TCASII LOGIC CAUSED CONFLICT BTWN ACR Y AND CPR X. EVASIVE ACTION TAKEN. SEE AND AVOID CONCEPT.

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CPR X (SW3) WAS VFR EBOUND 25 SW ERI AT 11500. ACR Y (F100) CLBOUT FROM ERI ON 220 DEGS HDG TO 14000. TFC WAS EXCHANGED AT 15 NM TO Y AND 12 NM TO X. Y ADVISED HE HAD TFC ON TCASII. TFC EXCHANGED AGAIN AT 5 NM. Y (SHOWING 11300 ON MODE C) ADVISED HE WAS RECEIVING A TCASII RA AND WOULD LEVEL BELOW TFC. I ISSUED TFC AGAIN TO X AT 12 O'CLOCK 3 NM 11400 AND I BELIEVE HE'S DSNDING. NEXT RADAR UPDATE SHOWED Y AT 11600 AND I TOLD X HE SHOWED 11600 AND CLBING. X RESPONDED HE WAS NOW PAST HIM. NEXT RADAR UPDATE SHOWED Y AT 11100; AND CLR OF X. SUBSEQUENT PHONE CONVERSATION WITH X (A WELL-SEASONED CORPORATE CAPT) CONFIRMED THAT Y WAS JUST AHEAD AND TO L OF SW3 AND ABOVE HIS ALT; WHEN Y'S NOSE DROPPED AND DSNDED BACK THROUGH HIS ALT AND CROSSED HIS NOSE AT APPROX 1 MI AND APPROX 400 FT BELOW. I BELIEVE THE TCASII RA CAUSED THIS EVENT TO HAPPEN. AN UNINTERRUPTED CLIMB IN MY OPINION WOULD HAVE PRESENTED NO CONFLICTION. THE TCASII RA INSTRUCTED THE F100 TO DSND; HOWEVER; BEFORE CLB ENERGY COULD BE ARRESTED AND DSCNT STARTED; THE ACFT WAS ALREADY ABOVE THE CONFLICTING TFC. THE DSCNT REQUIRED BY THE TCASII ALERT PLACED THE ACFT IN HARMS WAY. I BELIEVE MOST OF THE NEW TECHNICAL AUTOMATION TO BE A SPLENDID AID TO AVIATION; HOWEVER; THE HUMAN LOGIC MUST BE PERMITTED TO OVERRIDE OBVIOUS SHORTCOMINGS. TCASII HAS A HISTORY OF PROBS WITH THE SOFTWARE LOGIC. WE SHOULD RECONSIDER THE REQUIREMENT FOR CAPTS TO REACT TO INSTRUCTIONS THAT ARE QUESTIONABLE; UNTIL THE TCASII IS MUCH MORE RELIABLE.