A method and system or determining a flight path for an aircraft between an initial point and a destination point are described. An airspace is divide into a set of grid cells, and a flight path is established between the initial point and the destination point. A cumulative threat risk value for each of the grid cells is calculated, and an allowable threat risk per grid cell is established based on a rule set. An intermediate point from which to deviate the flight path is identified based upon an analysis of the cumulative threat risk values of the grid cells, if the threat risk value of any of the grid cells intersecting the flight path exceeds the allowable threat risk. A deviation from the intermediate point to a new intermediate point is determined such that the new intermediate point has a cumulative threat risk value lower than or equal to the allowable threat risk per grid cell. The flight path is adjusted between the initial point and the destination point to pass through the new intermediate point, thereby providing a modified flight path.