FLC RPTS ERRONEOUS COMMAND FROM TCASII RA; COMMANDING A DSCNT INTO XING TFC BELOW PRIOR TO ORDERING A REVERSAL CLB COMMAND AFTER ACFT Y'S PASSAGE.

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WHILE APCHING PHL INTL FOR LNDG; ATC CALLED TFC 11 O'CLOCK; L TO R AT 6500 FT. WE WERE THROUGH 8000 FT FOR 7000 FT. TCASII THEN CALLED 'TFC; TFC.' TFC WAS NOT SPOTTED VISUALLY YET. TCASII CALLED 'DSND' AND EFIS DISPLAYED A DSCNT ATTITUDE TO AVOID INCURSION. AUTOPLT WAS DISCONNECTED AND DSCNT BEGAN. THEN I MADE VISUAL CONTACT WITH AN SMA Y. DSCNT WAS STOPPED DUE TO ACFT BELOW US. TCASII LATER CALLED 'CLB; CLB' WHEN TFC WAS 1 MI AWAY. IN SHORT; TCASII HAD INSTRUCTED US TO DSND INTO ANOTHER ACFT. BECAUSE I HAVE BEEN TRAINED TO FOLLOW TCASII ADVISORIES (RA'S); I FOLLOWED THE DSCNT INSTRUCTIONS; DESPITE KNOWING THAT AN ACFT WAS BELOW. OUR TRAINING HAS SUGGESTED THAT TCASII CAN TRACK MULTIPLE ACFT SIMULTANEOUSLY AND THEREFORE I INITIALLY BELIEVED THE MACHINE AND SURMISED THAT THERE MAY HAVE BEEN A SECOND ACFT NEARBY AND THAT A DSCNT WAS NECESSARY. SUPPLEMENTAL INFO FROM ACN 233440: NO ALTDEV OCCURRED. HAVING VISUAL ON THE INTRUDING ACFT; WE ELECTED TO MAINTAIN 7000 FT. I DO NOT PRETEND TO UNDERSTAND THE ELECTRONIC FUNCTIONS OF THE TCASII BUT STILL WONDER WHY IT COMMANDED WHAT WOULD HAVE BEEN A MIDAIR COLLISION IF ITS (TCASII) DIRECTIVES HAD BEEN FOLLOWED.