WHILE ON A LOC APCH AFTER THE FAF INBOUND; ANOTHER ACFT APPEARED ON THIS RPTING CAPT'S TCASI. THE TARGET WAS WITHIN 4 MI AND WAS NOT TALKING TO THE INBOUND E120 CREW OR EVEN XMITTING IN THE BLIND; ON THE CTAF. THE FLC EXPEDITED THEIR DSCNT THROUGH THE ALT OF THE CLBING AC69 EVEN THOUGH THEY ARE PROHIBITED FROM USING THE TCASI TO INITIATE AN EVASIVE MANEUVER. THIS RPTR SURMISES THAT TCASI EQUIP IS NOT SAFE ENOUGH.

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WE WERE CLRED FOR THE SBP LOC RWY 11 APCH. AS WE PASSED DOBRA INTXN ZLA CLRED US TO SBP CTAF AND DID NOT ADVISE US OF ANY TFC. I CALLED OUR POS ON THE CTAF AND; AS EXPECTED; GOT NO RESPONSE. WE WERE IN SOLID IMC WITH BASES AT 1200 FT AGL. ABOUT THIS TIME (2200 FT MSL) A PA TARGET APPEARED ON OUR TCASI AT 1 O'CLOCK; MINUS 800 FT AND CLBING; AT 4 MI. THIS WAS EXTREMELY DISCONCERTING. WE COULD NOT TELL IMMEDIATELY WHICH WAY THE TFC WAS GOING. WE WERE NO LONGER TALKING TO CTR; AND WE ARE PROHIBITED FROM USING THE TCASI TO INITATE AN EVASIVE MANEUVER. I FELT I HAD 3 OPTIONS: 1) PRETEND WE DIDN'T HAVE TCASI (THE HEAD IN THE SAND APCH) -- UNACCEPTABLE; 2) ABANDON THE APCH AND CLB TO OUR MISSED APCH ALT OF 3000 FT AND HOPE IT WAS VFR -- NOT GOOD; THE OTHER TFC WAS ALSO CLBING; OR 3) EXPEDITE THE DSCNT THROUGH THE CONFLICTING TFC'S ALT -- BEST CHOICE. SHORTLY AFTER DSNDING BELOW THE TFC'S ALT; AN AERO COMMANDER CALLED ON CTAF 'LEAVING SBP AREA.' THIS ACFT THEN CLBED TO 3000 FT; PROCEEDED SSE TOWARD SMX AND CALLED ZLA ABOUT THE TIME WE TOUCHED DOWN. SBP DOES HAVE A 'CTL ZONE;' BUT THE FIELD WAS NOT IFR (BARELY). I'M NOT SURE HOW THE AERO COMMANDER CLBED VFR TO 3000 FT; BUT IN HIS DEFENSE; WE DID SEE HIM VISUALLY ONCE WE WERE ON THE GND. I FEEL THE AERO COMMANDER PLT USED VERY POOR JUDGEMENT; CONSIDERING THE WX AND HVY COMMUTER TFC; IN NOT TALKING TO CTR BEFORE TAKING OFF. ONE LEVEL OF SAFETY DEMANDS ALL ACRS (INCLUDING COMMUTERS) HAVE TCASII.