Monitoring the Use of TCAS

Monitoring the Use of TCAS

34TH ANNUAL CONFERENCE, Jerusalem, Israel, 27-31 March 1995

WP No. 138

Monitoring the Use of TCAS


During the 33rd Annual Conference in Ottawa working papers were presented detailing new areas of Air Traffic Control in which it was proposed that TCAS could be used. SC7 were tasked with monitoring the use of TCAS including these new developments from a legal viewpoint.


TCAS II was designed to provide aircraft with traffic advisories and resolution advisories on traffic in the vicinity which may come into conflict with the subject aircraft. TCAS is not intended as a substitute for Air Traffic Control, but as a final safety net in the prevention of collisions. The carriage of TCAS became mandatory in the USA for all passenger carrying aircraft with more than 30 passenger seats in December 1993. Because of this requirement, many aircraft are using this system throughout the world, and it is likely that the carriage of TCAS will be required in countries other than the USA. As a consequence of this, controllers outside of the USA will inevitably have to deal with incidents in which one or both of the aircraft involved are TCAS equipped. In this event it is likely that a controller’s actions would be enhanced if he were to be aware of those aircraft able to utilise this equipment. This information could be made available by insertion on the flight plan submitted to Air Traffic Control.

Since the introduction of TCAS many problems have been encountered, these mainly originate from the logic used to decide a course of action, and the fact that the system does not know the actual intentions of individual aircraft. Various changes have been made to the programming, but some problems still remain. In view of these difficulties, and in order that controllers responsibilities are clearly defined, some states have written statements into their operational orders and manuals of ATC defining a controller’s obligations in situations where an aircraft departs from its Air Traffic Control Clearance because of a TCAS resolution advisory.

IFATCA policy states that:

“A Controller shall not be held liable for incidents in which a loss of separation occurs due to a resolution advisory’ issued by an automated system.”


Oceanic In Trail Climb

Despite the fact that problems still exist with the operational use of TCAS, trials have recently taken place in which TCAS has been used as the primary method of separation. This procedure is known as “Oceanic In Trail Climb”. The principal motivation behind these trials is commercial, in that at present aircraft are sometimes not assigned the most economical cruising level because under existing ICAO criteria the required separation between aircraft could not be guaranteed particularly during the climb phase. It has been proposed that TCAS could be used to guarantee separation between two aircraft whilst a climb to the required cruising level is canned out. IFATCA policy in this area is:

“IFATCA recognises that the development of airborne collision avoidance systems should be encouraged. However, it must be accepted that the primary means of collision avoidance within a controlled airspace environment must continue to be the Air Traffic Control system which should be totally independent of airborne emergency devices such as ACAS. Autonomous airborne devices should not be a consideration in the provision of adequate air traffic services.”


ATCOs using the above procedure could be put in a difficult position as they might potentially have to “second guess” the information given to them by pilots. From a legal point of view, the issue in any incident will be: “Should the controller have realised that the information provided to him by the pilot was in error?” In most cases it is unlikely that blame would be put on the controller concerned, as he will not have any method of checking the information on which he based his course of action. IFATCA policy states:

“A Controller shall not be held liable for incidents that may occur due to the use of inaccurate data if he is unable to check the integrity of the information received.”


If there is an accident due to the use of incorrect TCAS information then it is to be expected that the lawyers involved in the resulting litigation would attack the overall system, its approval procedure, and the reasons for its being put into operation. The litigation may well involve the ATC organisations responsible, but in most countries, the individual controller would not be involved. However, this situation may vary from country to country, and it cannot therefore be assumed that controllers will be completely free from legal action.

Although it seems that considerable care has been taken to ensure that potential TCAS errors have been taken into account. The oceanic in trail climb procedure relies heavily on the fact that there is likely to be only one aircraft visible on the TCAS display.

However, there may be occasions when this is not the case, and in this situation the potential for misidentification is increased significantly. The method of identification used is of a passive nature, in that it requires the leading aircraft to deselect the aircraft transponder, which should result in the relevant response disappearing from the following aircraft’s TCAS display. Unfortunately, other factors such as masking or equipment failure, may cause the same effect, thus leading to possible misidentification. Because of the shortcomings of this identification procedure, Air Traffic Control only employs it when no other means of establishing aircraft identity is available. ICAO documents advise that extreme caution should be used.

If a pilot in command did relay erroneous TCAS information to Air Traffic Control thou the brunt of any civil litigation would be aimed at the airline concerned. Nevertheless one problem with litigation, particularly in the United States, is that claimants lawyers will sue anyone with a potential involvement. This obviously causes anxiety and cost to those implicated.


In consideration of the fact that the use of TCAS is widespread, it is essential that all MAs have a written agreement with the relevant authority that: – “Controllers will not be liable for any loss of separation that might occur if an aircraft departs from it’s air traffic control clearance because of a TCAS resolution advisory”.

The flight plan should indicate if an aircraft is equipped with TCAS.

IFATCA policy does not support the use of TCAS as the primary means of collision avoidance in a controlled airspace environment.

Controllers should consider very carefully the problems associated with the use of the “Oceanic In Trail Climb” procedure before employing it operationally. In particular, the fact that no method of verification of the data is available must which heavily against its use unless there is an overriding safety reason.


It is recommended to conference that this paper be accepted as information material.


Oceanic in trail climb – Issues Summary V 3.1 – TCAS Separation Assistance Working, Group, FAA.

Last Update: September 28, 2020  

February 11, 2020   212   Jean-Francois Lepage    1995    

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