A Piper Seminole was being flown from Essendon airport, which is located 5 NM south-east of Melbourne airport, to Bendigo. The extended centrelines of the respective northerly runways, which were being used for arrivals and departures, are approximately 3 NM apart. The wind at the time was northerly at 25 kts.
The pilot of the Seminole was instructed to maintain runway heading of 350 degrees and climb to 6,000 ft. After take-off the aircraft was identified on radar by the Melbourne departures north (DEPN) controller.
Six minutes later the crew of a Boeing 767 (B767) was cleared to take-off from runway 34 at Melbourne, for Sydney, and to maintain runway heading on climb to 5,000 ft. After take-off the aircraft was identified on radar by the DEPN controller who cleared the crew to climb to flight level (FL) 200.
Traffic was light, with four departing and one arriving aircraft being managed by the DEPN controller. The controller was aware of the need to maintain either vertical or lateral separation between aircraft departing from Melbourne and the Seminole, and was also conscious that the present heading of the Seminole was not in the direction of Bendigo. He had previously radar vectored a departing Boeing 737 ahead of the Seminole, and planned to do the same with the B767, however, he observed that the B767 did not appear to be climbing as fast as he had expected. Consequently, the controller would have to take both the B767 and the Seminole further to the north before achieving sufficient vertical or lateral separation to enable him to allow the aircraft to resume their respective planned routes. The minimum required separation was either 3 NM lateral or 1,000 ft vertical.
When the B767 was approximately 7 NM to the south-west of the Seminole the controller believed he could vector the B767 to pass behind it, thus minimising any delay to both aircraft. The controller did not issue instructions to ensure vertical separation prior to turning the B767.
The B767 was vectored right, onto a heading of 040 degrees. Shortly after, the Seminole was vectored left onto a heading of 270 degrees. The DEPN controller then instructed the B767 crew to continue the right turn onto 060 degrees as the aircraft was passing approximately 4,500 ft, and instructed the pilot of the Seminole to turn right onto 290 degrees. The B767 crew was requested to expedite the turn onto 090 degrees. Shortly after, the crew reported receiving a traffic alert and collision avoidance system (TCAS) resolution advisory, indicating the crew should maintain the current altitude of approximately 5,400 ft due to a conflicting aircraft. The crew complied, and advised the DEPN controller accordingly. The controller advised the crew that they had passed another aircraft that was to their left. Separation was reduced to 1.5 NM lateral and 600 ft vertical. The B767 crew did not sight the other aircraft.
The B767 crew subsequently reported that the wind at 5,000 ft was westerly at 50 kts. This would have had the effect of increasing the groundspeed of the B767 as it turned towards the east. Consequently, the rate of closure between the B767 and the Seminole would have been greater than anticipated by the controller, and it was probably at that stage he became aware that the aircraft might pass with less than the required separation. However, his subsequent instructions were unable to rectify the situation. Had the controller continued to employ separation assurance techniques, the occurrence would probably not have eventuated.