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An Airbus Industries A330 (A330) aircraft was en-route from Melbourne, Vic. to Denpasar, Indonesia. The Yarowee sector air traffic controller had issued the crew of the A330 with a clearance to climb to flight level (FL) 340. The A330 was on a crossing flight path with a Boeing 737 (737) that was en-route from Adelaide, SA to Melbourne, Vic. at FL310. The 737 was under the control of the adjacent Canty sector controller. Both the Yarowee and Canty sector controllers had recognised that there was a potential confliction between the two aircraft. The controllers discussed the potential confliction at 1119 EST and estimated that the aircraft would pass close to the boundary between the two sectors. The Yarowee controller accepted the responsibility for separation. That controller reported a reasonably low traffic level, which allowed him the opportunity to monitor the separation between the two aircraft. To ensure that the minimum horizontal separation standard distance of 5 NM was maintained, the Yarowee controller advised the Canty controller that the A330 would travel to the left of the proposed track and pass to the west of the 737.

Subsequent to that decision, the Canty controller reassessed the separation between the two aircraft and concluded that the potential confliction would occur inside the Canty sector. At 1121 the Canty controller instructed the Yarowee sector controller to assign the A330 crew FL300 on climb. Maintaining that altitude would ensure that the vertical separation minimum of 1,000 ft would not be infringed. The Yarowee controller entered FL300 into The Australian Advanced Air Traffic System (TAAATS) as the amended cleared flight level (CFL) for the A330. That entry changed the colour of the CFL in the label of the A330 that was displayed to the Yarowee controller on his air situation display. According to TAAATS procedures, the different colour provided a memory prompt to the Yarowee controller indicating that he had not received a correct amended CFL read back by the crew of the A330. Normally, once a controller had assigned a CFL to a crew and received a correct read back of that CFL, the controller would cancel the prompt and the CFL in the aircraft label would return to its regular colour.

There was a distraction at a neighbouring console just after the Yarowee controller had entered FL300 into TAAATS but before he had assigned the crew of the A330 the amended CFL. When the attention of the Yarowee controller returned to the Yarowee air situation display, the cleared flight level for the A330 was highlighted to prompt him to confirm that FL300 had been issued to the A330 crew. The controller believed he had already issued the crew of the A330 the amended CFL and that he had received a correct read back of the amended CFL from the crew prior to the distraction. The Yarowee controller cancelled the prompt. Analysis of the voice recording confirmed that the crew of the A330 had not been issued FL300.

The Yarowee controller incorrectly believed the A330 crew had been assigned FL300 and consequently that a vertical separation standard of 1,000 ft had been established between the A330 and the 737. At 1127, the controller approved the A330 crew to track direct to Woomera. That action placed the two aircraft on crossing flight paths with no prescribed lateral separation.

At 1131, as the A330 approached the Yarowee sector boundary, the Yarowee controller transferred jurisdiction of the A330 to the Canty controller. The Canty controller accepted jurisdiction of the A330 at FL295 and waited for the A330 to call on the Canty frequency.

At 1132, the Canty controller received a TAAATS Cleared Level Adherence Monitor (CLAM) alarm when the A330 passed FL303 on climb to FL340. A short time later, when the A330 and the 737 were 12.5 NM apart, both the Canty and the Yarowee controllers received a Short Term Conflict Alert (STCA) alarm from TAAATS. This alarm indicated to the controllers that the two aircraft were in potential conflict. Vertical separation reduced to 900 ft while horizontal separation reduced to 7.5 NM. That horizontal distance exceeded the minimum separation standard. There was no infringement of separation standards.

At the time of the CLAM and STCA alarms, the crew of the A330 was still on the Yarowee sector frequency. The Yarowee controller reported observing the A330 pass FL303 and resumed jurisdiction over the A330. That controller then queried the A330 crew about their assigned flight level. He did not pass traffic information to the crew of the A330 about the 737 because he did not believe there would be an infringement of separation standards. Nonetheless, he subsequently issued a turn instruction to the A330 crew to ensure that separation was maintained. The A330 crew acknowledged traffic following that controller's query regarding their CFL.

The Canty controller was unable to contact the crew of the A330. The controller was aware that the A330 had climbed through what the controller believed was the assigned level of FL300 and issued turn instructions to the 737 crew to ensure that separation was not infringed. The Canty controller did not inform the 737 crew of the reason the vector was issued, however, the 737 crew subsequently acknowledged sighting the A330.

The Canty sector was combined with a low-level sector at the time of the incident and the controller reported that there was a medium traffic level at the time of the occurrence.

The Yarowee controller had recently returned from leave. He had completed the three required familiarisation shifts, had a two-day break, and completed one unsupervised shift during the day prior to the incident shift.

 

The Yarowee sector controller had entered the amended CFL into TAAATS. He believed he had received the required information from the crew of the A330 and consequently cancelled the CFL prompt in the displayed label of the A330. The distraction at a nearby console interrupted the Yarowee sector controller before he had completed the task of issuing the amended clearance. The occurrence highlights the importance of controller vigilance in completing necessary TAAATS' interface action to ensure system data integrity.

At the time of the CLAM and STCA alarms, the Canty sector controller had accepted jurisdiction of the A330, however, the Yarowee controller had not advised the crew of the A330 to change to the Canty frequency. The crews of the A330 and the 737 were on different frequencies and were therefore unaware of the potential conflict. Neither crew was given a traffic advisory despite the STCA alarm.

Both controllers used effective scanning and monitoring techniques which enabled timely detection and resolution of the conflict before there was an infringement of separation standards.

 
  1. The Yarowee sector controller was distracted by activity at a neighbouring console before completing the actions required to assign the amended flight level to the crew of the A330.
  2. The Yarowee sector controller cancelled the cleared flight level prompt on the air situation display prior to receiving a correct read back.
  3. The aircraft crews were operating on different frequencies at the time of the potential confliction and were therefore unaware of the situation.


 

ATSB safety action

As a result of a previous occurrence, the ATSB is investigating a safety deficiency (BS/20020004) relating to the limitations of self-checking of data inputs by controllers. The circumstances involved in this investigation also relate to the same deficiency. Any outcome from the investigation of the safety deficiency will be published on the ATSB website .

 
General details
Date: 09 June 2002 Investigation status: Completed 
Time: 1130 hours EST Investigation type: Occurrence Investigation 
Location   (show map):185 km NW Melbourne, (VOR)  
State: Victoria Occurrence class: Airspace 
Release date: 12 May 2003 Occurrence category: Incident 
Report status: Final Highest injury level: None 
 
Aircraft 1 details
Aircraft manufacturer: Airbus Industrie 
Aircraft model: A330 
Aircraft registration: PK-GPE 
Type of operation: Air Transport High Capacity 
Damage to aircraft: Nil 
Departure point:Melbourne , VIC
Destination:Denpasar, INDONESIA
Aircraft 2 details
Aircraft manufacturer: The Boeing Company 
Aircraft model: 737 
Aircraft registration: VH-TJT 
Serial number: 24445 
Type of operation: Air Transport High Capacity 
Damage to aircraft: Nil 
Departure point:Adelaide, SA
Destination:Melbourne, VIC
 
 
 
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Last update 13 May 2014