Two Boeing 737 jets hurtling towards each other on the same flight path came too close after civilian and military air traffic controllers struggled to co-ordinate a last-minute change due to weather.
The October, 2018, incident saw a Qantas 737 inbound to Brisbane from Melbourne on a reciprocal track to an outbound Virgin flight headed to the Queensland leisure destination of Proserpine.
The jets breached separation standards when they came within 2.1 nautical miles of each other horizontally and 650ft vertically.
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The minimum lateral separation allowed is three nautical miles and aircraft are supposed to maintain a vertical separation of 1000ft.
An investigation by the Australian Transport Safety Bureau found a combination of factors led to the breach, including thunderstorms and a global parachuting championship that prompted Airservices Australia to reconfigure its Brisbane control center.
The Qantas aircraft was descending through military-controlled airspace near Amberley Air Force Base while the departing Virgin 737 was being managed by the civilian controllers at the Airservices Brisbane terminal control unit.
Controllers in the two systems are currently required to share information manually while a joint system is under development.
The thunderstorms meant the Virgin plane was flying radar headings assigned by Brisbane air traffic control rather than using a standard instrument departure.
The Brisbane departures controller identified the Virgin plane as it approached Amberley airspace and was told by the military controller of the inbound Qantas jet.
Brisbane replied that the Virgin aircraft would soon be turning right, and this meant it would avoid Amberley airspace
However, when advised to turn right, the crew of the Virgin aircraft asked to continue on their current heading for another 70 or 80 nautical miles due to the weather.
This led to the Virgin jet entering Amberley airspace without a hand-off from Brisbane and operating on a different radio frequency to the military controller and the Qantas 737.
Compounding the problem was the fact that Airservices had not advised Amberley it had changed the terminal control unit configuration as a result of the parachuting event being put on hold.
This caused a 17-second delay in Amberley ATC being able to contact the correct controller during which time the distance between the planes reduced by 6kms.
“This delay in communication between Amberley and Brisbane ATC reduced the amount of time available to recover the impending loss of separation while the aircraft were closing on reciprocal tracks in opposite directions,” the report said.
“If this configuration change was immediately relayed to Amberley ATC, it would have allowed the Amberley approach controller more time to resolve the impending conflict between the two aircraft, and separation requirements may have been maintained.”
Once communication was established, the Virgin aircraft was transferred to the Amberley frequency and the two aircraft were diverted away from each other.
Investigators noted both aircraft were fitted with traffic collision avoidance systems that would have allowed pilots to resolve the conflict had ATC not done so.
The incident resulted in both Brisbane and Amberley ATC taking steps to improve communication between their two systems.
“This investigation highlights the importance of clear communication and coordination between air traffic controllers operating in different, yet immediately adjacent airspace, and the need for a clear understanding of the responsibility for separation assurance, especially when operating without a shared traffic picture,” ATSB director transport safety Dr Stuart Godley said.
“The ATSB welcomes the new dedicated communications pathway between the Amberley approach and Brisbane departures south positions, and the implementation of an airspace release that controls the risk that short notice deviations present across the two non-linked systems.”
Dr Godley said the successful recovery of separation also illustrated the effectiveness of the conflict resolution training received by air traffic controllers in loss of separation events.
