Misinterpretation of TCAS RA Aural Annunciation Messages

Misinterpretation of TCAS RA Aural Annunciation Messages

EURCONTROL Mode S & ACAS Programme Reminder

  • There have been a number of instances of incorrect pilot responses to Airborne Collision Avoidance System (ACAS) Resolution Advisories (RAs), apparently due to misinterpretation of TCAS RA aural annunciations and RA displays. Some of these incorrect responses have led to serious incidents.
  • TCAS II provides vertical RA manoeuvre indications to pilots by means of a dedicated RA display, either as a pitch cue on the PFD, or indications on an Instantaneous Vertical Speed Indicator (IVSI), together with an associated aural annunciation.
  • The following RAs have occasionally been misinterpreted:

"Adjust Vertical Speed, Adjust"

"Monitor Vertical Speed"

"Maintain Vertical Speed, Maintain"

"Maintain Vertical Speed, Crossing, Maintain"

  • Therefore, flight crews are reminded that:
    • “Adjust Vertical Speed, Adjust” RAs always require the pilot to reduce the vertical rate of the aircraft, i.e. to climb more slowly, or to descend more slowly, or to level off. The required vertical rate is indicated on the RA display.
    • “Monitor Vertical Speed” RAs always require the pilot to avoid the vertical speeds prohibited on the RA display. Typically, a “Monitor Vertical Speed” RA does not require a change in vertical speed.
    • “Maintain Vertical Speed, Maintain” RAs always require the pilot to maintain a climb or to maintain a descent in accordance with the indications on the RA display. Note: if a “Maintain Vertical Speed, Maintain” RA is generated when the pilot is in the process of changing the vertical speed, it is possible that the current vertical speed will not satisfy the RA. Therefore, to ensure that the vertical speed required by the RA is achieved, it is essential to follow the indications on the RA display.
    • “Maintain Vertical Speed, Crossing, Maintain” RAs should be flown in the same way as “Maintain Vertical Speed, Maintain” RAs. The inclusion of the word "crossing" in the aural annunciation tells the pilot that his/her own aircraft will pass through the altitude of the intruder aircraft.
  • Check the RA display - follow the RA! Extensive TCAS safety information is available from: EUROCONTROL Mode S & ACAS Programme.


© European Organisation for Safety of Air Navigation (EUROCONTROL) May 2007. This alert is published by EUROCONTROL for information purposes. It may be copied in whole or in part, provided that EUROCONTROL is mentioned as the source and to the extent justified by the non-commercial use (not for sale). The information in this document may not be modified without prior written permission from EUROCONTROL. The use of the document is at the user’s sole risk and responsibility. EUROCONTROL expressly disclaim any and all warranties with respect to any content within the alert, express or implied.


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