ANSP info/procedural error involving a Boeing 747-438, VH-OJG, 8 km west of Townsville, Queensland, on 3 September 1993

Both aircraft were in communication with Townsville Arrivals and being tracked to the west of Townsville on approximately reciprocal tracks. The northbound aircraft at FL390 (39,000ft) and the southbound aircraft at FL370 (37,000ft). The Arrivals Controller had some six aircraft under his jurisdiction and was moderately busy. When the two aircraft were within a few minutes of passing each other, a co-ordination message was received from the Cairns Arrivals Controller with descent instructions for the northbound aircraft.

The instructions specified a higher than normal descent speed and involved delaying the time of descent. The controller was aware that the crew of the aircraft needed the information as soon as possible to allow the reprogramming of the flight management computer. If there is no conflicting traffic, descent instructions are normally passed to the aircraft together with a clearance to descend. The controller passed the information to the aircraft and also gave a clearance to descend to FL230 despite the lower opposing traffic.

The crews of both aircraft realised that the controller had made a mistake and commented as they passed and saw each other. It was then that the controller realised that he had made an error. The higher aircraft did not descend until six minutes later, at its designated descent point. There was no breakdown in separation. Later, the controller could not explain his mistake, commenting that it was probably force of habit that he passed the clearance at the same time as the descent instructions.