Two Qantas jets carrying hundreds of passengers came within 215m of colliding over the Great Australian Bight yesterday, sparking a major investigation.
Qantas flight 576 was heading from Perth to Sydney at 39,000ft when the Airbus A330's pilots received an alert from the plane's traffic collision avoidance system.
It climbed suddenly to avoid Qantas flight 581, which had been given an instruction from air traffic control to climb.
Flight 581, also an A330, was heading from Sydney to Perth at 38,000ft when its pilots asked for and received permission to climb.
That permission was given by Airservices Melbourne Air Traffic Control centre but was rescinded shortly afterwards when the error was apparently detected, but not before the TCAS alerts were activated.
Flight 576 passenger Graeme Vincent said the pilots applied "quite extreme power" to the engines to climb away from a possible collision.
"I knew something was wrong and I was quite nervous," Mr Vincent said.
Passengers on the other plane did not notice anything unusual.
Qantas confirmed the incident and said its pilots followed ATC instructions.
Qantas chief pilot Phil Green said the airline was "full of praise for the pilots involved".
"These pilots have years of experience and handled the situation exactly as they have been trained to," Capt. Green said.
The Australian Transport Safety Bureau called the near miss serious and has started an investigation.
The incident was about 19km west of Adelaide at 10.13am.
Airservices confirmed that flight 581 "was cleared to climb from 38,000ft to 40,000ft and the aircraft commenced the climb".
"Soon after, the controller cancelled the clearance and the aircraft descended back to 38,000ft," Airservices told the ATSB.
The investigation will review and analyse recorded radar and audio data and air traffic control procedures.
Air traffic controllers and flight crew will be interviewed and it is expected the investigation will be completed by September next year.
Loss of separation of aircraft is common because of either pilot or air traffic control error.
However, the development of TCAS has provided an almost fail-safe backup for pilots in such circumstances.TCAS is fitted to all commercial passenger planes to monitors other nearby flights, which are shown on the pilots' primary flight displays.