TCASII RA; SIGHT TFC AND TURN AWAY.

Narrative

ON APCH TO ARPT; MARGINAL VISIBILITY; WX RADAR ON FOR LIGHT PRECIPITATION; TCASII ISSUED A TA; RADAR WAS OPERATIONAL ON 20 MI RANGE AND TARGET WAS OBSERVED SOMEWHERE NEAR CTR OF ACFT. WAS BUSY SETTING UP FOR APCH AND TALKING TO APCH CTL. SECOND TA ISSUED BY TCASII. IMMEDIATELY WENT TO 5 MI TCASII ONLY MODE AND OBSERVED TARGET MOVING FROM 1-2 O'CLOCK; LESS THAN 1 MI. WENT VISUAL AND OBSERVED SMALL ACFT OFF R AT SAME ALT. ADDED THRUST AND BANKED L DEVIATING FROM APCH INSTRUCTIONS. ADVISED ATC AND RETURNED TO PROFILE WHEN CLR OF TFC. ATC WAS VERY BUSY; HOWEVER; THEY RPTED NO TFC AT OUR ALT AND GAVE NO TA'S. TCASII SHOWED NO ALT RPTING ON TARGET. HAVE HAD NUMEROUS CLOSE CALLS IN THIS AREA DUE TO NUMEROUS ARPTS OPERATING IN AREA; AND SEVERAL UNIDENTIFIED ACFT. ALSO; TCASII SOFTWARE WOULD BE VERY MUCH MORE USEFUL IF; WHEN A TA IS ISSUED; IT WOULD AUTOMATICALLY SWITCH TO TCASII ONLY 5 MI RANGE; AS THIS IS THE MOST USEFUL DISPLAY WHEN SEARCHING FOR ACFT. IN TCASII WX MODE IN GREATER THAN 10 MI TFC TARGET READOUT RELATIVE TO ACFT IS USELESS.