I am an air traffic controller and I had the following situation. Traffic maintaining FL360 and below him traffic at FL350 requesting FL370. Once they crossed each other I gave clearance to climb FL370 with 5.1 nmi separation. The guy read back the clearance and said he received a TCAS RA, the guy at FL360 said he received a TCAS TA. Does this make sense?
It's hard to give a definitive answer without more information. My go-to for TCAS details is the FAA's Intro to TCAS II. In general, an RA will try to coordinate with the other aircraft, but such coordination doesn't require the other aircraft also issue an RA, it only listens for other RA "intent" messages and makes sure it doesn't issue a conflicting RA.
Here's some possibilities:
- A marginal RA encounter. Each airplane has slight differences in calculation , so, as the FAA manual linked above reads, "In a majority of the TCAS/TCAS encounters, the two aircraft will declare the other aircraft to be a threat at slightly different times"
- The TCAS of one aircraft was set to TA-ONLY, although this normally only happens if the airspace is busy
- One had an inaccurate pressure altimeter reading
- Thresholds are larger at higher altitudes and this could have contributed to the higher altitude pilot receiving only a TA
- One airplane was running TCAS I instead of TCAS II and couldn't get RA's. This is probably not the case if the aircraft was commercial.
- They're running different versions of TCAS with slightly different algorithms