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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.

Download Final Report
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General details
Date: 18 July 1991 Investigation status: Completed 
Time: 1734 hours Investigation type: Occurrence Investigation 
Location   (show map):40 km ENE Eildon Weir  
State: Victoria  
Release date: 28 May 1992 Occurrence category: Incident 
Report status: Final Highest injury level: None 
 
Aircraft 1 details
Aircraft manufacturer: Airbus Industrie 
Aircraft model: A320 
Aircraft registration: VH-HYB 
Serial number: 23 
Type of operation: Air Transport High Capacity 
Damage to aircraft: Nil 
Departure point:Sydney NSW
Departure time:U/K
Destination:Melbourne Vic
Aircraft 2 details
Aircraft manufacturer: The Boeing Company 
Aircraft model: 727 
Aircraft registration: VH-TBN 
Serial number: 21479 
Type of operation: Air Transport High Capacity 
Damage to aircraft: Nil 
Departure point:Canberra ACT
 
 
 
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Last update 13 May 2014