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.