Airbus A330-341, PK-GPE

ATSB safety action

As a result of a previous occurrence, the ATSB is investigating
a safety deficiency (BS/20020004) relating to the limitations of
self-checking of data inputs by controllers. The circumstances
involved in this investigation also relate to the same deficiency.
Any outcome from the investigation of the safety deficiency will be
published on the ATSB website .

The Yarowee sector controller was distracted by activity at a
neighbouring console before completing the actions required to
assign the amended flight level to the crew of the A330.
The Yarowee sector controller cancelled the cleared flight
level prompt on the air situation display prior to receiving a
correct read back.
The aircraft crews were operating on different frequencies at
the time of the potential confliction and were therefore unaware of
the situation.

The Yarowee sector controller had entered the amended CFL into
TAAATS. He believed he had received the required information from
the crew of the A330 and consequently cancelled the CFL prompt in
the displayed label of the A330. The distraction at a nearby
console interrupted the Yarowee sector controller before he had
completed the task of issuing the amended clearance. The occurrence
highlights the importance of controller vigilance in completing
necessary TAAATS' interface action to ensure system data
integrity.

At the time of the CLAM and STCA alarms, the Canty sector
controller had accepted jurisdiction of the A330, however, the
Yarowee controller had not advised the crew of the A330 to change
to the Canty frequency. The crews of the A330 and the 737 were on
different frequencies and were therefore unaware of the potential
conflict. Neither crew was given a traffic advisory despite the
STCA alarm.

Both controllers used effective scanning and monitoring
techniques which enabled timely detection and resolution of the
conflict before there was an infringement of separation
standards.

An Airbus Industries A330 (A330) aircraft was en-route from
Melbourne, Vic. to Denpasar, Indonesia. The Yarowee sector air
traffic controller had issued the crew of the A330 with a clearance
to climb to flight level (FL) 340. The A330 was on a crossing
flight path with a Boeing 737 (737) that was en-route from
Adelaide, SA to Melbourne, Vic. at FL310. The 737 was under the
control of the adjacent Canty sector controller. Both the Yarowee
and Canty sector controllers had recognised that there was a
potential confliction between the two aircraft. The controllers
discussed the potential confliction at 1119 EST and estimated that
the aircraft would pass close to the boundary between the two
sectors. The Yarowee controller accepted the responsibility for
separation. That controller reported a reasonably low traffic
level, which allowed him the opportunity to monitor the separation
between the two aircraft. To ensure that the minimum horizontal
separation standard distance of 5 NM was maintained, the Yarowee
controller advised the Canty controller that the A330 would travel
to the left of the proposed track and pass to the west of the
737.

Subsequent to that decision, the Canty controller reassessed the
separation between the two aircraft and concluded that the
potential confliction would occur inside the Canty sector. At 1121
the Canty controller instructed the Yarowee sector controller to
assign the A330 crew FL300 on climb. Maintaining that altitude
would ensure that the vertical separation minimum of 1,000 ft would
not be infringed. The Yarowee controller entered FL300 into The
Australian Advanced Air Traffic System (TAAATS) as the amended
cleared flight level (CFL) for the A330. That entry changed the
colour of the CFL in the label of the A330 that was displayed to
the Yarowee controller on his air situation display. According to
TAAATS procedures, the different colour provided a memory prompt to
the Yarowee controller indicating that he had not received a
correct amended CFL read back by the crew of the A330. Normally,
once a controller had assigned a CFL to a crew and received a
correct read back of that CFL, the controller would cancel the
prompt and the CFL in the aircraft label would return to its
regular colour.

There was a distraction at a neighbouring console just after the
Yarowee controller had entered FL300 into TAAATS but before he had
assigned the crew of the A330 the amended CFL. When the attention
of the Yarowee controller returned to the Yarowee air situation
display, the cleared flight level for the A330 was highlighted to
prompt him to confirm that FL300 had been issued to the A330 crew.
The controller believed he had already issued the crew of the A330
the amended CFL and that he had received a correct read back of the
amended CFL from the crew prior to the distraction. The Yarowee
controller cancelled the prompt. Analysis of the voice recording
confirmed that the crew of the A330 had not been issued FL300.

The Yarowee controller incorrectly believed the A330 crew had
been assigned FL300 and consequently that a vertical separation
standard of 1,000 ft had been established between the A330 and the
737. At 1127, the controller approved the A330 crew to track direct
to Woomera. That action placed the two aircraft on crossing flight
paths with no prescribed lateral separation.

At 1131, as the A330 approached the Yarowee sector boundary, the
Yarowee controller transferred jurisdiction of the A330 to the
Canty controller. The Canty controller accepted jurisdiction of the
A330 at FL295 and waited for the A330 to call on the Canty
frequency.

At 1132, the Canty controller received a TAAATS Cleared Level
Adherence Monitor (CLAM) alarm when the A330 passed FL303 on climb
to FL340. A short time later, when the A330 and the 737 were 12.5
NM apart, both the Canty and the Yarowee controllers received a
Short Term Conflict Alert (STCA) alarm from TAAATS. This alarm
indicated to the controllers that the two aircraft were in
potential conflict. Vertical separation reduced to 900 ft while
horizontal separation reduced to 7.5 NM. That horizontal distance
exceeded the minimum separation standard. There was no infringement
of separation standards.

At the time of the CLAM and STCA alarms, the crew of the A330
was still on the Yarowee sector frequency. The Yarowee controller
reported observing the A330 pass FL303 and resumed jurisdiction
over the A330. That controller then queried the A330 crew about
their assigned flight level. He did not pass traffic information to
the crew of the A330 about the 737 because he did not believe there
would be an infringement of separation standards. Nonetheless, he
subsequently issued a turn instruction to the A330 crew to ensure
that separation was maintained. The A330 crew acknowledged traffic
following that controller's query regarding their CFL.

The Canty controller was unable to contact the crew of the A330.
The controller was aware that the A330 had climbed through what the
controller believed was the assigned level of FL300 and issued turn
instructions to the 737 crew to ensure that separation was not
infringed. The Canty controller did not inform the 737 crew of the
reason the vector was issued, however, the 737 crew subsequently
acknowledged sighting the A330.

The Canty sector was combined with a low-level sector at the
time of the incident and the controller reported that there was a
medium traffic level at the time of the occurrence.

The Yarowee controller had recently returned from leave. He had
completed the three required familiarisation shifts, had a two-day
break, and completed one unsupervised shift during the day prior to
the incident shift.