"Roger Halstead" wrote in message ...
This arrangement has caused accidents in homebuilts in the past,
and it's the reason that certified airplanes having interconnected
tanks (the "both" position) must also, by law, have interconnected
tank venting to keep vent pressures equal.

I think you will find even certified planes like Bonanzas have an
either, but not both.

So? That has nothing to do with the venting. The problem is that if you
can't gravity feed to a common point, then you are in deep dodo if you try
to pump and one tank is full of air. High wings generally have no problem
gravity feeding (and frequently have a both position, and in sometimes
only a both position). Low wings generally don't have a BOTH, although
my Navion for example drains both wing tanks to a common low point
and pumps from there. The (stock) Navion fuel valve is OFF-ON.

The Cessnas with a BOTH position have a bent line that runs between the
two tanks just behind the pilot's head. This line is actually the source of
some interesting fuel flows that cause the Cessna tanks to drain unevenly
when filled above the vent line. Likewise the Navion fuel vent line runs
between the two tanks (forward of the pilot).