A de Havilland Canada Dash 8-102 (northbound Dash 8) was en route from Brisbane to Gladstone at flight level (FL) 140. A second de Havilland Canada Dash 8-102 (southbound Dash 8) was tracking from Gladstone to Brisbane via Maleny (MLY) also at FL140. Those routes placed the aircraft on reciprocal tracks with approximately 1.25 NM lateral displacement. The Keppel (KPL) and Alma (LMA) sectors were combined. The controller was responsible for the airspace that extended from approximately 80 NM north of Brisbane to approximately 70 NM north of Rockhampton and from the coast to approximately 90 NM to the west. The KPL/LMA sector controller received a Short Term Conflict Alert (STCA) between the aircraft when they were approximately 10 NM apart. The STCA was a collision avoidance tool in The Australian Advanced Air Traffic System (TAAATS). The controller instructed the crews of both aircraft to turn 90 degrees to the right and issued traffic information to the crew of the northbound Dash 8 about the southbound Dash 8. The aircraft passed 5.4 NM abeam each other. The radar separation standard was 5 NM. Each crew had the other aircraft in sight. There was no infringement of separation standards.
Traffic levels at the time of the occurrence were considered low. There were two aircraft to the northwest of Brisbane that required the attention of, and coordination between, the KPL/LMA controller and the Burnett (BUR) controller.
The KPL/LMA controller had assigned the crew of the southbound Dash 8 FL190. The crew subsequently requested and was assigned FL150. Eight minutes later, the crew requested "amended non-standard FL140". Use of the standard levels for the track flown would normally provide a degree of separation assurance between aircraft. A non-standard level was a level that was not in accordance with the table of cruising levels detailed in the Manual of Air Traffic Services. The controller approved the crew's request for the level change. Shortly after, the southbound Dash 8 was maintaining FL140. The controller then conducted a handover/takeover and another controller assumed responsibility for the sectors.
The crew of the southbound Dash 8 reported a layer of stratus at FL150 extending from Gladstone almost to Brisbane. The crew requested FL140 to remain beneath the cloud and avoid any associated turbulence; although the crew did not provide a reason to the controller at the time of the request. Flight level 130 was available and was the standard level but was not requested because the crew thought the base of controlled airspace was FL130 in that area and was therefore outside controlled airspace. The base of controlled airspace was FL125. Flight level 130 would have kept the aircraft within controlled airspace and would have provided a minimum of 500ft vertical separation with aircraft operating outside controlled airspace.
The crew of the northbound Dash 8 had been cleared to FL180 on departure from Brisbane. The crew reported on climb to FL180 and requested amended FL140 from the BUR sector controller. That controller re-cleared the crew of the northbound Dash 8 at FL140, a standard level. The controller also re-cleared the crew direct to Gladstone when the aircraft was clear of conflicting traffic in the BUR sector. Local instructions enabled controllers to approve direct tracking subject to a number of conditions. One of those conditions stated that direct tracks were available on "northbound tracks that are east of and will not cross a line MLY-RK [Rockhampton]: landing within the lateral limits of Fraser airspace". The track of the northbound Dash 8 met those requirements.
The BUR sector controller then transferred control responsibility of the northbound Dash 8 to the KPL/LMA controller. A transfer of control responsibility, or hand off, was performed when one controller highlighted the subject aircraft on another controller's Air Situation Display (ASD). The symbol then changed colour to indicate the intention to transfer control responsibility. The symbol changed colour again when control responsibility had been accepted. Crews were instructed to change to the next control frequency following acceptance of the hand off. Following the hand off, the northbound Dash 8 entered KPL/LMA sector. Both the northbound Dash 8 and the southbound Dash 8 were maintaining FL140. Neither the KPL/LMA nor the BUR controllers were required to voice coordinate level changes or changes in tracking. That information was entered into TAAATS by the responsible controller and was updated automatically at all relevant consoles.
The assignment of the non-standard level to the southbound Dash 8 was not considered significant by the first KPL/LMA controller because the aircraft was within radar coverage. There was a general awareness of standard levels among the KPL/LMA controllers interviewed, but none considered the use of standard levels essential for aircraft within radar coverage. The use of standard levels often did not achieve separation assurance because much of the sector's traffic climbed and descended into and out of coastal ports and controllers had to monitor aircraft altitude or levels to ensure vertical separation was maintained. The controller assigned FL140 to the southbound Dash 8 as there were no conflictions at that time and the use of a non-standard level was not uncommon.
The second controller believed he had missed the significance of the non-standard level because he had not issued the level himself and did not use any scanning techniques after the takeover that may have highlighted the conflict. He advised that he would have normally highlighted the label of an aircraft at a non-standard level using the individual quick look (IQL) function available on TAAATS. The IQL function was used to check hidden track label details but also changed the colour of a label on an individual console. The controller could not recall whether the first controller had drawn his attention to the non-standard level during the handover/takeover. The first controller did not believe he had mentioned the non-standard level during the handover/takeover.
The use of direct tracking by the BUR controller meant that the northbound Dash 8 would require either a different level or radar vectoring to maintain separation with the southbound Dash 8. The flight-planned route for northbound traffic provided a segregated two-way route structure between southbound aircraft on the Gladstone-Maleny track, and northbound traffic. However, the segregated two-way route structure was not usually used because direct tracking provided separation assurance between succeeding northbound aircraft departing Brisbane and facilitated traffic management along the coast.
The southbound Dash 8 had maintained FL140 for approximately twelve minutes prior to the time the KPL/LMA controller received the STCA. The crew of the northbound Dash 8 had reported maintaining FL140 three minutes prior to the time the KPL/LMA controller received the STCA.
The KPL/LMA controller reported that he had maintained radar surveillance, had not been distracted and was aware of both aircraft. The controller was fit for work and all equipment was serviceable. The controller advised however, that he had taken his family on a short trip to relax. He had returned two days before the occurrence but had been unable to relax. He also stated that he usually stayed up late at night and preferred to sleep until mid-morning. The night before the occurrence the controller stayed up until midnight but had awoken in sufficient time to commence duty at 7:00am on the day of the occurrence. He also reported that he had been discussing the inbound sequence with other controllers immediately prior to the occurrence.