Aviation safety investigations & reports

Boeing Co 767-336, VH-ZXA

Investigation number:
Status: Completed
Investigation completed


A Boeing 767-336 (B767) was en route from Sydney to Melbourne and was maintaining flight level (FL) 350. A Boeing 737-800 (B737) was en route from Melbourne to Nadi, Fiji, and was also maintaining FL350. The aircraft were on segregated routes that provided lateral separation until the crew of the B737 was provided with track shortening. That decision placed the two aircraft on conflicting flight paths at the same level. The Eildon Weir/Benalla (ELW/BLA) sector controllers saw the impending conflict and alerted the Wollongong/Jervis (WOL/JVS) controller. Both controllers then issued traffic information and instructions to the crews for avoiding action. Both crews received Traffic Alert and Collision Avoidance System (TCAS) Resolution Advisories (RA) and the controllers received a Short Term Conflict Alert (STCA) from The Australian Advanced Air Traffic System (TAAATS). The aircraft passed within 4.8 NM laterally and 800 ft vertically. The required separation standard was either 5 NM laterally or 2,000 ft vertically. There was an infringement of separation standards.

The B737 crew had flight planned, and had been initially cleared to, FL330. That was a standard level for the direction of flight. Levels were assigned depending on a number of factors including the track of the aircraft in accordance with the Table of Cruising levels in the Manual of Air Traffic Services (MATS). Aircraft operating at altitudes and flight levels in accordance with the table were considered to be at standard levels, and those operating at altitudes and levels not in accordance with the tables were considered to be at non-standard levels. The crew of the B737 subsequently requested, and was assigned, non-standard FL350. At the time of the level change, both the crew of the B767 and the crew of the B737 had been issued airways clearances on one-way, designated air routes that formed a segregated race-track pattern between Sydney and Melbourne.

The controller responsible for the ELW/BLA sectors was instructing a student training on the ELW sector. Both were concentrating on the sequencing and separation of two jet aircraft in a step descent into Melbourne. A step descent allowed aircraft to simultaneously descend to vertically separated levels provided that the higher aircraft was progressively assigned levels that provided vertical separation with the lower aircraft. The step descent was occurring in the bottom left quadrant of the controller's Air Situation Display (ASD). The instructor was positioned behind and to the left of the student so that he could readily view the ELW sector and the traffic on the left side of the ASD. The B767 and the B737 were displayed in the top right quadrant of the ASD. Once the step descent had been established, the instructor noticed that the B737 was about to conflict with the B767. By that time the B737 was within the WOL/JVS combined sector and the instructor advised the WOL/JVS sector controller that he would be turning the B737. The student instructed the crew of the B737 to turn right to avoid the opposite direction B767.

The B737 was within the WOL/JVS control area, but under the jurisdiction (and control) of the ELW/BLA controller. The Australian Advanced Air Traffic System Human Machine Interface (HMI) used different coloured tracks and labels to aid situational awareness. The track and label colour of the B737 was green to the ELW/BLA controller, and blue to the WOL/JVS controller. The B737 crew should have been transferred to the WOL/JVS sector prior to crossing the boundary between those sectors and the ELW/BLA sectors. Had the transfer been made the track label and symbol would have been green on the WOL/JVS ASD.

The WOL/JVS controller initiated the amended route clearance for the B737 at FL350, and verbally coordinated the change with the ELW/BLA controller. At that time, the southbound B767 was on climb to FL280. The crew of the B767 contacted the WOL/JVS controller and was assigned FL350. The amended route for the B737 was direct to ALLOC, a waypoint located 77NM east-north-east of Sydney.

The workload on the WOL/JVS sectors at the time of the occurrence was reported by the controller to be low. The controller was providing a directed traffic information (DTI) service to military helicopters operating under visual flight rules (VFR) in class "G" airspace. The provision of DTI to VFR aircraft was available on request and subject to controller workload.

The WOL/JVS controller reported that the ASD at that console had recently "been faulted" and found to be outside the acceptable parameters for contrast. Subsequent to the occurrence, the screen was again checked and was found to be below acceptable contrast parameters. The controller reported difficulty distinguishing the blue track colours from the grey background of the screen; they appeared faded. A track was blue to indicate that the track was about to become the responsibility of that controller. Tracks will appear (in this case) blue to only one console at a time. The same track symbol and label was a different colour to all other controllers to indicate the relevance of that track to each control position.

The WOL/JVS controller stated that a smaller information screen that was open on the WOL/JVS ASD initially obscured the track symbol and label of the B737. He had used the route function available in TAAATS to determine the cleared route of the B737. The controller also stated that he normally used the text message box in TAAATS as a visual reminder of potential conflicts. He did not use the text box on this occasion. He also indicated that he would have preferred using strips because they better facilitated the acquisition and maintenance of the traffic picture.

The ELW/BLA instructor commented that the angle of view from behind and to one side of a student made it difficult to see some areas of the ASD and to monitor keyboard entries.


The crew of the B737 was recleared from a standard level to a non-standard level and, in order to provide track shortening, from a track that would have provided separation with the B767, to one that conflicted with the B767. The allocation of a non-standard flight level on a one-way route does not guarantee separation from opposite traffic on other, crossing, one-way routes. However, the planned routes of these two aircraft did not cross and were laterally separated. The conflict would have been avoided had the B737 continued on its planned route or had it maintained a standard level.

The B767 crew was recleared to F350 after the B737 crew had already been cleared direct to ALLOC and had been assigned FL350. Had an intermediate flight level below the B737 been assigned to the B767 crew, until the aircraft passed, the conflict would have been avoided.

The ELW/BLA instructor was concentrating on the student. Neither controller realised that the B737 had entered the WOL/JVS sectors. Had the transfer of control and jurisdiction of the B737 been initiated before that aircraft crossed the boundary between the sectors, as it should have been, the WOL/JVS controller may have become aware of the imminent conflict.

The WOL/JVS controller had been distracted by a low priority task. Also the contrast on the WOL/JVS ASD may have been below specification and that possibly impaired the ability of that controller to maintain situational awareness.

Neither the WOL/JVS controller nor the ELW/BLA controller effectively employed the tools available in TAAATS to highlight the non-standard nature of the B737 flight; either the non-standard level, or the direct route. Use of a standardised method of highlighting the non-standard nature of a flight may assist controllers with conflict recognition.

When operating sectors that have been combined, diverse scenarios and increasing workloads can quickly distract controllers. Controllers need to be vigilant and recognise the need to separate sectors ahead of the requirement to do so.

All the controllers were distracted by events occurring on their ASD's away from where the conflict occurred. The instructor eventually detected the conflict using effective scanning techniques. Although scanning in this case was not done in time to avoid the conflict, it allowed for timely avoiding action. It also demonstrated the importance of effective scanning not only in conflict recognition, but in recognising when other actions are due, or over due, especially during, and after, busy periods and in larger sectors with multiple crossing points.

Significant Factors

  1. The controllers approved route and level changes that eliminated effective separation assurance strategies.

Safety Action

Local Safety Action

Airservices Australia advised that the WOL/JVS ASD was replaced after being declared unsuitable for continued use due to reduced brightness levels.

General details
Date: 18 July 2001   Investigation status: Completed  
Time: 1428 hours EST    
Location   (show map): 28 km E Canberra, (VOR)    
State: Australian Capital Territory   Occurrence type: Loss of separation  
Release date: 15 April 2002   Occurrence category: Incident  
Report status: Final   Highest injury level: None  

Aircraft 1 details

Aircraft 1 details
Aircraft manufacturer The Boeing Company  
Aircraft model 767  
Aircraft registration VH-ZXA  
Serial number 24337  
Type of operation Air Transport High Capacity  
Damage to aircraft Nil  
Departure point Sydney, NSW  
Destination Melbourne, VIC  

Aircraft 2 details

Aircraft 2 details
Aircraft manufacturer The Boeing Company  
Aircraft model 737  
Aircraft registration DQFJH  
Type of operation Air Transport High Capacity  
Damage to aircraft Nil  
Departure point Melbourne, VIC  
Departure time 1350 hours EST  
Destination Nadi Fiji Islands  
Last update 13 May 2014