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Todd Bartrim wrote:
This may be a little off-topic and more appropriate for the RV-list as it deals with leaking RV tanks, but since most on this list use mogas, I figured I'd rather ask here. I brought a few gas cans up to the airport today and topped up the main tanks and since I had an extra couple of gallons, I just dumped it into the auxiliary outboard tanks. I haven't used these tanks since last summer and they've been empty of fuel since then. Before adding the fuel to the aux. tanks, I checked the water drains. One of the tanks was bone dry, while the other had a surprising amount of water in it (plane is not hangered). I filled my fuel sampler tube 4 times with water, then the 5th was about 1/3 water followed by 1/2 tube of fuel, then the tank was empty. I made a note to change the o-ring on the fuel cap and proceeded to dump several gallons of fuel into this tank.
I then proceeded to work on other items such as the heater valve & looking for an electron leak in my dimmer circuit. Close to an hour after I'd added the fuel, I suddenly heard a large amount of liquid falling to the ground and quickly climbed out from the cockpit to find a very large flow flow of fuel pouring from the entire wing root area. After the initial barrage of expletives I grabbed a gas can and funnel and stuck it under the area of heaviest rainfall of fuel, while I hurriedly removed the screws from the top wing root fairing. This didn't reveal much, but it appeared the fuel was flowing somewhere along the main spar. I incorrectly surmised that the main tank was leaking possibly from somewhere in the top seam since I'd just filled this tank to capacity and was guessing that possibly the sun was causing vapours, which somehow were unable to vent, to pressurize the tank... etc. So I quickly removed the drain and began draining directly from the main tank into a gas can, after draining several gallons out of this main tank, the leaking fuel from the wing root suddenly stopped, so I assumed I'd gotten the level below the leak. Now I was considering draining all of the fuel from this main tank and putting it into the auxiliary tanks, but first I checked the drain on the auxiliary tank that I'd found the water in and found it was empty! And I'd just dumped several gallons into it an hour previously. So the main tank wasn't leaking after all, so I put all of the drained fuel back into it and found no leaks.
Now what exactly happened there? It didn't leak a drop for close to an hour, then suddenly it let several gallons leak out in less than 10 minutes. It must have been an aggressive chemical reaction that dissolved the fuel tank sealant. The fuel that I use is premium unleaded from a card lock, that is supposed to be free from alcohol as it is marked for use with off-road vehicles, which typically do not approve the use of alcohol or ethanol additives in the fuel. I recall that alcohol only becomes corrosive when mixed with water, but I did a quick search through the archives and couldn't find confirmation of this. I did find mention that Jim was researching mogas for a presentation, so I'd be curious as to your take on this Jim.
I'm hoping that it is confined to the one tank that I'd found water in, otherwise I may find both full main tanks have leaked out next time I go up there. And with the ridiculous price of fuel these days that's allot of $$$, not to mention the work to re-seal these tanks. Since the fuel seemed to be running down the main spar, I expect that the leak is somewhere long the bottom of the rear baffle plate, but the tank will have to be removed in order to inspect this (no small job). I don't yet no which is the best way to repair a leak of this magnitude.
Todd Bartrim
RV9Endurance
13B Turbo Rotary
C-FSTB
1st step would be to check the datasheet on whatever sealant you used in the tanks. The Flamemaster brand stuff Van's sells now is supposed to be immune to avgas and mogas. It seems unlikely that a few minutes of exposure would cause such a major leak, even with the old mogas-vulnerable sealant. Did you use access covers on the aux tank like the mains? If so, did you use the cork gaskets? A LOT of people have leaks around the cork gaskets after months or years of flying.
I'm just about finished with my RV-7 tanks (remember my aux tank questions about a year ago?) & I had to seal a few leaks. I called Flamemaster to ask about a compatible, pourable sealant & they recommended cs 3600, a 1 part pourable compound. I swapped a few emails with their help person because some of the datasheets indicate use as a pourable 'slosh' type sealant, other (later) datasheets omit that use in the product description. In emails, they maintained its suitability for use on top of the 2 part paste that Van's sells. This stuff is not the same as the milky white stuff that caused so much grief in the past. It's a translucent red color & acts about like pourable contact cement. In fact, the datasheet describes it's primary use similar to contact cement application.
I was able to 'inject' it to the problem areas by sucking it into some nylon tubing & routing the tube toward the problem seam before releasing the fluid & then pushing it out of the tube with very low pressure air. I was trying to minimize the unneeded sheeting over large areas of the tank where no holes existed. The tanks now hold air, but it will obviously be a while before I can vouch for long term reliability.
Several folks on the RV-list mentioned that most peeling problems with 'slosh' were probably caused by the tank's interior not being completely clean before sloshing. That makes sense to me, since I very carefully cleaned the areas where I knew I'd be putting sealant but didn't work too hard on the rest of the skin inside the tanks.
I also got a couple of other recommendations on pourable sealants I can send you if you want.
Charlie
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