Update 26 February 2013
On 2 October 2012, a loss of separation occurred between a Boeing 717 (717) aircraft, registered VH-NXQ and operating a scheduled passenger service from Alice Springs to Darwin, Northern Territory, and a Boeing 737 (737), registered VH-VXM, operating a scheduled passenger service from Darwin to Melbourne, Victoria. The aircraft were under the jurisdiction of military air traffic control (ATC) at the time of the occurrence.
At 1338 Central Standard Time, Brisbane Centre handed over control of the 717 to Darwin ATC. The crew had been cleared to descend from flight level (FL) 320 to 10,000 ft and track direct to Darwin for runway 29.
At about this time, the call sign label for the 717 was assigned to another aircraft in the Australian Defence Air Traffic System (ADATS) and displayed on the Approach controller’s radar display (as ‘NXQ’). The other aircraft was overflying the airspace at FL 260 and not on the Approach controller’s frequency, or under approach control at the time. The actual radar return for the 717 was displayed on the controller’s display as an unlabelled track.
At 1341, the 737 departed runway 11 on climb to FL 130 on a heading of 170 degrees to separate it from another aircraft. The departure was the last movement on runway 11 before a switch to runway 29.
At 1342, the 717 was passing FL 112 and the crew requested further descent. The Approach controller queried the aircraft’s flight level and the crew advised they were at 10,500 ft. The controller responded that their displayed altitude was FL 260. At that stage, the controller thought that the 717 was the aircraft incorrectly displayed as NXQ and located to the south-west of Darwin at FL 260. The flight crew were requested to maintain 10,000 ft and to recycle their transponder. After further communications, the controller cleared the crew to descend to 7,000 ft. At 1344:01 the crew advised that that they were 18 NM (33 km) from Darwin and leaving 10,000 ft.
The traffic confliction was detected by the 717’s flight crew and they did not initiate their descent from 10,000 ft. Concurrently with the activation of ADATS’ predictive conflict alerts, the Approach Supervisor identified the conflict and directed the Approach controller to commence separation recovery procedures. At 1344:20, the Approach controller instructed the 737 crew to stop their climb at 9,000 ft. At that stage the aircraft was displaying 8,400 ft and climbing. At 1444:32 the crew of the 717 advised the controller of conflicting traffic below them and the controller instructed them to stop descent.
Radar data indicated that the 737 climbed to 9,100 ft before descending back to 9,000 ft and the 717 remained at 10,000 ft. The separation reduced to about 900 ft vertically as the 717 passed directly overhead the 737 on a reciprocal track. The required separation standards were either 1,000 ft vertical separation or 3 NM (5.56 km) radar separation.
ATSB investigators have interviewed Darwin air traffic controllers and conducted a site visit to Darwin air traffic control. The investigation is continuing and will include:
- further analysis of the radar and audio data
- review of the functionality of ADATS and related systems, including the messaging interface with the civilian air traffic computer system
- review of the workspace equipment layout for control positions in Darwin
- review of air traffic control procedures, documentation and training.
The ATSB aims to finalise this investigation in October 2013.
The information contained in this web update is released in accordance with section 25 of the Transport Safety Investigation Act 2003 and is derived from initial investigation of the occurrence. Readers are cautioned that new evidence will become available as the investigation progresses that will enhance the ATSB’s understanding of the occurrence as outlined in the web update. As such, no analysis or findings are included in this update.
Update 11 October 2012
The call sign label for a Boeing 717 aircraft inbound to Darwin was assigned to another aircraft in the air traffic computer system and displayed on the approach controller’s radar display. The other aircraft was overflying Darwin and not on the approach controller’s frequency, or under approach control at the time. The actual radar return for the 717 was displayed on the controller’s display as an unlabelled track. Based on the displayed information, the 717 was cleared to descend through the level of a Boeing 737 that was outbound from Darwin.
Preliminary investigations indicate that the incorrect call sign label assignment was identified by the approach controller, who commenced a compromised separation recovery procedure that placed level restrictions on both aircraft.
The investigation is continuing.
2 October 2012
A Boeing 717 inbound to Darwin was cleared to descend through the level of an outbound Boeing 737 near Darwin. ATC subsequently cancelled the clearance and there was no breakdown of separation standards however a loss of separation assurance occurred. The investigation is continuing.
