TCAS TA & RA question
 

Hello!

Is it possible for one aircraft to receive a TA alert while the other gets an RA alert? I was leveling off from a climb while the other aircraft was leveling off from a decent (1000 ft separation). Other aircraft claimed they received a RA, we only got a temporary TA. Both aircraft were A320s. I thought RAs would be triggered on both aircraft. Our manuals are not very clear on this.

Thanks.

If two TCAS equipped aircraft are in an encounter the two systems should both issue and RA (and they are coordinated). They won’t necessarily be corrective RAs, one or both could be preventative.

A few other considerations.

1. At the risk of stating the obvious, did you possibly have TA ONLY selected? (Finger trouble on line up)
2. Could there have been another aircraft involved? I.e. giving one another an RA while you were a bystander?
3. Possibly a non TCAS light aircraft on a VFR squawk suppressed by ATC but blundered into CAS? (This has happened historically)
3. TCAS has different sensitivities by altitude, perhaps you were both either side of a boundary? My understanding is that regardless if Either box triggers an RA it attempts to initiate the coordination data linking., but maybe I’m wrong.

I had exactly this just recently. We were descending to a level 1000’ above a lower aircraft. We got an RA (AP TCAS worked very well, thanks Airbus). The other aircraft said they only received a TA. Both A320s, no other TCAS traffic within 20 NM. I’ve also been on the other side where another aircraft got an RA and we only got a TA with TCAS blue on the PFD. Whatever is supposed to happen, it certainly seems like just one aircraft can get an RA.

What I think: Maybe the TCAS was in RA Mode on one of the planes (Unlikely, since it would be apparent on the MFD)

Or... maybe more probably: The TCAS systems were configured slightly differently with different parameters/software versions

Well, in my case they were both company aircraft with identical TCAS units as far as I know. If one of them was in TA mode, you’re correct that they would see it on the PFD and they would not have TCAS blue on the PFD either.