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How Does Tcas Handle A 3 Way Conflict?  
User currently offlinebueb0g From United Kingdom, joined Jul 2010, 642 posts, RR: 0
Posted (3 years 3 days 15 hours ago) and read 3382 times:

I was thinking about a hypothetical (and thoroughly unlikely, but possible) scenario the other day, and I was wondering how TCAS would handle it:

Three aircraft, A, B and C, the type doesn't matter but they are all airliners, all equipped with the same version of TCAS.

Aircraft A is climbing through FL 355, aircraft B is cruising on FL 360, and aircraft C is descending through FL 365. Their flightpaths will converge essentially on aircraft B at FL360.

How does TCAS respond to this? Will it recognise the three way conflict, "talk" between all the transponders and then issue RA's agreeing with the other two aircraft (e.g a/c A will recognise a 3 way conflict, the crew will receive an RA to descend, a/c B to maintain vertical speed, and a/c C to climb), or will each TCAS on the different aircraft only recognise it as a "normal" two way collision, and react to only one of the conflicts? In this case, I guess a/c A would still receive an RA to descend, and a/c C would get one to climb but would a/c B get one to climb/descend, disregarding one of the two aircraft?

In this case, (let's say B gets an RA to climb, the TCAS only recognising a/c A as a conflict) once it was clear of conflict from aircraft A, would the crew get another RA telling them to descend, this time in relation to aircraft C?

Was just wondering if TCAS was designed with a three-way contingency in mind.


Roger roger, what's our vector, victor?
8 replies: All unread, jump to last
User currently offlinegingersnap From United Kingdom, joined Aug 2010, 893 posts, RR: 5
Reply 1, posted (3 years 3 days 15 hours ago) and read 3373 times:

I'd assume the aircraft af FL365 would get a call to climb call. The lower aircraft at FL355 would get a descend now call. The aircraft at FL360 would be told to maintain vertical speed.

That's my uneducated guess. Similar to what you suggested I believe.

Flown on: A306 A319/20/21 A332 B732/3/4/5/7/8 B742/4 B752 B762/3 B772/W C152 E195 F70/100 MD-82 Q400
User currently offlineDashTrash From United States of America, joined Aug 2006, 1527 posts, RR: 2
Reply 2, posted (3 years 3 days 15 hours ago) and read 3351 times:

Happened to me one time, and asked me to crash into the aircraft in level flight below us as it though another aircraft was descending into our altitude.

Not sure what the actual programming logic is, but I'd say it doesn't work too well.

User currently offlineTPAJAY From United States of America, joined Aug 2010, 35 posts, RR: 0
Reply 3, posted (3 years 3 days 12 hours ago) and read 3230 times:

Very carefully, only happened once in College! Usually it ends with someones feelings hurt, lol.

User currently offlinehorstroad From Germany, joined Apr 2010, 263 posts, RR: 0
Reply 4, posted (3 years 3 days 11 hours ago) and read 3199 times:

i was told the decision whether the aircraft has to climb or descend is dependent on the s/n of the TCAS system installed. the TCAS with the lower serial No (the older device) tells the pilot to descend, the newer one with the higher number tells the pilot to climb. I have no information if TCAS can handle multiple conflicts or not. I guess each TCAS would handle it as two different conflicts rather than one three way conflict. but i have no idea how it would react in detail

User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 5, posted (3 years 3 days 10 hours ago) and read 3154 times:

Quoting bueb0g (Thread starter):
Was just wondering if TCAS was designed with a three-way contingency in mind.

I don't know the answer, but I'll be shocked if it was designed to handle three-way conflicts. A true three way (as opposed to two two-ways in close proximity) is so phenomenally unlikely that the effort to develop and certify it, coupled with it being a *far* more complex problem, makes me see no way you could build a business case for that.


User currently offlineDashTrash From United States of America, joined Aug 2006, 1527 posts, RR: 2
Reply 6, posted (3 years 2 days 22 hours ago) and read 2986 times:

What I referenced earlier:

We were at 9000 with jet traffic overtaking and descending to 10. Opposing traffic climbing like a bat out of hell to 8000. We first got a TA, on the climbing traffic, then another TA on the overtaking / descending traffic which busted his altitude turning the TA into a descending RA. (Sounds like an Abbott and Costello skit, huh?) The descending RA would have put us head on with the climbing traffic, who could have possibly gotten a climbing RA sending him into the descending traffic who busted altitude. The descending jet also got an RA, but I don't know what it told him to do. Probably a climbing RA, but I didn't read his ASAP report.

Either way, it could have been the world's first 3 way midair....   

User currently offline26point2 From United States of America, joined Mar 2010, 821 posts, RR: 0
Reply 7, posted (3 years 2 days 21 hours ago) and read 2980 times:

TCAS vs. 3 way? Happened here... partially the fault of the of TCAS logic.

Climbing from departure airport got an RA on a VFR target. Responded to RA command to climb which sent us toward further VFR traffic and another RA which commanded an immediate descent. TCAS is nice but not fool proof.
BTW was later told the TCAS should not have allowed this? Happened to me though.

User currently offlineB747FE From Hong Kong, joined Jun 2004, 230 posts, RR: 4
Reply 8, posted (3 years 2 days 7 hours ago) and read 2750 times:

Quoting bueb0g (Thread starter):

As far as I know, it's logic is designed to handle multiple encounters. It will however issue a single RA attempting to provide separation from all the designated intruders.

Quoting horstroad (Reply 4):

In an encounter between two TCAS, they will normally declare each other as threats at different times.
From that point, the first TCAS declaring the other will choose the direction of the RA (based on the particular encounter geometry). The second TCAS, having received the intentions of the first one will choose a complementary RA.
In case of a simultaneous declaration, the RA will be chosen based on geometry by both. If both TCAS sense the selection of the same RA, the equipment with the higher mode S address will the one reversing it's direction.
(I believe the last version of TCAS II has some modifications on the reversal logic)

I'm not sure how the coordination would work in case of multiple TCAS encounter.

Quoting 26point2 (Reply 7):
Climbing from departure airport got an RA on a VFR target. Responded to RA command to climb which sent us toward further VFR traffic and another RA which commanded an immediate descent. TCAS is nice but not fool proof.

Well, keep in mind that in certain conditions such as high/low altitude or aeroplane configuration (gear/flaps) due to performance limitation the number of RA's to choose from may be limited.
Also based on the way the intruder maneuvers, an RA can be increased in strength or reversed completely.


"Flying is more than a sport and more than a job; flying is pure passion and desire, which fill a lifetime"
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