A system and method for flight planning determines an optimal route by setting an initial departure weight or range of such weights for an aircraft traveling from a departure airport to a destination airport, defining a network of nodes for potentially legal routes, labeling each node with a graph of fuel needed to reach that node either as a function of flight duration or as a function of flight duration and departure weight, selecting or discarding routes when conditions at a node of that route are favorable or violated, selecting a preferred route, departure weight and duration for the desired payload and minimizing fuel for the desired route and payload.