Hi,

>Not on Hercs. Herc fuel tanks are vented to atmosphere. Unlike a fighter which requires a pressured fuel system to keep fuel flowing in aerobatics a Herc's is just pumped from the tank to the engines. Their is almost no pressure in the tanks at all. That is why the external tanks have two pumps, keep fuel flowing if one fails.

A by-effect of what Dax is saying is that you might have a different problem though - fuel "boil-off" during rapid climbs. Since the tanks are effectively unpressurized, a rapid climb might cause air trapped (and dilluted) in the fuel to expand, causing fuel "foam" to be created (sort of like the effect you get when you shake a coca cola bottle and then rapidly unscrew the cap).
This can cause some loss of engine power, mainly when the fuel is gravity fed. The actual effect depends on various factors (which I do not recall - initial fuel temprature and rate of climb spring into mind). The AFM is pretty detailed on this subject.

Regards,