TCASII SAVES THE DAY AGAIN BY TURNING A POSSIBLE NMAC INTO A POTENTIAL CONFLICT.

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FO WAS FLYING THE ZIGGY 3 ARR INTO ONT. AFTER HITOP (PALMDALE 104 DEG RADIAL AT 41 MI); WE WERE GIVEN A HDG OF 150 DEGS FOR THE RWY 26L LOC AND A DSCNT TO 8000 FT. DURING THE ARR; WE WERE PREVIOUSLY GIVEN SEVERAL ALT RESTRS FOR TFC; MOST OF WHICH WE DID NOT SEE. WE WERE HELD AT 8000 FT; AND THEN GIVEN A HDG OF 230 DEGS TO INTERCEPT THE LOC AND A DSCNT TO 4200 FT. WE RECEIVED A TA AT 10 O'CLOCK AND 400 FT BELOW. I WAS LOOKING FOR THE UNRPTED TFC IN THE HAZE; AND WE RECEIVED AN RA OF 'CLB; CLB.' THE FO ADDED PWR TO STOP THE DSCNT AND COMMENCED CLBING AND TURNING AWAY FROM THE UNSEEN THREAT. I RPTED OUR ACTION TO ONT APCH AND THEY ASKED US IF WE WERE ABLE TO GET DOWN (7500 FT 14 MI) AND THE FO SAID 'YES' AND TURNED BACK TO INTERCEPT THE LOC. DSNDING THROUGH 6500 FT; WE RECEIVED ANOTHER TA AT 9-10 O'CLOCK AT OUR ALT. I SAW A SMALL PLANE CONVERGING AND TOLD THE FO TO LEVEL OFF AND COME R. WE CLRED THIS THREAT AND I TOLD APCH THAT WE HAD ANOTHER CONFLICT. APCH SAID; 'THAT WAS THE TFC AT 2 O'CLOCK AND 2 MI THAT I RPTED.' (WE NEVER HEARD THAT XMISSION.) I REPLIED; 'NO; IT WAS 10 O'CLOCK AND A MI.' THE FO REJOINED THE LOC; INCREASED THE RATE OF DSCNT AND MADE A STABILIZED APCH TO AN UNEVENTFUL LNDG. THE ONT AREA IS EXTREMELY HAZARDOUS. THE WX IS USUALLY MARGINAL VFR (HAZE; SMOKE) WITH HIGH TERRAIN IN THE AREA. THERE ARE MANY OTHER ACFT; BOTH VFR AND IFR THAT AIR TFC DOES NOT SEE OR RPT. IN ADDITION; ONT APCH OFTEN CLRS ACFT FOR A 'SLAM-DUNK' ILS AND THEN TELLS THEM TO MAINTAIN VISUAL SEPARATION WITH OTHER TFC; WHICH IS USUALLY UNSEEN. WITHOUT TCASII; WE WOULD PROBABLY HAVE HAD A MIDAIR COLLISION. I WAS SURPRISED TO HEAR THAT A PART 121 CARGO CARRIER (WHO FREQUENTS ONT) DOES NOT HAVE TCASII INSTALLED!