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VH-HYF departed Adelaide Airport on an 'Adelaide East 2 Mildura' Standard Instrument Departure (SID), with an unrestricted climb to its cruising level of Flight Level (FL) 370, then via the 'Tango 77' route to Brisbane. Because a Boeing 747 from Melbourne would be overflying Mildura at FL 310, Melbourne had co-ordinated with Adelaide a requirement for VH-HYF to reach FL 330 by 37 km south-west of Mildura. VH-EAK was flying from Sydney to Adelaide via Mildura, cruising at FL 310. It was on the reciprocal heading to VH-HYF, and appeared on the Adelaide Control radar display at 235 km from Adelaide. When VH-HYF was transferred to Adelaide Control, the controller requested that the aircraft maintain its best rate of climb to FL 330. His objective was to climb VH-HYF above VH-EAK, using radar to maintain separation instead of procedural control. This would allow VH-EAK to follow its preferred descent profile to Adelaide, and for VH-HYF not to be held at a lower level until procedurally separated with the overflying Boeing 747. Approaching FL 310, VH-HYF experienced an increase in ground speed due to a strong westerly windshear increasing the closing speed between the aircraft. While maintaining maximum rate of climb, the air speed of VH-HYF fell below the minimum manoeuvering speed. With the airspeed trend indicator fluctuating due to turbulence, the Captain reduced the climb angle to accelerate the aircraft without advising the controller of the reduction in climb performance. VH-HYF passed VH-EAK 200 km north-east of Adelaide, with 800 ft vertical and 0.4 nm horizontal separation with less than the required standard of 2000 ft vertical or 7 nm horizontal separation. VH-HYF had regained its normal rate of climb as the aircraft passed. The controller had been operating two sectors on combine, with a subsequent increase in co-ordination workload. He had monitored VH-HYF during its climb to FL 310, at which time it was 76 km to the west of VH-EAK. At its initial climb rate he expected VH-HYF should have passed FL 340 by the estimated time of passing. The controller continued his scan of the radar display. When his gaze returned to these aircraft,the noticed that the radar returns were merged, which would be normal as they passed. When the radar returns separated, the controller noted that the altitude indication for VH-HYF was showing the aircraft to be at FL 330. He therefore had no reason to believe that a breakdown in separation had occurred. A discussion had also been in progress during this period with other controllers regarding industrial matters. This situation was causing the controller considerable amount of anxiety and may have been a factor in this occurrence.

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General details
Date: 17 July 1991 Investigation status: Completed 
Time: 1258  
Location   (show map):200 km NE of Adelaide Investigation type: Occurrence Investigation 
State: South Australia  
Release date: 02 July 1992  
Report status: Final Occurrence category: Incident 
 Highest injury level: None 
Aircraft 1 details
Aircraft manufacturer: Airbus Industrie 
Aircraft model: A320 
Aircraft registration: VH-HYF 
Serial number: 27 
Type of operation: Air Transport High Capacity 
Damage to aircraft: Nil 
Departure point:Adelaide SA
Departure time:1242
Destination:Brisbane Qld
Aircraft 2 details
Aircraft manufacturer: The Boeing Company 
Aircraft model: 767 
Aircraft registration: VH-EAK 
Serial number: 23305 
Type of operation: Air Transport High Capacity 
Damage to aircraft: Nil 
Departure point:Sydney NSW
Departure time:1145
Destination:Adelaide SA
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Last update 13 May 2014