Can one mix some mogas / car petrol into avgas, for emergency use?

I’m sort of running on the principle that these are fairly basic engines and not likely to be that fussy about what they drink.

Peter wrote:

You can do ethanol tests before you use the stuff, but if it comes up as containing it, do you drive down to the petrol station down the road??

I am yet to see a positive test for ethanol, so I don’t know! Apparently the fuel company certifies it as ethanol-free, so in theory one could return it for a refund. Whether one could be bothered with this is another matter.

Would I put it in the aeroplane anyway? If I owned it outright, then probably. As those with Africa experience tell us, you just get on with it. One dose of ethanol isn’t going to hurt, and if the next can from the same filling station tested positive too then one would look for a different supplier or ask the company about their spec.

But it’s a group-owned aircraft and I have a responsibility to the other guys to do the right thing – so no, in this instance I would not put it in the aeroplane.

Sorry for repeating myself, ethanol is NOT your problem. Ethanol is not aggressive to rubberlike material as in seals or hoses, I suggest doing your own test. Just put an NBR o-ring in a glass of whisky – or any brandy you´d have at home and see in a few days what that does. I say – no reaction – don´t know about effecting the taste of the drink though. At the same time get a glass of unleaded super 95 or 98 and E 10 from your filling station and do same test . You will see in short time a lot of swelling and softening of the rubber in each sort of fuel, no dissolvement, a typical shaft seal will grow that much so the lip will no longer contact the rotating shaft and you´d have a leak there. So for a fuel pump you will use Viton seals only, no NBR will do. It is the solvents like acetone, butane, toluol, in modern gasoline that is very different to avgas with its certified content and will affect seals, ethanol is not a factor at all. One can discuss if E 10 might be safer for aviation as ethanol can dissolve water contamination in fuel so it does not form droplets that can freeze at altitude. But then you will have to do tests to find out how much of water content is actually in the tank. The typical test for water will not show droplets at all. Auto fuel will not contain ethanol at all if it does not have to like in E 10 as bio ethanol is more costly to produce so they will not add it and choose cheap thinners for anti-knock properties instead. Anyway, we have super 98 in our radial since 13 years and avgas only when it is free in a show, no problems ever – apart from vapour locks initially till we got electric fuel pumps for the low wing aircraft and once a leak from a very soft o-ring, now Viton . Vic