For those engines that require auto gas from the local off-airport "service" station, here in California the spectre of ethanol rears its ugly head. The Internet runneth over with conflicting advice. For example, here's an infomercial (despite protestations to the contrary) for the VP Power line of products.

Is there a fuel/engine expert in the house that can provide the last word on how to and how not to fuel a Rotax?

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Not exactly. Here in Cloverdale there is an ethanol free gas station and 30 miles away there is ethanol free gas available at Ukiah Airport.

Interesting. I searched a while back on the Internutcase and couldn't find any. Do you know if there are other places that sell it?

Gary, the local supplier says their fuel contains "up to" ten percent ethanol. He was evasive at first ("a small amount"); I had to press him very hard to pry the truth out of him. Should he be reported to the website? He told me to go to the VP Fuels website; I sent them an email.

Does the ethanol "up" the octane reading?


I really have zero knowledge on this subject other than the reference web page someone had pointed me to.

I've used auto gas in my Zenith with a Rotax now for 6 years and 400 hours with no problems.  I'm based in AZ where ethanol is required in all auto gas.  I would be more afraid of putting additives in the fuel than I would be of alcohol.  Just make sure you fuel system is compatible with ethanol.  This is pretty easy to do. 

I've heard that some pilots drain fuel that has been (or will be) in the tanks more than a week or so, so serious is the deterioration problem. Some fuels (like 100LL) are apparently more resistant to deterioration that others, especially those containing ethanol. Any experience with or knowledge about this?


If I did that I would have to drain one tank every week and leave the plane's tanks empty.  Sounds like a lot of trouble.  The longest my plane has sat idle has been one month.  The idle time was during August, really hot here during this period.  Got in the airplane with no prep and flew with no problems.  I used a bout 6 gallons on that flight. Topped off with fresh fuel in that tank.  Next week flew again.  Now the fuel in tank I used was over 5 weeks "old."  Again no problem.  Topped off that tank with 5 gallons of fresh gas.  Came back a week later and flew again no problems.  I see no easy economical way to make sure that the fuel I'm flying with is less than a week old.

Some fuels are apparently more subject to deterioration than others, and water (attracted by ethanol?) apparently intensifies it. This is not based on technical knowledge on my part, so if anyone is actually qualified to speak on this subject, I'm all ears.


The ethanol contaminated fuel causes problems in three ways. One is the chemical attacks ethanol can make on many components (fiberglas, many rubber and plastic compunds, etc). The ethanol can directly damage or destroy non ethanol tolerant components in your fuel system. That is what folks have been telling you when they say you need EVERYTHING in your fuel system made of ethanol tolerant materials - hoses, tanks, pump diaphragms, O-rings, seals, etc etc etc.

Second is shelf life. Ethanol contaminated auto gas spoils in about 2 months (less if in a vented container, more on that in discussion three), Ethanol free auto gas spoils in about two years and Avgas spoils in three to five years. Aircraft are expected to sit for extended periods so the fuel is specified to have a long shelf life.

Third is moisture. Ethanol, like all alcohols, loves water and absorbs it readily, even eagerly. Modern automotive fuel systems are sealed - the tank is vented through the emissions control system and the gas cap is airtight when installed. Therefore atmospheric water vapor cannot get to the top of the fuel in the tank. In older vehicles and in any airplane the fuel tanks and carburetor float chambers are vented to atmosphere. Therefore the air at the top of the fuel in the tanks and inside the carb float chamber is atmospheric air - with water vapor in it. The alcohol in the fuel sucks that moisture out the air and takes it into the alcohol, making it a water and alcohol solution in the gasolene. Every time more air enters the vented tank or carb bowl the alcohol absorbs the newly arrived water vapor and adds water to the water/alcohol solution. Eventually the alcohol can absorb no more water and any newly arrived water gets sort of absorbed and becomes a yucky slimy mess inside the fuel system. This is called phase seperation. This does lots of damage. The phase seperation mess also will eat aluminum so even your aluminum tank and carb fuel bowl are not safe.

So, if you have an ethanol tolerant fuel system and you burn the fuel out of the tanks very soon after putting it into the tanks you will have no trouble burning ethanol contaminated auto fuel. If you do not have ethanol tolerant fuel system components you will have trouble very promptly as your compenents get attacked by the ethanol. If you have ethanol tolerant components but leave the fuel in the tanks (and carb) for extended periods you will have phase seperation and problems.

What some folks do is have one tank that never sees any fuel that is not ethanol free. It only sees Avgas or ethanol free auto gas That is the tank they take off and land with and is also the tank they use for a long enough period before shutdown that there is only ethanol free fuel in the plumbing and the carb. They use a different tank for the ethanol contaminated fuel and only use it for airborne use. If the plane is not going to fly again right away they drain that tank and use up the fuel in their car. You do not want to leave ethanol contaminated fuel in your vented tank for more than a few days or a week or two - nobody knows exactly when phase seperation will occur, but it will occur. It depends on how much humidity is in the air, how many cycles of barometric pressure cause the tank to "breath in" new air with new moisture in it, etc etc.

All of the above is somewhat over simplified, but that is it in a nutshell.

Rotax engines come from the factory ethanol tolerant, by the way. All the components on the engine that are exposed to fuel are made of ethanol tolerant materials. However, the rest of the fuel system (tanks, lines, hoses, gaskets, seals, plastic fittings and floats, etc etc etc) also need to be ethanol tolerant in order to be safe.

Also, by the way, the ethanol tolerant term has come to be used because that term means the part can stand up to exposure to ethanol. They don't say ethanol proof because the tolerance is only for exposure to the ethanol. When phase seperation happens you get a whole new problem - a problem that can even eat aluminum - and no amount of ethanol tolerance protects you from that.

Short version - if you have an ethanol tolerant engine and aircraft fuel system you can safely use ethanol contaminated fuel but only until phase seperation happens. Then your are no longer safe. To prevent phase seperation do not allow ethanol contaminated fuel to stay in the tanks or the carb for more than short periods. Either burn it or drain it. If you drain it do not then put it back into the plane later - it is still absorbing moisture as it sits in the fuel can. Burn it in your car and get fresh ethanol contaminated fuel to put into the plane next time.

Yes, thanks much!

I got some ethanol/lead-free 91 octane in AZ at about the same price as 100LL. I have it in the airplane and in plastic jerry-cans and a 30-gallon barrel with CO2 pump. I wonder how long this stuff can sit around without deterioration, and what you think of the strategy of having one tank dedicated to 100LL and the other to 91 with ethanol and no-lead? Should I "save" the no-ethanol, no lead to mix with the ethanol or the 100LL?

What are the proper terms for the various automobile fuels? I've heard that some of these "racing" fuels (100, 110, 115 octane?) should NOT be used in aircraft engines. True or false and why? 




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