The Cessna 310 (C310) was the first aircraft in a busy arrival sequence and had been programmed by the flow controller for a visual approach to right base for runway 15. The weather had been fluctuating around marginal visual meteorological conditions for some hours and although the cloud base was generally 2,500 ft, it was lower in passing, heavier showers. The visibility was generally 8 km but reduced to 3,000 - 4,000 m in the showers.
When the pilot of the C310 reported that he was unable to establish visual contact with the ground, the approach controller had to change the arrival plan and vector the aircraft to the east of the aerodrome to facilitate further descent. This amendment to the traffic management plan was intended to assist the pilot to become clear of cloud but it necessitated extra track miles for the aircraft. Consequently, an adjustment to the arrival sequence was required and the C310 became number three in the landing order. This change also meant that the timing of sequential landings had fallen behind that which the flow controller had planned and following aircraft would need to be delayed.
The Aero Commander 500 S (AC50) was the fourth aircraft in the sequence and had been held outside controlled airspace by the approach controller until a separation standard could be guaranteed with the other aircraft.
During the course of these events, several verbal exchanges took place between various tower and terminal control area staff regarding the availability of visual approaches in the deteriorating weather conditions. It was agreed that all jet aircraft, and other aircraft arriving from the north and west, would be processed for instrument landing system (ILS) approaches. While these conversations were taking place, the weather had deteriorated such that the cloud base was 1,500 ft and the visibility was generally 3,000 - 4,000 m.
The terminology of what was "north and west" was never positively determined. The flow controller thought that the track of the C310 (via Copperlode Dam - approximately 210 degrees from Cairns) was south-west and not west. Other controllers considered anything west of 180 degrees was "west" and would not have used this track for a visual approach under the prevailing weather conditions.
When the pilot of the C310 reported that he was "visual", the approach controller authorised a visual approach and transferred the pilot to the tower control frequency. Shortly after, he issued a clearance for the AC50 to enter controlled airspace at 1,000 ft. He judged that, as their observed ground speeds were similar, he would be able to maintain the required radar standard of 3 NM while keeping the aircraft comparatively close to each other in order to minimise delays in the landing sequence.
As he was monitoring the progress of the aircraft, the approach controller noticed that the groundspeed of the C310 had reduced on final approach and that the 3 NM separation with the AC50 was unlikely to be maintained. He asked the aerodrome controller if he could use the "sight and follow" procedure, which would allow the pilot of the AC50 to establish and maintain visual separation with the C310 and, consequently, allow the distance between the aircraft to reduce to below 3 NM. However, due to the deteriorating weather and his resultant inability to sight the aircraft, the aerodrome controller declined the request.
As the AC50 closed on the C310, the approach controller realised that the 3 NM standard was not going to be maintained and asked the aerodrome controller to visually separate the two aircraft, a procedure that would again allow the aircraft to proceed with less than the required radar standard. Once again, the aerodrome controller declined the request as he could not see the aircraft in the prevailing weather conditions.
The approach controller then decided to re-sequence the AC50 and issued instructions for the pilot to turn away from the approach. As the aircraft turned, the horizontal separation reduced to 2.8 NM. An infringement of separation standards had occurred.