TCASII CHANGED ITS MIND IN MID SIT.

Narrative

WE WERE CLRED FOR A DSCNT MAINTAINING VISUAL SEPARATION FROM ANOTHER ACFT. THE SIGHTED TFC SEEMED TO BE FAR ENOUGH AWAY THAT IT WOULD NOT BE A FACTOR. WHILE DSNDING AT ABOUT 1500 - 2000 FPM; THE TCASII ISSUED A CLB (ABOUT 1200-1500 FPM) RA. THE DSCNT WAS SLOWED AS WE BEGAN TO COMPLY WITH THE TCASII ADVISORY; AND INDEED THE SIGHTED TFC DID SEEM MUCH CLOSER -- AND CLOSING FAST. HOWEVER; IT CLRLY WAS ALREADY ABOVE OUR ALT. WE WERE JUST ABOUT IN LEVEL FLT WHEN THE TCASII CHANGED ITS ADVISORY TO DSND. THAT AREA BEING VISUALLY CLR; WE RESUMED THE DSCNT AT ABOUT 1500-2000 FPM AND THE TFC PASSED US; IN THE OPPOSITE DIRECTION; OFF OUR R ABOUT 500 FT AND 1000 FT ABOVE. THE TFC'S LIGHTS WERE ALWAYS CLRLY VISIBLE BUT GND LIGHTS BELOW AND BEHIND THEM MAY HAVE ADDED TO THE ILLUSION THAT THE ACFT WAS NOT MOVING TOWARD US AND THUS WAS NOT A FACTOR. HOWEVER; SINCE THE TFC WAS MOVING DIRECTLY IN THE OPPOSITE DIRECTION. THE CLOSURE WAS VERY FAST. THAT; COUPLED WITH THE FAST DSCNT; MAY HAVE MOMENTARILY OUTPACED THE TCASII; WHICH THEN ISSUED THE CLB ADVISORY. AT THAT POINT; WE SHOULD HAVE BEEN DSNDING FASTER. OUR STARTING TO COMPLY WITH THE TCASII CLB ADVISORY ACTUALLY KEPT US NEARER THE THREAT.