Tau area around TCAS

[SP]

Any of you guys know what is the tau area inside the surveillance area of a TCAS ? I have opened up the good old ATPL groundschool books and nothing is mentioned in there!
Thanks a lot
Sp

[QAVION]

SP...

"TAU" is a protective area around an aircraft which is defined not purely as a distance between your aircraft and an intruder aircraft, but one based on time to impact (or closest point of passing).

If you think of a simple area, x nautical miles all around an aircraft, the area defined would be a perfect circle. However, with Tau, you are considering the relative closure rates of two aircraft. The protective "circle" is distorted by this effect.

If you have two aircraft converging at high speed, say head on, the circle would have to extend far in front of the aircraft (distance-wise) to ensure protection for these aircraft (detection, reaction and manoeuvre times have to be considered at these greater speeds). TCAS warnings are given when the aircraft are quite far apart.

If you have an intruder aircraft coming up behind your aircraft, the closure rate should be a lot less, so TCAS will allow the intruder aircraft to get much closer to you before it sounds a warning (The protection zone is squashed at the rear (distance-wise).

In the latter case, there are additional safeguards built into the TCAS circuits for very slow closure rates (Absolute distance between aircraft must also be considered).

Hope this helps.
Rgds.
Q.

[Rumbo de Pista]

Basically, TCAS evaluates Range by Range Rate.

In the head-on case, Range 10nm by Range Rate 15nm/min is bad news, in the tailchase case, Range 10nm by Range Rate 1nm/min is nothing to worry about.

There is more to it than that, of course, especially with V7.0, but this should help you to get a handle on it. Think of Tau as a limit value for Range by Range Rate and you're almost there!

[SP]

Thanks for the answer. Very impressed by the expertise... I suppose I could ask another question then. What is the density of a/c per square mile a TCAS system can handle? I have seen this question in a tech quizz before, and I am afraid I haven´t got a clue!
Help much appreciated
Thanks

[QAVION]

"What is the density of a/c per square mile a TCAS system can handle?"

I don't have a density value as such, but, last time I read the specs, TCAS can track up to 50 aircraft.

Hope this helps.
Cheers.
Q.

[SP]

Thanks for the help
Sp