John Slade wrote:

Dave,
An hour of fuel "consumption" is obviously NOT an hour of fuel draw. 

Agreed..

Return to the sump
Risk pressurizing the sump. Needs overflow vent to the mains.

Pressurizing would be bad... I would have to say "no check valves" in order to allow the sump to "backfill".. truth be known, as long as the sump has a slight pressure differential above the gravity feed from the wing tanks, it should be enough to prevent further feed from the tanks to the sump... But again.. if you are pumping 30 GPH FROM the sump, and returning 15 of it TO the sump, you are still drawing 15 GPH from the mains... intuitively you would think the sump wouldnt pressurize. You also are having the thermal dilution of the return fuel by a continuous inflow of cool wing-tank fuel.

Return to the mains
Risk negative pressure in the sump and failure to gravity feed fast enough.

With AN-6 lines, I would hope the entire tank could empty in minutes by gravity, rendering this scenario unlikely., The previously mentioned nose down scenario (evacuating the sump) appears to hold water, but you would have to be near empty, and in a prolonged descent, to develop that problem. Unfortunately, if you DO develop that prob, you are in the least likely situation to be able to fix it easily (climb attitude to refil the sump)

John

Good points.. thanks for the feedback,
Dave