sfilipowicz
Posts: 290
Joined: Wed Jul 17, 2002 3:59 pm

How Does The Tcas Work?

Mon Feb 02, 2004 9:41 pm

Hi All,

I was wondering if somebody could tell me how the TCAS works?!

Does it work in combination with the transponder and with the ATC?
Would the TCAS be able to 'see' different aircraft when flying above the oceans?

Thanks!
 
Ikarus
Posts: 3391
Joined: Mon Jan 08, 2001 10:18 pm

RE: How Does The Tcas Work?

Mon Feb 02, 2004 10:24 pm

Please add on to that questions:

How does TCAS work at low altitudes (i.e. when a downwards diversion would be problematic) and during final approach?

Technically, I believe it uses transponder information to trace other aircraft and spot any risk of collision. Correct me if I'm wrong....

Cheers

Ikarus
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: How Does The Tcas Work?

Mon Feb 02, 2004 10:31 pm

It uses timing for direction, mode C information for altitude and a directional antenna for a (quite unreliable) indication of direction.

It then uses the rate of closure to decide on a tau value, or time to collision. The tau value used to trigger traffic alerts and resolution advisories is dependant upon altitude. At lot altitudes, the inability to go down will be considered. Eventually, all RAs will be inhibited.

The aircraft will coordinate their RAs over the mode S datalink.

That is a very brief description. It is all rather complicated, and I do not even claim to know all the intricasies, especially when it comes to performance and the effect on RAs issued. I'm an engineer, much more concerned with the bit sequencing than the algorithms used.  Big grin

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
ben
Posts: 1369
Joined: Fri Aug 06, 1999 9:27 pm

RE: How Does The Tcas Work?

Mon Feb 02, 2004 10:41 pm

Light aircraft now have a similar system but it doesn't coordinate RA's because there is no data link to allow them to talk to eachother. That's why it's now being advised that all aircraft have mode-c squawking, even when in the pattern/circuit.

Light aircraft transponders which do not use mode-c can trigger alerts on airliners flying far above, because without the altitude encoding, TCAS can't determine your vertical separation.
 
sfilipowicz
Posts: 290
Joined: Wed Jul 17, 2002 3:59 pm

RE: How Does The Tcas Work?

Mon Feb 02, 2004 11:17 pm

Hi Fred,

Thanks for your answer!
What are RA's ???

Is there a new system in the making for TCAS?

Thanks again!

Steven.
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: How Does The Tcas Work?

Tue Feb 03, 2004 12:03 am

RAs are Resolution Advisories, vertical path guidance commands issued by the TCAS system to avoid a potential collision.

TCAS II provides this. TCAS III was supposed to include lateral guidance (turn commands) as well, but has now been scrapped to the best of my knowledge.

The new system in the making is Automatic Dependent Surveillance Broadcast, ADS-B, where the aircraft and ground stations are all communicating over TCP/IP inspired datalinks, telling each other where they are and what they are up to. Thus, no radars are required. Each pilot can have the full picture presented right there in the cockpit, just as if they had a radar scope today (if not better). This will open entirely new possibilities as far as airspace management goes! How about free flight? Or even self-separating traffic in trail of one another along the airways?

TCAS II requires a mode S transponder, something you won't see in many light aircraft.

There is also a "faux TCAS" available for GA, which is not a TCAS at all since it doesn't interrogate the transponders of other aircraft. It merely listens in on the responses transmitted to other interrogations, and tries do determine the distance based on other parameters such as signal strength. Anyone who has tried it and would like to chime in with how well it works?

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
buckfifty
Posts: 1278
Joined: Tue Oct 16, 2001 4:05 pm

RE: How Does The Tcas Work?

Tue Feb 03, 2004 12:09 am

RA's are resolution advisaries (considered a warning). Which are basically instructions from the TCAS telling you to either climb or descend with traffic approaching. TA's are traffic advisories (considered only a caution), which is when the TCAS will visually and aurally warn you of approaching traffic, but will not instruct you to do anything.

I've forgotten when the range of TA's and RA's are activated, but I believe TA's are when the target is in a probable collision course, at 40 seconds, and RA's are activated when that same target is within 20 seconds. Someone please correct me if I'm wrong.

And an aircraft equipped with Mode C only will still generate a TA/RA on an aircraft equipped with TCAS and Mode S. It's just that it won't be reciprocated.

The system in use for light aircraft is called TCAD. It's based on Mode C transponders, and will give traffic advisories in terms of height above or below, and the rate of vertical approach or separation.
 
buckfifty
Posts: 1278
Joined: Tue Oct 16, 2001 4:05 pm

RE: How Does The Tcas Work?

Tue Feb 03, 2004 12:14 am

There is also a "faux TCAS" available for GA, which is not a TCAS at all since it doesn't interrogate the transponders of other aircraft. It merely listens in on the responses transmitted to other interrogations, and tries do determine the distance based on other parameters such as signal strength. Anyone who has tried it and would like to chime in with how well it works?

Hehe. It's crap, it's annonying, and I think it's dangerous, especially to low hour pilots. It dings an awful lot, especially in crowded training areas, and people I think tend to look less carefully at other traffic with this device on board. And it can give a lot of heart attacks too, because it generates a lot of nuisance and false warnings that'll really give you a scare.
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: How Does The Tcas Work?

Tue Feb 03, 2004 12:30 am

Buckfifty,
as I said above, the tau value limits are altitude dependant. I'm fast, am I not? Corrected you before you posted! Big grin

Interesting to hear that those not-really-TCAS-systems live up to my expectations - crap.  Big grin

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
buckfifty
Posts: 1278
Joined: Tue Oct 16, 2001 4:05 pm

RE: How Does The Tcas Work?

Tue Feb 03, 2004 12:37 am

Buckfifty,
as I said above, the tau value limits are altitude dependant. I'm fast, am I not? Corrected you before you posted!


Hehe. Well, I'm always being corrected, even in my past life, it seems. Big grin
 
BMAbound
Posts: 654
Joined: Fri Nov 14, 2003 5:59 am

RE: How Does The Tcas Work?

Tue Feb 03, 2004 12:40 am

Fred, I know, I should know the answer (  Big grin ) , but does the TCAS on the heavy iron pick up light A/C that has squawked VFR?

cheers

johan
Altitude is Insurance - Get Insured
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: How Does The Tcas Work?

Tue Feb 03, 2004 1:30 am

I should know too, but I could just picture that they have put in a filter to avoid nuisance alerts. Would be tricky though, as the VFR code isn't the same all over...

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
ben
Posts: 1369
Joined: Fri Aug 06, 1999 9:27 pm

RE: How Does The Tcas Work?

Tue Feb 03, 2004 1:57 am

The "faux TCAS" in my experience isn't reliable even when working.

The systems for displaying/alerting are inconsistent between models.

I have had one which was just 2 red L.E.D. numbers that indicate the distance in nautical miles to the nearest 'threat'. Not all that useful.... if you notice the numbers counting down, it can be distracting as you frantically search around in every direction for the other aircraft. Most of the time it is just ignored because you're too busy actually flying and looking out.

A lot of light aircraft either dont have or dont use their Mode C so it wont spot them anyway.

I usually fly with it turned off, if one is installed, but always make sure I squawk VFR Mode C which is 7000 in the UK.

[Edited 2004-02-02 18:00:00]
 
jumbojet
Posts: 1180
Joined: Mon Dec 29, 2003 3:01 am

RE: How Does The Tcas Work?

Sat Mar 12, 2005 5:29 pm

picking up where this left off, will TCAS work over the open oceans?
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: How Does The Tcas Work?

Sat Mar 12, 2005 7:19 pm

For the record, this question was answered in another thread just a short while ago.  Smile
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
airplay
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

RE: How Does The Tcas Work?

Mon Mar 14, 2005 12:29 am

This is classic A.net stupidity.....

Because somebody (I have my suspicions) cryed, the only post on this entire thread that contained correct information got deleted...

I guess you guys prefer to wallow in ignorance.

[Edited 2005-03-13 16:36:42]
 
KAUSpilot
Posts: 1659
Joined: Mon Jan 28, 2002 2:15 pm

RE: How Does The Tcas Work?

Mon Mar 14, 2005 3:47 am

Fred, I know, I should know the answer ( ) , but does the TCAS on the heavy iron pick up light A/C that has squawked VFR?


TCAS definitely picks up VFR traffic in the aircraft I fly. Just the other day we got an RA from an aircraft flying 500 ft above class B airspace, obviously VFR with his xponder turned on.

Last week we got a TA from a VFR aircraft doing aerobatics off our right side on approach to MHT. If there is a feature in TCAS which blocks VFR targets, I'm certainly not aware of it.

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