Airspace related - Other involving a Beech Aircraft Corp 1900C, VH-OST, 56 km north of Williamtown Aerodrome, New South Wales, on 26 November 1998

As the aircraft descended through 7,500 ft, the controller informed the pilot that radar services were terminated. Below 8,500 ft the aircraft was in `Class G demonstration airspace' and the pilot was responsible for separation from other aircraft. To assist him in this task, he was advised of radar-observed traffic by the controller, within the limits of radar coverage and as the controller's workload permitted. The pilot had expected that such assistance would not be terminated at 7,500 ft but would be provided to a much lower altitude. Recorded data from three radar sites covering the area was examined during the investigation. 

Returns from the aircraft to one site were reliable down to 7,500 ft, while returns from the other two sites were broken for varying periods of time on a number of occasions during the descent. The radar system software utilised a built-in matrix to determine which radar data was used in specific geographical areas. The matrix had recently been changed as technicians attempted to obtain the best possible coverage for the area. The investigation found that radar coverage in the area the aircraft was operating was unreliable below about 7,500 ft.