"Tanks on both" checklist item

Koopas Ly wrote:

Would you mind re-explaining the mechanics you just described? I
don't get the "overflow through the vents" part".

On the Maule, each tank has two 1/8" tubes which run up through the bottom of
the tank and end just below the top, inside the tank. These are vents intended
to allow air to enter the tank as fuel is burned. If the tank is overfilled, the
excess will also run out the vents. My old 150 had only one of these vents, but
it also had vented fuel tank caps. The vent of the 150 is larger than those on
Maules.

The Maule has a fuel system in which there is a fuel line coming out of one
of the lowest spots in the tank. IIRC, the right tank line crosses the cabin
just behind the top of the windshield, and both left and right fuel lines then
run down inside the left front door post to a valve on the lower left side
panel. A single fuel line then runs from here to the engine. In the 150, the
valve is in the floor, and I'm not sure if the right line crosses above or below
the cabin. The 150 also had a separate line between the two tanks which ran
above the cabin to allow air flow between the tanks to try to equalize fuel
burn, and fuel flows through this if either tank is more than about 3/8 full.

In either case, if the valve is "on" in the 150 or on "both" in the Maule, and
you park the plane sideways on an incline, fuel will run downhill to the lower
tank through the valve. If both tanks are full, fuel will discharge through the
vent tubes on that tank. In my 150, this was only a problem if the plane was
parked so that the vent was on the downhill side.

Some aircraft have anti-syphon devices on the vent tubes to prevent this sort of
thing. Perhaps the 172 does.

This problem also occurs in my Maule when the valve is "off", but not when it is
set to "left" or "right". I do not understand why this happens, but apparently
it is not unique to Maules. I got the suggestion of setting it to one tank from
an article in either AOPA Pilot or Sport Aviation some time back. The author
was advising someone else who had the same problem.

George Patterson
Some people think they hear a call to the priesthood when what they really
hear is a tiny voice whispering "It's indoor work with no heavy lifting".