VH-MZX was inbound to Sydney from the north-west and had been given a clearance to descend to 3,000 ft. Thai International 986 (THA 986), which was outbound to the north-west, had just departed runway 34 at Sydney. The aircraft had been instructed to turn left onto a heading of 330 degrees and cleared to climb to 3,000 ft. Subsequently, THA 986 was turned onto a heading of 270 degrees. When the controller responsible for the control of THA 986 realised that both aircraft were at 3,000 ft he instructed THA 986 to turn right. The closest point of approach of the aircraft was laterally 0.9 NM and vertically 200 ft. The required separation was either 3 NM laterally or 1,000 ft vertically. Control of aircraft in the terminal area at Sydney can be divided among up to four different controllers.
In some cases, it is necessary for aircraft under the control of one controller to temporarily enter the airspace of another controller. In such cases, it is normal practice for the aircraft to remain on one radio frequency under the control of the former controller. To assist with the processing of the aircraft, the controller responsible for each aircraft co-ordinates the passage of aircraft under his control with the controllers responsible for the adjoining airspace. This adds an internal co-ordination workload for any controller involved in this procedure.
On this occasion, VH-MZX was being controlled by the Approach (North) controller and THA 986 by a trainee Departures controller who was being supervised by a rated Departures controller. As THA 986 became airborne, the trainee controller asked the Approach (North) controller if he could vector and climb THA 986 clear of VH-MZX through the Approach (North) airspace. The Approach (North) controller concurred with his request. It was then the responsibility of the trainee Departures controller to separate THA 986 from VH-MZX. The trainee Departures controller believed that the inbound aircraft would only be cleared to descend to 4,000 ft and planned to vector THA 986 below this aircraft at 3,000 ft. This belief stemmed from the fact that this was the technique normally used by his training officer. The realisation that both aircraft were at the same altitude was delayed because the radar screen labels that indicate aircraft altitude had become cluttered because of the number and proximity of aircraft in the area. When the Approach (North) controller observed that the aircraft were in confliction he instructed VH-MZX to turn south.
The aircraft did not respond to the instruction. The pilot of VH-MZX later reported that he had not heard the callsign and the beginning of the instruction and was therefore unaware that the instruction was directed to him. He also advised that he had not seen the other aircraft until it was established in the right turn. This was probably because it was shielded behind a cockpit pillar. The controller supervising the Departures controller also had the expectation that VH-MZX would only descend to 4,000 ft. This expectation was apparently based on his normal technique but neither he nor the trainee had requested the Approach controller to limit the descent of VH-MZX to 4,000 ft. However, the supervisor believed that the trainee was going to vector THA 986 behind VH-MZX and by the time he realised that both aircraft were at about the same level the trainee had taken appropriate action to resolve the conflict and regain separation. The pilot of THA 986 saw the light aircraft as his aircraft was turned to the north, and was satisfied that the aircraft were clear of each other.
Significant Factors:
1. The airspace structure and air traffic control procedures applying in the Sydney Terminal area resulted in an unnecessary rise in co-ordination workload.
2. The control technique used by the trainee was inappropriate for the traffic disposition in the Sydney terminal area.
3. The Departures Controller did not monitor the techniques and instructions given by his trainee to the extent necessary to ensure adequate separation was maintained.
Safety Action:
The division of airspace responsibilities within the approach/departures cell of Sydney AACC is to be changed following trials in the ATS simulator.
These "Structured Airspace" proposals should be implemented during 1994 and are expected to result in procedures whereby the internal co-ordination safety deficiency identified in this report will be alleviated.