A method of determining a descent profile , the method comprising: receiving, at a controller module , a final model of performance characteristics for an aircraft at a final approach fix for a target destination; determining, by the controller module , a first aircraft energy level of the final model, the first aircraft energy level defining a combined kinetic and potential energy of the aircraft at the final model; repeatedly back-calculating, by the controller module , a set of subsequent models based on successively larger aircraft energy levels, wherein each successive back-calculating determines a variable thrust parameter value of the respective subsequent model based on a minimizing cost profile; determining a descent profile based on the repeated back-calculating the set of subsequent models the method being further characterised by determining an estimated descent profile aircraft weight at an initial descent point based on the determined descent profile , the set of variable thrust parameter values, an estimated horizontal distance of the determined descent profile , and an estimated final approach aircraft weight of the final model; determining an estimated flight plan aircraft weight at the initial descent point based on a current flight plan of the aircraft ; comparing the estimated flight plan aircraft weight with the estimated descent profile aircraft weight; upon satisfaction of the comparing, updating the estimated final approach aircraft weight of the final model, and repeating determining the first aircraft energy level and repeatedly back-calculating the set of subsequent model until convergence is achieved; and operating the aircraft in accordance with the determined descent profile . receiving, at a controller module , a final model of performance characteristics for an aircraft at a final approach fix for a target destination; determining, by the controller module , a first aircraft energy level of the final model, the first aircraft energy level defining a combined kinetic and potential energy of the aircraft at the final model; repeatedly back-calculating, by the controller module , a set of subsequent models based on successively larger aircraft energy levels, wherein each successive back-calculating determines a variable thrust parameter value of the respective subsequent model based on a minimizing cost profile; determining a descent profile based on the repeated back-calculating the set of subsequent models the method being further characterised by determining an estimated descent profile aircraft weight at an initial descent point based on the determined descent profile , the set of variable thrust parameter values, an estimated horizontal distance of the determined descent profile , and an estimated final approach aircraft weight of the final model; determining an estimated flight plan aircraft weight at the initial descent point based on a current flight plan of the aircraft ; comparing the estimated flight plan aircraft weight with the estimated descent profile aircraft weight; upon satisfaction of the comparing, updating the estimated final approach aircraft weight of the final model, and repeating determining the first aircraft energy level and repeatedly back-calculating the set of subsequent model until convergence is achieved; and operating the aircraft in accordance with the determined descent profile . <EOS>