ACAS II Bulletin - Issue 24

Occasionally, reports are filed that suggest that TCAS II may not have performed as specified. That includes cases when no RA was triggered despite close proximity to the threat aircraft or an RA was triggered in the absence of any credible threat, the RA was inappropriate (e.g. towards rather than away from the threat), unusual RA sequences or technical anomalies.

If it is found that TCAS II performed in such a way, investigations are typically carried out by Aircraft Operators, ATC authorities and, in particularly serious cases, by State investigation bodies. The purpose of these investigations is to find out what happened, establish whether TCAS II has performed as designed, look into all the contributing factors, and finally formulate recommendations in order to prevent similar events in the future.

Investigations should be carried out using all possible data sources.

These comprise airborne recordings (e.g. Quick Access Recorders or dedicated TCAS recorders that are available on some aircraft), ATC radar data (including messages downlinked by the aircraft to the ground during the RA), and pilot and controller reports. Only a complete view of the event allows a full analysis of what really happened and makes it possible to draw conclusions. Limiting available evidence to, for instance, only pilot reports is unlikely to provide the answers. If it is found that TCAS II has not performed as required, the investigation should seek to establish how the problem needs to be addressed. Additionally, the investigation may identify training needs or improvements. Occasionally, pilots or controllers draw their own conclusions on the usefulness of specific RAs from their perspective however, a full assessment can be done only through the investigation process examining recordings and determining whether TCAS performed as specified.

In order to illustrate these key points we have selected some real-life cases that have been reported and investigated. These investigations allow us to establish what really happened in these events.


ACAS II bulletin - Issue 24