This assumes no gasahol - a separate issue in both our states (MN & IA)


I add a couple of tablespoons full of yellow can (isopropyl) HEET every
now and then when the temps drop below freezing. Pure gasoline will
dissolve a small amount of water that will come out of solution and
freeze as the temperature drops below freezing. It looks like very
light snow but it won't go thru a gascolator screen. Consider that in
the winter, gas is usually colder that it has ever been since it was
made down south. It will be cooled in your airplane even more when it
comes out of the ground tanks at say 45 deg F. Like air, warm gas will
dissolve more water than cold gas. The difference on chilling, shows
as snow. The gascolator provides a way that the very smallest amount
of H2O can interrupt the fuel flow - especially at full throttle.

In my case I took a previously fueled airplane from someone else that
had already flown it an hour on that -20 deg F morning. It probably
had the gascolator fully iced from that flight, but there was enough
fuel flow and reserve in the carb bowl to do a normal run up so away we
innocently went. However there was insufficient fuel flow to sustain
takeoff power & we only got above and a little beyond the end of the
runway when it just faded away over only a couple of seconds.

The FAA wasn't able to explain it at the time except to say that "there
are some things we just don't really know about fuels". I found some
data from Amoco that gave the water solubility in gasolines, cranked
some numbers, blended in enough knowledge of chemistry & meteorology to
be dangerous, and yep, that would explain it.

Another FAA fellow had seen the same thing happen to others but didn't
have a good explanation. We went together and published an article in
the Sport Aviation December 1986 issue. It should be required reading
for anyone flying in colder climates.

In the mean time - BEWARE of fuel that has been severely chilled since
it was last filtered!