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Both aircraft crews acted in accordance with company procedures and followed the TCAS resolution advisories.

The incident occurred at a well-known point of conflict within an en-route sector. The experienced controller was operating a control position with which he was familiar and with traffic volume and complexity that should have been well within his ability. However, he did not take action to ensure separation between two aircraft that he had earlier recognised as being in potential conflict. That was most likely the result of a number of predisposing factors, including the effects of stress, limited recency, distraction, and not using a memory aid.

At the time of the occurrence the controller was testing the serviceability of the secondary radio transmitter. That was a routine and relatively unimportant task. However, the controller felt under pressure, largely self-imposed, to complete it without undue delay. Initially the controller was mistaken as to the exact test required and that led to some confusion on his part. Consequently, he was distracted for longer than anticipated and his awareness of the developing traffic situation was compromised.

Correct prioritisation is fundamental to any complex operating task. The controller was faced with several competing demands for his attention. In addition to monitoring the conflicting aircraft he carried out routine tasks such as issuing instructions to aircraft and transferring aircraft to or from other control sectors, and attempted to assist the supervisor with the radio checks.

Inadequate prioritisation committed the controller to remembering the unresolved aircraft conflict in order to deal with it in a timely manner. However, distraction and a subsequent memory lapse left the conflict unresolved until the activation of the short-term conflict alert.

There were other aspects of the controller's performance that, while not significant in isolation, were possibly indicative of a lower general level of performance at the time of the occurrence. When the southbound aircraft entered the controller's airspace, he did not appreciate that the aircraft was at a non-standard flight level, even though he was well familiar with the airways route. Later, approximately 10 minutes before the incident, the controller did not correct an oversight by a Dash 8 crew. Further, when asked by the supervisor to carry out a frequency check, the controller had some difficulty with what was a relatively straightforward task.


Using low traffic movement periods to combine sectors in order to maintain adequate work levels may result in an individual meeting the formal recency requirements, but not actually having sufficient exposure to a particular sector to warrant meaningful traffic practice. In this occurrence, the amount and type of recent control work that the controller had completed may not have been sufficient to ensure performance to the standard that the recency requirements were intended to maintain; taking into consideration the variation in the level/complexity of traffic during different periods. It is possible that the controller's performance was affected by the relatively short time he had worked in the control position during the previous fortnight.

Stress and performance

At the time of the incident the performance of the Inverell controller may have been adversely affected by stress. Recent significant personal factors may have been likely to cause him considerable chronic stress. The extent to which stress related to non-work factors can affect work performance is often underestimated. Major life events can markedly affect stress levels. In addition, the acute stress of leaving work to attend to an urgent personal matter may also have influenced the controller's performance.

Individuals are often unaware of the extent to which their performance is affected by stress. They may try to "work on" despite problems or difficulties. Individuals may be reluctant to admit, even to themselves, that they are suffering from stress because of a perception that this will be seen as a sign of weakness or failure.

Research has shown that stress can produce errors such as inappropriately delaying necessary actions and forgetting to carry out required actions at a time of high workload or distraction. Stress can result in perceptual and cognitive narrowing, where attention and decision making are focussed on a restricted range of information and tasks. For example, a controller's scan pattern may be disrupted. Stress can also lead to task shedding. This can result in the neglect of crucial matters while time may be spent on tasks of lesser importance. Memory can be significantly inhibited by stress.

In this incident, stress may have reduced the controller's capacity to handle what would normally have been a moderate workload. The controller allowed himself to be distracted by testing the radio equipment, to the detriment of his primary task; that of managing air traffic.

Memory aids

Because the potential conflict remained unresolved, the controller had to keep the task in short-term memory. Omitting to carry out planned actions - a failure of prospective memory - is one of the most common forms of memory lapse. A necessary condition for a memory lapse to occur is that attention is captured by either an external distraction or an internal preoccupation. The use of an appropriate memory aid would have guarded against the separation task being forgotten and a number of such methods were available to controllers.

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