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Summary

Summary

VH-UZP departed Townsville on the Townsville-Collinsville track but was then cleared to track direct to Mackay. VH-JJY, a faster aircraft, departed Townsville six minutes later and climbed initially on the Townsville-Bowen track before also being given a direct track to Mackay. The aircraft were thus on converging tracks. VH-JJY was subsequently cleared to divert up to 10 NM right of track to avoid thunderstorm cells. This brought the aircraft into potential conflict with VH-UZP, so the controller instituted a stepped climb procedure between the two aircraft. He then noted that VH-JJY was not diverting far off track so he cleared that aircraft to climb through the level of VH-UZP which by this time was level at FL210. The controller instructed VH-JJY to expedite climb to FL 220. However, VH-JJY then diverted further right of track (but still within 10 NM of track) and did not climb at the rate the controller expected. This resulted in a breakdown of separation standards between the aircraft. Recorded radar data indicated that when the aircraft were 3 NM apart, there was 200 ft vertical separation. The prescribed standard is 5 NM or 1,000 ft. Neither crew was given traffic information on the other aircraft. The controller, on realising that separation standards could be infringed, gave the leading aircraft (VH-UZP) a heading change from 125 degrees to 140 degrees. This was not of sufficient magnitude to ensure that the separation would be maintained. Significant Factors The following factors are considered relevant to the development of this incident: 1. Weather avoidance considerations resulted in the aircraft diverting off track. 2. Appropriate separation standards were not applied following the diversion of VH-JJY off track.
 
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