ORIGINAL: DenverJayhawk
This doesn't happen in flight because when nose down the inertia of accelaration pushes the fuel to the back of the tank, where the clunk will still be able to draw fuel. Think of swinging a bucket of water in a complete loop above your head.
Absolute rubbish!
There is a nice video of fuel in a visible tank posted on these forums that completely disproves this myth, which unfortunately is often repeated even by very experienced flyers.
It is simply UNTRUE.