The Australian Transportation Safety Bureau (ATSB) has published its final report into a loss of separation incident involving two Qantas Airways Airbus A330-200s on 20 September 2013, and the failure of one aircraft’s traffic collision avoidance system (TCAS).
The report corroborates details of the incident initially published in the preliminary report in October 2013 about the “serious incident,” which occurred at midday 17km west of Adelaide. It also lists a number of remedial actions that were subsequently undertaken.
Remedial actions included an Airservices Australia analysis of route structures in the country with an eye to assessing areas of the greatest risk, examining ways to reduce the impact of human error, and a look at emerging risks presented by traffic growth. Airservices is also looking at opportunities for better route management provided by technologies such as automatic dependent surveillance broadcast (ADS-B) and the Global Navigation Satellite System.
After analysis of VH-EBO’s TCAS system, the manufacturer was unable to determine the cause of the failure. It deemed the system’s failure as a one-off “unique event,” and said that it is not aware of other, similar occurrences. The TCAS system aboard the A330 is produced by US firm Honeywell.
During the incident, an air traffic control (ATC) operator erred in allowing the west-bound aircraft, VH-EBO, to climb from 38,000 to 40,000 feet, creating a loss of separation with an east-bound aircraft, VH-EBS, which was at 39,000 feet.
The crew of EBS received a TCAS warning and ascended. The ATC computer system also alerted the controller to the loss of separation, and he contacted EBO accordingly. The crew of EBO, however, received no TCAS warning, as its unit not function until the aircraft landed at Perth airport hours later.
Data from both aircraft show the minimal vertical separation was 650 feet with the two aircraft 4.1nm apart laterally. Minimal lateral separation occurred 14 seconds later, when the two A330s were 1.6nm apart with a vertical separation of 870ft.
The ATSB says the incident underlined the importance of redundant systems as a defence against safety incidents.
“The malfunction of EBO’s TCAS removed the effectiveness of that defence in this occurrence,” says the ATSB. “However, this was a very rare occurrence and as EBO’s transponder was functioning correctly, the aircraft remained an eligible and identifiable target for the defences provided by other aircraft’s TCAS and the ATC computer system’s conflict alerting function.”