ATC Collision Avoidance Techniques

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ATC Collision Avoidance Techniques

43RD ANNUAL CONFERENCE, Hong Kong, China (SAR), 22-26 March 2004

WP No. 97

ATC Collision Avoidance Techniques

Presented by SC1


1.1. The purpose of this paper is to advise controllers on techniques that can be used for collision avoidance instructions when they are aware that ACAS RAs could be in progress.

1.2. Following the Ueberlingen collision in July 2002, a review of the interactions between ATC and ACAS were debated and, among them, the notion collision avoidance techniques possibly conflicting with ACAS was raised. The same discussions were already taking place in various Groups and Countries, even prior to July 2002.

1.3. For instance, the UK CAA came up in January 2002 with a very interesting document called “Radar Control – collision avoidance concepts “(CAP717), coming from the “Avoiding Action Working group”. SC1 found the document to be very mature and some of the recommendations it contains are worth considering. The text and the conclusions below are widely taken directly from the UK CAA CAP717 document, with their permission.


2.1. By collision avoidance instruction we mean last minute instructions (clearances) given to pilots to prevent a collision. These are not the normal day-to-day instructions controllers give to pilots to resolve conflicts. The paper also refers to positive (e.g. Radar) control environment.

2.2. To understand how the interactions between ACAS and ATC can have dramatic consequences , one has to understand the existing RADAR LIMITATIONS : First, ACAS reacts instantly to other traffic and issues immediately RAs to pilots. But, the situation the controller sees on his radar display is delayed information. For instance in case of Mosaic radar being used, and taking an average radar with a 10 rpm rotation antenna; this means an update every 6 seconds. An aircraft flying at 420 Kts ground speed will travel ¾ of a NM in that period, therefore could be well ahead of what is displayed. Similarly an aircraft climbing at 3000 ft/min with that same radar could actually be 300 ft higher than indicated on radar. Processing delays associated with the transmission of data between radars equipment sites can also add to this time lag.

Therefore, the traffic picture as understood by the controller can be significant mismatch with reality at any given time.

It is the task of the controller to prevent collisions between aircraft by applying techniques which ensure that the separation provided does not fall below the required minima. A controller will base separation on the aircraft positions on the radar display which, as described above, may not be the actual exact position of the aircraft. When the aircraft are very close to each other, or very fast, or if they are on reciprocal tracks closing head on, the time available for the controller in which to make a decision is much reduced. With little time and possibly wrong (delayed) positions, the possibility for errors is increased

2.3. When separation is in the process of being lost, and the controller perceives a risk of collision, in practical terms there is no longer an issue of providing standard separation. The essential task is avoiding the collision.

Instructions passed by the controller in these circumstances must, of necessity, be passed without delay and must be phrased as an urgent instruction.

In that case ACAS system will almost certainly interfere and possibly issue RAs, which are conflicting with the controller instructions.

2.4. Today a pilot faced with conflicting instructions from a controller and ACAS, should follow the ACAS advice. However since the current generation of ACAS provide resolution advisories only in the vertical plane, it may be useful for controllers to give collision avoidance instructions in the horizontal plane, ensuring that the pilot does not have an unnecessary decision to make: (for instance: if the pilot is already descending he can also probably turn), thus the two pieces of information become complementary instead of potentially contradictory. Equally controllers should be prepared for pilots electing to ignore the ACAS RAs. (this high percentage of non-compliance is unfortunately confirmed in many reports)


3.1. It becomes obvious, especially after recent accidents and incidents, that there are potentially dangerous interfaces between ACAS RAs and controllers collision avoidance instructions.

3.2. Since the ACAS systems currently in use only gives avoidance advices (RAs) in the vertical plane, controllers should consider to give collision avoidance instructions in the horizontal plane, in order to complement, rather than oppose ACAS logic.


It is recommended that;

4.1 In a situation where an ACAS RA is likely to occur between aircraft under radar control, and a collision avoidance instruction need to be issued, controllers should consider horizontal movements (i.e. turns) to avoid contradictory instructions to an RA that may be issued.


CAP 717 RADAR CONTROL Collision avoidance concepts. An output from the avoiding Action Working group published by the UK CAA on 18 January 2002.

Last Update: September 29, 2020  

March 24, 2020   136   Jean-Francois Lepage    2004    

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