AN A-320 CAPT WONDERS ABOUT THE OP AND LOGIC OF THE TCASII.

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OUR FLT WAS RNAV DIRECT PGY IN ANTICIPATION OF VECTORS TO ILS 9 AT SAN. AT 6000 FT; APPROX 23 NM FROM THE VORTAC; APCH ADVISED US OF OPPOSITE DIRECTION VFR TFC LEVEL AT 5500 FT. FO SIGHTED TFC IMMEDIATELY; FOLLOWED VERY SHORTLY BY CAPT. CREW ANTICIPATED A TCASII ADVISORY BECAUSE OF 500 FT VERT SEPARATION. IT WAS OBVIOUS VISUALLY THAT THE TFC WOULD PASS BELOW AND WELL CLR OFF THE L SIDE. THERE WERE NO OTHER ACFT ON TCASII; CALLED BY APCH OR VISUALLY SEEN. AS THE TFC PASSED JUST FORWARD OF DIRECTLY ABEAM; TCASII ISSUED AN RA 'CLB; CLB.' CAPT (PF) SAID 'DISREGARD' AND MAINTAINED LEVEL FLT. RA WAS ALMOST IMMEDIATELY FOLLOWED BY 'CLR OF CONFLICT' FROM TCASII AS ACFT PASSED ABEAM AND THE TARGET LEFT OUR TCASII SCREENS. AT NO TIME WAS THERE ANY HAZARD TO FLT. THE PROB IS THIS: 1) TCASII RA WAS ISSUED TOO LATE TO DO ANY GOOD EVEN IF CREW DID NOT SEE THE TFC. 2) CONFUSION EXISTS ON THE LINE REGARDING PROPER RESPONSE TO TCASII WHEN INTRUDER IS IN POSITIVE VISUAL CONTACT. OUR MANUALS ARE VAGUE; THOUGH WE ARE TOLD TO REACT TO ALL RA'S. AN INFORMAL AND THOROUGHLY UNSCIENTIFIC POLL WAS TAKEN OF SEVERAL COMPANY PLTS REGARDING THIS TYPE OF ENCOUNTER (RA IN A TCA WITH POSITIVE VISUAL CONTACT). 50 PERCENT OF THE PLTS SAID THEY WOULD REACT; THE REST WOULD 'DISREGARD.' VIRTUALLY 100 PERCENT INDICATED THAT AT ONE TIME OR ANOTHER THEY HAD DISREGARDED AN RA IN SIMILAR CIRCUMSTANCES. THIS APPEARS TO BE MORE AN ISSUE OF LEGALITIES THROUGH SAFETY IN THIS INSTANCE; SINCE; PRIOR TO TCASII INSTALLATION; THIS WOULD HAVE BEEN A COMPLETELY ROUTINE AND UNMEMORABLE ENCOUNTER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR IS A CAPT FOR A SMALL MAJOR CARRIER FLYING THE AIRBUS A-320. THERE WAS AN FAA INSPECTOR SITTING ON THE JUMPSEAT WATCHING THIS INCIDENT. THE INTRUDING ACFT WAS A LIGHT GA ACFT; 'POSSIBLY A CHEROKEE' WHO WAS FLYING LEGALLY VFR ON TOP. AS THE FLC VISUALLY SIGHTED THE ACFT BEFORE AN RA WAS ISSUED; THE RPTR DOES NOT BELIEVE THAT HE WAS REQUIRED TO FOLLOW THE RA. HE HAD ALREADY SOLVED THE PROB. THE A-320 TCASII IS DISPLAYED ON THE PFD BUT ONLY AFTER BEING FILTERED BY A COMPUTER IN THE ACFT. NOT ALL TFC IS DISPLAYED ON THE PFD; ONLY THAT WHICH THE COMPUTER FINDS WORTHY. THE RPTR'S ACR ALLOWS THEIR AIR CREWS TO TURN OFF THE TCASII WHEN IN AN APCH SIT; BUT THE RPTR AGREES THAT THIS WOULD HAVE BEEN A LITTLE FAR OUT TO DO THAT.