Hi Richard,
I reached out to a petroleum engine and airplane owner I know asking how long it avgas good for? Does it have a finite usable life.
Here’s what he said:
“What I recall is that the spec requires two year life… but practically, there’s no limit. The avgas in Glacier Girl sat in the airplane for 50 years on the Greenland ice cap, and was still usable. Cold is good, though, to retard chemical reactions that degrade fuels… but it was exposed to oxygen.
The degradation process begins with the weathering off of the light components, butane and isopentane, which are added to achieve the 7 pound vapor pressure requirement. Once those are gone, vapor pressure will be about half that… making winter starts difficult.
Then, upon exposure to oxygen in the air, the anti-oxidant additive will eventually be used up… once the antioxidant is gone, theoretically oxygen in the air can start forming gums and varnish with the remaining components. BUT, the avgas spec requires very stable components. Practically speaking, the most likely thing to oxidize is the blue dye… so the avgas may turn clear. The aviation alklyate and aromatics will hang around a long time… some will oxidize a little perhaps… but generally not enough to be an operational problem.
I’ve seen communication from some avgas blenders advising that avgas that exceeds the two-year life spec can be requalified by sending a sample for testing. I would not expect any such sample to ever have failed the test, particularly is stored in an air tight container.”
So the official answer is a usable life of 2 years.
Steve
