Australian authorities were Saturday investigating how two Qantas passenger planes got so close that it triggered a collision warning alert, forcing them to take evasive action.
The incident happened over southern Australia on Friday and involved two Airbus A330s travelling in opposite directions between Sydney and Perth.
The Australian Transport Safety Bureau (ATSB), which is investigating, said early reports were that the eastbound plane was cruising at 39,000 feet when the westbound plane was given permission to climb from 38,000 feet to 40,000 feet, triggering one of the plane's traffic collision avoidance systems.
"One of the aircraft, the lower one, asked for clearance to climb; the air traffic controller gave that clearance," Richard Woodward, vice-president of the Australian and International Pilots Association, told the ABC.
"And as they started the climb, they got a collision warning alert from the aircraft and they took evasive action."
Woodward, a Qantas pilot, said the incident was worrying because the collision warning system was "sort of the last line of defence".
"At that altitude and that speed it's very difficult for the crews to see and take avoiding action. The aircraft's doing a kilometre every two seconds or so when they're closing on each other like that," he said.
Qantas said that a "loss of separation", the technical term when planes breach the distance they are required to maintain between each other, happened near Adelaide.
"Indications are that the loss of separation occurred when one of the Qantas aircraft received clearance to climb from air traffic control," the airline said in a statement on Friday.
"Our pilots followed standard operating procedures in re-establishing the required separation distance following the alert from the onboard notification system. There was no impact to passengers."
One passenger told the ABC after arriving in Perth: "I saw a plane going underneath us to the left, somewhere in South Australia I'd suggest.
"I don't think anybody knew, I saw it and didn't care. They know what they're doing".