Study Reporting of TCAS RAs

Study Reporting of TCAS RAs

45TH ANNUAL CONFERENCE, Kaohsiung, Taiwan, 27-31 March 2006

WP No. 84

Study Reporting of TCAS RAs

Presented by TOC


1.1.  After a near mid air collision (NMAC) over Japan (January 31st, 2001) and the tragic mid-air collision near Ueberlingen, Germany, (July 1st, 2002) where the Traffic Collision Avoidance System (TCAS) – procedures have played a role in the chain of events, ICAO reacted very swiftly in reviewing and clarifying previously published procedures. This clarification was laid down in Amendment 12 of ICAO-document 8168 (PANS-OPS) which went into force in May 2003. It now reads that TCAS advisories not only take precedence over (conflicting) ATC instructions, but also that pilots are required to report all TCAS Resolution Advisories (RA) as soon as practical to ATC.

1.2.  After the May 2003 implementation, operational monitoring and operator feedback started to flow in. It revealed that the effected changes weren’t only having the positive effects that were expected, but also had significant drawbacks. This feedback was taken over by ICAO’s Surveillance and Conflict Resolution Systems Panel (SCRSP) at its plenary meeting in November 2004. There, it was decided to proceed with a complete review of the relevant TCAS procedures. The panel wanted to find out what the operational effects of the recent PANS-OPS changes were, and so determine where improvements are needed. The result was a series of recommendations issued by SCRSP 1 to the Air Navigation Commission (ANC) of ICAO for action. In July 2005 ICAO reacted to these recommendations and sent out parts of the SCRSP TCAS proposals for consultation. Other recommendations were referred to other ICAO bodies or panels for additional work and supplementary studies, as the ANC wasn’t convinced of their maturity, or its relevance.


2.1. The operation of TCAS in busy and complex airspace poses a real challenge. From the beginning TCAS was designed to function as an independent airborne Safety Net (SN) and therefore shall be of no consideration for the provision of ATM service. This means that ATC must offer services to each flight, regardless if the aircraft is carrying TCAS or not. For controllers, therefore, TCAS remains an uncoordinated system that is not transparent to ATC.

2.2.  In theory, and in design, TCAS was meant to function as a last-ditch safety net, i.e. after the provision of ATC separation has failed. However, practice has shown that while the pilot is responding to an RA, the ATCO, not knowing what the pilot is doing at the same moment, might issue contradictory ATC instructions (in particular dangerous when in the vertical plan). This has led to several incidents and has been a factor in the chain of events of one accident. Therefore, ATC must be informed by R/T, as soon as possible, about aircraft deviating from ATC clearances.

2.3.  At the beginning, when TCAS was introduced, RAs were flown with significant deviations from the acknowledged ATC clearance. Improved pilot training and crew awareness, coupled with a refinement of the TCAS Software Logic has permitted a considerable reduction of the magnitude of the RA-deviations. The introduction of Collision Avoidance System (CAS) software version 7.0 has further improved the situation, as it reduced the alert thresholds even further. Version 7.0 was also introduced to enable Reduced Vertical Separation Minima (RVSM) operations. One of the means to achieve this reduction in magnitude in the RA-responses is to reduce the time frame these RAs are active. Starting as early as with TCAS version 6.04A, TCAS logic was able to issue so-called Preventive RAs. A preventive RA is a resolution advisory that advises the pilots not to deviate from the current manoeuvre in the vertical plan. In other words, to maintain the actual vertical flight path in response to a TCAS RA. This manoeuvre (even if no change is required) is clearly a TCAS RA, although “resolution” doesn’t mean in all cases manoeuvring.

2.4.  Despite a considerable effort of providing full and complete information about the functioning of TCAS, there is still a widespread belief within the aviation community and the international decision makers regulating the ATM system that a TCAS RA always means a positive pilot control input and always a deviation from ATC clearance. This is coupled with the strong opinion “that TCAS RAs can only happen when ATC separation has already failed” or when “ATC made an error”. This is not true for all cases but it is very hard to fight against these misconceptions. It’s certainly under this wrong impression and suffering from immense time pressure to act (after the serious NMAC in Japan and the mid-air disaster of Ueberlingen) that the urgent PANS-OPS changes went through the ICAO process with record speed. This amendment of document 8168 reinforced the TCAS-procedures for pilots and tried to address several uncertainties and ambiguities that were brought to light by incidents and accidents. It made it clear that pilots must always follow a TCAS RA, even if a simultaneous ATC instruction, contrary to the direction of the RA posted in the cockpit, is transmitted and received.

2.5.  The change in Amendment 12 of ICAO Doc 8168 (PANS-OPS) determines that all TCAS RAs must be reported over R/T:

“Pilots shall, as soon as permitted by flight crew workload, notify the appropriate ATC unit of the RA, including the direction of any deviation from the current ATC instruction or clearance”.

This provision leaves no room for pilots but to report all TCAS RAs to ATC via the frequency. Controllers, especially those working in high density airspace, where R/T loads sometimes reach critical limits, started to encounter many disturbing preventive RA-reports that were of no added value to them. In some cases, important ATC clearances were blocked or couldn’t be transmitted in due time as the frequency was occupied by crews reporting a Preventive RA, or the ensuing “Clear of Conflict” message. All this despite the fact that the RAs posted were very short and that the aircraft never manoeuvred, and/or never left the assigned ATC clearance.

2.6.  Most Air Traffic Controllers and many pilots agree that R/T reports of preventive RAs without deviating from ATC clearance has no added value and is a nuisance for ATC. In September 2005 ICAO has accepted this view and published a State Letter proposing another change of the TCAS RA-reporting requirement. This time ICAO proposes that only TCAS RAs that are a deviation from ATC clearance must be reported on the frequency. It remains to be seen what are the replies and reactions from States and the stakeholders addressed by this ICAO consultation process and how quickly the international organization can proceed with changing the PANS-OPS provisions.

2.7.  Another consequence of the systematic TCAS RA reporting over the frequency that must be tackled is the responsibility for separation provision during a TCAS RA. This is of utmost importance for IFATCA and the operational controllers that the Federation represents. All Air Traffic Controllers shall receive proper TCAS training as part of their training syllabus, or as recurrent training. ATCOs are required to “not try to attempt to modify the flight path of any aircraft involved in the RA”. ICAO PANS ATM (Doc 4444) clearly determines this under

“When a pilot reports a manoeuvre induced by an ACAS resolution advisory (RA), the controller shall not modify the aircraft flight path until the pilot reports returning to the terms of the current air traffic control instruction or clearance but shall provide traffic information as appropriate”.

2.8.  This ICAO provision for the TCAS-system behaviour is of utmost importance as the danger of pilots receiving contrary instructions from ATC and TCAS is a real one. Further analysis of the ICAO requirement to report all RAs to ATC as mandated through PANS-OPS amendment 12, provides cause for concern, especially with regards to controller responsibility. ICAO document 4444 (PANS-ATM), paragraph reads:

“Once an aircraft departs from its clearance in compliance with a resolution advisory (RA), the controller ceases to be responsible for providing separation between that aircraft and any other aircraft affected as a direct consequence of the manoeuvre induced by the resolution advisory.”

For controllers this statement is not at all satisfactory. The proposed systematic reporting of all TCAS RAs, including preventive RAs, will result in a situation whereby controllers have to remain “hands off” the traffic on very frequent occasions. In most preventive RA cases the aircraft don’t manoeuvre and/or don’t leave their assigned ATC clearance, however the preventive RA-report must still be made (current situation). When the initial part of paragraph, namely “that an aircraft departs from its clearance” isn’t fulfilled, as is the case in most of these situations, the controllers actually remain fully responsible for provision of separation. The result is that operational controllers are left in a very uncomfortable position by this conflicting legislation. Operational monitoring indicates that nowadays 75% of all RAs are of the preventive RA-type, which demonstrates how frequently these cases are happening.


3.1. Operational monitoring and operator feedback showed that the changes recently implemented by ICAO (in 2003) with amendment 12 did solve some existing and perceived problems with TCAS-operations and RA-reporting, but created new issues and shortcomings.

3.2.  Most pilots and controllers agree that only RAs where aircraft leave their assigned ATC clearance shall be reported to ATC. By doing this, unnecessary and possibly dangerous R/T frequency loading can be avoided. ICAO’s ANC has now accepted this view and has issued in September 2005 an ICAO State Letter proposing this change to document 8168 (PANS-OPS). According to Eurocontrol’s TCAS performance monitoring approximately 75% of all TCAS RAs are of the preventive type requiring no deviation from ATC clearance and no positive pilot input on the commands. Stakeholders are confident that this ICAO proposal will help to avoid unnecessary and possibly dangerous R/T frequency loading with preventive RAs.

3.3.  Operational controllers are also not satisfied with the determination of the separation responsibility during a TCAS RA. As far as ICAO regulations go, controllers are required to cease to transmit flight path modifying messages and clearances to all flights subject to a TCAS RA. According to PANS-ATM, the separation responsibility for controllers only ceases once the aircraft starts to manoeuvre in response to a TCAS RA. If the crews are not manoeuvring their aircraft, the controllers remain fully accountable.

3.4.  It is important to determine clearly that a flight crew, when reporting a TCAS RA over R/T is releasing the controllers of the responsibility of providing separation between that aircraft and any other aircraft under control until the reception of “Clear of conflict”. This must be automatic and unambiguous, and must happen irrespective of whether the aircraft is manoeuvring or not, and also irrespective of whether the aircraft is leaving the assigned ATC clearance or not.

3.5.  The points and problems reported in this working paper prove that part of the current ICAO TCAS procedures and the RA-reporting provisions are not satisfactory. ICAO has now issued a State Letter, and until ICAO policy is ratified, the IFATCA Policy is recommended for adoption. IFATCA, by changing and adapting its TCAS policies now, is taking a proactive stand which will help its representatives to make strong and important inputs into ICAO’s decision making process in the coming years.


It is recommended that;

4.1. IFATCA Policy is:

“Pilots shall only report TCAS RAs on the frequency that require a departure or deviation from the current ATC clearance or instruction.”

and is included on page 3 2 1 2 of the IFATCA Manual.

4.2. IFATCA Policy is:

“Once an aircraft departs or deviates from the current ATC clearance or instruction in response to a resolution advisory or when a pilot reports a TCAS RA, the controller ceases to be responsible for providing separation between that aircraft and any other aircraft under control until the pilot reports clear of conflict and separation has been established. This must be automatic and unambiguous, and must happen irrespective of whether the aircraft is manoeuvring or not, and also whether or not the ATC clearance is left.”

and is included on page 3 2 1 2 of the IFATCA Manual.

Last Update: September 29, 2020  

April 8, 2020   310   Jean-Francois Lepage    2006    

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