TCAS RA Survey


The EUROCONTROL Safety Improvement Sub-Group (SISG) identified TCAS RA Not Followed as one of the Top 5 ATM Operational Safety Priorities in 2017.

There was no reliable and representative data on the number of TCAS RA not followed. The results from different sources ranged from 1% to 35%. Therefore, a survey on the matter among pilots was conducted. It was voluntary and received 3800 responses from 90 countries.

The aim of the survey was to gather and analyze statistical data to better understand the factors that lead to TCAS RA non-compliance and the impact of the operational environment on such occurrences.

The time period covered by the survey was 5 years ending in 2017.

dedicated study combined the results of the survey with theoretical analysis of relevant generic scenarios and possible safety barriers.

Study Findings

General Statistics

  • About 36% of the participants reported to have “encountered a TCAS RA situation in the past 5 years”.
  • About 5% of the participants reported experiencing a TCAS RA that was not followed either at all or not as commanded.
  • About 15% of all the pilots that experienced TCAS RA participated in a TCAS RA not followed event.
  • About 12% of the participants reported having 3 or more TCAS RAs during the 5-year period.

The statistical analysis that took into account the number of events (i.e. more than one event per person) concluded that the percentage of RAs that are not followed is likely to be around 11%.

Reasons for TCAS RA Not Followed

A total of 18 reasons (out of 20 possible) were stated for not following a TCAS RA.

  • The most reported reason was "Decision not to follow due to visual acquisition and/or avoidance of the conflicting traffic" (45% of all non-follow events).
  • The second most reported reason was "Short duration RA (RA terminated before a response could be taken)" (15% of all non-follow events, i.e. 3 times less frequent than the first one).
  • The third most reported reason was "Proximity to the ground" (11% of all non-follow events, i.e. 4 times less frequent than the first one).

Reason Analysis

  • The Top 3 reasons account for 70% of all responses (45 + 15 + 10).
  • ICAO documents state that all RAs should be followed unless the safety of the aircraft is at risk. Three reasons (proximity to terrain, priority of other alerts and performance issues) with a total of 14% fall within this definition.
  • Three other reasons are considered valid for non-compliance (short duration, false/incorrect RAs and closely spaced runways). These account for 24 % of the answers.
  • About 38% (14 + 24) of RAs Not Followed might be for legitimate reasons.
  • The "Decision not to follow due to visual acquisition and/or avoidance of the conflicting traffic" is not considered a valid reason and account for about 70% of the non-legitimate reasons. This is the prevailing answer for all flight levels.
  • The level of inability to follow RAs due to their short duration is constant across all altitude bands.
  • About 46% of RAs not followed are below 3000ft. Most of these involve a decision not the follow the RA after visual acquisition of the apparent intruder.

Pilot TCAS Training

  • Pilots who, in training, are not pre-warned that a TCAS RA is coming, are disposed to react better in live situations.
  • No one type of TCAS RA is trained more than 80% each time. The less common, but perhaps more demanding RAs such as Crossing Climb/Descend are only trained around 30% of the time.
  • About 5% of pilots involved in TCAS RA Not Followed events report no TCAS training within the previous 5 years.

Study Recommendations

The dedicated study made 5 recommendations:

  • that IATAPilot AssociationsAircraft Operators and Regulators review the findings of this study and consider undertaking operational safety analysis and improvement activities for “TCAS RA Not Followed”.
  • that IATAPilot AssociationsAircraft Operators and Regulators consider actions to support an increased active use of FDM in the monitoring of TCAS RA compliance and the provision of feedback to training organisations and flight crew involved.
  • that European ANSPs and the EUROCONTROL Safety Improvement Sub-Group (SISG) monitor occurrences involving “TCAS RA Not Followed” to determine changes in frequency and severity.
  • that all European stakeholders monitor and support the development of tools and procedures that may assist in the prevention and/or mitigation of “TCAS RA Not Followed” events.
  • in particular, that all European stakeholders promote and emphasise the requirement and importance of following TCAS RA commands despite an apparent intruder being visually identified and monitored (subject to the overriding safety of the aircraft).

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