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A Boeing 737 (B737) departed runway 34R on an ENTRA ONE standard instrument departure (SID), on climb to 5,000 ft. Immediately after departure, the crew established communication with the Departures North (DN) controller.

A Metro, inbound to Sydney from the north, was being radar vectored for a wide downwind leg to runway 34R, maintaining 7,000 ft, having earlier been diverted to the east for sequencing with preceding slower traffic. Although this track placed the aircraft in DN airspace, the Metro crew, as instructed, remained in communication with the Approach North (AN) controller. On request, the AN controller had been granted approval by the DN controller for the Metro to transit through the DN controller's airspace. The weather conditions were fine, and runway 34 parallel operations had been in progress for 18 minutes.

The DN controller had elected to provide vertical separation between the two aircraft by issuing instructions that would enable the B737 to climb above the Metro. This plan was coordinated with the AN controller. A clearance to climb to flight level 280 was issued to the crew of the B737, with a request to expedite climb through 8,000 ft. As this transmission was being made by the DN controller, the AN controller instructed the crew of the Metro to turn right onto a heading of 170 degrees for the downwind leg.

Subsequently, it was perceived by the DN controller that the effective climb performance of the B737 would be insufficient to ensure that the required separation standard of 1,000 ft vertically or 3 NM horizontally between the two aircraft would be maintained. The controller amended the clearance and instructed the crew of the B737 to maintain 6,000 ft. However, because the B737 was climbing at 4,500 ft/min, it had climbed to 7,000 ft before the crew were able to stop the climb and commence descent. As there was now less than 3 NM between the aircraft, both controllers passed traffic information to the crew of their respective aircraft. The crew of the B737 sighted the Metro, passing beneath that aircraft with approximately 600 ft of vertical separation. The crew of the Metro did not sight the B737.

During the two years since the AN controller had been rated, the system in which he was working had changed significantly due to noise-sharing arrangements at Sydney (Kingsford-Smith) Airport. The controller's training and endorsement had been undertaken and achieved in a less complex environment. The unpredictability of the more complex arrangements required a high degree of coordination for which the controller had not been adequately trained.

Conversely, the DN controller had gained his initial rating in the days when air traffic control was more reactive and dynamic than the present more regulated system. In the previous, less structured air traffic control environment that the controller had been trained in, there had been undocumented procedures that everyone was aware of. Those procedures were passed on from controller to controller as skills were developed and refined. The DN controller expected that his intentions and plan would be readily interpreted by the AN controller.

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