ACAS II Bulletin - Issue 10

5 November 2007

The purpose of an air traffic control service is to prevent collisions and to maintain an orderly, expeditious flowof traffic.

The objective of TCAS II is simply to prevent mid-air collision – it is an independent safety net.It is inevitable that at times there will be some interaction between ATC instructions, often as a result of STCA warning, and TCAS II.

Where an ATC avoiding instruction conflicts with a TCAS RA, flight crews have sometimes responded to the ATC instruction and have not correctly followed the RA. It must be remembered that, for aircraft in close proximity, the TCAS II knowledge of the vertical situation is much better than that of ATC. Every second, TCAS II assesses if there is a risk of collision.It immediately communicates the necessary avoidance manoeuvre to the flight crew by an aural alert and the RA display. Furthermore, if both aircraft are operating TCAS II, the RAs are coordinated. The overriding action must be to “Follow the RA” and then,if required, to report the RA to ATCas soon as possible.

Therefore, it is essential that both pilots and controllers receive appropriate ACAS training.

TCAS II does not attempt to achieve ATC separation. It is the last resort collision avoidance safety net. Events described in this Bulletin show why it is crucial that the manner in which it can interact with ATC is well understood.

Readers should note that new ICAO rules relating to RAs are applicable from 22 Nov 2007 -see page 2 and page 3 of this Bulletin.

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ACAS II published bulletins regularly focusing on a different operational theme of interest to both aircrews and air traffic controllers.