ACR X TCASII RA NON ADHERENCE TO ATC CLRNC HAD NMAC LTSS FROM ACR Y. PLTDEV.

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ACR WAS CLRED TO 11000 FT ON THE ARR. APPROX 12 MI NW OF ATL ACR X ADVISED THAT HE HAD A TCASII AND HE WAS DSNDING. I TOLD ACR X THAT THE TFC WAS LEVEL AT 10000 FT AND DON'T DSND. HE STILL GOT OUT OF 11000 FT AND DOWN TO 10600 FT BEFORE HE STARTED BACK UP. THE OTHER ACR Y WAS RECEIVING AN RA AT THE SAME TIME TELLING HIM TO CLB. IF ACR Y HAD CLBED AS THE TCASII ADVISED; HE STATED THAT THEY WOULD HAVE COLLIDED WITH ACR X. SUPPLEMENTAL INFO FROM ACN 257130: I HEARD THE TCASII SAY; 'TFC; TFC.' I LOOKED OUTSIDE BUT WE WERE IN AND OUT OF CLOUDS AND I SAW NOTHING. BOTH OF US LOOKED AT THE TCASII DISPLAY ON THE RADAR SCREEN AND SAW A YELLOW SYMBOL WITH MINUS 2900 CLB TO BESIDE IT. I ASSUMED THIS WAS DEP TFC CLBING OUT OF ATLANTA BUT I WAS NOT TOTALLY SURE. WE WERE DSNDING AT A RATE OF APPROX 500 FPM WHEN I HEARD THE TCASII SAY; 'MONITOR VERT DSCNT.' I ASKED FO IF HE COULD SEE THE TFC IN BTWN THE CLOUD LAYERS. AS WE WERE LEVELING AT 11000 FT MSL; THE TCASII SAID; 'DSND; DSND.' WE WERE STILL IN THE CLOUDS AND THE VERT SPD DISPLAY COMMANDED A DSCNT OF 800- 900 FPM AND THEN COMMANDED A DSCNT RATE OF 1800-2000 FPM. THE TCASII SOUNDED MORE INSISTENT BY SAYING; 'DSND NOW.' THE TARGET ON THE TCASII SCREEN WAS RED; BUT THERE WAS SO MUCH RADIO CHATTER; TCASII CHATTER; AND PLT TO COPLT DISCUSSION; THAT I DID NOT NOTICE ANY MORE DETAILS ABOUT THE RED TARGET. EVEN THOUGH I INITIALLY THOUGHT THAT THIS TFC WOULD BE LEVELING OFF AT 10000 FT; I WAS IN THE CLOUDS WITH THE TCASII GOING CRAZY. I ELECTED TO DEPART 11000 FT BUT NOT AT THE GREAT RATE RECOMMENDED BY THE TCASII. I FINALLY GOT THROUGH TO ATLANTA APCH AT 10800 FT ADVISING HIM OF OUR TCASII COMMANDED DSCNT. HE IMMEDIATELY STATED THAT WE HAD TFC LEVEL AT 10000 FT RIGHT IN FRONT OF US! WE WERE AT APPROX 10600-10700 FT MSL. AT THAT POINT WE BROKE OUT OF THE CLOUDS AND THERE WAS ACR Y PASSING RIGHT BENEATH US! I TOLD ATLANTA APCH THAT WE HAD THE TFC IN SIGHT AND WERE CLBING BACK TO 11000 FT. I TRULY FEEL THAT HAD WE FOLLOWED THE TCASII COMMANDS; WE COULD HAVE HAD A MIDAIR COLLISION AND A MAJOR CATASTROPHE. WE DID NOT FOLLOW THE DIRECTIONS GIVEN FROM THE TCASII TO THE EXTENT COMMANDED ALTHOUGH WE DID DEPART 11000 FT DSNDING. I CAN ONLY SPECULATE THAT ACR Y TCASII WAS COMMANDING THEM TO 'CLB; CLB.' REST ASSURED THAT EVEN IN IMC; I WILL BE VERY CAUTIOUS ABOUT FOLLOWING TCASII ALERTS AND WILL BE MUCH MORE PRONE TO DISREGARD THE TCASII WARNINGS. THIS IS THE ONE TIME THE SYS COULD HAVE CAUSED RATHER THAN AVOIDED A MIDAIR COLLISION. THIS INCIDENT RAISES AN APPARENT FLAW IN THE TCASII SYS. IF THE OTHER ACFT IN THE SCENARIO I HAVE JUST DESCRIBED IS VMC AND RECEIVES AN RA SAYING 'CLB; CLB' GETS A TFC CALL OUT FROM ATC; AND SEES NO APPARENT CONFLICT VISUALLY; HE IS QUITE LEGAL TO DISREGARD THE RA. ON THE OTHER HAND; WE RECEIVED NO TA AND OUR ACFT WAS IMC WITH AN RA SAYING; 'DSND; DSND.' BY COMPANY POLICY; I COULD HAVE FLOWN THE COMMANDS OF THE TCASII SINCE WE WERE IMC WITH NO BETTER OUTSIDE INFO. WITH ONLY 1 OF THE 2 ACFT INVOLVED IN THE CONFLICT COMPLYING WITH THE AVOIDANCE COMMANDS; IT WOULD APPEAR YOU ARE BEING SET UP FOR A MIDAIR COLLISION WITH BOTH FLCS AND THE AIR TFC CTLR BEING LEGALLY RIGHT.