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Date: Mon, 14 Mar 2005 15:22:42 -0600
From: Charlie England <ceengland@bellsouth.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Leaking Mogas
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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