Sequence of events
On 3 June 2004, a Boeing 737-376 (737) registered VH-TJD was
tracking on a standard arrival route, descending on the downwind
leg for runway 34 at Melbourne Airport. Air traffic control (ATC)
had issued the crew with a clearance to descend to 6,000 ft. At the
same time, a de Havilland Dash 8 (Dash 8) registered VH-WZI was
departing from runway 34, on a standard instrument departure on
climb to 5,000 ft. As the 737 levelled at 6,000 ft and the Dash 8
was approaching 5,000 ft, the Dash 8 crew advised ATC that they had
received a brief traffic alert and collision avoidance system
(TCAS) resolution advisory (RA). The advisory was to climb the
aircraft. Before the crew could react, the TCAS indicated that the
confliction had been resolved. At the time of the TCAS RA, the
aircraft tracks had already crossed and the aircraft were
diverging.
The ATSB examined the recorded ATC radar data and the recorded
flight data from both aircraft. The data indicated that the minimum
lateral and vertical distances between the aircraft were 0.5 NM and
1,300 ft respectively. The required separation standard was either
3 NM or 1,000 ft. There was no infringement of separation
standards.
The TCAS manufacturer suggested that the occurrence was
indicative of a known problem with TCAS, termed `bump up'. When one
aircraft is climbing and approaching a specific level 1,000 ft
below another aircraft that is already level, or if an aircraft is
descending to and approaching a level 1,000 ft above another
aircraft that is already level, `bump up' may trigger a TCAS
advisory.
The TCAS tracks intruder aircraft and uses the data calculation
to determine the appropriate collision avoidance advisory
information. The manufacturer reported that the possibility of an
RA depends on the timing of the level-off and the relative distance
and vertical rates at that time. The manufacturer also advised that
the latest upgrade to the TCAS logic had reduced, but not
eliminated, `bump-up' occurrences.
Examination by the ATSB and the manufacturer of the actual
separation between the aircraft did not reveal any reason a TCAS RA
should have been issued. The reason the Dash 8 crew momentarily
received a climb advisory when they were climbing towards the level
of the 737 and diverging from it could not be determined.