A system and method of generating optimised aircraft flight trajectories on Flight Management Systems with limited computational power that takes into account developing operational conditions, air traffic constraints and aircraft performance in a timely manner on the flight management system that can allow tactical flight plan changes to be incorporated without unduly introducing operational or financial penalties to the operator. An example method involves parameterisation of optimal trajectories as functions of operational parameters thereby allowing computational systems to use such computed functions in the air to determine the optimal trajectory or flight profile required for the specific operating conditions quickly and accurately.