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Analysis

Summary

Controllers at Cairns considered that the aircraft were "like types" for the purposes of departure standards and neither the aerodrome controller nor the approach/departures controller considered increasing the separation requirements specified in Local Instructions. However, Boeing 737-400 series aircraft are known to normally out-perform the Boeing 737-300 series.

Separation standards used for consecutive departures allowed controllers to increase the minimum distance at their discretion. On this occasion it was not increased and, although the runway departure standard was not breached, the decision did not allow for any unexpected manoeuvre by either aircraft. Both crews acted in accordance with the standard instrument departure and the closure was the effect of aircraft performance.

The approach/departures controller elected to input data to the air traffic computer during the departure sequence. Although these actions were necessary, they were labour intensive and diverted his attention from the air situation display. They were not urgent and the decision resulted in the controller not having his full attention on the relative positions of the aircraft when they first appeared on the display.

The design of the SWIFT 2 standard instrument departure did not guarantee separation assurance. Whenever the second aircraft reached 4,000 ft prior to the first aircraft (whatever the reason) a reduction in horizontal separation was likely.

 
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