A method of determining a change in speed of an aircraft, comprising the steps of: defining a merge point and a tie point; monitoring the location of a first aircraft; and determining when the first aircraft passes the tie point, wherein after the first aircraft passes the tie point the method comprises: providing trajectory data associated with the first aircraft; providing trajectory data associated with a second aircraft; predicting a trajectory of the second aircraft using the trajectory data associated therewith; predicting a trajectory of the first aircraft using the trajectory data associated therewith; defining a minimum permissible longitudinal spacing; predicting a longitudinal spacing along the predicted trajectory of one of the first and second aircraft between the first and second aircraft at the merge point based on the predicted trajectories; and if the minimum permissible longitudinal spacing is greater than the predicted longitudinal spacing then calculating a proposed change in speed of the second aircraft that will result in the longitudinal spacing between the first and second aircraft at the merge point being greater than or equal to the minimum permissible longitudinal spacing. defining a merge point and a tie point; monitoring the location of a first aircraft; and determining when the first aircraft passes the tie point, wherein after the first aircraft passes the tie point the method comprises: providing trajectory data associated with the first aircraft; providing trajectory data associated with a second aircraft; predicting a trajectory of the second aircraft using the trajectory data associated therewith; predicting a trajectory of the first aircraft using the trajectory data associated therewith; defining a minimum permissible longitudinal spacing; predicting a longitudinal spacing along the predicted trajectory of one of the first and second aircraft between the first and second aircraft at the merge point based on the predicted trajectories; and if the minimum permissible longitudinal spacing is greater than the predicted longitudinal spacing then calculating a proposed change in speed of the second aircraft that will result in the longitudinal spacing between the first and second aircraft at the merge point being greater than or equal to the minimum permissible longitudinal spacing. providing trajectory data associated with the first aircraft; providing trajectory data associated with a second aircraft; predicting a trajectory of the second aircraft using the trajectory data associated therewith; predicting a trajectory of the first aircraft using the trajectory data associated therewith; defining a minimum permissible longitudinal spacing; predicting a longitudinal spacing along the predicted trajectory of one of the first and second aircraft between the first and second aircraft at the merge point based on the predicted trajectories; and if the minimum permissible longitudinal spacing is greater than the predicted longitudinal spacing then calculating a proposed change in speed of the second aircraft that will result in the longitudinal spacing between the first and second aircraft at the merge point being greater than or equal to the minimum permissible longitudinal spacing.