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The Cessna Aircraft Company 402C (Cessna) was established on final for a straight-in approach to runway 15 at Cairns. The pilot had been assigned a descent to 3,000 ft, due to a de Havilland Canada Dash 8 (Dash 8) that was on final approach ahead of the Cessna and was at or below 2,000 ft. The assigned altitude ensured that the 1,000 ft vertical separation standard was maintained. The Approach One controller did not notice the Cessna descend through 3,000 ft or that the ground speed of the Dash 8 had reduced such that the spacing between the aircraft was less than the required 3 NM radar separation standard. There was an infringement of separation standards. The weather was reported to be fine with a clear sky.

Cairns approach control was managed by two control positions; Approach One and Approach Two. The Approach One area of responsibility was over the sea and included the final approach for runway 15 and all departures. Approach Two was responsible for the area over land.

The Approach One controller had accepted responsibility for the position about 10 minutes before the occurrence. The controller had been the Flow controller for the afternoon and was due to finish his shift at 1830 Eastern Standard Time. Prior to leaving, he offered to take over Approach One to enable another controller to take a short break. A handover/takeover was conducted at the Approach One position and he was aware of the six aircraft in the arrival sequence; having established the sequence himself while in the Flow position. He was also aware of another four aircraft taxiing for departure.

The initial aircraft in the arrival sequence were a Twin Otter from Mackay, the Dash 8, the Cessna and then an Embraer Bandeirante from the west. The Approach One controller was responsible for the Twin Otter, Dash 8 and the Cessna, while the Bandeirante was under the control of the Approach Two controller.

The Twin Otter crew had been assigned a visual approach for left base to runway 15 but subsequently requested an instrument landing system (ILS) approach. Approval of the ILS would entail additional track miles and reduce the spacing between the Twin Otter and the Dash 8. The controller believed sufficient spacing for separation and sequencing would be maintained with some minor track adjustments. He instructed the Dash 8 crew to turn left heading 330 degrees and to descend to 3,000 ft to position the aircraft east of the coast and 6 miles behind the Twin Otter. About two minutes later the controller instructed the Dash 8 crew to descend to 2,000 ft and the pilot of the Cessna to descend to "3000 (ft) visual". Those instructions were appropriately acknowledged by the crews. As the Cessna was from the north, and effectively on a long final, the controller issued the instructions to establish the vertical separation standard between the aircraft.

The Approach One controller instructed the Dash 8 crew to turn left heading 250 degrees and to descend to 1,500 ft. The crew was aware of aircraft arriving from the north. As the Dash 8 approached the runway 15 extended centreline, the crew thought the approach controller may have forgotten them and they discussed whether or not to contact the aerodrome controller direct. The crew could see the terrain to the west and reduced the aircraft's speed to reduce the rate of closure with the terrain. Shortly after, the Approach One controller instructed them to make a visual approach. That required them to turn the Dash 8 approximately 140 degrees to the left and then turn back to the right to establish the aircraft on final for runway 15. The crew did not report, nor were they required to report, their reduced speed. The controller could monitor aircraft ground speeds via the air situation display radar track labels.

The pilot of the Cessna had been assigned a descent to 3,000 ft and was instructed to maintain the best approach speed. He had been given traffic information on the Dash 8 and was expecting to be instructed to sight and follow that aircraft. Use of the sight and follow procedure by air traffic control (ATC) was common and transferred separation responsibility from ATC to a pilot. The Cessna pilot could see the Dash 8 ahead on final and continued descent below 3,000 ft without a clearance. He later stated that he only became aware that he had not complied with his clearance after he transferred to the aerodrome control frequency. The pilot also said that he believed there was no risk of collision.

The Approach One controller was an experienced controller and was also a team leader. He was on the second shift of a four-day cycle and had commenced the shift at 1100. He had worked the same shift the previous day. The controller had two days off duty before commencing this shift cycle. On the day of the occurrence he had visited a relative in hospital at 0800 and then returned home to do some paperwork before going to work. The relative had entered hospital about a month previously and the controller had visited regularly over that period. The controller was aware of the need to not allow the relative's illness to impact on his work. He believed he had achieved a balance between work and non-work commitments.

The controller spoke briefly with the Approach Two controller and gained the impression that the Cessna was under the control of the Approach Two controller. Consequently, as he saw the distance reducing between the Dash 8 and the Cessna he was waiting for a response from the Approach Two controller despite the fact that both pilots were operating on the Approach One radio frequency.

The Aerodrome controller was concerned with the reduction in spacing between the aircraft, and asked the Approach One controller to slow the Cessna to provide sufficient time for the Dash 8 to vacate the runway. That prompted the controller to ask the pilot of the Cessna if he could see the Dash 8. The pilot replied that he could, and was then instructed to contact the Aerodrome controller, who instructed the pilot to make an orbit, as there was insufficient spacing to ensure that the runway standard would be met.

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