First, the particulars about my plane:
415-C, S/N 4313
9 gallon aluminum wing tanks, vented caps
Skyport Rain-pruf nose tank guage, vented
fuel pump is less than one year old, and has about 15 hours on it.
Bear with me for the background.
Went flying yesterday and my passenger noticed fuel siphoning around the cap on
the right wing tank, about 15 minutes after takeoff. (The wing tanks were
about 3/4 full, and the fuel level was the same on each side before takeoff.)
I did a 180 to return to the airport and, as we were on our way there, I
noticed that the gauge in the left wing tank had dropped to "0". Then, a few
minutes later, the nose tank float gauge (a Skyport Rain-pruf version) started
dropping: not drastically, but definitely noticeable. Shortly before we got
back to the airport, the siphoning had stopped, but the left tank gauge still
read "0". After we landed and were taxiing in, the nose tank float gauge
gradually returned to the "Full" mark on the glass tube.
We parked on the ramp and started investigating the problem. We noticed that
the fuel levels in the wing tanks had stabilized and were again equal (but down
to about 1/2 full after the flight and the fuel siphoning).
I thought I had a cap-sealing problem on the right tank. Not so: the problem
was the tank filler neck flange, which was cracked about a third of the way
around its circumference. The fuel was siphoning in the small area between the
outer edge of the cap gasket and the upper skin of the tanks. In flight, it
looks like it's coming out from under the gasket. The crack isn't visible from
the top: you need a mirror in the fuel filler hole, and a flashlight.
OK, so that's the story. But I'm puzzled. I understand that the fuel will
siphon because there is a pressure differential between the tanks, as a result
of the crack. Low-pressure area and all that. But what I don't understand is,
Why would the nose tank level drop? After all, there was still plenty of fuel
in the right tank ("pulled" over there from the left tank by the pressure
differential.) Is it because the suction affecting the right tank, through the
cracked flange, is more than the fuel pump's suction can overcome?
Thanks for listening!
Bill T.
