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.
