I've installed a 12-gallon fuel cell aux tank in the tailcone of my
4-place conventional low-wing plane -- intended for go-fast trimming
when flying solo more than for its extra fuel capacity. As you know,
a typical 4-place is terribly nose-heavy with only front-seat
passengers and no baggage. The tank would be emptied if flying with a
full load of passengers and baggage.

Now I'm considering how to plumb it. My engine is gasoline, not
fuel-injected.

The simplest approach would be to vent the tank it to the outside, and
run a single 3/8 hard tube between my left main and the aux tank. The
aux tank is positioned towards the ceiling of the turtledeck so
there's a fair amount of gravity-feed available for it in normal
flying attitude to drain back into that same tank. I'd have a single
electric pump to pump from the main uphill to the aux tank, with a
shut-off valve in the line to keep it there. Opening the valve would
let it gravity-feed back into the main tank ... slowly. That is, if
it's OK to allow it to gravity feed "backwards" through the fuel pump.

(I need to consider the possibility of the main tank gravity-feeding
back into the aux tank in a prolonged steep climb. I'll need to
remember to shut off the valve whenever a fuel transfer isn't wanted.)

I'm wondering about installing a second electric pump in series at the
aux tank end, pointing back towards the main tank, to considerably
speed up the draining. Only one pump would be run at a time of course
-- either to fill or to drain the aux tank. But when either pump was
on, it would be pumping fuel "backwards" through the other one. I
understand that the simple Facet pumps have no check-valve so this
should be possible. But is it harmful to such pumps?

I've got a more elegant approach (a little complicated to draw here)
that would use a single pump and a double-stacked selector valve as
used on a fuel-injected engine. The selector valve makes it so that
in the "fill" position, the pump would pump fuel from the main to the
aux, and in the "drain" position, that same pump would pump fuel from
the aux to the main. This is the slick set-up, but is considerably
more complex/expensive/heavy to implement.

The fourth approach would use separate fill and return lines, with
separate pumps, check valves, and shut-off valves. This would avoid
venting overboard. But it's even more complex, and I hope to not have
to go there.

Surely others have solved this simple plumbing challenge before me. I
welcome your ideas. :-)

Greg