There is something about this business of adding check valves in the fuel
tank vent lines that I don't understand. Hopefully, someone on the LML can
explain it.

The primary purpose of the fuel tank vent is to allow air to flow into the
tank as fuel is withdrawn. However, the vent also has the secondary, but no
less important task, of equalizing pressure inside the tank with ambient
conditions. For example, if the plane is refueled at an airport which is at
sea level, the air pressure within the tank will be at equilibrium with the
atmosphere at roughly 14.7 psia. If the plane then departs and climbs to say
10,000 ft, atmospheric pressure will have dropped to roughly 10.1 psia. If
there is a check valve in the vent line, air inside the tank cannot flow out
the vent during the climb to equalize the pressure. Under these conditions,
the pressure inside the tank will still be 14.7 psia and the pilot will be
performing an inadvertent pressure test on the plane's fuel tanks with a
differential pressure of 4.6 psid (i.e., 14.7 psia inside the tank minus 10.1
psia outside).  I have read of builders rupturing their tanks at
substantially lower differential pressures than this during leak testing. At
20,000 ft the theoretical differential pressure is about 7.9 psid. Implicit
in this analysis are several assumptions:

1) A significant quantity of fuel is not withdrawn from the tank during the
climb.
2) The fuel cap doesn't leak or depart the plane.
3) The fuel tank doesn't leak.
4) The check valve doesn't leak.
5) Temperature effects are presumed to be negligible.

Is there something wrong with this analysis? What am I missing? Does everyone
who installs a check valve in their vent line just assume that the check
valve or gas cap will leak a sufficient amount to prevent overpressurization
of the tank? Seems like a gross rationalization to me. If you are wrong, you
could lose a helluva lot more than a few pints of petrol.

Until someone can explain to my satisfaction how overpressurization is not an
issue, I think I will continue to build my LNC2 without check valves in the
vent lines and put up with the occasional loss of fuel.

Jim McKibbin

PS - A float valve makes a lot more sense to me than a check valve from a
functional standpoint but I haven't a clue where you might get a suitable one
or how to install it.