On 23 April 2011, at 1644 Eastern Standard Time, a loss of separation assurance (LOSA) occurred between a Boeing B737-8BK (737), registered VH-VOB, and an Airbus Industrie A320-232 (A320), registered VH-VGZ, near Armidale Airport, New South Wales (NSW).

Both aircraft were conducting scheduled passenger services at flight level 320 on the same one way air route, H62, until position Mount Sandon, where the leading 737 aircraft would continue tracking south for Sydney, while the following A320 aircraft was flight planned to track left to Williamtown.

As the 737 was required to enter a holding pattern for sequencing into Sydney, the air traffic controller approved a speed reduction for the aircraft, but did not identify that the longitudinal separation between it and the following A320, was closing.

When the 737 flight crew provided the controller with their aircraft's speed information for the benefit of the following traffic, the controller identified the potential confliction and issued the A320 with a vectoring instruction to maintain separation. Although the separation between the two aircraft did not reduce below the required standard, a LOSA had occurred due to the closing longitudinal proximity between the two aircraft.

Airservices Australia (Airservices) advised that a review of LOSA and breakdown of separation occurrences would be undertaken to determine whether speed differential and aircraft performance were significant contributing factors and if additional refresher training was required for controllers.

This occurrence highlights the need for awareness of the effects of low workload on performance, identification of lowered vigilance and subsequent action or implementation of strategies to maintain safe operations. It also demonstrates how appropriate flight crew communication enhanced the situational awareness of the controller. In addition, the knowledge and application of effective compromised separation recovery techniques by the controller was integral in managing the situation.