Aviation safety investigations & reports

Breakdown of separation - VH-YVA/VH-CGF, 59 km NE Armidale, NSW, 8 October 2011

Investigation number:
Status: Completed
Investigation completed


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At 1500:51 Eastern Daylight-saving Time on 8 October 2011, a breakdown of separation (BOS) occurred 59 km north-east of Armidale, New South Wales between a Boeing Company 737-8FE (737), registered VH-YVA, and a Gulfstream Aerospace Corporation Gulfstream IV (G-IV), registered VH-CGF.

Both aircraft were under radar surveillance and subject to an air traffic control (ATC) service. The aircraft were on reciprocal tracks on air routes that intersected about 35 NM (65 km) north-east of Armidale.

The Australian Transport Safety Bureau (ATSB) established that the controller's mental model for separation correctly identified the situation and included a plan to manage the traffic. However, the instructions that were issued to the pilot of the G-IV contradicted that mental model in that the controller cleared the G-IV for descent through and below the level being maintained by the 737. The progression towards the BOS continued when the controller did not recognise the error during the G-IV pilot's read-back of the clearance.

Ultimately, the controller's earlier correct level input into The Australian Advanced Air Traffic System allowed a system alerting function to activate. In response to that alert, the controller initiated compromised separation recovery actions to recover the required separation standard.

The ATSB identified a number of human factors and individual work processes that contributed to the occurrence. In addition, a safety issue was identified in respect of differences in the traffic alert phraseology between the Manual of Air Traffic Services and Aeronautical Information Publication (AIP). These differences increased the risk of non-standard advice being provided to pilots by controllers during compromised separation recoveries.

In response to this safety issue, Airservices Australia (Airservices) amended the AIP to enhance understanding of the criticality of any safety alerts and avoiding actions being provided to flight crew. This amendment came into effect on 28 June 12.

Download Final Report
[Download  PDF: 537KB]
Alternate: [Download  DOC: 592KB]
General details
Date: 08 October 2011   Investigation status: Completed  
Time: 1500:51    
Location   (show map): 59 km NE Armidale   Investigation type: Occurrence Investigation  
State: New South Wales   Occurrence type: Loss of separation  
Release date: 31 July 2012   Occurrence class: Airspace  
Report status: Final   Occurrence category: Incident  
  Highest injury level: None  

Aircraft 1 details

Aircraft 1 details
Aircraft manufacturer The Boeing Company  
Aircraft model 737  
Aircraft registration VH-YVA  
Serial number 40995  
Operator Virgin  
Type of operation Air Transport High Capacity  
Damage to aircraft Nil  
Departure point Brisbane, Qld  
Destination Sydney, NSW  

Aircraft 2 details

Aircraft 2 details
Aircraft manufacturer Gulfstream Aerospace Corp  
Aircraft model G-IV  
Aircraft registration VH-CGF  
Serial number 1083  
Type of operation General Aviation-Unknown  
Damage to aircraft Nil  
Departure point Melbourne, Vic.  
Destination Coolangatta, Qld  
Last update 14 November 2018