"Dan Luke" wrote in message
...
The practice of topping the fuel tanks after each flight rests on the
notion that air contains a certain amount of moisture, and that the
water will condense out of the air contained in partially emptied
tanks and contaminate the aircraft's fuel system.

Which is an old wives tale. The amount of water contained in 20-30
gallons of air is insignificant.

I would agree that condensation is practically never an issue, but like
Larry I would not say that it could NEVER be an issue. Most of the time,
even when the air is very humid, the likelihood of the temperature changing
enough to cause any significant amount of the water vapor to condense is
incredibly small, and the total amount of water is also small. But to say
that it simply cannot ever be a source of water in a fuel tank seems
short-sighted to me. A tiny amount of water, in the wrong place at the
wrong time, can cause all sorts of trouble.

That said, there's another reason to try to keep the tanks topped off if you
can, when the airplane is equipped with rubber fuel bladders. My
understanding is that having the rubber "wetted" by the fuel helps extend
their lifetime. In my plane, I'm "fortunate" enough to have just one fuel
tank with a rubber bladder, with the other four being sealed aluminum
structures. So I always top of the one rubber bladder tank, and leave the
others partially or entirely empty.

Pete