TCAS

[OldAg84]

In light of the recent JAL vs. JAL near-miss; I have some questions. I am not an aviation professional- so please humor me.

How does TCAS work from a technical standpoint?

How far apart will it work?

Assuming no confusion or other factors, ATC weather, etc.- with a nose to nose cruising speed and angle- how much warning in seconds can a crew expect?

Thanks in advance to all you professionals?

Fly Safe.

OldAG84

[Mac the Knife]

I'm not a professional, but http://www.caasd.org/proj/tcas/
told me what I wanted to know.

[OldAg84]

Thanks, Mac. That link answered most of my questions. Do any pilots have insights/opinions on how well it works in practice?

[ft]

As for the technical side that seemed to be left out in that article... The operating range is 10 to 20 nm. As soon as an a/c is found within range it will be databased and continuously tracked.

Tracking is done by ATCRBS replies, mode C/S. Altitude of an intruder comes from the altitude encoding, the distance from the timing as per usual for secondary radars and the angle from using a directional antenna.

TA/RAs are generated based on a value called the tau value. From two subsequent distance measurements a closing speed is calculated. This and the distance gives a time to impact value, namely the tau. When this goes below a certain number of seconds a TA is generated. Under a second threshold, an RA. These thresholds vary depending on altitude AGL.

Alt TA tau RA tau
0-500 20 inhibited
500-2500 35 20
...
20000+ 48 35

I hope that answered the rest of your questions. :)

IIRC, the level of nuisance alerts is about 2/3rds of the alerts which is quite low for such a simple system. I plowed through a paper or two on conflict detection in ATC systems and it can get quite a lot more complicated without decreasing the nuisance alert level all that much it seems. ATC have some pretty neat systems in operation to show potential conflicts far ahead in time and it's getting better all the time... Add ADS-B and we'll be able to cram so many planes into the skies that they will darken... :)

Cheers,
/ft

[411A]

TCAS was slow to be accepted in Europe but now required, wonder why it took so long (not invented here perhaps) as the system works very well, generally.

[ft]

You'll have to excuse me if I find it amusing that you, as an American, makes references to the "not invented here"-syndrome, particularly when speaking about airborne traffic detection and avoidance. You *have* been following the circus surrounding the ADS-B system I mentioned above, haven't you? :)

Cheers,
/ft

[411A]

FT
Yes indeed have been following the ADS-B information and trials but, as TCAS has been available for a long time and been fitted to most aircarrier aircraft at considerable cost, it will be a LONG time for airlines to equip with yet another box. Unless of course, it results in direct routings. But then the very big problem is ATC priorities.

[Smurfjet]

ft

totally unrlated, but from your profile I see you're an AE student, what school do you go to in Sweden? Drop me an Email if you like, just wondering what kind of program you have.

Regards
SJ

[PPRuNe Radar]

OldAge84

You can download a document about the latest version of TCAS from the URL below. Not had a look myself yet so I don't know if it will be too technical for you, but as it's written for pilots you'll probably be OK :)


http://www.arinc.com/tcas/

------------------
PPRuNe Radar
ATC Forum Moderator
[email protected]

[Roadtrip]

All the TCAS II that I fly have selectable ranges of 5-10-20-40 nm.

[OldAg84]

Thanks to everyone! Now, I can't wait to corner somebody at a party and talk thier ear off about TCAS!! "Well the tau is driven by......."


Fly Safe.