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.