Classic British Flight Sim

Operationally fuel is only loaded into centre tank after wing tanks are full, otherwise any centre tank fuel becomes "ballast fuel" and is included in the zero fuel weight of the aircraft as "payload". Centre tank fuel should be transferred out to keep wings full.

When the aircraft is on the ground, if the wings are not full, you don't have enough weight outboard of the pivot point, the MLG legs, thereby increasing the bending moment on the wing roots. As the fuselage is effectively hung between the wings, this bending moment is what gives the Max zero fuel weight limit. Full wings...weight outboard due the dihedral...counterbalances the centre tank fuel. Transferring it to the wings relieves this moment.

Very simplified explanantion, and also explains why aircraft with sponson mounted gear don't have a Zero Fuel Weight limitation.

Nigel

Thanks to all, I did get it slightly muddled, by saying wings to centre, however Now I have it. I still get the low fuel warning, but I think that is a bug with FSX. Not a big deal. However, DM, Nick and Nigel a huge thanks to you. Also a special thanks to DM for putting together such great machines. Really, really fantastic stuff sir. What's more is the great support I get here, no matter how stupid the question may seem.