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The Hamilton Island Aerodrome Controller (ADC) had a plan to separate the aircraft, but did not clearly communicate the plan to the pilots and consequently it was not executed correctly. Having the 737 pass behind the 717 was going to present some difficulties due to the intended tracks of the aircraft, and required the ADC to advise the crew of the 737 as early as possible of that tracking requirement. While traffic information about the 717 was provided to the crew of the 737, the ADC did not communicate an important aspect; that is, that after reporting seeing the 717 they would have to pass behind it. Had that been the case, the crew of the 737 probably would have been able to turn right in sufficient time to safely pass behind the 717. Alternatively, they may have requested another means of separation as the position of the aircraft may have prevented them from passing behind it.

The use of visual separation, either by controllers or pilots, increases the likelihood of an apparent traffic alert and collision avoidance system (TCAS) alert between aircraft. Apparent alerts result from aircraft being within the TCAS alert parameters while complying with an air traffic control (ATC) clearance. In this occurrence, if the 717 crew had not turned away, it is possible that one or both of the aircraft's systems would have issued a traffic alert (TA) or a resolution advisory (RA) In the case of the latter, the pilots would have had to comply. That would have increased crew workload, particularly for the departing 717 crew, when the aircraft was in a critical stage of flight. As it was, the crew of the 717 had to descend to avoid the 737.

The limitations in using visual separation, highlighted in the Manual of Air Traffic Services, applied similarly to the ADC and both crews. The fact that the aircraft were tracking on almost reciprocal tracks, with little or no divergence when viewed by the ADC or the crews, made it difficult for those involved to obtain adequate cues about the situation. However, the crew of the 717 was able to appreciate the potential for conflict by using available visual and system information.

Despite the regulations stating that in a situation where aircraft are approaching head on a pilot shall alter the aircraft's heading to the right, the actions by the crew of the 717, in turning left, could be seen to be reasonable in the circumstances. A turn to the right may have increased the risk of collision. Similarly, if the crew of the 737 had turned their aircraft to the right in accordance with procedures, as the 717 turned left, the risk of a collision may have increased.

Once the ADC was under the impression that responsibility for separation had been transferred to the crew of the 737 and had issued instructions to the crews to climb/descend, the protection afforded by the vertical separation standard was lost. From that point on, the only defences available to the crews to prevent them being in close proximity were their awareness of the other aircraft and the use of TCAS. As they could clearly see the 717, the crew of the 737 did not perceive that there was a problem. The crew of the 717 were concerned at the developing situation and turned away from the 737. That action ensured that sufficient spacing was maintained between the two aircraft and probably prevented a subsequent TCAS RA.

Had the ADC maintained the 1,000 ft vertical separation standard between the aircraft until they had definitely passed, or else confirmed that one of the crews could maintain separation with the other aircraft, it is likely that the occurrence would have been prevented. The use of vertical or lateral separation standards instead of a visual standard would have also limited the possibility of a TCAS alert.

The occurrence highlighted the importance of using unambiguous radiotelephony phraseology to avoid misunderstandings and the need for pilots and controllers to remain vigilant at all times, especially when the dynamics of a situation require action to be implemented early to ensure that the safety of aircraft is not compromised.

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