European ACAS Operational Monitoring 2002 Report

The report analyses a total of 1054 European RAs from 16 states, mostly provided by pilot and controller reports.

The figure below shows the distribution of RA events by altitude for the European airspace. A peak is evident in altitudes between FL100 -130. This is mainly the result of the airspace organization at these levels, coupled with high vertical rates in standard level-off procedures. The peak seen in previous years between FL200-300 is no longer evident following the introduction of RVSM. However there is a bulge between FL180 and FL280 reflecting traffic climbing and descending at those levels. The peak at about FL30 corresponds to interactions between controlled and uncontrolled airspace.

In the vast majority of cases (97% - an improvement on previous years) the pilot reported following their RA. This may show increased awareness of the importance of following TCAS RAs among the airlines.

Still the biggest issue remains the nuisance level offs. Flight levels separated by 1000ft, with either or both climbing and descending traffic levelling off, leads to a considerable number of these cases.

Adjust Vertical Speed incorrect response

One new issue was discovered: some pilots have been misinterpreting RAs with the aural annunciation “Adjust Vertical Speed Adjust”. An example of such a misinterpretation is given below:

AC1 is climbing to FL310. AC2 is descending to FL320 in the opposite direction. As they close TCAS provides both aircraft with a traffic advisory followed by a resolution advisory with the annunciation “Adjust vertical speed adjust”. AC1 correctly decreases the rate of climb approaching the cleared level. AC2 incorrectly increased the rate of descent significantly, continuing through its cleared level before TCAS provided an advisory to climb, which was followed. As AC2 increased the rate of descent TCAS issued AC1 a resolution advisory to descend which was followed. AC2 reported visual with intruder descending due to TCAS as it passed the cleared level. This is one of a number of encounters where this TCAS advisory has been misunderstood when visual with the intruder. In nearly all these cases visual presentation of the advisory has been on a vertical speed tape.

Full report

For further information send an email to acas@eurocontrol.fr