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The investigation could not discover the reason for the power outage in the Sydney TCU.

Equipment issues

Although the initial timing of the UPS maintenance was scheduled for between 1500 and 1800, the duration of the works plan, 27 hours, would have left the TCU exposed to a single point of failure during busy traffic periods. The significance of running the TCU on a single UPS for 27 hours did not seem to be understood by the parties approving the works plan. Consideration should be given to conduct maintenance on critical equipment when the level of operations is low.

Maintenance documentation

In western culture, people are taught to read from left to right. Information is processed in this order due to this learned behaviour. The converse display of the UPS system with the "A" system on the right and the "B" system on the left has the ability to confuse an operator as to which system they should be switching at any given time. Switching errors may occur due to the transposition of the schematic diagram as opposed to the actual machinery.

The Airways engineering instructions (AEIs) that related to the UPS equipment were inadequate for use by the electrical technical officers. The AEIs had insufficient procedures to ensure safe operations while conducting the required tasks. That was because of the broad parameters of those documents. The electrical technical officers were expected to know how things were done. The AEIs did not adequately direct how activities and tasks were to be carried out.

The practice of putting safety defences in place is a measure that is used to mitigate human error. Two of the functions of defences are to create understanding and awareness of the local hazards and to give clear guidance on how to operate (Reason, 1997). The AEIs for UPS equipment failed to meet those two basic safety defence functions. That proved to be a latent failure in the Airservices system.

There are many different types of equipment the technicians need to be familiar with. This multiple array of equipment does not allow the technicians to gain enough competency and currency on all types of equipment they are needed to repair and service. This lack of familiarisation with the equipment makes competency errors in maintenance procedures more likely. Greater clarification of the correct procedural steps to be undertaken by the technical officers would lessen the possibility of recurrence.

Equipment serviced by Airservices electrical technical officers has grown to a point where the use of a defined specific task list for each different type of equipment is needed. This is to correct the lack of safety defences by creating understanding and awareness of the local hazards and giving clear guidance needed by the technicians to conduct their tasks. This in turn would help in ensuring that occupational health and safety is not compromised. Specific guidelines, which include the steps involved in the processes needed to complete a task, need to be developed. This will decrease the possibility of an error occurring because of an incorrect sequence of events or procedure being carried out.

Communications issues

The Tower Traffic Management (TTM) Coordinator's view of the TCU Team Leader's advice of "we've lost everything down here" meant something different from the meaning inferred by the TTM Coordinator. This was probably because of prior experiences the TTM Coordinator held in terms of the "loss" of incoming information during degraded modes operations within TAAATS. Language interpretation is often based on learned expectations. When words or phrases are spoken, there is often a semantic and grammatical interpretation made of those words or phrases. In complex control and operation tasks, this interpretation is based on the context in which the information has usually been received. Misunderstandings can arise when faulty diagnosis of the underlying inferred information is assumed.

The information given by the TCU Team Leader appeared to be interpreted in a filtered manner because of previously learned expectations of the TTM Coordinator. This misunderstanding prevented the TCU controllers from getting help from the tower controllers. The misunderstanding by the TTM Coordinator may have contributed to the lack of a broadcast being made on the Computerised Automatic Terminal Information System.

The assumption made by the TCU Team Leader that the TTM Coordinator had lost a similar amount of facilities was based on his interpretation of the underlying problem. This assumption was not correct. There was no mechanism in the procedure of describing types or levels of emergencies that allowed either party to gain further clarification of the correct meaning. In flight operations, an example of this is the ability to rank an emergency as either a Pan call or a Mayday call. This needs to be addressed in the ATC environment.

Teamwork issues

The notion that managing all the available resources - information, equipment and people - at any given time in the most effective and efficient manner is not new in the aviation industry. The concepts of Team Resource Management (TRM) are the same as those for Crew Resource Management (CRM). However, there seems to be little consideration of this aspect for the work conducted by electrical technicians.

There is very little research on the use of TRM with technicians in the aviation industry. However, based on the successful implementation of CRM with flight crew, air traffic controllers and aircraft maintenance engineers, there is no reason to believe that this success cannot be transferred to technicians.

To achieve this efficient working relationship it is necessary to analyse what jobs technicians perform and which tasks need two people to accomplish. Team Resource Management can be designed to help in achieving the best use of resources to gain the desired outcomes.

Although technicians work together when conducting potentially life-threatening work, there were no procedures or practices in place to manage two sets of resources in unison, with each having specifically demarcated roles and responsibilities to enable work input and output to be managed to achieve the desired goals in the most effective manner.

Role clarification

There appeared to be some overlap in the tasks conducted by each technician at the Sydney Terminal Control Unit (TCU). This can lead to confusion as to who is to do what, and when each task is to be performed. Role clarification and demarcation of tasks is needed for technicians to decrease the chance of something being done twice or not being done at all. To do this successfully it is necessary to find out what the tasks are, when they are to be done, and which person should be completing them. A process review of the tasks performed and the level of skills and expertise needed for each task could accomplish this.

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