Before Reduced Vertical Separation Minima (RVSM) operations commenced in European airspace, work was carried out to ensure that Airborne Collision Avoidance System (ACAS) II would be compatible with RVSM and would deliver the expected safety benefit.

TCAS II Version 7 successfully addressed compatibility issues related to the reduced vertical separation. As importantly, EUROCONTROL conducted an extensive safety study which used modelling of TCAS II encounters in RVSM. It concluded that, although there were instances when RAs could occur due to high vertical rates in level-off manoeuvres with 1000ft separation or due to poor altitude station keeping, TCAS II would provide the expected additional safety benefit.

Following more than four years experience of TCAS II operations in European RVSM airspace, a further safety study (ASARP - ACAS Safety Analysis post-RVSM Project) was completed using actual recorded TCAS operational encounters. This study confirmed that TCAS II provides substantial safety benefit in European RVSM airspace.

Related Articles

• Full list of ACAS Bulletins published by EUROCONTROL

Further Reading

EUROCONTROL

• ACAS Bulletin 8 "TCAS II Operations in European RVSM Airspace"; The real events described in this Bulletin not only illustrate the safety performance of TCAS II in RVSM airspace, but also confirm that, as in any other airspace, the reduction in collision risk afforded by TCAS II relies on prompt and accurate responses to RAs.

• ACAS Bulletin 1; ATC Avoiding Instructions Opposite to RA;

• High Rates of Climb or Descent;

• ‘Climb’ RAs and Performance Limitations;