TCAS II Logic Question

[DCS99]

Does anyone know what is the TCAS logic for the decision *which* aircraft he will issue a Climb RA to and to which he will issue a Descend RA?

It shouldn't make any difference as long as one goes up and one goes down. But how does he decide which one gets the Climb instruction and which one the Descend?

Shall I put it into context:

I'm thinking about the mid-air that happened up the road from me. If TCAS had told the DHL to climb, the accident would never have happened. Then the Bashkirian Tupolev would have got a TCAS descent RA which syncd with his ATC instruction.

Let's assume hypothetically, there was no other traffic around and both aircraft were travelling at the same speed, why did TCAS tell the Tupolev to climb and the DHL to descend and not vice-versa?

Hope that makes sense,
Schönenabig und Grüess from Sunny Switzerland

[SimJock]

Amongst other things, TCAS is aware of the performance limitations of the aircraft that it is fitted to, and various RA can be inhibited under certain conditions. If you are at or near your service ceiling, or have an engine out, or iceing (on some systems), or just reduced performance, TCAS will issue a Descend RA for your aircraft in preference to a Climb RA.

Perhaps TCAS thought that a three engined aircraft is more capable of climbing than one with two engines in this instance ? and coordinated the Mode S response accordingly..

[Golden Rivet]

Occasionally, the two aircraft declare each other as threats simultaneously, and therefore, both aircraft will select their RA sense based on the encounter geometry. In these encounters, there is a chance that both aircraft will select the same sense. When this happens, the aircraft with the higher Mode S address will detect the selection of the same sense and will reverse its sense.
Version 7 includes the capability for TCAS to issue RA reversals in coordinated encounters if the encounter geometry changes after the initial RA is issued. The RA reversals in coordinated encounters are annunciated to the pilot in the same way as RA reversals against non-TCAS intruders. In a coordinated encounter, if the aircraft with the low Mode S address has Version 7 installed, the low Mode S address can reverse the sense of its initial RA and communicate this to the high Mode S address aircraft. The high Mode S address aircraft will then reverse its displayed RA. The aircraft with the high Mode S address can be equipped with either Version 6.04 or Version 7.
In a coordinated encounter, only one RA reversal based on changes in the encounter geometry can be issued. The initial RA sense will not be reversed until it has been displayed for at least nine seconds, unless the low Mode S address aircraft has a vertical rate higher than 2500 feet per minute and acts contrary to the RA. This delay is included in the design to allow sufficient time for the two aircraft to initiate a response to the initial RA.

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

[DCS99]

Much appreciated esp Golden Rivet, I had to read your post a few times before I could understand: and I thought writing a program to calculate a loadsheet was complicated...Cheers