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Date: Mon, 14 Mar 2005 10:40:05 -0600
From: Jim Sower <canarder@frontiernet.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Leaking Mogas
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Todd,
Most of what I learned about Mogas was gleaned from the EAA web site.  A 
lot of the stuff is kind of dated - particularly discussions of 
volatility - and is of somewhat questionable utility.

As to the corrosiveness of Mogas with alcohol, the word I got is that 
neither Mogas or alcohol is damaging by itself, but when mixed (which is 
inevitable) the combination is very damaging.  Good news is that this 
damaging effect is mostly limited to poorly prepared composite fuel 
tanks.  Like the wet wing (strakes) on plastic airplanes.  If you have a 
composite tank, you could very easily have issues around this, but if 
not, you probably don't.

First step is to determine if you have alcohol.  Aside from corn 
producing states, alcohol as an oxygenator is pretty much limited to 
large metro areas that aren't making their EPA clean air numbers.  You 
can easily test your source:
Get a largish "test tube" and mark it off in 11 graduations.
Fill it up to level 10 with the Mogas you want to sample
Top it off with one unit of water
Shake it vigorously for a longer time than you really feel is necessary.
If you now have 9 - 9.5 units of gas and 2 - 1.5 units of water, you 
have alcohol.

I can't visualize your fuel system so I'm clueless as to how your aux 
tank could be filled and suddenly turn up empty.  I don't understand the 
scenario, so I can't help there.  I do not believe your problem is 
alcohol related since it slowly corrodes tanks that are not properly 
protected - it does NOT dissolve them catastrophically.

Hope this helps ... Jim S.

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