A method of controlling aircraft arrivals at a merging waypoint, comprising the steps of: (a) defining a tie waypoint (60a, 60b, 110a, 110b) which is at a first distance D OPD from the merging waypoint (10, 10'); (b) generating a first parameter set of flight instructions for aircraft arriving at the tie waypoint (60a, 60b, 110a, 110b), the first parameter set including an aircraft speed instruction for a target separation between a first aircraft arriving at a target waypoint downstream of the tie waypoint (60a, 60b, 110a, 110b) and a second aircraft arriving at the target waypoint; (c) defining a diversionary flight path (130a, 130b) which is at a distance D PM from the merging waypoint (10, 10'), wherein D PM < D OPD; (d) generating a second parameter set of flight instructions for aircraft arriving at the diversionary flight path (130a, 130b), the second parameter set instructing the first aircraft to follow at least a part of the diversionary flight path before flying to the merging waypoint (10, 10'), whereby the total flight path of the first aircraft is increased, when it is determined that the expected arrival of the first and second aircraft at the target waypoint (10, 10') will have a separation that is shorter than a threshold separation; (e) recalculating the tie waypoint (60a, 60b, 110a, 110b) and the first parameter set so as to optimize aircraft fuel consumption whilst maintaining a separation at the target waypoint which exceeds the threshold separation, wherein the diversionary flight path (130a, 130b) and the second parameter set are fixed whilst the tie waypoint distance D OPD and the first parameter set are recalculated; and (f) recalculating the diversionary flight path (130a, 130b) and the second parameter set so as to optimize the aircraft fuel consumption whilst maintaining a separation at the target waypoint which exceeds the threshold separation, wherein the recalculated tie waypoint (60a, 60b, 110a, 110b) and the recalculated first parameter set are fixed whilst the diversionary flight path and the second parameter set are in turn recalculated. (a) defining a tie waypoint (60a, 60b, 110a, 110b) which is at a first distance D OPD from the merging waypoint (10, 10'); (b) generating a first parameter set of flight instructions for aircraft arriving at the tie waypoint (60a, 60b, 110a, 110b), the first parameter set including an aircraft speed instruction for a target separation between a first aircraft arriving at a target waypoint downstream of the tie waypoint (60a, 60b, 110a, 110b) and a second aircraft arriving at the target waypoint; (c) defining a diversionary flight path (130a, 130b) which is at a distance D PM from the merging waypoint (10, 10'), wherein D PM < D OPD; (d) generating a second parameter set of flight instructions for aircraft arriving at the diversionary flight path (130a, 130b), the second parameter set instructing the first aircraft to follow at least a part of the diversionary flight path before flying to the merging waypoint (10, 10'), whereby the total flight path of the first aircraft is increased, when it is determined that the expected arrival of the first and second aircraft at the target waypoint (10, 10') will have a separation that is shorter than a threshold separation; (e) recalculating the tie waypoint (60a, 60b, 110a, 110b) and the first parameter set so as to optimize aircraft fuel consumption whilst maintaining a separation at the target waypoint which exceeds the threshold separation, wherein the diversionary flight path (130a, 130b) and the second parameter set are fixed whilst the tie waypoint distance D OPD and the first parameter set are recalculated; and (f) recalculating the diversionary flight path (130a, 130b) and the second parameter set so as to optimize the aircraft fuel consumption whilst maintaining a separation at the target waypoint which exceeds the threshold separation, wherein the recalculated tie waypoint (60a, 60b, 110a, 110b) and the recalculated first parameter set are fixed whilst the diversionary flight path and the second parameter set are in turn recalculated.