THE CREW OF A C550 ON A VECTOR BY SCT HAS AN NMAC WITH A VISUALLY ACQUIRED SMA THAT DID A 180 DEG TURN AND PASSED 200 FT BEHIND THEM WHILE THEY WERE ON DOWNWIND TO CRQ; CA.

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ON DEC/MON/99; WE WERE ON THE LAST LEG OF A 4 LEG TRIP FROM SNA TO CRQ. THIS WAS A PART 135 REPOSITIONING LEG TO HOME BASE. WE WERE ON AN IFR FLT PLAN UTILIZING SOCAL TRACON ENRTE ROUTING. AFTER XING OCN VOR; WE WERE GIVEN A HEADING OF 070 DEGS FOR RADAR VECTORS TO THE ILS RWY 24 AT CRQ. WE WERE MAINTAINING AN ASSIGNED ALT OF 4000 FT MSL. WHILE ON A DOWNWIND LEG ATC ADVISED US OF TFC AT OUR 11 O'CLOCK POS; 2-3 MI; TYPE UNKNOWN; AS ATC WAS NOT TALKING WITH THIS ACFT. ATC ALSO ADVISED THAT THE ACFT WAS ALSO AT 4000 FT MOVING FROM OUR 11 TO 12 O'CLOCK POS. WE ACQUIRED THE ACFT VISUALLY; AS WE ARE NOT EQUIPPED WITH TCASII; AND INFORMED ATC. WHILE ON ASSIGNED HEADING OF 070 DEGS; WE INFORMED ATC THAT WE WILL PASS BEHIND INTRUDER ACFT; SAID THAT IT SHOULD NOT BE A FACTOR. AS THE INTRUDER ACFT MOVED FROM OUR 12 TO 1 O'CLOCK POS; IT BEGAN A L TURN WHILE REMAINING AT OUR ALT; 4000 FT. THE ACFT CONTINUED THE TURN UNTIL REACHING 180 DEGS IN THE TURN DIRECTLY BACK TOWARDS US AT OUR ALT. WHILE THE ACFT WAS IN THE L TURN BACK TOWARDS US; I INFORMED ATC OF THE SIT; AND ADVISED THE CAPT TO PREPARE FOR EVASIVE MANEUVER. ATC THEN AUTH US TO MANEUVER AS NECESSARY TO AVOID COLLISION. IT IS ALSO IMPORTANT TO NOTE WE WERE ON AN ASSIGNED AIRSPD OF 180 KIAS. WE HAD OUR STROBE AND RECOGNITION LIGHTS ON AT THIS TIME. AS WE ARE VERY FAMILIAR WITH THIS PARTICULAR ROUTING; AND KEEPING IN MIND THE TFC DENSITY OF THE SOCAL AREA WE BEGAN DISCUSSING AN EVASIVE MANEUVER. THE INTRUDER ACFT STOPPED ITS TURN AT OUR 2-3 O'CLOCK POS AT OUR ALT; ABOUT 1-1.5 MI AWAY. CLOSELY WATCHING THE ACFT WE ELECTED NOT TO DEVIATE FROM HEADING AND ALT BECAUSE WE HAD DETERMINED THE ACFT WILL PASS BEHIND US; ALBEIT VERY CLOSELY BEHIND US. AS THE ACFT PASSED US; IT IS IN OUR OPINION; IT WAS EXACTLY AT OUR ALT AND PASSED BEHIND WITH A DISTANCE OF NO MORE THAN 200 FT HORIZ. HAVING FLOWN ACFT EQUIPPED WITH TCASII; IT IS MY OPINION THIS INCIDENT WOULD HAVE GENERATED AN RA. AFTER THIS ENCOUNTER WE CONTINUED ON THE APCH AND LANDED UNEVENTFULLY. CONTRIBUTING FACTORS: TFC DENSITY IN SOCAL; VAST MIX OF ACFT IN SOCAL AIRSPACE; MIX OF IFR AND VFR TFC IN SOCAL; APPARENT LACK OF OPERATING PROCS BY INTRUDER ACFT; LACK OF INSTALLED TCASII IN OUR ACFT; ATC SATURATION IN SOCAL. CORRECTIVE ACTION: I BELIEVE ALL TURBOJET ACFT SHOULD BE EQUIPPED TO HAVE TCASII; AND ALL ACFT IN SOCAL AIRSPACE BE REQUIRED TO FILE IFR FLT PLANS.