leaking fuel tank

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Anonymous

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I have a 1977 Citabria 7GCBC and have a fuel leak appearing on the underside of my right fuel tank. I verified that it was not fuel that was spilled around the filler neck and am fairly positive that I have a leak in the tank. Has anyone had this experience and what is the usual cause? Vibration,fatique? Is there any particulars to be concerned with in the removal and repair of the tank and associated fabric?

Thanks
n1047e
 
We also have a fuel leak in our 74 7ECA that we are trying to track down. The right tank leaks and the fuel is evident around the drain and also on the trailing edge of the wing. It leaks when we fill the tank and stops leaking when the fuel level falls to about 3" below the filler cap. At present we are checking to see if the one way check valve has failed in the vent system allowing fuel to enter the vent tubes and leak from there. We are starting here because it is the cheapest and easiest place to start and it quits leaking when the level falls below the vent outlet.
If it is not the vent system, then it looks like we may have to tear into the wing to find where the tank is leaking. I have heard that where the hold down bands cross the tank is a wear area to look at. I guess that will be next.

Ron86654
 
Thanks for the info. Unfortunately for me the leak starts with only a few gallons of fuel in the tank. I will keep you posted once we pull the tank out for inspection.

N1047E
 
I am having the same problem with my 1968 7KCAB. Mine only does it if it is full or if I slip for a long distance. It does not leak when I do aerobatics. The leak is extremely small, but persistant. My mechanic and I have just resealed the neck of the fuel tank. I just patched the fabric there. This slowed, but did not stop the leak. We are presently looking at the vent tube. If this doesn't do it, we will look at the fuel drain on that side. I have been told that the tank sometimes gets hairline crackes around where the fuel drain attaches to it. Thanks for the tip about the hold-down bands.

Colette 1889G
 
It had been my experience that the leaking fuel tank have something in common. They always leak (crack) on the top side of the tank. The tank is made of aluminum and is formed of two half shells welded together. To reinforce the large flat areas of of aluminum, 4 stiffening dimples were rolled or pressed into the metal to give it stiffness. It is at the corner of this stiffening dimple that the crack develops.

It is always the second stiffening dimple that cracks and mostly the inboard side that cracks first, although I just repaired my tank for the second time, the outboard end cracking this time along the dimple that I repaired about 4 years ago. The aircraft (a 74 KCAB)has about 6000 hrs on it.

To find the crack, do this: (I'll see if I can take some pics and post them on this tread to help you visualize the location of the trouble area)

Starting in front of the filler cap, push down on the fabric over the fuel tank and feel for the first stiffening dimple. Once you have located it, move aft 3 to 4 inches and feel for the second dimple. The dimples run most of the width of the tank, but the first and third ones are shorter than the second. Again, the second one is the culpret and the area that it cracks is just out board of the outboard tank hold-down strap which runs fore and aft, or inboard of the inboard most one.

Once you have located these two positions, the outboard and inboard end of the dimple....and again, it will be the right one if it is just aft of the filler neck.....you take out your razor blade and make a 2 inch dia circle cut-out to be able to look at the tank suface. Later these will be patched with a small fabric repair...no big deal.


What you are looking for is easy to spot. A 1 to 2 inch crack that spans the end of the stiffening dimple. Again, in my experience they alway crack in the same place.

This crack CANNNOT be weld repaired, don't even try it!! The metal will immediately work harden from the heat of the weld and crack again right through the weld. I know, I've gone this rout. I had a 8GCBC with long range tanks that needed all 4 tanks replaced, but not until after I wasted a lot of effort removing them and welding, only to remove them again!

So here is how I repair this crack. It's not exactly spelled out in AC 43.13 so get an I/A you can work with on this!

I had several tanks with cracks laying around so this is the idea I came up with.

I took a scrap tank, one that was cracked and removed from the Scout, and used a metal saw to cut out a piece of the tank from the same location as the portion of the tank to be repaired. Yes, I picked a location (stiffening dimple end) that was not cracked. This piece is about 1 inch on all sides larger than the area to be repaired and incorporated the stiffening dimple end.

The ends of the patch are smoothed and tapered, the general shape is roundish. Both the area of the tank to be repaired and the patch are abraided with scotch bright, preferrably using a motorized tool. If you want, you can etch both with phosphoric acid (Metal Etch brand) to further enhance adheasion.

Using a good brand of metal filled epoxy, bond the patch over the cracked tank using enough epoxy to fill any voids that might occur.

Using a small cardboard box, place the box over the repair area and insert a hair dryer into the side of the box. To do this, cut a small round hole, smaller than the dia of the hair dryer nozzle and cut radially outward several slits. The hair dryer will fit snugly and stay put. Turn it on low heat to keep the repair warm (not hot)while it cures. Some epoxys will take overnight to cure, even when warm, some will cook off much quicker. Do not use 5 minute epoxy. Use a Devcon brand of metal filled epoxy.

Once the epoxy has cured, the hole in the fabric is repaired and painted.

If you would like, you can call me direct for more info on this repair.
925-456-0430 work.

Steve
 
Sorry about that! I checked my spare tank today and took some measurements to make sure of the location of the stiffener dimple. It is not the second dimple but the third that has the cracks and it is located 9 1/2 inches aft of the aft edge of the filler neck. The patch I made the other day was on the outboard end of that stiffener and was located dead aft of the filler neck.

I hope this helps.

Steve
 
Steve

We found the source of the right wing fuel leak in our 74 7ECA. It was under the out board side of the inboard hold down strap at the 3rd(from the front) stiffener groove or dimple. These 4 dimples run laterally from near the outboard end of the tank to near the inboard end of the tank. We cut the first hole in the fabric at the 3rd dimple aft of the filler cap as you suggested, because we had no clue where to start and that was as good a place as any. All was dry there so we moved inboard to a point even with our first cut and just outboard of the innermost rib to stay in the dimple. This is where we found that the outboard edge of the hold down strap had rubbed a 1 1/2 inch long groove in the curvature of the dimple. There is a neoprene looking strap under the hold down strap that had worked to the inboard side of the aluminum strap allowing it to contact the tank.
Your directions and suggestions were instrumental in our finding the leak with minimal cutting of the fabric and I thank you for the leads.
The only difference we saw from your experience is that the leak is near the inboard end of the 3rd dimple.

Ron86654
 
Hi:

We found the leak on my 77' 7GCBC to be on the bottom side of the tank on the outer edge of the third dimple. Thanks for all the information, it was very helpful.

Bill
N1047E
 
We had almost the same experience as Ron, except that this leak had already be welded and was leaking around the weld. My mechanic is a little leary of using the cut out dimples from other fuel tanks because I fly aerobatics and he wasn't sure if these patches would be up to that kind of stress. Anyone got an opinion on this? In any case, I thnk I have located two used tanks. I find that I cannot buy a new tank for a 68 7KCAB. They simply don't make them anymore. If I do decide to change the fuel tank, any ideas on where I can get a good used fuel tank? Thanks to evryone for your help and suggestions. Colette
 
Worst come to worst you can have a tank made. I think if the part is no longer made the owner can have the part constructed and if the mechanic finds it suitable for a replacement part it is totaly legal. I would also say you could probably have one made cheaper than a bought new tank if one were available.
 
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