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Fuel Replace after 6 weeks?

WJaviation

Active Member
My 12is has been sitting for about 6 weeks. Not run or flown in that time. its got about 14 gallons of 93 zero Ethanol fuel in the tank. Air temp has been 85-90 with 80% humidity during the days over that same time frame.

Should I drain the tank and replace with fresh? or just mix in some fresh 93 octane fuel and take her flying? or is there a more elegant way to check the gas is still good to go?
 
That seems pretty extreme. I don't know much about rotax engines, but I can't imagine any gas going bad enough that it would cause a problem in only 6 weeks.

If it were me, I'd just sump it really well, top it off and not worry about it.

personal opinion only. In this case I don't have anything to back it up with.
 
I use 93 octane zero ethanol in my IO-360 and was told by the supplier, it is good for use for up to 5 years. I'm sure the fuel in my nurse tank has often exceeded 6 weeks.

The only caution is do not use winter mix fuel in summer weather. the blend used during winter has lower vapor pressure and is prone to vapor lock in warmer weather.
 
I completely agree that 6 weeks is nothing....not enough time to cause significant octane loss. However, I had the impression that the OP might also be worried about condensation, therefore the possibility of water contamination. I think it's unlikely that there could be enough water to be capable of phase-separation, but that can be tested with a chemical test. I think that thorough attention to the sump drains on the tanks and any other low spot in the fuel system should demonstrate whether or not there's water there (let the sample stand for several minutes).
 
The only caution is do not use winter mix fuel in summer weather. the blend used during winter has lower vapor pressure and is prone to vapor lock in warmer weather.

Correct, but OP has fuel-injected 912 which runs higher fuel pressure. Should be OK with winter blend fuel. The real concern is the carbureted 912 which is low fuel pressure delivery. In-line electric pump and fuel return to the tank help, but no guarantee. RV-12 is tightly-cowled with minimal air flow on the ground. Several accidents can be attributed to vapor lock...
 
At Rotax school we were told to drain carb bowls if they were going to sit more than three weeks. That was because of the fuel deteriorating and leaving deposits in the bowls. You don't have carb bowls so that's not an issue, but they also said mogas with ethanol can lose octane over time. I never have been able to determine what the limit on that time is, but the 100 HP Rotax is barely at it's octane limit with fresh premium unleaded anyway. For me, mogas in that ballpark of 6 weeks is drained and used in cars. Another reason to look forward to an unleaded 100LL replacement is it will be stable in storage.
 
Ethanol is alcohol which is hygroscopic. (Absorbs water)
Shelf life is short.
Ethanol free lasts much longer.
 
Living where it's common to put motorcycles away for 5-6 months at a time over winter, the procedure was always to fill the tank to the top, drain the carbs, and park it. Come spring, turn the fuel on, let the carbs fill, and fire it up. The fuel was always fine and the carbs never gummed up with this process.

Ethanol-enriched fuels absorb water from the air in the tank. They can't absorb water from air that isn't there... If it's full, there's (almost) no air. Even a vented tank won't circulate anything of significance through the vented cap in 6 months... 6 weeks is nothing.
 
I had a really long annual this year with many SBs to catch up on and a number of mods I came up with. The whole thing took me 8 weeks working about 8 hours a week. I had about 14 gallons sitting in the tank. I just filled up with 100LL and had no issues. I’ve done about 5 hours flying since annual.
 
For the past 4 years or about 300 hours I have added 1 oz of Seafoam to my 93 octane 10% alcohol fuel with no issues to keep my fuel preserved, I've heard of others using Stabil as well.
 
OK, some folks posting here clearly didn't read the OP's question or operating conditions while others have posted helpful feedback. Let's hope this response falls into the category of helpful feedback.

As others have mentioned, oxygenated fuels (that translates to gas with alcohol) have a short shelf life. We've already seen comments about the alcohol potentially absorbing moisture and potentially losing octane. Bottom line is that alcohol-gas mixtures don't last a long time in the tank, especially when the tank is vented to the atmosphere as our aircraft tanks are.

Pure gas doesn't suffer the same rate of deterioration as oxygenated gas. I have been burning pure mogas for about 16 years now in one of my aircraft. Through these years of experience I have learned a few things.

My chief point of learning is that pure gas in a sealed container (jerry can) will keep for six months without much change in performance characteristics. Put that same gas in a vented aircraft fuel tank and it will change characteristics perceptible to the human. It's odour changes, it's colour darkens. How long this takes is directly impacted by temperature - the hotter it is, the faster the gas changes.

Through experience I have learned that leaving gas in the tank from late spring through mid-summer produced no change in operation of the aircraft. This aircraft is carb'd with no fuel pump on a gravity-fed fuel system - very simple.

I've also learned that gas left through an entire summer is not going to smell fresh and will be somewhat yellow in colour. Again, from my experience, this gas burned well.

A key factor in storage conditions is accessibility of outside air. A full tank is much less likely to degrade than a half tank. If the tank is only half full, fill it up with fresh gas so you at least have a 50/50 mixture of old and new. Go fly.

Another anecdote which reinforces the above statements... We recently have been lashed by a nasty storm which took out electrical power for a large portion of our city. My gas-powered generator sits in the uninsulated garage year-round. Its tank is full of pure 91 gas without any inhibitor or other additive. In this case the gas in the generator was at least a year old. While a generator may not be as critical an application as an aircraft engine, the truth cannot be denied. The generator burned that year old gas without any trouble. Keeping the tank full and the carb drained are two "must do" acts to preserve functionality in longer-term storage.
 
My 12is has been sitting for about 6 weeks. Not run or flown in that time. its got about 14 gallons of 93 zero Ethanol fuel in the tank. Air temp has been 85-90 with 80% humidity during the days over that same time frame.

Should I drain the tank and replace with fresh? or just mix in some fresh 93 octane fuel and take her flying? or is there a more elegant way to check the gas is still good to go?

I know the OP isn't considering E-gas, but it's contained in this quote from a fuel supplier re: "road grade" gasoline -

"Compared to traditional gas blends like E10 that have 10% ethanol, non-ethanol gas lasts much longer. If stored properly, ethanol-free gas can last up to six months. It is not as likely to succumb to oxidation or evaporation. E10 gas has a three-month shelf life maximum. This makes ethanol-free gas ideal for storage compared to E10."

When I was around "the business", auto gas was spec'd to be good for 1 year. AVGAS had a 2 year shelf life - as told to me by a Chevron engineer.

Storage conditions have to be factored in to some extent, but the straight "oil" based gasoline doesn't suffer nearly as much as ethanol mix gasoline does. The same goes for road vs AV gas.

My rec, FWIW, top off with fresh gas and "fly the snot out of it" as my buddy in Colorado says ;)
 
Plenty of info on Auto fuel and the Rotax family of engines can be found on line by some of the best experts that have extensive experience with these engines

Rainbowaviation.com

Lockwood.aero
 
^^^^^^ I had to look at your VAF Profile. It was bugging me and I needed to see your background. Move along... nothing to see here....
 
I think people WAY over think this ... just fly a lot (2 or 3 times a week or two), keep the gas moving and (at least I) don't have any problems with MOGAS
 
Snowbird RV-12

I don't have any facts for specific knowledge, but my RV-12 sits in my heated hangar in Western Washington for five months each year without flying and when I get back I do a good pre-flight and runup and go fly. Never had a problem using non-ethanol unleaded 92.
 
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