Loss of separation involving a Cessna 441, VH-ANJ and Fokker B.V. F28 MK 4000, VH-FKI, 74 km east-north-east of Perth, Western Australia, on 14 January 1993

Arrivals radar was being operated by a trainee controller under the supervision of a rated controller. VH-ANJ and VH-FKI were both tracking to Perth via Clackline. The trainee controller had given both heading and altitude instructions to the aircraft in an attempt to manage their separation. A breakdown in separation standards occurred when the aircraft approached to within 3 nautical miles and 500 feet vertically before the instructions took effect. The supervising controller took over and provided VH-FKI with traffic information on VH-ANJ which allowed visual separation to be maintained.

Although the trainee controller recognised that there would be a separation problem the instructions given to the aircraft were not given in time nor were they sufficient to separate the aircraft under all possible conditions.

The supervising controller was slow to take control of the situation because he wished to give the trainee, who was at an advanced stage of training, the opportunity of recognising the situation and making the necessary corrections.

The trainee's task was made more difficult when VH-FKI did not descend as early as anticipated and drifted to the right of the runway 24 localiser, and towards VH-ANJ, after the pilot had reported established.

The following factors led to the development of this occurrence.

1. The trainee controller did not have sufficient expertise to be able to recognise and allow for all possible variables in the traffic conflict situation.

2. The supervising controller made an error of judgement when he delayed his intervention and was unable to prevent the breakdown in separation standards.

3. Whilst not a direct factor, the flight path of VH-FKI complicated the situation.