Loss of separation assurance involving a Boeing 737, VH-XZA and a Fairchild SA227, VH-ANW, near Darwin Airport, Northern Territory, on 2 June 2014

On 2 June 2014, at about 1200 Central Standard Time (CST), the approach controller at Darwin Airport, Northern Territory was processing the arrival of a Qantas Boeing 737 aircraft, registered VH-XZA (XZA), and an Airnorth Fairchild SA227, registered VH-ANW (ANW). When about 34 NM south-east of Darwin on a standard arrival route, XZA was cleared by the approach controller to descend to 3,000 ft for an approach to runway 11.

About 3 minutes later, when about 34 NM SE of Darwin, ANW was cleared by the approach controller to descend to 3,000 ft. This resulted in a loss of separation assurance as both aircraft were at a similar distance, tracking for runway 11, assigned the same altitude, with no assurance that vertical or radar separation would be maintained.

The approach controller then handed over to another approach controller, explained that both aircraft were on descent to 3,000 ft, and advised the incoming controller to monitor the situation.

When ANW was about 19 NM from the airfield, the controller instructed the pilot to turn left onto a heading of 360° and about 20 seconds later advised the pilot of ANW that relevant traffic was a Qantas 737, about 6 NM ahead, and to report sighting that aircraft.

The controller then received a ‘predicted conflict alert’ (PCA) on their situation data display. The pilot of ANW then reported having the 737 in sight and the controller instructed the pilot of ANW to follow the 737 and cleared ANW for a visual approach to runway 11.

When the PCA sounded, about 1,300 ft of vertical separation and 4.5 NM laterally existed between the two aircraft. As the radar separation standard of 3 NM laterally and 1,000 ft vertically applied at the time, a loss of separation between the aircraft did not occur.

This incident highlights the importance of having tactical separation assurance in place at all times.

Aviation Short Investigations Bulletin - Issue 34