A Boeing 737 (B737), VH-TJT, was en route from Darwin to
Adelaide via air route J187 at flight level (FL) 370. Another B737,
VH-CZN, was en route from Ayers Rock to Sydney via air route G222,
also at FL370, which was a standard level for the direction being
flown by both aircraft. The air routes converged, and eventually
crossed at Oodnadatta. Both aircraft were under procedural
(non-radar) control by Melbourne Sector 1. The sector console was
staffed by a controller who was undergoing re-familiarisation prior
to being checked for return to controlling. He was being supervised
by a controller who had recently returned after a six month break
from full-time operational controlling.
The controller had calculated estimates for Oodnadatta of 1518
and 1520 respectively for CZN and TJT. At 1515, when the crew of
TJT reported at SARAH, 40 NM north-north-west of Oodnadatta, the
controller became aware that there was no separation standard being
applied between the two aircraft, so he instructed the crew of TJT
to descend immediately to FL350. The crew reported on descent and
indicated that their traffic alerting and collision avoidance
system (TCAS) showed an aircraft 33 NM ahead, over Oodnadatta. The
crew of TJT subsequently reported maintaining FL350 at 1520. As a
result, there was a breakdown of the required vertical separation
standard of 2,000 ft.
Both controllers had earlier noted the possibility for conflict
between the two aircraft and annotated their respective flight
progress strips with an Oodnadatta position, and the calculated
estimates for that position. The estimates were annotated at
different locations on the strips. There was no local instruction
for the standardisation of the annotation of the Oodnadatta
position or similar positions that marked the intersections of the
various route crossings in the area. During the following 90
minutes, the traffic level steadily increased and the two
controllers had discussed the need for a coordinator. The sector
was normally operated as a solo operator position; however, there
were periods when there were significant or complex traffic levels
and there were procedures to enable the controller to be assisted
by a coordinator in such cases.
Approximately 10 minutes prior to the occurrence, the
controllers requested the assistance of a coordinator and as a
consequence, a third controller joined them at the console. At this
stage, the traffic situation was very busy, with approximately 70
flight progress strips being monitored and with additional strips
being discarded or added to the board as flights progressed through
the sector. The coordinator controller had little opportunity to
obtain an adequate handover/takeover brief from the two controllers
already at the position. The third controller assumed the
coordinator position and endeavoured to action any items that he
considered required attention as he attempted to gain some idea of
the traffic disposition. He was unable to monitor the Sector 1
controller's air-ground-air program on a continuous basis due to
the conduct of coordination actions. The work at the console was
difficult, with three controllers working in an area normally used
by only two controllers.
As a result of a combination a factors, the potential
confliction between CZN and TJT, although recognised earlier by
both controllers, was subsequently overlooked as the sector
workload increased. It was only when the crew of TJT reported at
SARAH that action was undertaken resolve the situation.
Airservices Australia carried out an investigation of this
occurrence under the overall supervision of a Bureau officer. The
significant factors that contributed to this occurrence were found
to be;
- The two controllers manning the Sector 1 position were not
operating at their optimum level of performance due to a lack of
recency.
- Following an increase in Sector 1 traffic density, the
controllers' recognition that they required coordination assistance
to manage the increased workload was too late to provide any
effective relief.
- The amount and disposition of traffic in the sector led the
controllers to be distracted to a point where they were unable to
adequately monitor all sector traffic.
- The controllers' annotation of flight progress strips did not
alert them to the possibility for a conflict between CZN and
TJT.
