The specific fuel consumption (SFC) characterizes the efficiency of an aircraft's engine. For a jet engine, the thrust specific fuel consumption is defined as the fuel mass consumed per unit thrust and per unit time (kg/N/s). SFC changes with aircraft speed (V) and altitude (h). The knowledge of SFC is fundamental for calculations in flight mechanics and aircraft design. In aircraft design also engine mass is important, because it adds to the aircraft overall mass. Engine size is important for engine integration.