ACAS II Bulletin no. 3

ACAS operational monitoring programmes have shown that the TCAS RA display is occasionally misinterpreted by flight crews. Occurrences seem to be linked to the type of RA with the aural, ‘AdjustVertical Speed, Adjust’. This RA is posted either as an initial RA, or as a weakening RA which can follow a “Climb” or “Descend” RA. “Adjust Vertical Speed” RAs are the most frequent RAs triggered by TCAS II. It is essential that these RAs are followed accurately. This ACAS Bulletin, through explanations and illustration of actual events, seeks to address this operational issue. It shows clearly the flight crew responses which are expected, both to initial reduce vertical speed RAs, and to weakening RAs. In addition, it highlights the risks which can be associated with inaccurate responses to these RAs. Finally, earlier Bulletins have emphasised the need to ‘Follow the RA’. Amendment 12 to the ICAO PANS-OPS, Doc 8168, includes reinforced ACAS provisions which make this clear. Adherence to the PANS-OPS ACAS provisions will help ensure that the ACAS safety benefit is maximised.

John Law
ACAS Programme Manager,
EUROCONTROL
October 2003