Overview of the investigation
Just after midnight on 5 April 2023, a Boeing 737-838, registered VH‑VXH, was being operated by Qantas on a passenger air transport flight from Darwin, Northern Territory to Melbourne, Victoria. An Airbus A320, registered VH-VGV, was being operated by Jetstar on a passenger air transport flight from Sydney, New South Wales, to Darwin.
A temporary restricted area (TRA) was in place between 2130–0530 each night due to Royal Australian Air Force (RAAF) air traffic control services not being available in the airspace around Darwin Airport below 8,500 ft. As a result, aircraft flight crews were required to coordinate their own separation when arriving and departing the airport. Flight crews were also required to gain permission from the RAAF Darwin flightwatch prior to operating within the TRA. With the TRA in place, Airservices Australia was responsible for managing the airspace above 8,500 ft through its Brisbane Centre.
At 0009:56, the 737 first officer (FO) contacted the Darwin flightwatch and received permission to operate within the TRA. Darwin flightwatch also advised of opposite direction traffic inbound to Darwin (this referred to the A320 and another subsequent flight).
At 0013:43, the 737 FO contacted Brisbane Centre and advised that the aircraft was taxying at Darwin Airport and would depart using runway 11. The Brisbane Centre controller acknowledged the FO’s radio call and advised of the 2 inbound aircraft, giving details of the arrival routes (both were VEGPU SEVEN standard arrivals from the south-east and landing on runway 29). The A320 was due to land at Darwin Airport at about 0027:00.
The flight crew of the 737 recalled considering the inbound traffic and believed there was sufficient spacing to safely depart ahead of the A320 using runway 11 (the reciprocal to runway 29).
At 0016:16, the 737 flight crew commenced the take-off from runway 11 and, at 0018:04, the captain made a departure call on the Darwin flightwatch frequency. Shortly after, the Darwin flightwatch controller queried when the 737 would commence a turn and stated the A320 was now 30 NM to the south. At 0018:37 the 737 captain responded they were conducting the PALGA SEVEN standard instrument departure and they would contact Brisbane Centre shortly to get a clearance to enter the overlaying airspace.
At 0019:22, and climbing through 4,500 ft, the 737 captain contacted the A320 flight crew directly on the Darwin flightwatch frequency and offered to level off at 7,000 feet to maintain separation. By this time, the flight crew of the A320 had the 737 displayed on the aircraft’s Traffic Collision Avoidance System (TCAS) and believed the aircraft did not present a conflict. At this time, the aircraft were about 26 NM apart on converging courses, with the A320 at 8,500 ft. The A320 FO replied on the Darwin flightwatch frequency that the 737 could continue the climb.
At 0019:42, the 737 captain contacted Brisbane Centre to advise the aircraft had departed Darwin. The Brisbane Centre controller informed the 737 captain they could expect a clearance to enter the overlaying airspace on a heading of 060° and further stated, ‘suggest left heading 060 now’. The 737 captain confirmed the heading. The 737 commenced a left turn onto a heading of 060° at about 0020:00, with the aircraft about 20 NM apart. The tracks diverged and the aircraft crossed the same level a few seconds later.
At about 0022:06, the lateral separation between the 737 and the A320 decreased to its closest point of approach of about 10.6 NM, with vertical separation reducing to about 4,500 ft (Figure 1).
Figure 1: 737 and A320 locations at 0019:46, 0021:06 and 0022:06
Source: Airservices Australia, annotated by the ATSB
The ATSB received an initial occurrence report on 5 April 2023 and commenced an investigation on the same date. As part of the investigation, the ATSB interviewed the 737 flight crew, A320 flight crew, and the Brisbane Centre air traffic controller, and reviewed:
- data from the 737 and A320 quick access recorders
- weather information
- recorded air traffic control audio and surveillance data
- information provided by the aircraft operators and air traffic control services.
The investigation identified the following:
- The practice of activating daily TRAs at Darwin Airport commenced in July 2022 due to controller availability preventing the delivery of a continuous approach control service. The RAAF advised that daily TRAs were planned to continue to be utilised at Darwin Airport until approximately the first quarter of 2024.
- Both flight crews described an increased level of workload associated with the Darwin Airport TRA that was similar in complexity to common traffic advisory frequency (CTAF) procedures.
- Although experiencing increased workload the crews felt that there was no concern regarding separation in this instance.
- The 737 flight crew elected to depart from runway 11 for noise abatement and operational reasons, believing there would be adequate separation between the 737 and the arriving A320.
- At the time the 737 departed, the automatic terminal information service for Darwin Airport stated the preferred runway was runway 29. The windspeed and direction recorded at the time was 2-3 kts from between 150–191° magnetic, which favoured runway 11.
- The A320 and 737 flight crews coordinated their separation on the Darwin flightwatch frequency and were sufficiently aware of each other’s position, level and track to maintain separation.
- Neither the 737 or A320 flight crew received a TCAS alert or advisory.
- The Brisbane Centre controller recalled having concerns about the proximity of the 2 aircraft; however, the controller recalled not knowing the actual separation between the 2 aircraft due to the scale of the display.
- The Brisbane Centre controller’s decision to turn the 737 was to provide increased separation; however, this purpose was not communicated to the 737 flight crew.
- The lateral and vertical separation between the 737 and A320 at the closest point of approach met the separation standards required had the airspace been controlled at the time. It is probable the same separation standards would have been met had the 737’s flightpath continued without the turn.
Reasons for the discontinuation
Based on a review of the available evidence, the ATSB considered that there was no safety-related separation issue in this instance and it was unlikely that further investigation would identify any systemic safety issues or important safety lessons. Consequently, the ATSB has discontinued this investigation.
The evidence collected during this investigation remains available to be used in future investigations or safety studies. The ATSB will also monitor for any related occurrences that may indicate a need to undertake a further safety investigation, noting the potential for increased risk in terminal airspace that is made non-controlled for certain periods.