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Analysis

Summary

Although there was no breakdown in separation, this incident involved a serious breakdown in coordination that resulted in a B737 not being under effective air traffic control from 0858 EST until after the radar return was identified at 0947. Had the Perth controller not detected the problem, there would have been a breakdown of separation. Although the two aircraft would have both been to the east of TAMOD at the time of passing, and therefore on different routes, this situation was not planned.

The incident directly resulted from a combination of active failures. The two most significant errors were: (a) the omission of a hard copy departure message via the AFTN; and (b) the omission of any coordination on the aircraft from the Melbourne Sector 1 controller.

Processing the Hard Copy Departure Message

Had the trainee ADSO sent the departure message via the AFTN, Perth control would have activated the flight progress strips and, therefore, been better prepared to detect the missing coordination on an aircraft from Melbourne Sector 1. This error consequently removed an important safety defence.

Factors that contributed to the missing AFTN message included:

  1. the aircraft movement advice form being incomplete, and therefore not providing an appropriate cue; high workload;
  2. insufficiently detailed task procedures; and
  3. a lack of direct supervision of the trainee ADSO.

It is likely that the supervising ADSO became too involved in the operation of departure processing tasks to be able to effectively monitor the trainee's performance.

Passing Coordination of the TAPAX Position Report

The omission of the oncoming Sector 1 controller to pass coordination on the B737 to the Perth outer controller meant that the Perth controller had no forewarning of the arrival of that aircraft into his airspace. Therefore, this error increased the likelihood of a breakdown of separation standards. Although the Perth controller did detect the problem in sufficient time to avoid a breakdown in separation, it is likely that such a detection would not usually occur.

Factors that contributed to the omission included the absence of clear communication between the departing Sector 1 controller and the oncoming controller as to who would pass the coordination, and the inability of the departing controller to remain available to ensure the oncoming controller was fully briefed. More specific handover/takeover procedures could have reduced the likelihood of these factors. The development of appropriate teamwork and team resource management programs would also help reduce the likelihood of such inter-controller coordination and communication failures.

The coordination omission was also partly a result of the inherent nature of the task. It was probably an action slip, or an error where the controller's actions did not proceed as planned. More specifically, the error appeared to be a substitution of one highly automatic task (asking the B737's flight crew to transfer frequency at TAMOD) for another (passing coordination on the B737 crew's TAPAX report to the Perth Outer controller). Such substitution slips are usually associated with a period of inattention or distraction. In this case, the controller may have been distracted by the call from another flight crew immediately before she performed the task. As the controller had only been working on the sector for a few minutes, she could also have been distracted by her scanning and familiarisation activities.

An increase in the likelihood of skill-based errors is often correlated with fatigue. In this case, the controller had a less than normal amount of sleep in the 2 nights prior to the incident. However, the investigation was unable to determine if fatigue influenced the controller's performance at the time of the occurrence.

There were cues available to alert the Sector 1 controller to the error, such as the times on the TAMOD strip and the absence of a tick in the last box of the TAPAX strip. However, having developed the intention to conduct a frequency-transfer task, it is unlikely that the controller would have considered checking times on a strip. In addition, research has shown that the absence of a cue (such as a tick) is often not detected. Shortly after the error occurred, the controller disposed of the flight strips and effectively removed any remaining cues she could use to detect the error.

With the introduction of The Australian Advanced Air Traffic System (TAAATS) throughout Australia in 1998 and 1999, the likelihood of this specific type of error should be reduced. Transfers of aircraft between sectors will be conducted automatically. In addition, for procedural control sectors such as Sector 1, the spatial positions of aircraft will be pictorially displayed. It is reasonable to expect that these changes will reduce the likelihood of a controller incorrectly perceiving the position of an aircraft.

Flight Levels

The westbound A320, VH-HYA, was maintaining a non-standard flight level (FL370) which resulted in an eastbound A320, VH-HYR, being given a non-standard level (FL350) for the initial level clearance. This consequential action had the effect of placing VH-HYR in direct conflict with the B737. Had standard levels been applied on the two-way route system that was under procedural control, a safety net would have been put in place. This net would have become prominent had the Perth controller not observed the radar paint of the B737.

 
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