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.