History of the flights
A Metroliner had departed Darwin on a flight to Dili, East Timor. The crew had climbed to flight level (FL) 180 and were maintaining that altitude. They had been under the control of Darwin Air Traffic Control for the first 60 NM and had transferred to Brisbane Control in accordance with normal procedures. They had established two-way radio communication with Brisbane sector control.
The Airbus A330 (A330) was on a flight from Kuala Lumpur, Malaysia to Darwin and the crew had commenced descent to FL210 in accordance with instructions from Brisbane Control.
When the A330 was approximately 97 NM from Darwin, the crew requested further descent. The Brisbane controller immediately activated the intercom line to Darwin Approach and informed the Darwin controller of the position of the A330. The Metroliner was 81 NM from Darwin at that time.
The Darwin controller, who was under training, only had access to a 60 NM radius display screen. He had already handed off the outbound Metroliner to the Brisbane controller and authorised descent to FL 120 for the A330 in the belief that the Brisbane controller would separate the two aircraft. The trainee was not immediately aware of the relative positions of the aircraft as the conflict was not shown on his display.
The training officer immediately realised that separation had not been assured by the coordination interaction and looked to one side where he had set up a second screen display on an adjacent console. He had selected a greater range scale on the console, that showed the position of the two aircraft. As he was briefing the trainee on urgent action to remedy the situation, the crew of the A330 reported on the Darwin Approach radio frequency that they were on descent to FL120. While that radio exchange was occurring, the trainee had selected the longer range scale on his console and the display had now appeared on his screen. It showed that the aircraft were approximately 9 NM apart in a nose-to-nose conflict. He immediately issued an amended instruction to the crew of the A330 to maintain FL190. The crew was able to carry out that instruction before the vertical separation standard of 1,000 ft was infringed.
A few moments later the captain of the Metroliner saw the A330 pass immediately overhead. He reported that there was insufficient time to draw the co-pilots attention to the aircraft before it passed. He was of the opinion that there would have been insufficient time to initiate an evasive manoeuvre if it had been required.
The A330 crew saw the Metroliner but took no action as the aircraft were obviously going to miss. The traffic alert and collision-avoidance system was serviceable but did not activate.
Radar analysis indicated that the aircraft passed with approximately 1,300 ft vertical separation but with no horizontal difference.
At the Brisbane sector console, the short term conflict alert activated approximately 20 seconds prior to the time of passing. The controller immediately checked the radar display and found that a separation standard was being applied by Darwin approach control and took no further action.
Darwin approach control
The Darwin approach unit controlled airspace to 60 NM, in the north-west quadrant, up to and including FL200. At the time of the coordination from Brisbane sector, both aircraft were outside those parameters and, therefore, not subject to direct control by Darwin.
Local procedures required that arriving aircraft be assigned no lower than FL210 by Brisbane Control. Therefore Darwin Control was required to approve any further descent despite the aircraft being in Brisbane's airspace. When the Brisbane controller coordinated the position of the A330, the trainee read into the tone of the voice that a request was being made for further descent even though the actual words used did not state that intent. The trainee was not immediately aware of the confliction because it was outside his area of jurisdiction and not within the range of his selected display. The trainee also had the impression that, as the aircraft were in Brisbane's airspace, he was only approving descent in Darwin's airspace and that the Brisbane controller was still responsible for the separation of the aircraft.
Radar hand-off was the subject of a specific instruction in the Manual of Air Traffic Services Supplement (MATS Supp) which stated: "Aircraft handed off by radar between Darwin ATC and Brisbane/Tindall Sectors shall be subject to `no restrictions' unless otherwise advised."
The adjacent console was not in use and had been positioned so that the controllers could view the display screen, albeit at a distance of 2 - 3 m. The range was selected at a longer distance than the 60 NM in use at their primary screen. That selection meant that the position of the aircraft could be seen but the details in the information blocks could not be read. A software change was scheduled for introduction the next day that would have allowed the trainee to view the longer range (second look) on his own console rather than having to look across to the second screen.
When the trainee had approved descent for the A330, the Brisbane controller de-selected the intercom line before the training officer could intervene to cancel the approval. The training officer immediately briefed the trainee to maintain the A330 at FL190 because he expected the crew to change frequency quickly as, in his experience, they normally did. The training officer was prepared to go back to Brisbane via the intercom line, but the broadcast by the crew of the A330 made that option redundant.
Brisbane sector control
The controller coordinated the position of the A330 with Darwin Control and expected an instruction from Darwin regarding descent. When they issued approval for FL120 he assumed that they were separating the aircraft, even though he thought that he may not have looked at his display at the time of carrying out the coordination and may have, momentarily, overlooked the actual position of the Metroliner. He then issued the descent instruction and told the crew to change radio frequency to Darwin Approach.
Shortly after, the short term conflict alert activated and the controller immediately checked the display. He saw that there was no infringement of separation standards and took no further action.
The controller had been rated on The Australian Advanced Air Traffic System (TAAATS) for 4 months but had just returned from a month's leave. He received 2 days familiarisation and had performed 4 active shifts prior to the occurrence. He stated that he was well rested and that the workload was moderate. However, he said that he was a bit `rusty' and that the 2 days familiarisation was a bare minimum for adequacy.
Short term conflict alert
The activation of the alert was to warn the controller that the aircraft were predicted to pass within 600 ft and 4.1 NM. Analysis of the performance of the alert indicates that it operated in accordance with the parameters and provided approximately 20 seconds warning of the conflict.
