottom of the wing skin was found to be missing sealant on the spar side near that front access plate and we added sealant to this area when we sealed the inspection plates. Sealant on all other areas of the doubler's seams was intact and looked to be in good shape.

Although I went to Home Depot looking for anything that might be useful to remove the old sealant, eventually the most useful tools were several plexiglas hand tools we cut from a sheet of plexiglas (1/8" thick) of different lengths and widths with a sharpened edge on one end. Eventually, practically all the Home Depot products were returned.

We spent a day removing the outboard (fore and aft) inspection plates, cleaning these items and their wing mounting areas, inspecting and getting a clear idea of the areas of the wing/spar/rib to be cleaned and putting the left wing main wheel on a 4X4. The 4X4 made it more comfortable to work under the wing and sit somewhat upright during the cleaning process (less strain on the back and neck). Even with this position I could only manage at most about 6 hours under the wing per day and this was with breaks every 60 minutes. The inspection of the spar area revealed that the maintenance shop that had done the last repair had put new sealant over old and had dumped a significant amount of sealant at the lower front area of the spar. We also discovered that the sealant at the top of the spar had the consistency of sticky peanut butter (not had setup or hardened in any way). The sealant (new or old) at the rear lower area of the spar was highly suspect. But, since the maintenance shop had only removed the forward tank inspection plate, we couldn't tell if this was new or old sealant.

Prior to sealant removal and at the end of each day I took digital pictures of the spar/wing/rib areas. This was useful as when I uploaded the pictures to the computer I could zoom into areas around the spar and identify areas that needed additional attention. As the work progressed I made drawings of the spar/wing/rib from these pictures and marked in red ink areas where sealant was present and took these drawings to the plane to provide additional direction for my work.

Day 1 was the beginning of the scraping/cleaning process. After considerable time nibbling at the large blob of sealant at the lower front area of the spar, the two of us took a long blade knife with a semi-sharp edge and with him on one side of the spar and me on the other, we ran the knife back and forth above the lower wing skin but parallel with the rib to loosen this area of sealant. I suspect this effort was similar to Bluejay's use of a hacksaw blade, but in a different location and a different type of problem. But it was not until the first half of the second day that we were able to completely remove this large chunk of sealant (about 1" tall and 3" long and that was after the earlier nibbling attempts) but on Day 1 we could begin to see some metal under the old sealant in a few small areas.

Days 2-5 consisted of more and more scraping/cleaning (similar to Bluejay's activities). The most time consuming area was the sticky peanut butter area at the top of the spar that had to be scooped (not scrapped) out with a tool my A&P made from a soft pliable and slim piece of "metal" that was 6" long and about 1/4" wide. Any large amount of pressure on this tool would cause it to bend. Tedious work but it did the job. Another "tool," if you can call it a tool, which was extremely useful were my fingernails to dig into the sealant. They were especially useful along the edges and corners of the spar's flange and on flat parts of the rib. In reality Day 5 was "make sure day." Make sure that all old sealant was removed from every nook and cranny. Attention to the small "tongue" area of the spar's flange (where the flange joins the spar at the top and bottom) was made because old sealant was especially difficult to see without the computer images and hard to dig out. I found that a brush typically used for spark plug cleaning was useful here and around the bolts and rivets on the spar flange. We put MEK on a scotch bright pad and ran it across and underneath the spar in a back and forth manner and that assured us that the area was clean when we also used a rag with MEK in the same manner and it came back clean. The end of day 5 was also devoted to cleanup inside the wing of all the bits and pieces of sealant that had gotten spread around inside the wing. Vacuuming the inside of the wing didn't work. So I took some clear tape (used for boxes) looped it with the sticky side out around my 4 fingers and dabbed the inside of the wing until no sealant particles could be seen on the tape.

Day 6 was devoted to sealing the spar/wing/rib areas. Tongue depressors and a large irrigation syringe from a local healthcare supply store were the main tools for applying the sealant. The syringe was utilized to inject the sealant into the area above and below the spar, between the wing skin and the spar. I also used a silicone spatula along with the tongue depressors to spread the sealant on the smooth areas of the spar, rib and along the lip of the rib that mates with the wing (we had cleaned out from the spar several inches along the rib top and bottom). The spatula and depressors eliminated the problem or look of "sealant stalactites and stalagmites" as the sealant was applied. At the end of the day we took plastic screening (held in place with the inspection cover screws) to cover the open inspection panels and allow the sealant to set up and "breathe".

Three days later the sealant was inspected and found to have virtually completely cured, so the two inspection plates were installed. For the next 7 days (no real hurry and not wanting to do it again), I went out to the plane and opened the fuel cap to allow the tank to breathe for several hours. On the eight day I added fuel to the tab (19 gals) at 5 gal intervals and looking for leaks after each addition, none was found. My A&P was out of town for a week and I waited for his return to flight test the plane and to take the fuel to the top, let it set for at least an hour and inspect for leaks, none found. We flew the plane back to my home base and again checked for leaks, none found. After a couple more short flights the fuel filter was removed and inspected for pieces of old sealant, none found.