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This whole discussion has happened before. There are as many different ways
to handle the problem of pumping fuel out of the overflow as there are
builders.

The first time I knew I had a problem with overfilling the header, it was
because I could smell the unmistakable aroma of $2.20 per gallon go juice in
the cockhouse. It immediately brought my attention to the fact that I'd
switched on the transfer pumps and forgotten about it. I didn't like the
potential consequences of 100 octane blowing into the slipstream so I
decided I needed a visual warning when my header was full.

It's a simple thing to design and build a bridge amplifier to sense the
relative changes in two thermistors' temperatures. I simply put one in the
ambient under the panel and the other inside the overflow/vent tube. As soon
as the header is full and begins to dump fuel overboard, the thermistor in
the tube is cooled drastically and the bridge amp turns on and lights a
flashing LED (from Radio Shack). I have the LED right in front of my nose on
the panel so it's hard to miss. I now never pump more than a drop or two
over board before I catch it and turn off the pumps. The only problem I've
had is explaining to a nervous passenger that the flashing red light doesn't
mean an imminent crash.

I've got the circuit somewhere in my new hangar (probably frozen though) and
if any one is interested, as soon as things thaw out, I'll dig it up for
them. I know I've promised this before but I didn't figure on just how cold
it gets here in my new home. I seem to be able to work in the hangar for
less than 15 minutes before I freeze!

In re the pump configuration, I put two pumps in series and use a selector
valve to select the source tank. I run both simultaneously (each pump is
fused separately and electrically in parallel), though each are the
pump-thru variety. Should one fail, the other will pump fuel from either
wing (depending on the selector valve position). I did this because I have
had one of the electronic Facet pumps fail in flight. With only one, or one
dedicated to one side, you can see the problem. Either no fuel transfer or
transfer from one side only.

Dan Schaefer


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