The present invention relates to the definition, in a flight plan, of the horizontal profile of an air route with vertical flight and speed profile prescribed on departure and/or on arrival, by a stringing together of check-points and/or turn points associated with local flight constraints and called “D-Fix” because they are not listed in a published navigation database like those called “Waypoints”. It consists in charting, on curvilinear distance maps, a direct curvilinear path joining the departure point to the destination point of the air route while complying with vertical flight and speed profiles prescribed on departure and/or on arrival and while guaranteeing a circumnavigation of the surrounding reliefs and compliance with regulated overfly zones, then in approximating the series of points of the direct curvilinear path by a sequence of straight segments complying with an arbitrary maximum deviation threshold relative to the points of the series and an arbitrary minimum lateral deviation threshold relative to the set of obstacles to be circumnavigated and in adopting as “D-Fix” points the points of the intermediate intersections of the rectilinear segments.