Just thinking...The engines sucking in fuel via the inletport of the prime pump.....transfer pumps delivering the fuel from the main tank to the supplytank. Assuming the transfer fuel lines exits situated somewhere on/near the bottom of the supply tank...the fuel flow generated by the transferpumps is way larger than the suction flow to the engine. Is this creating some swirl effect in the supply tank.....when you suddenly stop the supply from the transfer pumps; the swirl will continue for a couple more minutes, creating static pressure drop at the exit of the fuel transfer line, and thus continue pulling fuel from the main tank. The swirl effect slows in time and after some minutes the static pressure drop ceases to exist and the engine starts sucking from the supply tank........
So I think a lot of sucking is involved
so if you did a similar check without switching on the transfer pumps at the beginning of the sequence....no swirl created,.... the supply tanks would start emptying rightaway. (assuming the main tank Qty level is below the overflow channels)
Yellowbird, I think thats the first plausible explanation since I started this long debate with my simple question...
Thanks!
skadi