TCAS and ACAS Procedures

TCAS and ACAS Procedures

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

WP No. 88

TCAS and ACAS Procedures


This working paper will outline 3 additions to the current policy statement, as well as a revision to an existing policy statement, their necessity having become apparent as a result of substantial exposure to the ACAS systems of today. SC1 believes the balance of the policy should remain intact, in as much as it’s relevance in light of developments, has been re-inforced. Throughout this paper the terms ACAS and TCAS ( Traffic Collision Avoidance Systems), will be used interchangeably. TCAS being by far the most common ACAS model in use internationally today.


The pressure being placed on air traffic control as a result of the incorporation of TCAS systems have been enormous from 2 standpoints:

(i) the continuing inability of ACAS manufacturers to rectify deficient software algorithms that initiate nuisance traffic and resolution advisories (TA and RA). These are well known and have been extensively documented in both controller and pilot communities and have been the subject of working papers which have led to intense discussions at several forums including these conferences.

(ii) the perceived inevitable expansion of TCAS II’s role in aviation as witnessed by development programs with the FAA (USA). The prospect of TCAS serving as a tool in distance measuring and / or separation maintenance, is proving to be quite unsettling to the profession, to say the least. We believe we are witnessing a gradual erosion of flight order and safety, with a concomitant increase in controller workload.

For these reasons, it was felt that a thorough review of our policy on ACAS should be carried out by SC 1 for the purpose of unequivocally establishing our position on ACAS matters in light of the most recent developments.

The preamble of our policy on ACAS has been re-written and included in the report of the SC1 Technical Policy Co-ordinator. The Ottawa Conference felt and SC 1 agreed, that the preamble required a more rigorous and emphatic definition of our position in light of the expanded role envisioned for TCAS by certain civil administrations. Our opposition to these originally unforeseen uses must be absolute.

It is noted that one of the major mitigating circumstances of tactical disruptions from the use of TCAS, is the lack of training being offered to pilots and controllers.

On the pilot side, this is evidenced by:

(i) deviations from guidelines which dictate maximum vertical displacements during RA encounters;

(ii) failing to respond to RAs;

(iii) using TCAS displays as a means of reporting traffic sighting to ATC;

(iv) using excessively high rates of climb approaching cleared to altitudes.

On the controller side this lack of training is evidenced by:

(i) untimely exchange of traffic information;

(ii) interfering with RAs in process;

(iii) his ability to discern with certitude the level to which an aircraft is ACAS equipped, from the often bewildering array of software logic version, aircraft size, USA or non-USA, and flight plan equipment suffixes.

Civil Aviation Authorities (CAA’s) must be convinced as to the urgency of providing a comprehensive TCAS training package as part of controller refresher and ab-initio training. Controllers require a subtle appreciation of the effects TCAS has on a cockpit environment. As well , pilots require to know that TCAS traffic information is not valid for purposes of ATC. Pilots require a similar subtle understanding of the impact TCAS is having on the controllers work environment, in a way that would allow them to better appreciate the importance of timely communication of RA events, and of excessive vertical deviations, as just 2 examples.

As a result of the rather lackadaisical approach taken to operational training by the vast majority of CAA’s, a patchwork of TCAS procedures has resulted from the administrations of countries operating out of concert with guidelines provided by ICAO’s SICASP ( SSR Improvement and Collision Avoidance System Panel ). Pilots have been observed to react in quite a number of different ways to common TCAS occurrences, and may employ their own on-board TCAS displays quite differently depending on their country of origin. This further compounds the workload aspect of the control environment where more and more, the controller is being left in the dark as to what he may expect of a pilot in specific circumstances. Uncertainty of any kind is a major contribution to increased stress levels as well as impacting directly on workload. In as much as the FAA is today the only civil aviation administration which is mandating the use of TCAS, it is becoming increasingly apparent that other nations are instituting standards, rules and guidelines without proper direction from a single informed authority. It is felt that a global coordination of procedures for the purpose of standardising the way pilots and controllers interface with TCAS, would be the only solution to this problem, and that information regarding region specific anomalies in TCAS use, form part of the training packages offered to controllers who deal with those carriers in question. Consequently, it is felt a statement to this effect would be appropriate for inclusion as supplementary policy.

It is currently IFATCA policy that civil aviation authorities be urged to implement procedures which absolve controllers of liability for losses of separation caused by RA events. There is here considerable ambiguity, it is felt, as to when, or at what point, the controller should resume responsibility for separation. If deviation used in responding to the RA, was of such a magnitude, that the controller would prefer an alternative to a pilot resuming his original clearance. We believe the policy should be expanded for the purpose of absolving the controller for separation until such time as the traffic is cleared to a specific altitude or lateral spacing, and the traffic reaches the said clearance. In this way, losses of separation imparted onto third parties, indirectly by an RA, would in the same way not be the liability of the controller. This is especially significant where the loss with the third party is imminent, and the speed with which the controller can rectify the situation, is dependant upon timely advice from the pilot experiencing the RA. At a certain point, another non-RA loss of separation may exist. In such a situation, our present policy would appear to be somewhat vague. SC1 felt that a policy statement which emphatically specifies an altitude or lateral separation for resumption of responsibility would be in order here.

The pros and cons of downlinking of RA information were examined. Given the potential for large delays (some have cited as much as 18 sec.) in downlinking via Mode S, there was a case made for its being superfluous.

The pros being:

(i) minimising the shock induced by the after-the -fact realisation that a change in the traffic situation had occurred and;

(ii) the ability that down-linking would afford the controller in assisting the pilot in locating any conflicting traffic.

The cons being:

(i) the potential for confusion, where the controller will not have been made aware of an RA event by either the pilots or a timely down-link message, and his being victimised by a Mode S message indicating an RA event which had already occurred and been resolved.

It was felt that the pros of downlinking this information far outweighed the cons. Delays of the magnitude of 18 sec. can be expected to be rather rare. SC 1 felt that this supplementary policy should stand as it is.

As regards to Simultaneous Intersecting Runway Operations (SIRO) it is felt that TCAS represents a particular hazard to an operation whose margin of safety can little afford the mitigating interference of an airborne device whose RA’s hold the potential for diverting the flight trajectories of aircraft on converging flight paths, thereby compromising the overall safety of the SIRO. Consider that it is deemed mandatory for a pilot to adhere to the instructions of an RA regardless of his appreciation of the traffic situation. Several airlines have already taken it upon their own initiative to operate their TCAS units in “TA Mode” only when operating in the vicinity of busy airports. IFATCA should support the extension of this practice as being mandatory for aircraft operating within designated areas and terminal areas where SIRO’s are taking place

In regards to the balance of our policy, SC 1 could find no other basis for amendment.

In light of the above discussions, our policy could best be brought up to date by the addition of 3 supplementary policy statements and by the replacement of our existing policy statement on controller liability, by an updated version.


IFATCA believes that a central body under the auspices of ICAO should be established, charged with the task of gathering regional ACAS operating practices and procedures, and disseminating to all regions information pertaining to the non-compliance by a region to any internationally accepted ACAS operating standard or practice.


All member associations should urge their national administration to assemble, disseminate, administer and maintain a comprehensive ACAS training package for ab-initio and regular refresher training.

All member associations should ask their national administration to introduce as a matter of urgency, procedures to absolve their controllers of responsibility for any losses of separation caused by an aircraft(s) executing an RA from the point in time where the RA was initiated to that point in time where the aircraft(s) concerned have been cleared to, and are maintaining new altitudes and/or lateral spacings.( These altitudes may well be the pre-RA altitudes of the aircraft concerned).

That at airports where SIRO’s are occurring, aircraft on departure and arrivals inside the final approach fix, TCAS should be operated in the TA mode only.

Due to the fact that there is currently a lack of compatibility between ACAS and the ATC system, and the risk that controller workload therefore could be increased by unnecessary information, IFATCA is opposed to the downlinking of any advisories generated by ACAS.

Last Update: September 28, 2020  

December 24, 2019   278   Jean-Francois Lepage    1995    

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