Aztec = Stupidest fuel cap/hatch cover design that I've seen. Cap has a
concave in the center to allow room for the over-center thermos type
expansion mechanism, it holds about a shot glass of water. Also directly in
the center is an air inlet hole, to allow air into the tank. The fill holes
and caps are below a flush type access door, with a rubber gasket sandwiched
between the two door surfaces. Want water in your fuel? Noooo problem...
just let the rubber gasket get old, or it may not fit well, so it leaks, let
the plane sit out in a rain storm and water will settle directly on top of
your fuel cap, which has... remember? a HOLE in the middle of it! The
interior of the hatch is vented overboard, such as Frank mentions, and is
subject to frost and freezing if plugged, but first the center of the cap
needs to fill up with water because it's lower than the flange of the cap or
the vent/drain.

We keep our caps covered with an FAA Approved PMA'd plastic cottage cheese
container cover, that only fits if "bent" so it will shed any water directly
into the drain rather than settle onto the cap, yet also allow air to enter
the tank.

Jim

"Frank Ch. Eigler" wrote in message
...

Jim Macklin wrote:

Vents are not installed in the direct impact slipstream, so
ice is not usually a problem in a well designed system.

Entertainingly, I had a fuel/ice encounter on an Aztec some years
back. Rather than suffer airborne icing, plain rain water got in
under the cap, and froze in the fuel vent line leading to the
high-pressure underside of the wing. That caused an impressive fuel
leak out the fuel cap as soon as we were airborne (= low pressure
formed above the wing).

- FCHE