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.