TCAS - one RA and one TA: is it possible?

[Philthy]

Can anybody please tell me authoritatively whether it is possible in an encounter between two Mode C, TCAS-equipped aircraft for one to receive an RA as the other simultaneously receives only a TA?

I've had a look at other TCAS threads and none seem to address this specific question.

Any answers gratefully received.

Philthy

[Denti]

If both are only Mode C equipped that is certainly possible. For coordinated RAs you need Mode S on both.

[jb5000]

Happened to us the other week.

We were AMS-LGW, and as we were reaching ToC over the North Sea we got a TA for an a/c descending to 1000' above us, whilst he reported a RA.

[fellman]

Firstly we need to clear up some nomenclature.
If you are talking about RAs then I assume that you are talking about TCAS II (TCAS II can issue TAs and RAs, TCAS I only issues TAs).
If you are talking about TCAS II then the aircraft is necessarily equipped with a Mode S transponder - a Mode S transponder on own aircraft is a requirement for TCAS II equipment to function (the other aircraft however can be a Mode C aircraft without TCAS).

So, in an encounter between two TCAS II aircaft (which are therefore Mode S aircraft), it is possible to get an RA in one aircraft and only a TA in the other.
In an encounter between two TCAS II aircraft there is no requirement for one to generate an RA just because the other has generated an RA (although this is what usually happens). IF they both generate RAs then those RAs will be coordinated.

[Philthy]

Thanks Fellman. Yes, TCAS II and Mode S (Mode C was really a reference to squawking alt).

Any chance you can point me at a reference for all this?

Philthy

[fellman]

Hi Philthy
The EUROCONTROL Brochure describing ACAS II operation principles is a good starting place for information on questions like this.
It is available on their website at:
www.eurocontrol.int/msa/public/standard_page/ACAS_training.html

Cheers
Fellman