How does TCAS know....

[380]

Hi,

Am I right assuming that everything that comes closer than 5nm within 1000ft above and below is considered an intruder by TCAS?

But how can TCAS then know about 2000ft separation above FL410? And how would it know about reduced separation in terminal areas or on the ILS?

thx for any answers

380

[NigelOnDraft]

I cannot confirm the figures you give - I probably should know though. TCAS works more on "Closest Approach" point, and a time of "Closest Approach".

What it does not work on is ATC type rules. It just wants the aircraft to "miss" each other by a set amount - and that amount is not a lot. I have had a TCAS RA in a 767 near FRA with a mode C bizjet. We flew straight over it, only 500' above with the TCAS screaming "Maintain V/S" (of zero).

So your points about FL410 and TMAs are I think not applicable...

NoD

[Capt Pit Bull]

Am I right assuming that everything that comes closer than 5nm within 1000ft above and below is considered an intruder by TCAS?

No, you're not.

TCAS advisory triggers are nothing to do with standard ATC seperations.

Very briefly:

TCAS predicts flight paths a certain ammount of time in the future. If a conflict is predicted, advisories will be issued. The time triggers are of the order of a few tens of seconds and vary with altitude, higher altitudes giving more time.

As far as distance goes:

Vertical triggers are of the order of several hundred feet, depending on the type of advisory and the altitude again.

Horizontal triggers are based on a calculation involving rate of closure. The higher the rate, the bigger a horizontal miss TCAS is looking for to classify the intruder as non-threat. There is however a bare minimum to provide protection against traffic converging very slowly. This is of the order of a few tenths of a mile, rising to just over a mile, again depending on altitude.

And remember, TCAS is predictive.

THEREFORE:

A) It is entirely possible to have TCAS issue a RA when standard ATC seperation has not yet been lost.

e.g. medium level (say time trigger of 30 seconds), intruder 2,000 feet above, descending at 3,000 feet per minute. So, TCAS predicts in 30 seconds intruder will descend by 1500 feet, placing it 500 feet above and close enough to generate an advisory. (even though usually the intruder in question will legitimately level off 1,000 above).

This happens quite often, though less these days, as folks have finally started to get the idea that big rates close to cleared level are best avoided.


B) It is also entirely possible to have a very serious loss of ATC seperation, without TCAS doing anything.

Put 2 aircraft at the same altitude on gradually converging tracks, at low altitude. They will get to within a mile before any advisory is issued. (I don't have a book handy to look up the trigger, but its something like 0.35 miles for a TA in the lowest altitude band, with the RA coming at 0.3 miles)


If you want exact figures you'll need a manufactures manual or the JAR guidance leaflet on ACAS.

I hope that is of some use.

CPB
(former TCAS instructor)

[380]

thanks a lot great answer CPB