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?

  • $\begingroup$ I am an ATCo as well, I have never seen, controlled, faced any similar situation before. $\endgroup$ Apr 6 '18 at 13:19
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    $\begingroup$ Without knowing the geometry (opposing, crossing, in-trail) of the encounter, whether there were other aircraft in the vicinity, when the RA/TA occurred, and what the RA guidance was; it would be hard to say whether it made sense or not. Normally, an RA between 2 TCAS equipped a/c will be coordinated and exist for both. The RA may have been for another a/c (and he didn't report it as it was a 'Maintain' and did not have to take any action). $\endgroup$
    – Gerry
    Apr 6 '18 at 13:24
  • $\begingroup$ It is very strange other traffic were away from those 2 aircrafts might TCAS system has messed up.anyway I filled report waiting for the results.thanks guys. $\endgroup$
    – Jamal
    Apr 6 '18 at 15:48
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    $\begingroup$ Normally TCAS RA for one, means RA for the other one as well unless the later has set is system to TA only and forgot to set it back to TA/RA. $\endgroup$ Apr 6 '18 at 17:27
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    $\begingroup$ Aircrafts involved B777 received TA & B737 received RA $\endgroup$
    – Jamal
    Apr 7 '18 at 5:54

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
  • $\begingroup$ Good answer, although third point I disagree with. Whatever altitude the TCAS thinks it's at, is what it's going to use. If it's wrong (i.e. they're at FL352 not 350 as shown, or vice versa), the indications will be something other than they ought to be, but each aircraft has its singular altitude that the system is working with. (My bet would be that your second point is what happened -- TA Only set when TA/RA was intended. Just my guess, though.) $\endgroup$
    – Ralph J
    Apr 7 '18 at 2:55

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