AN ACR B737-500 FLC RPTS TAKING EVASIVE ACTION AFTER RECEIVING A TCASII RA AT 5000 FT; BTWN THE SXC AND THE SLI VORS. THE OTHER ACFT WAS A SINGLE ENG; GA AIRPLANE THAT WAS NOT IN CONTACT WITH THE APCH CTLR. THE B737 DSNDED BELOW THE OTHER ACFT AS IT PASSED 200 FT OVERHEAD; AND ONE HALF MI AWAY.

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THE FOLLOWING EVENT TOOK PLACE ON A FLT FROM SJC TO SNA USING A B737-500. AS WE APCHED THE SXC VOR FROM THE N ON THE TANDY 2 ARR; WE WERE CLRED TO DSND FROM 14000 FT TO 5000 FT BY SOCAL APCH. AS WE APCHED THE VOR; WE WERE ALSO GIVEN A L TURN TO A HDG OF 090 DEGS; SHORTLY FOLLOWED BY ANOTHER VECTOR TO A 065 DEG HDG. THE CTLR WAS BUSY AND; AS WE LATER FOUND OUT; HE WAS WORKING THIS SECTOR BY HIMSELF. AT AN ALT OF 5000 FT AND ON A HDG OF 065 DEGS AT 250 KIAS; WE WERE NOW ABOUT HALFWAY BTWN SANTA CATALINA ISLAND AND THE SHORELINE. ACFT WAS ON AUTOPLT; AND THE CAPT AND I WERE BUSY LOOKING OUTSIDE FOR TFC. SUDDENLY; WE RECEIVED A TCASII TA 'TFC; TFC;' SHORTLY FOLLOWED BY ANOTHER. THE TFC WAS AT 12 O'CLOCK OPPOSITE DIRECTION; AND DSNDING FROM ABOVE OUR ALT. NEITHER ONE OF US HAD HIM IN SIGHT. WE THEN RECEIVED A TCASII RA 'DSND; DSND NOW.' THE CAPT TOOK OVER CTL OF THE ACFT; DISENGAGED THE AUTOPLT; AND BEGAN A L; DSNDING TURN. THE TFC PASSED OFF OUR R; APPROX 200 FT HIGHER; 1/2 MI AWAY. WE THEN RECEIVED A 'MONITOR VERT SPD' AURAL WARNING FOLLOWED BY 'CLR OF TFC.' WE BELIEVE THE OTHER TFC; A SMALL SINGLE ENG GA ACFT; NEVER SAW US. OUR LOWEST ALT WAS 4300 FT MSL. THE SOCAL APCH SUPVR LATER ADMITTED THAT; FOLLOWING OUR DEV; 3 CTLRS WERE ASSIGNED TO THAT SECTOR. ALTHOUGH SOMEWHAT HAZY; VISIBILITY AT OUR ALT WAS AT LEAST 5 MI. SUPPLEMENTAL INFO FROM ACN 336122: THE GA ACFT WAS NOT TALKING TO APCH CTL. BEING UNABLE TO GET THROUGH TO SOCAL APCH; WE FOLLOWED THE TCASII INSTRUCTION AND MADE A DSNDING L TURN. DURING THE DEV TFC WAS OBTAINED VISUALLY AND VISUAL SEPARATION WAS MAINTAINED. THE SIT WAS DISCUSSED WITH THE SOCAL SUPVR AND I WAS TOLD THE ACFT WAS 'UN-TAGGED' AND NOT IN CLASS B AIRSPACE. I FEEL VERY STRONGLY THAT THE WHOLE LA AREA SHOULD BE AT LEAST CLASS C AIRSPACE. IT IS UNSAFE TO HAVE UNCTLED GA ACFT IN SUCH A HIGHLY CONGESTED AIRSPACE!