Loss of separation involving a de Havilland Canada DHC-8-102, VH-TQP and de Havilland Canada DHC-8-201, VH-TQG, 19 km south of Williamtown Aerodrome, New South Wales, on 30 December 1997

A Dash 8 aircraft, VH-TQP departed Sydney for Coffs Harbour on climb to FL190. Shortly after this aircraft departed, a second Dash 8, VH-TQG departed Sydney for Port Macquarie on climb to FL170. The second aircraft in the departure sequence was a later model Dash 8, capable of maintaining a high-speed cruise climb. To maintain separation between the two aircraft, both were restricted to 160 KIAS on climb.

As TQG approached top of climb (FL170), the controller observed TQP, the slower Dash 8 ahead, still well below FL170. To expedite the departure of the faster aircraft and achieve coordination with a southbound aircraft, the controller decided to restrict the climb of TQP to FL160. The crew understood their instruction was to FL170, selected this level with their assigned altitude indicator and read this altitude back to the controller. The controller did not detect the discrepancy between the assigned altitude and the crew's read back of the instruction.

The controller was then distracted for a short period of time with other sector controllers involved with an aircraft experiencing communication difficulties. During this period the mode C readout from TQP was noticed at FL168, in close proximity to TQG maintaining FL170.

TQP was instructed to immediately descend to FL160 and TQG initiated a turn to increase the separation between the two aircraft. The aircraft proceeded safely to their destinations following this occurrence.