ACAS II Bulletin - Issue 6 (English and French version)

2 March 2005

TCAS II is a last resort safety net designed to prevent mid-air collisions. It alerts the flight crew and provides Resolution Advisory (RA) manoeuvre indications when it computes a risk of collision. The correct use of TCAS II increases the safety of air transport.
The TCAS II traffic display is provided for the purpose of assisting the flight crew in the visual acquisition of aircraft in the vicinity. Of course, it also helps to improve flight crew situational awareness.

However, experience has shown that in some cases, flight crew are tempted to make their own traffic assessment based on the traffic display information, and to manoeuvre in anticipation of ATC instructions.

The TCAS II traffic display can be misinterpreted, since it provides only partial information, it has limited accuracy, and it is based upon a moving reference. It has not been designed for the purposes of self-separation or sequencing, and using it for these purposes is inappropriate, and could also be hazardous.

This ACAS Bulletin includes some actual events where problems arose due to misinterpretation of the TCAS II traffic display, and provides some insight into why these events occurred.


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