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The crews of all four aircraft had planned to use the same air route between Port Macquarie and Taree. VH-IMA and VH-TQO were arriving at Port Macquarie while VH-IMH and VH-SVV were departing. Due to minor deviations in track-keeping, all four aircraft were to the west of the nominal track.

Weather conditions were such that a layer of cloud existed upwards from approximately 7,000 ft.

Although the flight service officer provided a timely and up-to-date directed traffic information service to all crews, radio communications between the crews of IMA and IMH, and between IMH and SVV were insufficient to ensure self-separation between their aircraft. The high number of radio transmissions on the various frequencies, when combined with the frequency management requirements of each crew, limited the opportunities for adequate radio contact.

The crews of IMA and IMH maintained altitudes in Class "G" airspace without broadcasting their intentions on the flight service frequency. In addition, while both crews were maintaining a listening watch on the flight service frequency for 8 minutes prior to their aircraft passing, neither crew made radio contact with the other.

Analysis of the recorded radar data indicated that the aircraft passed within 1,000 m horizontally and 200 ft vertically while IMA was maintaining 8,000 ft.

None of the aircraft were fitted with an Airborne Collision Avoidance System (ACAS). An ACAS would, most probably, have improved the situational awareness of the crews to the extent that a more effective traffic management plan may have been undertaken.

The lack of adequate situational awareness of the crews of IMA and IMH resulted in two regular public transport aircraft coming into relatively close proximity without either crew carrying out a positive separation plan.

The investigation identified safety deficiencies in respect to timely conflict alerting and self-separation procedures and contains seven recommendations to the Civil Aviation Safety Authority addressing those deficiencies.

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