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The tracks for aircraft operating in non-radar airspace, being monitored by automatic dependent surveillance (ADS) in the Melbourne Air Traffic Control Centre, were not updated at controller air situation displays in the Bight and West Groups. A controller noticed the problem following a missed position report alarm for a foreign registered Boeing 747 aircraft. An Airservices Australia investigation found that controller workstations were not being updated with ADS data after a problem in the air ground data processor (AGDP) of The Australian Advanced Air Traffic Control System (TAAATS). There were no infringements of separation standards.

There were no visual or aural alerts to indicate that the AGDP had failed. The ADS tracks froze for approximately 20 minutes before recovery action was initiated and completed by controllers.

The Airservices' investigation found that an error in address tables, as a result of maintenance by the service provider responsible for transferring data between Airservices and nominated aircraft, stopped the flow of data to TAAATS. A combination of the stopped data flow and an uncommanded switch of the AGDPs caused the ADS tracks to freeze.

Those actions also appeared to have caused the main processor to "loop". Normally, one of the dual processors would have recognised that there was a problem and would have assumed the master role. However, that action did not occur and the standby processor remained in standby mode believing that the other processor was operating correctly in the master mode. Airservices has corrected the looping problem and it will be included in Version 7.3.56 of the TAAATS software. In the interim, Airservices have enhanced the indicator that shows when there has been a AGDP switch to prompt technical and operational staff to check that the system is operating correctly.

The investigation also found that controllers' recovery actions that had worked during previous similar events did not work on this occasion. Controllers were constrained by the lack of appropriate procedures in the Degraded Modes Handbook.

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