On 30 August 2013, a loss of separation assurance occurred between an Airbus A320, registered VH‑VNQ (VNQ), and a Boeing 737, registered VH‑VZB (VZB), 50 km SSE of Hay airport, NSW. VNQ was conducting a passenger flight from Melbourne, Victoria to Cairns, Queensland under the instrument flight rules (IFR). VZB was also conducting an IFR passenger flight in the opposite direction, from Cairns to Melbourne.

Both aircraft were operating in Class A airspace, under the control of an Airservices air traffic controller. The required separation standard in the portion of Class A airspace covered by radar surveillance was 5 NM laterally or 1,000 ft vertically. The majority of air routes in the airspace over which the controller had jurisdiction were one-way routes – aircraft could only operate in the direction marked on the aeronautical charts, but the Melbourne to Cairns route was a two-way route (T139). The airspace also contained a number of east/west routes that crossed T139.

Flight crews were required to plan flights in accordance with levels based on the magnetic heading of the planned track. Within controlled airspace, air traffic control (ATC) may assign and pilots may request a level that does not conform to this requirement. The crew of VNQ had planned to operate at FL360, and the crew of VZB had planned to operate at FL360 to a position inland and abeam Emerald, Queensland, and then at FL370 to Melbourne. However, the change of level planned by VZB had not been initiated, resulting in both aircraft converging at the same flight level.

The controller reported that their focus was on monitoring VNQ’s climb through the levels of a number of aircraft on crossing air routes and only became aware of the aircraft converging at the same level when the Short Term Conflict Alert (STCA) activated. The controller applied compromised separation techniques so that, while there was a loss of separation assurance, radar separation was not infringed.

As a result of this occurrence, Airservices will review the Melbourne to Cairns air route with regard to creating one-way routes, and more generally review similar routes nationally. In addition, Airservices has issued a directive reminding controllers of their responsibilities regarding the application of non-standard levels and subsequent return to standard levels.

In this incident, the timely activation of the STCA and the controller correctly utilising compromised separation techniques ensured that the separation standards were not infringed.


Aviation Short Investigation Bulletin - Issue 26