Narrative:

While approaching phl international for landing, ATC called traffic 11 O'clock, left to right at 6500 ft. We were through 8000 ft for 7000 ft. TCASII then called 'traffic, traffic.' traffic was not spotted visually yet. TCASII called 'descend' and EFIS displayed a descent attitude to avoid incursion. Autoplt was disconnected and descent began. Then I made visual contact with an small aircraft Y. Descent was stopped due to aircraft below us. TCASII later called 'climb, climb' when traffic was 1 mi away. In short, TCASII had instructed us to descend into another aircraft. Because I have been trained to follow TCASII advisories (RA's), I followed the descent instructions, despite knowing that an aircraft was below. Our training has suggested that TCASII can track multiple aircraft simultaneously and therefore I initially believed the machine and surmised that there may have been a second aircraft nearby and that a descent was necessary. Supplemental information from acn 233440: no altitude deviation occurred. Having visual on the intruding aircraft, 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.

Google
 

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.