On 5-Sep-2004, Thomas Borchert wrote:

Doesn't leaving the tanks partially empty cause problems with
condensation
or something along those lines?


NO! That is another of the many OWTs in aviation (old wive's tales).
Cessna
did extensive experiments in a clima chamber. They could NOT produce any
noticable amount of water in a fuel tank no matter what they did to the
temperature. There are only two ways to get water in your tanks:

1. it's coming in with the fuel from the truck or depot tank.

2. it's been raining and your fuel caps leak.


Actually, there is a third way. and that is the condensation referred to.
Here is how it works:

Through its vent(s), the tank is open to the outside atmosphere. If the air
is humid, that puts water vapor in the tank. If the air cools, the water
vapor will condense (just like it does on outside surfaces, i.e. dew). Some
of the condensed water on the inside walls of the tank will drip into the
fuel. The cycle can be repeated for many warming/cooling cycles if the
plane is not flown for a while. Result: some water in the fuel. That is
the reason why we drain the sumps before flight.

HOWEVER: Despite the actual, albeit minimal, risk of water in the fuel,
there are overwhelming reasons for generally leaving less than full tanks on
most airplanes. The primary one is that usually when you return from a trip
you have no idea of the cabin load that will be needed for the next trip.
This is particularly true if the airplane is shared by multiple pilots.
With our Arrow, for example, if we topped the tanks (72 gal) between uses we
would be leaving an airplane with the ability to carry only 2 or 3 people.
What happens if the next user (we have 3 co-owners) wants to carry 4?. So
we leave the tanks filled to a total of 50 gal (usable), for which Piper
conveniently provided an indicator tab in each tank. That leaves a shade
under 700 lbs useful load, i.e. a 4-place airplane (that still has well over
500 nm range with 1 hr reserve.) If a user wants more range and has a
lighter load, he simply adds fuel before takeoff.
--
-Elliott Drucker