2.4. Maximum Structural Zero Fuel Weight (MZFW)
Bending moments, which apply at the wing root, are maximum when the quantity of fuel in the wings is minimum (see Figure B8). During flight, the quantity of fuel located in the wings, mWF, decreases. As a consequence, it is necessary to limit the weight when there is no fuel in the tanks. This limit value is called Maximum Zero Fuel Weight (MZFW).
Therefore, the limitation is defined by:
actual ZFW ≤ MZFW
The takeoff fuel is the sum of the trip fuel and the fuel reserves. Consequently:
actual TOW ≤ MZFW + Takeoff Fuel
Reference: Getting to grips with aircraft performance. 2.4