Loss of separation involving a Boeing 767-200, ZK-NBI and Mooney M20C, VH-UWW, 19 km east of Melbourne Aerodrome, Victoria, on 8 March 1996

ZK-NBI was inbound to the Plenty locator from Wonthaggi on descent to 5,000 ft. VH-UWW departed Essendon runway 17 with departure instructions to turn left heading 090 deg with an unrestricted climb to 5,500 ft. These instructions were issued by the approach controller who was also controlling ZK-NBI.

Approximately four minutes after VH-UWW departed Essendon, the controller realised that the two aircraft were on conflicting tracks with no vertical separation. Both aircraft were given heading changes, but their tracks closed to approximately two miles at the same altitude before radar separation was re-established.

Investigation revealed that the approach controller failed to ensure maintenance of separation between the two aircraft because he was distracted by separation requirements for other traffic. He had intended to turn VH-UWW to the northeast earlier to maintain separation, but due to the distraction this did not occur.

Two other factors contributed to the failure. The first one was that when the approach controller issued the departure instructions for VH-UWW, ZK-NBI was not displayed on his radar screen. The second was that ZK-NBI maintained an unusually high speed on descent. Even though the aircraft had been issued with no speed restriction below 10000 ft, an unusually high groundspeed was maintained.