ORIGINAL: rangerfredbob
with the engine running it'll be pulling air through the vent line, there is NO WAY for fuel to get out
In theroy you are right. But say you have the airplane real high & cut the throttle & start doing some maneuvers (possibly inverted sometimes)that places a good flow of air near the vent...fuel can be sucked out into the airstream from the vent to your tank Capt,n