The crew of VH-TAF, a Boeing 737, was taxiing for runway 18 at Maroochydore (MC) for a departure to Sydney. The route was flight planned, and subsequently cleared, via waypoint TRIKI (128 MC at 22 NM) on air route W196 at flight level (FL) 350. The crew had pre-briefed the noise abatement procedure that was detailed in AIP Australia as follows:
"2.2 Departing Runway 18 - MC to TRIKI: UNLESS OTHERWISE DIRECTED BY ATC. (a) Jet aircraft shall at 500FT, turn left heading 090 degrees M to intercept track. If not able to intercept track from this heading, then maintain heading 090 degrees until 4DME before turning to intercept track".
Another Boeing 737, VH-CZV, was inbound to Maroochydore from Sydney and tracking via TRIKI on W196. The Maroochydore aerodrome controller had received prior coordination on this aircraft, which was on descent to 5,000 ft. Although the BURNET enroute controller was responsible for CZV, the crew was not in two-way communication with the enroute controller. The enroute controller had agreed to allow the crew to transfer frequency direct from Brisbane Approach to Maroochydore Tower. The standard frequency transfer point in accordance with Letter of Agreement (LOA) ND 98/054 was TRIKI. CZV was roughly 32 NM from Maroochydore when the aerodrome controller contacted the enroute controller to coordinate a departure clearance for TAF.
Maroochydore was a non-radar tower and the aerodrome controller was responsible for providing procedural separation to aircraft below 4,500 ft. The enroute controller was responsible for the surrounding airspace overlying and to the south of Maroochydore as defined in the AIP Designated Airspace Handbook. The enroute controller was able to use radar or procedural means to provide air traffic control separation. Radar coverage in the Maroochydore area was generally available above 1,500 ft.
At 1236, the enroute controller advised the aerodrome controller to issue TAF a heading of 090 degrees M and to maintain FL150, a level that would provide vertical separation from a third aircraft overflying at FL160. The aerodrome controller recognised a potential conflict between TAF and CZV, and confirmed with enroute that the enroute controller would accept the responsibility for conflict resolution. The enroute controller accepted the responsibility for providing separation.
The aerodrome controller then issued the following departure instruction to the crew of TAF: "TAF, restriction on departure is to turn left and take up a heading of 090 magnetic, maintain FL 150, clear for take-off, make left turn". The crew correctly read back the instruction. The crew later reported that they interpreted the restriction on departure to be the altitude restriction and believed the heading instruction applied to the noise abatement procedure. At 1238, the crew was instructed by the aerodrome controller to contact the enroute controller to make a departure report.
CZV meanwhile, was approaching TRIKI and the crew reported to the Maroochydore aerodrome controller on descent to 5,000 ft. The aerodrome controller directed the crew to continue tracking direct to Maroochydore and to maintain 5,000 ft. Maroochydore tower then contacted the enroute controller seeking an unrestricted descent clearance for CZV, which was granted but not issued to the crew.
At the same time, TAF was turning right to intercept the 128 radial at 4 NM from Maroochydore and was passing 4,400 ft on climb. The crew contacted the enroute controller at 1239 and reported established on the 128 radial, climbing to FL 150. TAF and CZV were then on reciprocal tracks and closing. In an unsuccessful attempt to preserve the required separation, the crew of TAF was told to turn left onto a heading of 090 M and maintain 5,000 ft. The aerodrome controller who was in two-way communication with the enroute controller over the hotline, found that CZV was passing 5,400 ft on descent. The enroute controller asked the aerodrome controller to direct the crew of CZV to turn right on to 090 M, which was actioned. The enroute controller issued traffic information to the crew of TAF about the relative position of CZV and requested the crew to climb to FL 150 and maintain their best rate of climb. Traffic information was not issued to the crew of CZV about TAF.
At 1240, the required vertical separation of 1,000 ft between the two aircraft reduced to 400 ft when the minimum radar separation of 5 NM was infringed. The application of radar separation and the requirements for issuing clearances were detailed in chapters 4, 9 and 12 of the Manual of Air Traffic Services (MATS).
The enroute controller received a short-term conflict alert (STCA) on the radar display. However, neither of the flight crews reported receiving a traffic alerting and collision avoidance system (TCAS) warning from their respective aircraft's display.