Loss of separation involving a Cessna 441, VH-YFD and Cessna 441, VH-FMQ, Mount Magnet, Western Australia, on 22 January 1997

A Cessna Conquest aircraft (VH-YFD) was enroute Perth to Meekatharra via the Mount Magnet VOR at FL250. Another Conquest (VH-FMQ) was flying in the opposite direction, Meekatharra to Perth via the Mount Magnet VOR at FL240. Both aircraft had filed IFR flight plans, were inside Perth controlled airspace and their Mount Magnet position estimates were within 2 minutes of each other. YFD had advised a DME position for descent to Meekatharra and the Perth Centre controller subsequently calculated that the traffic would be passing and may require separation.

When YFD reached Mount Magnet, the pilot transmitted the position to Perth FIS and reported that he was planning to commence descent 2 minutes later. Perth FIS acknowledged the transmission and passed known traffic. Soon after, Perth Centre, unaware that YFD had changed to FIS frequency, unsuccessfully attempted to establish radio contact with YFD to arrange separation with FMQ. YFD had descended through FMQ's level by the time YFD had returned to the correct frequency.

The Perth Centre controller reviewed the aircraft positions and assessed that a breakdown of separation had probably occurred. Day VMC existed at the time of the incident.

The pilot of YFD reported that whilst he was within regulated duty hours, the day's flying program had changed a number of times with diversions and periods of waiting at various airfields in the heat of the day. He also reported that air traffic and company frequencies were very busy at the time of his transit to Mount Magnet.

FIS staff reported that Perth Centre regularly instructed aircraft to call FIS on descent to Meekatharra for traffic. It was also common for aircraft to call on the FIS frequency without prior notice from Perth Centre due to time constraints and workload.

The planning and attention required to cope with flight diversions and associated effects of waiting in the heat, probably fatigued the pilot and reduced his level of attention. His attention may have also been diverted by the radio traffic during the transit to Mount Magnet. A combination of fatigue, distraction and the regular practice of contacting FIS for descent may have influenced the pilot to contact FIS without direction from Perth Centre.

Due to the regular occurrence of aircraft appearing on frequency without prior notification, the FIS operator did not realise that the aircraft was descending without a clearance.

Perth FIS have reported that they and ATC are reviewing coordination procedures.