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ATSB analysis indicates no adverse safety trend since NAS stage 2b was introduced on 27 November 2003 based on the ATSB's preliminary review of its aviation safety occurrence database, including an examination of TCAS resolution advisory alerts.

Because of the significance of NAS airspace changes and public debate over the safety of their implementation, the ATSB has reviewed and categorised NAS-related occurrences and undertaken a comparative analysis of TCAS resolution advisory (RA) alerts in the 140 days from 27 November 2003 with 140 days a year earlier.

There were 37 RAs after NAS 2b was introduced and 38 RAs in the same period a year earlier - accordingly, no trend is apparent. One of the RAs in each period was indicative of a more serious safety situation. A second RA on 7 April 2004 in the latter period was still under investigation.

The one serious RA occurrence after NAS 2b was introduced occurred at Launceston on 24 December 2003. The ATSB assessed this as an 'airprox' incident and noted that: While a single occurrence does not provide the basis for a major change to the US-based NAS, which is yet to be fully implemented, the circumstances of this serious incident are indicative of a need for further review and analysis by CASA and Airservices Australia in consultation with industry. Such reviews have been initiated.

In the pre-NAS 2b period, the one more serious RA occurrence was associated with a 'breakdown in separation' in Class C airspace linked to an air traffic control oversight. At their closest, the two jet aircraft involved came within 1200 feet vertically and 2.5 NM laterally.

The ATSB is seeking comment on its Aviation Research Discussion Paper by 21 May 2004 with a view to incorporating suggestions for improvement in the context of a proposed publication in July containing six months of post-NAS 2b occurrence data.

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Last update 01 April 2011