A near-miss between two Qantas planes would have been even closer had the pilots waited for a confused RAAF air-traffic controller to respond to the collision alarm, a safety report has found.
The two Boeing aircraft came too close in October 2012 while flying near the shared civil-military airport at Darwin.
Due to a transponder coding mistake, the RAAF approach controller was unable to properly track the 717 twin-engine on the radar.
That meant when a predicted collision alarm went off, the RAAF controller had "a significant level of confusion" as to the most reliable information - resulting in a 15 second delay.
Fortunately the pilots of the 717 had already identified the potential threat from their own computer systems.
After visually spotting the approaching 737, the 717 pilots "proactively limited their aircraft's descent" to Darwin airport and informed the air-controller - who advised them to stay at 10,000ft.
Unable to pull out of its climb from Darwin airport with such limited notice, the 737 maintained its course for Melbourne.
Just 21 seconds after the 717 altered its descent the 737 passed 900ft "directly" underneath, less than the 1000ft minimum separation requirement.
An Australian Transport Safety Bureau report on Thursday found that "overall there was minimal risk of collision" due to the collision alarms alerting both sets of pilots.
However, the gap would have been smaller if not for the proactive response from the pilots.
The Air Force welcomed the report's recommendations regarding training systems and safety issues. It advised in a statement that it had already started implementing them.
At the time, the RAAF controller was temporarily suspended from duty pending investigations. An update has been sought from Defence.
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