ACAS II Bulletin - Issue 4

3 May 2004

The drive for TCAS II development in the US was from mid-air collisions involving light aircraft between a B727 and a Cessna 172 at San Diego in 1978, and between a DC9 and a Piper at Cerritos, California, in 1986. In Europe, extensive safety analyses showed that TCAS II systems would provide significant safety benefit in all the airspace.Resulting mandates mean that most airliners and many business jets are now equipped with TCAS II.

Operationally TCAS has proven to be very effective, and this includes encounters with VFR traffic squawking altitude. However, pilots and controllers often question the value of TCAS where IFR and VFR traffic is mixed:

  • Does TCAS only cause problems between IFR and VFR traffic or does it give good protection?
  • Does VFR traffic require a transponder for some TCAS protection?
  • Although IFR and VFR traffic are “correctly separated by 500 ft”, TCAS triggers alerts. Are these false alerts, or are they normal?
  • Does TCAS still work when aircraft are flying in the aerodrome traffic pattern?

The objective of this Bulletin is to provide answers to these sorts of questions.


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