Loss of separation involving a Boeing 747-438, VH-OJG and Boeing 737-377, VH-CZD, 37 km north of Wagga Wagga Aerodrome, New South Wales, on 16 April 1996

The Boeing 737 was northbound from Melbourne to Brisbane, via route H29. The air traffic controller cleared the Boeing 737 direct to Mudgee so that it would pass about 15 nautical miles west of Wagga. The Boeing 747 was eastbound from Perth to Sydney on route H31 tracking from Natya to Cullerin. This track crosses H29 about 20 miles north of Wagga. Both aircraft were maintaining flight level 370 as cleared by air traffic control.

Less than three minutes before the Boeing 747 reached the crossing point of the two tracks, the air traffic controller instructed the aircraft to descend to flight level 350. At the time the Boeing 747 commenced the descent, recorded radar data showed that the aircraft had a groundspeed of 530 knots and was 24 miles from the point of track intersection. At the same time the Boeing 737 had a groundspeed of 480 knots and was 20 miles from the point of track intersection. Subsequently both aircraft passed with 1,600 ft vertical separation and within 3 miles horizontally.

The time allowed by the controller for the descent was inadequate to ensure the required 2,000 feet vertical separation standard for aircraft within five miles of each other was maintained. The controller's resolution of the conflict was deemed to be inappropriate for the situation. Given the short time to the crossing point, a higher degree of expedition would have been required if descending the Boeing 747 was to be the only measure to achieve separation. For separation assurance, once the conflict was recognised, it was not enough for the controller to merely instruct the Boeing 747 to descend. A requirement or a radar vector was necessary given the proximity of both aircraft.

Weather was not a factor in the incident. The air traffic controller's workload was not busy. All facilities and equipment were serviceable. The controller was licensed and current. There was no evidence that the controller was suffering fatigue.

The following factor was considered relevant to the development of the incident:

1. The air traffic controller did not adequately plan ahead to ensure required separation standards were achieved.