Qantas says two jets that had a near miss over Adelaide were never on a crash course, despite coming within 3km of each other and a collision avoidance system on one of the planes not working.
The Australian Transport Safety Bureau (ATSB) began an investigation into the September 20 incident after a "loss of separation" between Airbus A330 VH-EBO (EBO), travelling from Sydney to Perth and Airbus A330 VH-EBS (EBS), travelling from Perth to Sydney.
EBS was cruising at 39,000 feet when the EBO pilot was cleared to climb to 40,000 feet, but the controller soon cancelled the clearance and the aircraft descended back to 38,000 feet.
The EBS crew received an alert from their traffic collision avoidance system but EBO's flight crew did not receive any indication on their system about EBS, the ATSB said.
Data from the two aircraft showed the minimum vertical separation was 650 feet when they were 8km apart laterally.
The minimum lateral separation was 3km when the aircraft were 870 feet apart vertically.
A Qantas spokesman said the traffic collision avoidance system failure mid-flight was not a contributing factor in the near miss because planes could function for a short time without it, due to other safety measures built into aviation systems.
"The aircraft were on diverging one-way routes and were never going to cross paths," he said.
Qantas also said the aircraft was compliant with Civil Aviation Safety Authority (CASA) regulations and it continued to provide accurate positional information to air traffic control and the other aircraft.
The faulty system has also been replaced.
ATSB's interim report noted that although all air transport aircraft were required to have a traffic collision avoidance system, it rarely failed.
In such situations, a flight crew is usually provided with a fault message, and the flight crew must advise the controller.
The ATSB investigation is continuing and will include examining radar and audio data, analysing the controller's actions, examining the traffic collision avoidance system computer and related components from EBO, and a review of the reliability and availability rates of traffic collision avoidance systems.The ATSB expects to complete its investigation by September 2014.