Methods and systems for determining a change of speed of an aircraft for enabling the avoidance of conflicts between aircraft trajectories. The method of determining a change in speed of an aircraft, comprises the steps of: defining a merge point and a tie point; monitoring a first aircraft; determining when the first aircraft passes the tie point; providing trajectory data; predicting a trajectory of a second aircraft using the trajectory data; defining a minimum permissible longitudinal spacing; predicting a longitudinal spacing between the first and second aircraft at the merge point based on the predicted trajectory; 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.