Aviation safety investigations & reports

Boeing Co 737-33A, VH-CZU, 28 km NW Wagga Wagga, (VOR), NSW, 18 October 1999

Investigation number:
Status: Completed
Investigation completed


VH-CZU a Boeing 737 (B737), was en route from Adelaide to Sydney at flight level (FL) 370 on air route H31. VH-CZS a B737 was tracking north on H29 en route from Melbourne to Brisbane at FL370. Their respective air routes intersected at a position approximately 25 NM north-east of Wagga and their relative positions and groundspeeds indicated that the radar separation standard of 5 NM would not be maintained.

The Melbourne Sector 6 controller coordinated with the Canty Sector controller for the crew of CZU to be issued with a requirement to descend to FL350 by 110 NM from Culin, a position approximately 34 NM north of Canberra. The intent of the instruction was to ensure that the vertical separation standard of 2,000 ft was established between the aircraft prior to the lateral distance between them reducing to less than the standard. Following the acknowledgment and read back of the requirement, the crew of CZU queried whether radar vectors would be available, as they preferred to maintain FL370 due to cloud and possible turbulence below that level. The Canty controller advised the crew to stand by and after conferring with the sector 6 controller instructed them to transfer to the sector 6 frequency. Once established on the sector 6 radio frequency the crew was instructed by the controller to turn right heading 130 degrees. The controller issued further instructions to the crew of CZU to turn onto 140 degrees and 150 degrees.

The controller then instructed the crew of CZS to turn right onto 060.

CZU passed 4 miles behind CZS while they were at the same level. There was an infringement of separation standards. The incident occurred during the period when Melbourne air traffic controllers were transitioning from the old centre that used the Australian Computer Air Traffic Control System radar and procedural flight strip bay facilities to new facilities using the Advanced Australian Air Traffic Control System (TAAATS). Sector 6 was in the old centre while Canty sector was a TAAATS position. TAAATS has a number of alarms to alert controllers of potential separation infringements. During the occurrence the short-term conflict alert operated at the Canty position. The sector 6 controller was busy at the time with a moderate level of traffic. The complexity of the management of sector 6 was compounded by weather that was causing flight crews to request advice of weather on various routes and also for diversions to avoid developing weather cells. Sector 6 had two control positions, radar and procedural and was normally operated in the combined configuration. The controller was managing both positions at the time. Another controller was available to assist at the position. This controller was not utilised until after the occurrence.

When CZS entered sector 6 airspace it was approximately 7 NM west of air route H29. This was normal practice as departure controllers were approved to instruct crews to track from their present position, within the terminal area (within 45 NM of Melbourne), direct to Mudgee. This required the sector 6 controller to calculate a specific solution for each potential crossing conflict with aircraft nominally tracking on H29 and aircraft on all intersecting routes. Sector controllers can use lateral separation diagrams to assist in the application of separation. However, the use of such diagrams was dependent upon aircraft operating within the navigational tracking tolerance of the air route being flown. Sector 6 did not have or use lateral separation diagrams.

Sector 6 was a joint radar/procedural sector with a majority of radar coverage. However, the size of the sector and the disposition of air routes within the sector meant that generally conflicts were resolved using procedural control methods. Radar vectoring was used to sequence aircraft for arrival into Sydney but was not generally used to establish separation between aircraft. Consequently controllers had limited opportunities to practice vectoring techniques.

The controller had returned to the sector 6 staff roster approximately two weeks prior to the occurrence after being rostered for familiarisation training on 30 September and 1 October 1999. The controller had agreed to return to sector 6, following his TAAATS transition training, to enable other controllers to be released for transition training. The sector 6 area of responsibility within TAAATS had been divided into two sectors, Parkes and Bourke. The management of traffic and coordination requirements for these sectors differed considerably from those required for sector 6. The controller had undergone radar vectoring exercises during the transition training but these had focused primarily on developing human-machine interface skills and not traffic management skills. Also, some exercises used nil-wind conditions and thus were not reflective of conditions likely to be experienced on the job. The controller was rostered for two familiarisation shifts on the return to sector 6. However, after five hours during the first period of familiarisation, the controller felt comfortable and believed he was capable of operating at a satisfactory level and was subsequently endorsed to operate the sector.

The crew reported that they believed that the requirement, to descend to FL350 by 110 NM Culin, had been cancelled when the Canty controller advised them to stand by. The sector 6 controller did not instruct the crew to maintain FL370 once he had decided to vector the aircraft. Neither the crew of CZS nor the controller queried or clarified the status of the level requirement until after the occurrence.


The disparity between the TAAATS training and management of sector 6, the differences in coordination and management between TAAATS and sector 6 and the limited opportunities to use radar vectoring, all contributed to limit the controller's ability to successfully resolve the conflict.

The ready provision of direct tracking to Mudgee for aircraft that had planned via H29 increased the controller workload. While this factor was minor in comparison to the others previously mentioned, it nevertheless resulted in some additional action by the controller. Had aircraft been required to intercept and rejoin air route H29 prior to the majority of the intersections with other routes it is likely that the complexity of the controller's task would have been reduced.

Prior to the crew requesting the availability of alternative separation methods the controller had formulated a traffic management plan that would ensure separation was maintained between the aircraft. After the crew's query, the controller adjusted his plan to compensate for the changed circumstances but was inadequately prepared to ensure maintenance of separation using the radar.

Safety Action

As a result of the investigation Airservices Australia Southern District:

  1. amended local instructions to minimise the use of direct tracking from positions within the terminal area to Mudgee;
  2. modified simulator exercises for controllers on the Parkes sector to:
    • include a significant wind component,
    • provide multiple traffic conflictions on air routes H29 and H31, and
    • provide opportunities to use radar vectoring of aircraft to resolve conflicts;
  3. briefed all controllers on the occurrence and the contributing factors; and
  4. included radar vectoring in annual controller refresher training.

Additionally, Airservices Australia Southern District is planning to add a non-compulsory reporting point on air route H29, south of the intersection with H31, so that aircraft can be tracked to this new position instead of Mudgee.

General details
Date: 18 October 1999   Investigation status: Completed  
Time: 1400 hours EST    
Location   (show map): 28 km NW Wagga Wagga, (VOR)    
State: New South Wales   Occurrence type: Loss of separation  
Release date: 22 May 2000   Occurrence category: Incident  
Report status: Final   Highest injury level: None  

Aircraft 1 details

Aircraft 1 details
Aircraft manufacturer The Boeing Company  
Aircraft model 737  
Aircraft registration VH-CZU  
Serial number 27267  
Type of operation Air Transport High Capacity  
Damage to aircraft Nil  
Departure point Adelaide, SA  
Destination Sydney, NSW  

Aircraft 2 details

Aircraft 2 details
Aircraft manufacturer The Boeing Company  
Aircraft model 737  
Aircraft registration VH-CZS  
Serial number 24030  
Type of operation Air Transport High Capacity  
Damage to aircraft Nil  
Departure point Melbourne, VIC  
Destination Brisbane, QLD  
Last update 13 May 2014