Loss of separation involving Airbus A320, VH-HYB and Boeing 727, VH-TBN, 40 km east-north-east of Eildon Weir, Victoria, on 18 July 1991

Circumstances:

Three aircraft, VH-TBN, VH-HYB and VH-ANF were tracking towards Eildon Weir (ELW) en route to Melbourne from the north-east. VH-TBN was a few miles ahead of the other two aircraft. VH-HYB was cruising at Flight Level 240 (FL 240). The other two aircraft were at higher levels, but VH-TBN was on descent to FL 160. VH-ANF and VH-HYB were required to enter the holding pattern at ELW to facilitate sequencing to Melbourne. VH-TBN was cleared to continue but instructed to reduce speed to 230 kts on descent. VH-TBN was paralleling track about 6 NM to the left to stay clear of the ELW holding pattern traffic. VH-ANF was scheduled to leave ELW before VH HYB and in preparation for this, the sector controller decided to descend VH-ANF below VH-HYB. Because there was only about 2 NM between these two aircraft on track, the sector controller instructed VH-HYB to turn 30` left onto 200`, which was towards the position of VH-TBN. The sector controller passed advice on the track and airspeed details for VH-TBN to the arrivals controller and then transferred control of VH-TBN to that controller. VH-ANF was initially cleared to commence descent to FL 250. Details on the track of VH-HYB and the reason for turning VH-HYB left were also passed to the arrivals controller. Control of VH-HYB was also handed off to the arrivals controller at that time. The crew of VH-HYB were then told to call arrivals control. In response, they acknowledged the instruction and asked the controller to advise the height of the aircraft in front of them. Realising that a loss of separation had developed between VH-HYB and VH-TBN, which had been placed under a speed restriction, the sector controller instructed VH-HYB to turn left onto 030`. He also told the crew that the other aircraft was about 200 ft below. The arrivals control position was being operated by a trainee under the supervision of an instructor. The relative positions of VH-HYB and VH-TBN were checked by the instructor when control of VH-HYB was transferred to them. At the time they were transferred, they were separated by about 5.7 NM. At that time, the minimum allowable separation was 5 NM horizontally or 1,000 ft vertically. The instructor and trainee then became involved in other aspects of the training management task. It was then noticed that the radar returns from VH-TBN and VH-HYB were in close proximity. VH-TBN, which was on descent to FL 160, was instructed to expedite descent. VH-HYB was instructed to make a turn, but the aircraft was still on sector frequency. Separation reduced to a minimum of about 200 ft vertically and one NM horizontally. In giving VH-HYB the initial instruction to turn left onto 200` the sector controller had inadvertently overlooked the potential conflict situation with VH TBN ahead but at a significantly reducing speed. The recorded radar data showed that as the incident developed, the closing speed between the two aircraft increased to about 180 kts. The radar screen labels for each aircraft included a read out of groundspeed. The arrival controllers did not detect the large closing speed. The traffic workload at the time was moderate. The sector controller had a low level of label brightness selected. The high rate of closure between the two aircraft was partly due to the fact that VH-TBN was operating at a reduced airspeed and partly because of the direction of the strong wind, which gave VH-HYB a reduced headwind component while being vectored left of track.

Significant Factors:

The following factors were considered relevant to the development of the incident

1. Significant oversight by the sector controller in vectoring VH-HYB towards VH-TBN, which was ahead and restricted to a slower speed.

2. Strong upper wind situation which increased the ground speed of VH-HYB when it was given a turn onto a heading of 200`.

3. The attention of the two arrivals controllers was diverted at a critical stage and they did not detect the situation of a high closing speed between the two aircraft. This incident is one of eight similar airmiss incidents which are being considered as a part of a special investigation aimed at identifying any ATS systemic deficiencies which may be contributing to airmisses. Any recommendations arising from this and the other reports will be addressed as part of the special investigation.