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A Boeing 737-476 (737) was en route from Melbourne to Coolangatta tracking northeast at flight level (FL) 310. A Cessna Aircraft Company C500 (Citation) was tracking southwest from Coolangatta to Essendon at the same level. The aircraft were on reciprocal tracks and the air traffic controller did not provide any assurance of separation between the two aircraft. The rate of closure of the aircraft was later calculated to be in excess of 800 kts. When the distance between the two aircraft was approximately 22 NM, the controller instructed the crews of both aircraft to alter heading and flight level. The controller's intervention ensured that a separation standard was established and maintained. The radar system's short-term conflict alert (STCA) activated. While the horizontal radar separation standard of 5 NM was never infringed, vertical separation, which assured separation for aircraft on reciprocal tracks, was not established until 25 seconds prior to the aircraft passing.

The 737 was being operated as a regular public transport flight and was flight planned at FL350. That aircraft departed Melbourne at 09:37 Eastern Standard Time and tracked via air route H66. That air route passed through the Benalla sector controller's area of responsibility. The Benalla sector controller was located in the Melbourne Air Traffic Control Centre. The crew of the 737 had requested FL290 on first contact with the Benalla sector controller, which was approved. At 09:52:51, the crew requested climb to FL310. The controller assigned FL310 at 09:55:59 and the recorded radar data indicated the aircraft reached that level at 09:57:43.

The Citation was operating a private flight at FL310 and departed Coolangatta at 08:16. Flight level 310 was a non-standard level for the track flown. Cruising levels were published in the Manual of Air Traffic Services (MATS) and the Aeronautical Information Publication (AIP). MATS also stated that aircraft operating at other than a level conforming to the table of cruising levels for the particular direction of flight or notifying intention to cruise at such a level, shall be advised accordingly and the pilot's intention sought. Non-standard levels were regularly used by air traffic control for maximum utilisation of available airspace.

The Citation pilot had flight planned to track via Parkes and TAREX, which was a non-standard route. The AIP stated that prior air traffic control approval was required for RNAV [area navigation] tracking on routes other than those published in AIP. An aircraft tracking from Coolangatta to Essendon would normally have tracked via air route Q94 and would have remained clear of the Benalla sector airspace. The pilot of the Citation had not received prior air traffic control approval to plan and conduct the flight via a non-standard route.

At 09:41:22, the crew of the 737 established radio contact with the Benalla sector controller. The controller was managing the combined airspace sectors of Eildon Weir and Benalla. The 737 crew was on climb FL200 and was recleared to the requested level of FL290. The controller was expecting the crew of the 737 on frequency and had earlier anticipated that the aircraft may be a problem at the planned flight level. However, at FL290 it would be separated from other aircraft.

Due to confusion caused by an anomaly with another aircraft, the Benalla controller was required to complete a significant period of coordination with another sector controller. The Benalla controller was discussing that problem at the console with the team leader when the crew of the 737 again made radio contact at 09:52:51. They reported maintaining FL290 and requested climb to FL310. The controller acknowledged the request and annotated the flight track with that request. The controller continued his discussion with the team leader and did not immediately coordinate the higher level request with the adjacent Parkes Sector.

At 09:53:37, the controller designated and accepted jurisdiction on the radar display of the Citation's flight track. That was before the pilot of the Citation established radio contact at 09:54:18 and reported maintaining FL310. The controller acknowledged that transmission and then transferred jurisdiction of two other aircraft to the Parkes sector controller.

At 09:55:56, about three minutes after receiving the 737 crew's change of level request, the controller coordinated the 737 with a request for FL310 with the Parkes sector controller. The Parkes sector controller concurred with the higher level and the Benalla controller cleared the 737 crew to climb. The controller did not recognise the potential conflict between the 737 and the Citation and continued his discussion with the team leader.

At 09:59:55, the controller activated the "individual quick look" function for the flight plan tracks of both aircraft. Almost simultaneously, the controller commenced action to ensure a separation standard was maintained between the two aircraft. The controller instructed the crew of the 737 to expedite descent to FL290 and instructed the pilot of the Citation to turn right 30 degrees. At 10:00:15, he further instructed the crew of the 737 to turn right 20 degrees. He also broadcast essential traffic information to the 737 crew about the position of the Citation. During that transmission, the short term conflict alert on the controller's radar display activated at 10:00:28 highlighting the close proximity of both aircraft. The traffic alerting and collision avoidance system (TCAS) of the 737 also activated, providing a traffic advisory warning to its crew. At 10:00:34, the controller told the pilot of the Citation to climb to FL320. At 10:00:57, a 2,000 ft vertical separation standard was established between the two aircraft when they were approximately 9 NM apart.

The controller had held a full performance rating within the group for a period just less than four years. He was completing the third day of a 5-day roster cycle. Although the controller was working a combined sector, the workload was considered light to moderate.

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