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The co-pilot had less than nine hours of interrupted sleep during the 48 hours preceding the incident. That small period of sleep suggests that the co-pilot may have been fatigued at the time of the incident despite feeling adequately rested. Fatigue may lead to impaired physical and mental performance in people and may explain why, when the controller requested confirmation of the aircraft's level, the co-pilot relied on his memory rather than performing the more demanding task of confirming the aircraft's level by looking at the altitude indicator.

Within a short time of waking from the period of controlled rest, the co-pilot had received a handover briefing and assumed control of the aircraft. Despite reporting the correct level (FL390) to the Bourke sector controller during a change of radio frequency, the co-pilot subsequently reported an incorrect level (FL370) a few minutes later with the APOMA position report.

It is likely that the co-pilot was suffering from the cumulative effects of fatigue and sleep inertia that resulted in the incorrect level being passed with the position report. The occurrence highlights that an understanding of fatigue and how to manage it are important defences to a human limitation.

Despite the aircraft being at flight levels that provided 2,000 ft vertical separation, the Bourke sector controller's provision of mutual traffic information to both the co-pilot and the pilot of the B747 was warranted. At that stage, the pilot reported information provided to the controller indicated that a separation standard was not being applied to the aircraft. The traffic information would have assisted both crews to assess the potential for conflict and would have provided a basis for pilot-initiated avoiding action. Under the circumstances, the provision by the controller of a safety alert that would have included a recommended action to either turn or to climb/descend, as well as the provision of traffic information, may have been a better option. That action would have ensured that the pilots clearly understood that the aircraft were in close proximity and at the same level, based on the reports provided to the controller.

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