Originally Posted by
FCeng84
It has always been a curiosity to me that for some models, center tank fuel must be counted as cargo weight when the wing tanks are not full, but is not considered for MZFW when the wing tanks are full. From the physics this makes no sense to me - weight in the fuselage is still weight that must be carried by the wing root regardless.
I believe that fuel in
full wing tanks - since it is a virtually incompressible fluid - is assumed to stiffen and reinforce the wing structure. Effectively increasing their strength and reducing flex (especially if the tanks extend to the wing tip). Thus they can carry more weight safely, just as though a stronger wing spar (or a mid-wing support strut, a la Cessna) had been installed.
That is
half the reason for burning off center/fuselage tank fuel before using any wing fuel - the other half being the more obvious and intuitive effect of reducing the fuel weight lifted through the wing root.