VH-CZU a Boeing 737 (B737), was en route from Adelaide to Sydney
at flight level (FL) 370 on air route H31. VH-CZS a B737 was
tracking north on H29 en route from Melbourne to Brisbane at FL370.
Their respective air routes intersected at a position approximately
25 NM north-east of Wagga and their relative positions and
groundspeeds indicated that the radar separation standard of 5 NM
would not be maintained.
The Melbourne Sector 6 controller coordinated with the Canty
Sector controller for the crew of CZU to be issued with a
requirement to descend to FL350 by 110 NM from Culin, a position
approximately 34 NM north of Canberra. The intent of the
instruction was to ensure that the vertical separation standard of
2,000 ft was established between the aircraft prior to the lateral
distance between them reducing to less than the standard. Following
the acknowledgment and read back of the requirement, the crew of
CZU queried whether radar vectors would be available, as they
preferred to maintain FL370 due to cloud and possible turbulence
below that level. The Canty controller advised the crew to stand by
and after conferring with the sector 6 controller instructed them
to transfer to the sector 6 frequency. Once established on the
sector 6 radio frequency the crew was instructed by the controller
to turn right heading 130 degrees. The controller issued further
instructions to the crew of CZU to turn onto 140 degrees and 150
degrees.
The controller then instructed the crew of CZS to turn right
onto 060.
CZU passed 4 miles behind CZS while they were at the same level.
There was an infringement of separation standards. The incident
occurred during the period when Melbourne air traffic controllers
were transitioning from the old centre that used the Australian
Computer Air Traffic Control System radar and procedural flight
strip bay facilities to new facilities using the Advanced
Australian Air Traffic Control System (TAAATS). Sector 6 was in the
old centre while Canty sector was a TAAATS position. TAAATS has a
number of alarms to alert controllers of potential separation
infringements. During the occurrence the short-term conflict alert
operated at the Canty position. The sector 6 controller was busy at
the time with a moderate level of traffic. The complexity of the
management of sector 6 was compounded by weather that was causing
flight crews to request advice of weather on various routes and
also for diversions to avoid developing weather cells. Sector 6 had
two control positions, radar and procedural and was normally
operated in the combined configuration. The controller was managing
both positions at the time. Another controller was available to
assist at the position. This controller was not utilised until
after the occurrence.
When CZS entered sector 6 airspace it was approximately 7 NM
west of air route H29. This was normal practice as departure
controllers were approved to instruct crews to track from their
present position, within the terminal area (within 45 NM of
Melbourne), direct to Mudgee. This required the sector 6 controller
to calculate a specific solution for each potential crossing
conflict with aircraft nominally tracking on H29 and aircraft on
all intersecting routes. Sector controllers can use lateral
separation diagrams to assist in the application of separation.
However, the use of such diagrams was dependent upon aircraft
operating within the navigational tracking tolerance of the air
route being flown. Sector 6 did not have or use lateral separation
diagrams.
Sector 6 was a joint radar/procedural sector with a majority of
radar coverage. However, the size of the sector and the disposition
of air routes within the sector meant that generally conflicts were
resolved using procedural control methods. Radar vectoring was used
to sequence aircraft for arrival into Sydney but was not generally
used to establish separation between aircraft. Consequently
controllers had limited opportunities to practice vectoring
techniques.
The controller had returned to the sector 6 staff roster
approximately two weeks prior to the occurrence after being
rostered for familiarisation training on 30 September and 1 October
1999. The controller had agreed to return to sector 6, following
his TAAATS transition training, to enable other controllers to be
released for transition training. The sector 6 area of
responsibility within TAAATS had been divided into two sectors,
Parkes and Bourke. The management of traffic and coordination
requirements for these sectors differed considerably from those
required for sector 6. The controller had undergone radar vectoring
exercises during the transition training but these had focused
primarily on developing human-machine interface skills and not
traffic management skills. Also, some exercises used nil-wind
conditions and thus were not reflective of conditions likely to be
experienced on the job. The controller was rostered for two
familiarisation shifts on the return to sector 6. However, after
five hours during the first period of familiarisation, the
controller felt comfortable and believed he was capable of
operating at a satisfactory level and was subsequently endorsed to
operate the sector.
The crew reported that they believed that the requirement, to
descend to FL350 by 110 NM Culin, had been cancelled when the Canty
controller advised them to stand by. The sector 6 controller did
not instruct the crew to maintain FL370 once he had decided to
vector the aircraft. Neither the crew of CZS nor the controller
queried or clarified the status of the level requirement until
after the occurrence.
