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What happened

At 1345 Central Standard Time on 2 October 2012, a loss of separation (LOS) occurred between a descending Boeing 717 aircraft, registered VH-NXQ (NXQ), operating a scheduled passenger service from Alice Springs to Darwin, Northern Territory, and a climbing Boeing 737, registered VH-VXM (VXM), operating a scheduled passenger service from Darwin to Melbourne, Victoria. The LOS occurred about 14 NM (26 km) south of Darwin, and the aircraft were under the jurisdiction of Department of Defence air traffic control (ATC) at the time of the occurrence.

Prior to the LOS, a predicted conflict alert was activated within the Australian Defence Air Traffic System (ADATS). After a short delay, the Approach controller instructed VXM’s flight crew to stop their climb at 9,000 ft. NXQ’s flight crew advised the controller of conflicting traffic below them and the controller instructed them to maintain 10,000 ft. Separation between the aircraft reduced to about 900 ft vertically as NXQ passed directly overhead VXM on a crossing track. The required separation standards were either 1,000 ft vertical separation or 3 NM (5.6 km) radar separation.

What the ATSB found

The ATSB determined that an already-assigned transponder code was allocated to the 717 in ADATS, which resulted in the 717’s call sign being incorrectly correlated in ADATS to an overflying aircraft that was in the general proximity of the 717. Manual processes to check the assigned transponder code with the code listed in ADATS were not conducted effectively. Due to local contextual factors and confirmation bias, the Darwin Approach controller and Approach Supervisor assumed that the radar return labelled as NXQ was correct, and they did not identify the error until after the conflict alert activated.

The ATSB identified safety issues relating to the Department of Defence’s (DoD’s) risk controls for ensuring transponder code changes were processed correctly, the expectancy in the Darwin approach environment about the relevance of radar returns with a limited data block, the risk assessment and review processes for the introduction of new equipment, and refresher training for compromised separation recovery actions.

What's been done as a result

The DoD issued a Safety Advisory to highlight to controllers the importance of the appropriate and timely actioning of all messages sent to the ADATS Problem Message Queue, for Planner controllers to confirm that correct transponder codes are allocated in the ADATS flight plan and to reinforce to controllers to take immediate action on all conflict alert and predicted conflict alert alarms. Following a September 2013 DoD review of the Comsoft Aeronautical Data Access System and its associated impact on the Planner role, Flight Data Operators have been introduced at a number of Defence air traffic control establishments to reduce workload in the Planner position.

The ATSB is not satisfied that the DoD has adequately addressed the safety issues regarding the provision of refresher training to air traffic controllers for the scanning of green radar returns and in compromised separation recovery requirements and techniques. As a result, the ATSB has made formal recommendations to the DoD to take further safety action on these issues.

Safety message

The ATSB reminds operational personnel such as controllers of the problems associated with confirmation bias when dealing with unusual situations and the importance of searching for anomalous indicators in such situations. The ATSB also reminds high-reliability organisations such as air traffic services providers that, even though they may have multiple levels of risk control in place to reduce safety risk, these controls need to be regularly evaluated to ensure that they are effective.

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Safety issues

AO-2012-131-SI-01 - AO-2012-131-SI-02 - AO-2012-131-SI-03 - AO-2012-131-SI-04 - AO-2012-131-SI-05 -  

Risk controls for manual processing of transponder code changes

The Australian Defence Air Traffic System (ADATS) did not automatically process all system messages generated by The Australian Advanced Air Traffic System. In cases where transponder code changes were not automatically processed, the risk controls in place were not able to effectively ensure that the changes were identified and manually processed.

Safety issue details
Issue number:AO-2012-131-SI-01
Who it affects:All Darwin Approach Supervisor, Approach and Planner rated Joint Battlefield Airspace Controllers
Status:Adequately addressed


 

Controller scan of green radar returns

Darwin Approach controllers were routinely exposed to green (limited data block) radar returns that were generally inconsequential in that Approach control environment, leading to a high level of expectancy that such tracks were not relevant for aircraft separation purposes. Refresher training did not emphasise the importance of scanning the green radar returns. 

Safety issue details
Issue number:AO-2012-131-SI-02
Who it affects:All Darwin Approach Supervisor, Approach and Planner rated Joint Battlefield Airspace Controllers
Status:Adequately addressed


 

CADAS risk assessment and review process

The Department of Defence’s risk assessment and review processes for the implementation of the Comsoft Aeronautical Data Access System and removal of the flight data position did not effectively identify or manage the risks associated with the resulting increased workload in the Darwin Approach environment, in particular with regard to the Planner position.

Safety issue details
Issue number:AO-2012-131-SI-03
Who it affects:All Darwin Approach Supervisor, Approach and Planner rated Joint Battlefield Airspace Controllers
Status:Adequately addressed


 

Long-range display effectiveness

The Darwin Approach long-range display was a low resolution screen that presented air traffic control system information with reduced clarity and resulted in it having diminished effectiveness as a situation awareness tool.

Safety issue details
Issue number:AO-2012-131-SI-04
Who it affects:All Darwin Approach rated Joint Battlefield Airspace Controllers
Status:Adequately addressed


 

Compromised separation recovery refresher training

The Department of Defence had not provided Darwin-based controllers with regular practical refresher training in identifying and responding to compromised separation scenarios.

Safety issue details
Issue number:AO-2012-131-SI-05
Who it affects:All Darwin-based Joint Battlefield Airspace Controllers
Status:Adequately addressed

 
General details
Date: 02 Oct 2012 Investigation status: Completed 
Time: 1445 CST Investigation type: Occurrence Investigation 
Location   (show map):near Darwin Airport Occurrence type:Loss of separation 
State: Northern Territory Occurrence class: Airspace 
Release date: 02 Oct 2014 Occurrence category: Incident 
Report status: Final Highest injury level: None 
 
Aircraft 1 details
Aircraft manufacturer: The Boeing Company 
Aircraft model: 717 
Aircraft registration: VH-NXQ 
Serial number: 55097 
Type of operation: Air Transport High Capacity 
Sector: Jet 
Damage to aircraft: Nil 
Destination:Darwin Airport, NT
Aircraft 2 details
Aircraft manufacturer: The Boeing Company 
Aircraft model: 737 
Aircraft registration: VH-VXM 
Serial number: 33483 
Type of operation: Air Transport High Capacity 
Sector: Jet 
Damage to aircraft: Nil 
Departure point:Darwin, NT
Destination:Melbourne, VIC
 
 
 
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Last update 20 July 2015