Zero Fuel Weight

The total weight of the airplane and all its contents (including unusable fuel) minus the total weight of the usable fuel on board. As a flight progresses and fuel is consumed, the total weight of the airplane reduces, but the ZFW remains constant. Maximum zero fuel weight (MZFW) is the maximum weight allowed before usable fuel and other specified usable agents (engine injection fluid, and other consumable propulsion agents) are loaded.