TCAS RA Not Followed


This article is intended to give an overview of the occurrences where a TCAS RA has not been followed (correctly) by the flight crew. It describes the possible scenarios and the safety barriers used to prevent or mitigate the safety effects of such occurrences.

TCAS is one of the last safety barriers to protect from an accident, the ones after it being pilot visual response (with limited applicability in Instrument Meteorological Conditions (IMC)) and providence (i.e. luck). Therefore, ICAO documents state that all RAs should be followed unless the safety of the aircraft is at risk.

One of the most common reasons for not following a TCAS RA is the visual acquisition of (what is perceived to be) the conflicting traffic. It is worth noting, however, that the flight crew might not be fully aware of the overall picture in complex traffic situations and therefore visual acquisition is not a valid excuse for procedural non-compliance.

Generic Scenarios

dedicated study was performed in 2017 by the EUROCONTROL Safety Improvement Sub-Group (SISG) after identifying "TCAS RA Not Followed" as one of the Top 5 ATM Operational Safety Priorities. The SISG's analysis examines the operational context (factors that can affect the situation) and establishes a set of generic scenarios that account for different situational developments. While the scenarios are not equally probable (and data collected from both A&I events and a dedicated survey confirm they are not) they do constitute a useful, concise representative subset that substitutes effectively for trying to imagine all possible scenarios. The scenarios are grouped in the following categories and subcategories:

  • Lack of pilot detection/awareness of the TCAS RA due to:
    • Human-machine interface (Human-Machine Interface (HMI)) issues (i.e., factors concerning the perception and interpretation of the TCAS HMI and alert annunciations);
    • Presence of other flight deck signals and alerts, or external distractions, occurring simultaneously with the TCAS RA; and/or
    • Flight deck mindset (i.e., any factor concerning the mind-set or expectations of the crew immediately beforehand).
  • Misinterpretation of TCAS RA due to:
  • Decision not to follow RA due to:
    • Other signals and alerts;
    • Contradictory ATC instructions (this situation also includes prior ATC instructions being actioned when the RA occurs);
    • TCAS credibility issues (e.g. if the RA is perceived as false/unreliable or due to rapid succession of different RAs);
    • Aircraft performance issues/concerns (actual or perceived aircraft performance limitations);
    • Flight deck mind-set;
    • Significant flight conditions (e.g. cockpit warnings, severe weather, terrain, etc.); and/or
    • Inadequate TCAS training.
  • TCAS RA response not executed correctly due to:
    • HMI Issues;
    • Late or inadequate pilot response;
    • Unintentional pilot response;
    • Aircraft performance issues/concerns;
    • Flight deck CRM;
    • Inadequate TCAS training;
    • Flight deck mind-set;
    • Duration of RA less than the pilot's reaction time; and/or
    • Flight deck control inputs and handling (e.g. inappropriate vertical speed when approaching the selected flight level).

Safety Barriers

Two major sets of barriers can reduce the risk associated with TCAS RA not followed events:

  • Prevention barriers: When deployed and employed correctly, these defences are capable of preventing a TCAS RA not being followed. Three prevention barriers have been identified:
    • Following a TCAS TA: TCAS RA Prevention Functionality by Auto Pilot reducing rates of climb/descent approaching selected level.
    • Enhanced TCAS training including experience with multiple tracks, conflict-aircraft crossing and reversal RAs; and,
    • Training and enforcement of standard operating procedures (SOPs) requiring TCAS RAs to be followed unless the safety of the aircraft would be compromised
  • Mitigation barriers: When deployed and employed correctly, these are capable of alerting pilots to a potential conflict during the initial stages of an event involving not following the commands of an activated TCAS RA, in sufficient time to act in order to prevent a collision. One mitigation barrier has been identified:

These barriers are identified in the TCAS-dedicated SISG study and are based on a wide literature search and consultation.

Analysis of the effectiveness of the safety barriers against the generic scenarios has shown that the three prevention barriers cited above achieve limited effectiveness (i.e. they are effective in a small number of situation types). However, the "Training and enforcement of SOPs requiring TCAS RAs to be followed" barrier could prevent the events where pilots detect and interpret the RA but decide not to follow it. Experience has shown that the decision not to follow a TCAS RA is the most common cause for such events. Therefore, this barrier could be beneficial from a practical point of view.

In contrast, the mitigation barrier cited above achieves high effectiveness (i.e. is effective for most of the generic scenarios). However, this capability currently is only available on new models from one manufacturer, with a retrofit being available for older fleet aircraft. For the time being, it is unlikely that this capability will become available from any other manufacturer due to patenting issues.

TCAS RA Not Followed A&I Examples

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