Boeing Co 747-412 aicraft, 9V-SPA and a Boeing 747-438, VH-OJL aircraft, 7 October 2001

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.