A Cessna 340 (C340) departed Sydney at 1036, on climb to flight
level (FL) 120, tracking via Shellys and Yass to Deniliquin. At
1047 a Saab 340B (Saab) departed Sydney for Canberra on climb to
FL120, also tracking via Shellys. Once both aircraft were
established in cruise there was a rate of closure of about 100 kt
between them. As the Saab passed over Shellys, the Melbourne Sector
12 controller observed on radar that the lateral separation between
the two aircraft was reducing to less than the required standard of
5 NM. The controller instructed the Saab to turn left though 90
degrees and advised the crew that there was traffic in their
2-o'clock position at 2 NM. The crew reported sighting the C340.
Subsequent radar analysis established that the lateral distance
between the aircraft had reduced to 2 NM.
The traffic level in the sector was low, with six aircraft on
frequency. The controller had been operating the radar display on
the 100 NM scale to enable him to readily observe boundary traffic
or approaching flights. The display was centred on Shellys at the
time of the occurrence. The controller was closely monitoring the
progress of three aircraft that were tracking between Wollongong
and Canberra. Two aircraft were in a step climb, while the third
was overtaking and outclimbing the first two. The controller
believed that he was maintaining an adequate scan of the radar
display and was unable to provide a reason for not appreciating the
effect of the high rate of closure between the two occurrence
aircraft.
The morning shift of the adjacent sector to the west of Sector
12 had transitioned to The Advanced Australian Air Traffic System
(TAAATS) approximately two and one-half weeks prior to the
occurrence. This had resulted in the implementation of a number of
changes to Sector 12 coordination procedures. Those changes were
only required during the morning shift. During the afternoon the
adjacent sector reverted to the same air traffic control system as
that being used by Sector 12. The controller subsequently reported
that while the amended coordination procedures were understood,
they required conscious thought to action them.
During the morning, and at the time of the occurrence,
coordination between the sector and Nowra Air Traffic Control had
been restricted by communications problems and the controller was
using a "hotline" and a telephone for coordination. There was no
telephone available on the Sector 12 console so the controller was
required to reach across to use one from an adjacent console.
The investigation did not establish the reason why the
controller was unaware of the impending conflict until there was
insufficient time to resolve the situation. It is probable that a
combination of low traffic levels, possible fixation on the
Wollongong - Canberra traffic situation, and some distraction due
to amended coordination requirements, was sufficient to reduce the
controller's normal level of vigilance.