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Airspace management

The general control technique to ensure separation on this route was for Darwin to assign FL200 (or below) for departures and for Brisbane to only assign FL210 for descent; therefore ensuring the 1,000ft separation standard.

In accordance with MATS Supp, when an aircraft was handed off from one controller to the other, that aircraft shall be subject to "no restrictions" unless otherwise stated. In the case of the A330, the Brisbane controller had not mentioned any restrictions for its descent and had, therefore, incorrectly coordinated the aircraft with Darwin. Darwin Control had correctly coordinated the Metroliner with Brisbane Control and had transferred the crew to the Brisbane controller's radio frequency before that aircraft had left their area of responsibility. As the Metroliner had passed outside 60 NM from Darwin, it was under the control of the Brisbane controller. Had the crew of the A330 been slow with their radio frequency transfer, neither controller would have been able to prevent the A330 descending through the level of the Metroliner.

Darwin approach control

The trainee controller made a decision to allow the A330 further descent based on a belief that the Brisbane controller would separate the aircraft that were in that controller's airspace. However reasonable that belief may have been, the approval left open a possibility for an infringement in separation standards. Phraseologies to ensure separation were available and would have clarified that the descent was subject to the Brisbane controller's separation of the aircraft.

The intended action of the training officer was appropriate and timely, but the broadcast from the crew of the A330 made his plans redundant.

Had the software change that was to be implemented the next day been available at the time, the trainee would have had a better opportunity to see the relative positions of the aircraft and, therefore, observe the developing air traffic situation and take more appropriate action.

Brisbane sector control

When the response from Darwin Control was for descent to FL120, the controller accepted the level and issued descent to that level to the crew of the A330 when the aircraft were approximately 16 NM apart and with a closing speed of approximately 12 NM per minute. Irrespective of the response from Darwin, the Brisbane controller still had responsibility for separation of the aircraft outside the 60 NM arc and the aircraft were obviously going to pass in that controller's airspace. The controller also had both aircraft on his radio frequency and the instruction to the crew of the A330 gave away both separation and radio contact. However, the crew of the A330 made a timely and successful change of frequency to the Darwin controller. Had this transfer taken longer to take place, the only solution would have been for the Brisbane controller to issue emergency instructions to the crew of the Metroliner. As the controller had considered that Darwin were separating the aircraft at the time of issuing the A330 descent, it is unlikely that such action would have been taken in time.

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