How much of an issue is this?
I think 100LL / 91UL has an indefinite life if stored in sealed drums, but that isn’t true for other fuels. I think Mogas / car petrol is especially bad in this respect.
Then there is water ingress into bowsers or underground tanks. I had an incident in 2014 (a brief engine stoppage, which I blamed on fuel servo icing) where I had the fuel lab tested and they found quite a bit of water in it.
Peter wrote:
How much of an issue is this?
Not a big issue in my experience (11 years, Rotax 914, car petrol station EN228) as long as you consume it within a month or so. Many of the Microlight pilots in Germany refuel from jerrycans.
I’ve often heard it said, but I don’t experience it.
My 1972 Triumph Spitfire doesn’t run for at least six months of the year and it just sits in the garage with whatever fuel in the tank it had at the end of the summer. Next spring it starts and runs without a problem, so the fuel that sat in the tank all winter is evidently ok.
100LL Avgas, and Mogas are both “fuel” for gasoline engines, but rather different liquids with different properties in terms of physics. Avgas is one refined product, as one might think of acetone. It is one composition, with one vapour pressure, and is not seasonally changed. This is demonstrated in that on your hand, it will evaporate entirely away. It has a long life in drums, if sealed. If the drum is vented, the fuel in the drum will still have the correct properties if some has evaporated, though now contamination is a risk. In northern Canada, one buys “a drum” (or more drums) of fuel. Once you’ve bought it, you pump it, and what you don’t use is wasted. This is to reduce contamination issues. You are shown the refinery seal on the drum as it is provided to you, and you break it to pump it yourself. I had planned my flying very carefully to assure I arrived with that much room in my tanks, while not running myself too short while getting to the very remote location!
Mogas, on the other hand, is a mixture of a number of liquids, each with different properties, and different vapour pressures. So the “light ends” in the Mogas will evaporate first, leaving the more oily constituents of the fuel. As the “light ends” are the elements which boil first, they are what makes starting easier, particularly on cold winter days. Mogas is seasonally changed to be most suitable in the local climate. If it is left open to the atmosphere for many months – it will (a), be out of season anyway, and (b) will have lost some of the light ends by evaporation, and be hard starting. I flew about 4000 hours on Mogas over 35 years in various GA types, some of which was formal Mogas evaluation in that type. It works in many engines and airframes – but not all – hence STC’s required.
In the Canadian north, it is common that an operator will cache fuel in drums along their routes, so they can land in to top off if they get stuck by weather or other factors. Caches decades old are reputed to have been used with no problem…
I have had issues in the past with mogas in small engines. Generators needed running more regularly, and lawnmowers needed the fuel changing and the carburettor cleaning. The local petrol station owner told me that automotive unleaded has a shelf life of ⩽6 months, which sounds about right based on my problems. I was recommended FuelFit by Briggs & Stratton, which stabilises petrol. From their site:
Todays’ ethanol-blend fuels can damage engines by attracting moisture. Ethanol readily bonds with atmospheric moisture creating an ethanol-water mix, which is highly corrosive for engine parts. Fuel also begins to degrade as soon as you pump it. Gums and varnishes are formed that can stick valves and clog fuel lines and carburettor jets, resulting in severe starting problems, poor engine performance, overheating, and fuel leakage.
It definitely works for me, but I can’t say if it’s any good for aviation use.
I think Mogas / car petrol is especially bad in this respect.
I just used 20L that was stored and sealed for 5 years, no issues (not in an IO360 
)
I would be surprised if fuel in sealed containers degraded, but the “containers” we see in GA aren’t sealed.
Bowsers and underground tanks are vented. One can also get water leaks and general moisture ingress via the same paths.
I’ve ordered that Fuel Fit stuff – many thanks for the tip Generators are a bastard to start after some months. I have sometimes had to squirt some IPA (isopropyl alcohol) into the air intake.
It is one composition, with one vapour pressure, and is not seasonally changed. This is demonstrated in that on your hand, it will evaporate entirely away. It has a long life in drums, if sealed. If the drum is vented, the fuel in the drum will still have the correct properties if some has evaporated
That’s really interesting. Was that composition intentional?
Don’t recommend it but my mechanic is a bit handy, and he threw a splash of out-of-date Mogas on the ground outside and lit it. Then showed me the flame off some fresh avgas. There is a difference.
I’ve had Mogas with ethanol separate, and create a brown caramel substance in the carbs and tanks of a C42 microlight over about 6-8 months. Strangely enough, we flew it with this in there, and it was all fine but only noticed it changing fuel lines at the permit renewal.
Plenty of them in the German-speaking countries.
