A method of formulating a specification for temporal and vertical required navigation performance is described. The method assesses various flight management guidance systems methods and allows a specification to be set for an airport. The specification may be used to increase aircraft traffic into the airport. The specification sets limits on deviations in time and altitude, for example, on approach into arrivals at the airport. The method comprises: calculating temporal and vertical deviations from a reference trajectory for one or more flight management guidance methods for one or more aircraft types; and comparing the temporal and vertical deviations with operational requirements of the airport. The deviations from a reference trajectory may result from uncertainties affecting trajectory prediction, such as wind and temperature prediction accuracy.