On Tue, 03 Oct 2006 01:40:46 GMT, "Allen"
wrote in :

I should have said the "tip tank" Twin Cessnas. Cessna designed it that
way. If you wanted to return the excess fuel to the same tank it was drawn
from you would need two sets of directional valves; one on the supply line
and another on the return line that were synched to each other.

What would prevent plumbing the fuel return line into the top of all
fuel tanks above the fuel level, so that the returning fuel would be
able to fill whatever tank space was available at the time? There
would be the added cost of labor and materials, and the added weight,
but the hidden hazard would be eliminated.

It is much easier just to tell the pilot to burn 90
minutes out of the mains before switching to the aux's.

It would seem a placard would be more prudent.