A Boeing 777 (B777) was tracking via an ARBEY Six standard arrival route (STAR) at 8,000 ft for a landing on runway 34 at Melbourne. A Boeing 737 (B737) departed runway 27 for Maroochydore and was cleared to track via the DOSEL Eight standard instrument departure (SID) on climb to 7,000 ft. Because the SID and STAR tracks crossed north of Melbourne aerodrome the departure controller planned to maintain the minimum vertical separation standard of 1,000 ft between the two aircraft until they had passed, and then clear the aircraft to climb/descend once the 3 NM lateral radar separation standard had been established between them. However, prior to the B737 reaching the crossing point the controller instructed the crew to cancel the SID and to track direct to Mudgee, believing that the track adopted by the B737 would result in the aircraft passing in front of the B777 with greater than the required radar standard.
When a Boeing 767 (B767) subsequently departed from runway 27 on climb to 5,000 ft the controller thought that he only had to separate that aircraft from the B777. Consequently, he instructed the crew of the B777 to descend to 6,000 ft. The crew acknowledged and advised that they had left 8,000 ft. The controller was about to instruct the crew of the B737 to climb to a higher level when he saw that the separation between the B737 and the B777 was going to reduce to less than the required standard. The controller issued instructions to both crews in order to maintain radar separation, however, the distance between the two aircraft reduced to 2.25 NM laterally and 200 ft vertically.
Both aircraft were fitted with traffic alert and collision avoidance systems (TCAS) and each crew received traffic advisories followed by a short resolution advisory. By the time the controller had issued turn instructions the advisories had ceased. Each crew had sighted the other aircraft just prior to the conflict. The short-term conflict alert (STCA) function of The Australian Advanced Air Traffic System (TAAATS) also operated during the occurrence.
The departure and approach control positions had recently transitioned to TAAATS, and the departure controller had been operating the new equipment for about 2.5 weeks. Immediately prior to the occurrence the controller had been discussing the operation of the route adherence monitor (RAM) function of TAAATS with another controller, and was attempting to establish the degree to which an aircraft would have to be off-track before the RAM activated.
When the controller instructed the B777 crew to descend, his impression was that he had resolved the possibility for conflict between the B737 and the B777. However, the track to Mudgee for the B737 did not provide the required lateral separation with the arriving B777. The controller had initially used separation assurance techniques to establish separation between all the aircraft. However, to facilitate the departure climb for the B737 the controller issued instructions to that crew which required him to more diligently monitor the track and altitudes of aircraft under his control. He then became distracted by non-essential manipulation and discussion of the system during a critical phase of the traffic management sequence. Consequently, he did not appreciate that the lateral distance between the B737 and the B777 was insufficient to maintain separation. Had the controller maintained vertical separation between the two aircraft, or had vectored the B737 behind the B777, it is unlikely that the incident would have occurred.
The situational awareness of both crews, the operation of the TCAS, and the STCA were all active defences in the incident.