A Boeing 767-338ER (B767) was maintaining flight level (FL) 370
and had been assigned FL330 to maintain separation with a Cessna
Citation 500 (C500), maintaining FL310, that was crossing the track
of the B767. The controller entered FL330 into The Australian
Advanced Air Traffic System (TAAATS). He subsequently, and
unintentionally, assigned the crew of the B767 descent to FL300.
The controller received a cleared level adherence monitor alarm
(CLAM) when the B767 descended through FL326. Vertical separation
between the B767 and the C500 reduced to 700 ft, and horizontal
separation reduced to 3.4 NM. The required separation standard was
2,000 ft or 5 NM. There was an infringement of separation
standards.
The controller had initially cleared the crew of the B767 to
descend from FL370 to FL330. The descent was to be commenced at the
discretion of the crew. He then entered FL330 into TAAATS as the
new cleared flight level (CFL). The controller reported that he had
made the necessary TAAATS entries on receipt of the correct level
read back from the crew of the B767. There were no subsequent
TAAATS entries required in relation to FL330 being assigned to the
crew of the B767.
The B767 crew reported leaving FL370 approximately five minutes
after they had been assigned FL330. The controller reported that he
had intended to confirm FL330 as the cleared flight level with the
B767 crew at that time, but he unintentionally assigned FL300. The
crew of the B767 read-back FL300 and continued descent through
FL330. The controller did not detect from the read back that he had
assigned an incorrect flight level. There were no subsequent
opportunities for the controller to realise the error until the
CLAM alarm from TAAATS.
The controller indicated that he considered his workload at the
time of the occurrence to be light. He was responsible for two
sectors of airspace but he did not believe that the increase in
workload caused by the combination of the two sectors contributed
to the error. The replay of the voice recording indicated that the
controller had up to ten aircraft under his control at the time of
the occurrence. Three of those aircraft had requested a shorter
route. The controller accommodated the requests because some of
those aircraft were involved in bush fire fighting operations. When
the routes for those aircraft were amended, the associated flight
data record in TAAATS also needed to be amended and the changes
needed to be coordinated with adjacent sectors. The controller did
not believe the extra workload generated by those tasks contributed
to the occurrence.
The controller was also the team leader on the shift. He
reported that there were no distracting team leader issues at the
time of the occurrence. The controller did not believe he was
fatigued.
