Interactions ATC/ACAS

Interactions ATC/ACAS

42ND ANNUAL CONFERENCE, Buenos Aires, Argentina, 17-21 March 2003

WP No. 97

Interactions ATC/ACAS

Presented by Philippe Domogala, IFATCA Representative to the IFALPA ATS Committee


1.1  The collision over Germany last July highlighted in the cruellest way the limits of the interactions between ATC and ACAS. Considerable discussions are taking place in various for a, but especially on the IFALPA ATS Committee on the interactions ACAS – ATC.

1.2  Two other incidents re-enforced the ATC / ACAS problems: a Near mid air collision over Japan in January 2001 involving a JAL B747 and another JAL DC10, and more recently, an Airprox over Mexico in October 2002, involving a Lufthansa B747 and a Mexicana A320.

1.3  The JAL/JAL case highlighted also the visual acquisition problem, and the Downlinking of RAs, which is now strongly pushed by the Japanese Aviation Authorities.

1.4  The Mexcicana / DLH case, while still under investigation, would also highlight another problem, that of controllers warning aircraft having reporting an RA, of the danger of other aircraft in the direct path of the RA (possibly not TCAS equipped)

1.5  ICAO has issued a State Letter (AN 11/10-02/82) to all States and International Organisations with some reminders and proposed changes to existing Documentation regarding ACAS operations and its interface with ATC.
Many comments to those changes are circulating at the time of preparing this paper, notably by IATA and IFALPA.


2.1  There are currently 3 major issues regarding ACAS / ATC interface that are in need of replies.


2.2.1 ICAO Documentation ( see 5.3 below) would indicate that controllers shall pass traffic information to pilots reporting an RA. This is currently debated within ICAO and there are opponents, arguing that this visual acquisition actually contradicts the new position taken to request pilots to always follow the RA.

2.2.2 QUESTION: Should controllers after having been advised that an aircraft is following an RA pass traffic information to the relevant aircraft?

2.2.3 BENEFITS: This will help the pilot assessing the threat and better adapt its manoeuvre to avoid the collision.


Legal responsibility for controllers in case that traffic information leads to a change in the speed or manner pilots are following their RAs.

Pilot could choose not to follow, or delay following his RA and try to avoid the threat visually (as current ICAO documentation suggest).

One must take also into account that the traffic information passed by the controller is likely to be based on radar returns which might be 10 seconds or more old, therefore not reflecting the real position and/or altitude of the threat aircraft.

There are also cases, where more than 2 aircraft are involved, not all of them necessarily TCAS equipped.

Pilot could identify the wrong aircraft (especially in case multi aircraft encounter).

Inability for the pilot to assess the other aircraft actual flight path or intentions (especially at night).

Taking pilot eyes away from TCAS display to the outside (looking for traffic) and as a consequence possibly under or overshooting TCAS requested rates of vertical manoeuvre.

2.2.5 POSSIBLE ANSWER: Controllers should nevertheless pass traffic info in case other aircraft (possibly not TCAS equipped) are in the vicinity. Special phraseology should be used such as: CAUTION, other a/c 1000 ft above ( or below).


2.3.1 QUESTION: Should IFATCA support and ask for RA s to be downlinked to ATC?

2.3.2 BENEFITS: This would provide controllers with information in case pilot does not do so in the R/T (or does it late)
Would clearly identify to controllers which pair (or trio) of aircraft are involved. If the sense of the RA is displayed it would help the controller understand what TCAS has chosen and what is happening.

2.3.3 DRAWBACKS: Necessity of Mode S radar coverage to have full benefits of this. Currently a very tiny portion of the world airspace is Mode S equipped. What will be downlinked in any case is the fact that TCAS generate an RA. Controllers will still not known if the pilots actually follow the RA.

2.3.4 TRANSMISSION DELAYS: The RA broadcast facility on board TCAS uses 8 seconds presently when using the 1030 MHz Mode S transmitter (not the 1090, and only the latest RA displayed to the pilot is coded and broadcasted every 8 sec).

Reception delays: (up to 12 seconds for long-range radar, but 10 sec for newer Mode S antennas) for controllers to receive the info. If one remembers that RAs are issued 15 to 35 seconds before closest point of approach and that corrective and reversal RAs have to be followed within 2,5 seconds by the pilot, the info received by ATC will be outdated and possibly incorrect by the time it is received.

2.3.5 RESPONSIBILITIES AND LEGAL ISSUES: At the moment the responsibility of the controller ceases once a pilot chooses to follow an RA. Controllers being advised of RA (via voice R/T) should stop issuing further instructions to pilots. With Data link this bring Human factors issues: would a controller stay silent if he watches something happening on its radar at the time? (i.e. an RA is generated, downlinked but the radar returns Altitude reporting is not confirming that the pilots follow the RA) and legal issues (would a Judge accepts that a controller had the information but said nothing to prevent a collision?) Bearing in mind that the information he sees on data link is most probably outdated and not reflect real situation at the time he speaks on the R/T?

[NOTE: I have read in IFALPA papers that there could be a technological solution to reduce reception delays, called fixed Mode S receivers, but I have no further information. Also to be noted that on the last SCRSP Working Group A meeting in Nov 2002, the Japanese rep announced that the plan to introduce Mode S data link in the next 5 years in Japan would not include the RA sense, only the fact that an RA has been triggered]


IFATCA continue to oppose the downlinking of RA in the current situation but would support studies on its possible introduction to include the following:

  • Transmission / reception times and nature of the data transmitted.
  • Responsibilities and legal issues for controllers receiving RA information via data link.
  • Implications on the ACAS procedures of such introduction.


2.4.1 QUESTION: Should controllers faced with a critical conflict between 2 aircraft with less that a minute before predicted CPA issue lateral manoeuvres (turns) instructions, instead of climb or descent instructions?

2.4.2 BENEFITS: This would drastically reduce if not possibly eliminate the actual collision.

2.4.3 DRAWBACKS: This is a less effective method to obtain legal separation (e.g. : 5NM/ 1000 ft). This reduces the controller choice in resolving a conflict and may not be always feasible due external factors (other a/c in vicinity, Mountains, Active restricted airspace, etc.) This will also complicates the pilot workload had he/she could have to follow RAs while performing turns.

2.4.4 POSSIBLE ANSWER: From a purely safety point of view, this simple procedure has the possibility to physically reduce or eliminate the actual collision. Therefore IFATCA should advise controllers to follow, when possible a similar procedure that the one issued to the UK controllers (see reference 5.1 below).


3.1 IFATCA has a duty to take the lead on safety issues and, after discussion both within SC1 and during Conference, should possibly issue policy on the 3 items above.


4.1 That this paper be accepted.


5.1 UK CAP717 on Radar Control, Collision avoidance concept: page 4 & 5 para 3.5:

“a pilot faced with conflicting instructions from a controller and an ACAS system should follow the ACAS advice. However since current generation ACAS systems provide RAs only in the vertical plane, it may be useful for controllers to give collision avoidance instructions in the horizontal plane for ensuring that the pilot does not have an unnecessary decision to make – if the pilot is already descending, for instance, he can also probably turn, thus the two pieces of information become complementary instead of potentially contradictory.”…

5.2. ICAO State letter AN1/19-02/82:

Annex B, item 10: […controllers is informed as soon as time and workload permits using standard phraseology…]

Item 11: […When possible an ATC clearance is complied with while responding to an RA (example on maintaining assigned altitude)…]

Item 12: […If pilot simultaneously receive instructions to manoeuvre from ATC and an RA which are in conflict, the pilot should follow the RA…] […ACAS should enhance visual acquisition of conflicting traffic…] […ACAS works on the concept of closest point of approach (CPA) rather than distance. Warning threshold varies with altitude. A TA occurs from 15 to 48 seconds and RA from 15 to 35 seconds before projected CPA…]

5.3. ICAO Doc 4444 PANS-ATM:

15.6.32. When a pilot report a manoeuvre induced by a TCAS RA, the controller […] shall provide traffic information as appropriate.

15.6.33. […once an aircraft departed from its clearance in compliance with an RA the controller ceases to be responsible for providing separation between that aircraft and any other aircraft affected…]

Last Update: September 29, 2020  

March 21, 2020   814   Jean-Francois Lepage    2003    

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