Aircraft seperation - Boeing 737-476, VH-TJT and Cessna C500, 324 km NNE Melbourne, 8 July 2002

A Boeing 737-476 (737) was en route from Melbourne to
Coolangatta tracking northeast at flight level (FL) 310. A Cessna
Aircraft Company C500 (Citation) was tracking southwest from
Coolangatta to Essendon at the same level. The aircraft were on
reciprocal tracks and the air traffic controller did not provide
any assurance of separation between the two aircraft. The rate of
closure of the aircraft was later calculated to be in excess of 800
kts. When the distance between the two aircraft was approximately
22 NM, the controller instructed the crews of both aircraft to
alter heading and flight level. The controller's intervention
ensured that a separation standard was established and maintained.
The radar system's short-term conflict alert (STCA) activated.
While the horizontal radar separation standard of 5 NM was never
infringed, vertical separation, which assured separation for
aircraft on reciprocal tracks, was not established until 25 seconds
prior to the aircraft passing.

The 737 was being operated as a regular public transport flight
and was flight planned at FL350. That aircraft departed Melbourne
at 09:37 Eastern Standard Time and tracked via air route H66. That
air route passed through the Benalla sector controller's area of
responsibility. The Benalla sector controller was located in the
Melbourne Air Traffic Control Centre. The crew of the 737 had
requested FL290 on first contact with the Benalla sector
controller, which was approved. At 09:52:51, the crew requested
climb to FL310. The controller assigned FL310 at 09:55:59 and the
recorded radar data indicated the aircraft reached that level at
09:57:43.

The Citation was operating a private flight at FL310 and
departed Coolangatta at 08:16. Flight level 310 was a non-standard
level for the track flown. Cruising levels were published in the
Manual of Air Traffic Services (MATS) and the Aeronautical
Information Publication (AIP). MATS also stated that aircraft
operating at other than a level conforming to the table of cruising
levels for the particular direction of flight or notifying
intention to cruise at such a level, shall be advised accordingly
and the pilot's intention sought. Non-standard levels were
regularly used by air traffic control for maximum utilisation of
available airspace.

The Citation pilot had flight planned to track via Parkes and
TAREX, which was a non-standard route. The AIP stated that prior
air traffic control approval was required for RNAV [area
navigation] tracking on routes other than those published in AIP.
An aircraft tracking from Coolangatta to Essendon would normally
have tracked via air route Q94 and would have remained clear of the
Benalla sector airspace. The pilot of the Citation had not received
prior air traffic control approval to plan and conduct the flight
via a non-standard route.

At 09:41:22, the crew of the 737 established radio contact with
the Benalla sector controller. The controller was managing the
combined airspace sectors of Eildon Weir and Benalla. The 737 crew
was on climb FL200 and was recleared to the requested level of
FL290. The controller was expecting the crew of the 737 on
frequency and had earlier anticipated that the aircraft may be a
problem at the planned flight level. However, at FL290 it would be
separated from other aircraft.

Due to confusion caused by an anomaly with another aircraft, the
Benalla controller was required to complete a significant period of
coordination with another sector controller. The Benalla controller
was discussing that problem at the console with the team leader
when the crew of the 737 again made radio contact at 09:52:51. They
reported maintaining FL290 and requested climb to FL310. The
controller acknowledged the request and annotated the flight track
with that request. The controller continued his discussion with the
team leader and did not immediately coordinate the higher level
request with the adjacent Parkes Sector.

At 09:53:37, the controller designated and accepted jurisdiction
on the radar display of the Citation's flight track. That was
before the pilot of the Citation established radio contact at
09:54:18 and reported maintaining FL310. The controller
acknowledged that transmission and then transferred jurisdiction of
two other aircraft to the Parkes sector controller.

At 09:55:56, about three minutes after receiving the 737 crew's
change of level request, the controller coordinated the 737 with a
request for FL310 with the Parkes sector controller. The Parkes
sector controller concurred with the higher level and the Benalla
controller cleared the 737 crew to climb. The controller did not
recognise the potential conflict between the 737 and the Citation
and continued his discussion with the team leader.

At 09:59:55, the controller activated the "individual quick
look" function for the flight plan tracks of both aircraft. Almost
simultaneously, the controller commenced action to ensure a
separation standard was maintained between the two aircraft. The
controller instructed the crew of the 737 to expedite descent to
FL290 and instructed the pilot of the Citation to turn right 30
degrees. At 10:00:15, he further instructed the crew of the 737 to
turn right 20 degrees. He also broadcast essential traffic
information to the 737 crew about the position of the Citation.
During that transmission, the short term conflict alert on the
controller's radar display activated at 10:00:28 highlighting the
close proximity of both aircraft. The traffic alerting and
collision avoidance system (TCAS) of the 737 also activated,
providing a traffic advisory warning to its crew. At 10:00:34, the
controller told the pilot of the Citation to climb to FL320. At
10:00:57, a 2,000 ft vertical separation standard was established
between the two aircraft when they were approximately 9 NM
apart.

The controller had held a full performance rating within the
group for a period just less than four years. He was completing the
third day of a 5-day roster cycle. Although the controller was
working a combined sector, the workload was considered light to
moderate.