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The weather at Cairns was visual meteorological conditions but with passing showers, especially over the western escarpment. The cloud base and visibility associated with the showers restricted visual flight in or near the showers. Tower control had advised the terminal area controllers that conditions were marginal for visual approaches from the north-west.

The base of controlled airspace in the north-western sector was 3,500 ft reducing to 2,500 ft at 12 NM from the Cairns airport, and ground level at about 6 NM west of the airport. In the south-western sector, the base remained at 3,500 ft until reducing to ground level at about 5 NM west. The terrain was mountainous, with average heights of more than 2,000 ft to within 5 NM west of the airport. The higher peaks in this general sector of airspace were over 3,000 ft, with some over 4,000 ft to the south-west.

An Embraer E110 (Bandeirante) was operating under the instrument flight rules and on descent to 5,500 ft on an arrival to Cairns from the north-west. Air traffic control approved the crew to divert right and left of track to avoid showers and to facilitate visual contact with the ground. On first contact, approach control gave the crew a radar vector of 060 degrees and descent to 4,000 ft, the lowest altitude the controller could assign with reference to the published radar lowest safe altitudes. Shortly after, the crew reported "visual" and were cleared to descend to 2,000 ft visually and to track direct to Cairns under their own navigation. This clearance required the crew to maintain their current track and continuous visual reference with the ground until within 5 NM of the aerodrome. It allowed flight below the lowest safe altitude and transferred the responsibility for terrain clearance to the crew.

A Piper PA24 (Comanche) was operating under the visual flight rules and maintaining 5,500 ft on approach to Cairns from the south-west. The pilot encountered showers in the area and requested descent to remain visual. The controller issued the pilot with a clearance limit of Stoney Creek (a visual reporting point approximately 6 NM due west of Cairns airport where the terrain contours form a "saddle" feature) and a maximum altitude of 5,500 ft. This clearance allowed the pilot to proceed both inside and outside controlled airspace and to track as required to Stoney Creek. The position of Stoney Creek was at the common boundary of the 3,500 ft, 2,500 ft and ground level control area steps. About 1 minute later, the controller observed that the Bandeirante had deviated left of track and, when this was confirmed by the crew, issued instructions for a visual approach to a right base to runway 15 at maximum speed. The instructions included a frequency change to aerodrome control at 5 NM. The crew were still required to maintain their track direct to the airport until within 5 NM.

The pilot of the Comanche reported at 3,000 ft and 5 NM south-west of Stoney Creek. Although this position was outside controlled airspace, the controller cleared the pilot to descend to 1,000 ft and asked him to report if any diversions were required to remain in visual conditions. No amendment to the clearance limit of Stoney Creek was issued.

Shortly after, the controller realised that the Bandeirante had diverted right of track and was converging with the Comanche to the extent that radar separation standards would not be maintained. The controller attempted to again radar vector the Bandeirante but the crew had radio difficulties and did not hear the instruction. After a brief check of the radio, they transferred to the aerodrome control frquency when about 7 NM from Cairns. The radar vector was for the crew to turn onto a heading of 060 degrees. The instruction was issued as the aircraft was descending through the base of controlled airspace and was below the radar lowest safe altitude.

Radar analysis subsequently indicated that, at that point, the Bandeirante was about 2 NM north of the Comanche, on a converging track and with both aircraft at approximately 2,300 ft. These positions placed both aircraft outside controlled airspace and below surrounding terrain.

The controller observed the altitude of the Bandeirante on the radar display screen but did not advise the crew that they had descended outside controlled airspace. MATS required a radar controller to advise a pilot when the aircraft was observed to deviate significantly from its cleared route or if the controller believed it would deviate from controlled airspace.

The controller then asked the pilot of the Comanche if he could see the Bandeirante. When the pilot answered "negative", the controller instructed him to make a right orbit. The pilot began the turn as directed but realised that the turn would place the aircraft close to terrain that was above his altitude. He requested a left turn and, although this direction initially placed the aircraft closer to the Bandeirante, he then saw that aircraft. The controller then formally instructed the pilot of the Comanche to maintain separation with the Bandeirante. No traffic information on the Comanche was passed to the crew of the Bandeirante.

As both aircraft were outside controlled airspace, there was no technical infringement of separation standards. Radar analysis indicated that the closest point was 1.3 NM when the aircraft were approximately 100 ft vertically apart.

The descent procedure used by the controller after the crew of the Bandeirante reported "visual", was allowed under the terms of visual approach procedures in both the Australian Manual of Air Traffic Services (MATS) and the Australian Aeronautical Information Publication (AIP). However, the AIP also required crews to maintain an altitude of not less than 500 ft above the lower limit of controlled airspace. MATS stated that a controller must provide this same buffer except at pilot request or when visual approach procedures applied.

Although cleared to track direct to the airport, the crew of the Bandeirante diverted initially 1 - 2 NM left of track. About 1 minute later, they diverted right of track and entered into conflict with the Comanche. Both turns were to avoid showers and to maintain visual contact with the ground but no request for track diversion was made to air traffic control. The AIP stated that a pilot must maintain track while on a visual approach and should not report visual until certain that such tracking can be achieved. If a pilot on a visual approach finds that such tracking is no longer appropriate, air traffic control must be informed immediately.

When the controller issued the second radar vector, the crew did not hear the instruction but thought the transmission was for their aircraft. They made an attempt to acknowledge but when they again could not hear the approach controller, and considering that they were only 2 - 3 NM from the nominated frequency change point, they elected to make an early change to the airport control frequency. The crew remained on that frequency and had no further communication problems. Why there was a period of poor air-ground communications could not be determined. The attempt to radar vector the aircraft onto a heading of 060 degrees occurred when the aircraft was descending through 2,500 ft (the base of controlled airspace) and in an area where the terrain rose above 2,000 ft.

During the descent from 2,500 ft to about 2,200 ft to track through the Stoney Creek gorge, the Bandeirante passed within 1,000 m of terrain spot heights of 2,139 ft and 2,119 ft.

As the controller realised that the aircraft were coming into conflict and that the crew of the Bandeirante were not receiving radio broadcasts, the pilot of the Comanche was instructed to make a right orbit. The intention of this instruction was to provide adequate separation between the aircraft, however, it would have resulted in the pilot flying in close proximity to terrain that was higher than his current altitude. Spot heights on the Cairns Visual Terminal Chart indicated heights of between 3,070 ft and 3,477 ft within 3 NM to the right of the track, with general terrain above 2,000 ft within 1 NM. Had the pilot not countered the instruction by changing direction to a left turn, the aircraft would have been placed in a potentially unsafe situation.

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