Boeing Co 737-476, VH-TJL

A Boeing 737-400 (737) registered VH-TJL was en route from
Brisbane to Townsville at FL340. Another 737 registered VH-TJF was
en route from Cairns to Brisbane at FL330. Both aircraft were in
the area of responsibility of the Brisbane Air Traffic Centre. TJL
was operating on the Tabletop Sector radio frequency (120.55 Mhz)
and TJF was operating on the Swampy Sector radio frequency (133.2
Mhz). The two sectors are adjacent to each other with the Swampy
Sector located south of the Tabletop Sector.

The Tabletop Sector controller issued instructions to the crew
of TJL to descend 'when ready' and shortly afterward, that crew
reported receiving a traffic alert and collision avoidance system
(TCAS) resolution advisory (RA), instructing them to climb. The
controller issued traffic information on TJF to the crew of TJL.
Shortly after, the controller received a short-term conflict alert
on The Australian Advanced Air Traffic System display.

The crew of TJF then advised the Swampy controller that they had
received a TCAS RA instructing them to descend. The controller
issued traffic information on TJL to that crew.

The horizontal distance between the aircraft reduced to 0.4 NM
while the vertical distance was 400 ft. The required radar or
vertical separation standard was respectively 5 NM or 1,000 ft.
There was an infringement of separation standards.

An Airservices Australia investigation found that:

1. the Tabletop and Swampy sectors had been de-combined about 4
   minutes before the occurrence;
2. the crews of both aircraft had been given direct tracking;
3. the crews were operating on different VHF radio frequencies;
   and
4. there were supervisory and operational control deficiencies
   during the period leading to the occurrence.

With regard to the use of direct tracking, the Airservices
investigation noted that the route structure was designed to
segregate traffic where conflicts may occur and that to some extent
direct tracking could reduce the separation assurance provided by
the route structure. Had the two aircraft operated on their
respective planned routes it was estimated that they would have
crossed about 50 NM south of Townsville and that their descent
profiles would have resulted in a vertical distance of 16,000 ft
between them. The investigation also estimated that the difference
in track length between the planned and actual routes was 1 NM.
Thus, the efficiencies achieved by the provision of direct tracking
were minimal compared with the increased risk to aircraft
associated with the reduction in separation assurance.

As a result of this and other occurrences, Brisbane Centre
implemented a trial of Aisle Supervisors that commenced 9 September
2002. Aisle Supervisor duties include operational command authority
for a group or groups in the aisle plus administration and
operational responsibilities.