Network Manager Operational Safety Study on traffic collision avoidance

10 November 2017

EUROCONTROL’s ultimate goal is to keep Europe’s ATM Network safe while increasing capacity and efficiency. The Network Manager identifies Network safety issues to help aviation stakeholders identify existing hazards and to anticipate and avoid new operational risks.

A study has been released on “TCAS RA not followed” (Traffic Collision and Avoidance System - Resolution Advisory) events. Although these events are infrequent, when they do occur, they tend to be severe  incidents so it was deemed necessary to raise awareness of the issue.

The study gives guidance for people working on operational safety analysis and improvement activities.

Pilot survey

For this study, it was concluded that the best way of understanding why pilots sometimes do not follow a TCAS RA was to find out from the pilots themselves. Consequently, a voluntary online survey was conducted.

The findings showed that around 36% of the pilots reported experiencing at least one TCAS RA situation in the five-year period covered by the survey. Of these 36%, around 15% reported not following an RA for various reasons. In addition, according to the survey, the percentage of TCAS RAs that are not followed is likely to be around 11%.

Conclusions and recommendations

Coordinated RA generation and response form an essential safety barrier. However, sometimes pilots fail to follow the RA correctly or even do not follow it at all.

This study identified just three safety barriers available to Flight Deck crew that could potentially prevent TCAS RAs from not being followed correctly. All these barriers are only effective for a small number of generic operational scenarios.

However, since one type of scenario is prevalent in the actual operation – that is, TCAS not followed due to visual acquisition (when the pilot flying thinks that he has seen and can avoid the “intruder” aircraft) – more training and greater pilot awareness could be effective.

The study concluded that the Autopilot/Flight Director’s capability of flying the RA is the only barrier that could potentially mitigate the impact of pilots not following TCAS RAs correctly.

The study recommends that the International Air Transport Association (IATA), pilot associations, aircraft operators and regulators consider promoting the more active use of flight data management systems (FDM) to monitor TCAS RA compliance; they could then provide feedback to training organisations and the flight crews involved.

Moreover, the study recommends that European air navigation service providers (ANSPs) and the EUROCONTROL Safety Improvement Sub-Group (SISG) monitor occurrences involving “TCAS RA Not Followed” to determine changes in frequency and/or severity.

In particular, it stresses the importance of following TCAS RAs even when the pilot thinks the “intruder” aircraft has been seen: it could happen that there is more traffic in close proximity that the pilot did not see, or that the other aircraft’s intentions are misread.

TCAS RAs have to be followed in all circumstances; they override ATC instructions. The only exceptions to following RAs are when critical warnings have also been given, such as stall warnings, wind shear and ground/terrain proximity warnings, especially when the aircraft is less than 2,500 feet above ground level.


To determine the Network Manager “Top 5”, the Safety Improvement Sub-Group first identified two broad areas for detailed review:

  • Runway incursions
  • Loss of separation en route.

They then came up with the detailed “Top 5” ATM operational safety priority areas:

  • Risk of operation without a transponder or a dysfunctional one.
  • Sudden high energy runway conflict.
  • Controller detection of potential runway conflict.
  • “Blind spot” - inefficient conflict detection with the closest aircraft.
  • TCAS RA not followed.

The latest study is on “TCAS RA not followed”.


Detection of occupied runway

Controller detection of occupied runway

Some Runway Incursion incidents could be prevented if controllers had better means to detect that the runway was occupied.
Risk of operation without transponder or with a dysfunctional one

Risk of operation without transponder or with a dysfunctional one

Operations without transponder or with a dysfunctional one constitute a single threat with a potential of “passing” through all the existing safety barriers up to “see and avoid”.

“Blind spot” – inefficient conflict detection with the closest aircraft

“Blind spot” – inefficient conflict detection with the closest aircraft

Loss of separation “Blind Spot” events are typically characterised by the controller not detecting a conflict with the closest aircraft. They usually occur when a controller is focussed on a “future situation” and has filtered out the most immediate aircraft.

Watch the SKYbrary clip

Sudden High Energy Runway Conflict (SHERC)

The scenario typically involves rapidly developing situation of runway entry in front of a high energy landing or taking-of aircraft at position where the available reaction time is close or less than the needed reaction time for detection, communication and collision avoidance manoeuvre.

TCAS RA not Followed

Losses of Separation in the En-Route environment sometimes involve “TCAS RA not followed by one or more flight crews”. Coordinated RA generation and response is an essential safety barrier; however, some events include a failure to follow the RA correctly or not following the RA at all.


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