The basic procedures used by most airlines to compile their flight plans are very similar, and are by no means as simple as they could be. When a choice of routes exists it is the normal practice to compile a series of flight plans from which the one giving the most advantageous route in the prevailing weather conditions is selected. In the absence of a direct approach to the problem of selecting the best route, the method of comparing different flight plans is improved by increasing the number of plans; it is therefore desirable that a method of speeding up the process of compilation should be evolved so that a greater number of plans can be prepared. Also the increased aircraft speeds which are to be expected in the future, and the requirement to reduce fuel loads, especially fuel reserves, to a minimum call for some rapid method of modifying the flight plan on receipt of in-flight forecasts and observations.