CL60 CREW ON RWY 26L ILS APCH INTO ONT HAVE NMAC WITH AN FA20 ON A RWY 26R VISUAL APCH TURNING ONTO A 2 MI FINAL.

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AN NMAC INCIDENT; INVOLVING A CANADAIR CHALLENGER JET AND A FALCON JET; IN THE VICINITY OF ONT ARPT; CA. THE WX CONDITIONS AT THE TIME WERE EXCELLENT. ONT ARPT ATIS INFO READ IN PART WIND 250 DEGS AT 8 KTS; 10 SM VISIBILITY; 17000 FT BROKEN; ILS RWY 26L APCH IN USE; LNDG RWY 26R. OUR ACFT WAS ON AN IFR FLT PLAN. I SUSPECT THE FALCON WAS ON AN IFR FLT PLAN AS WELL. I WAS ACTING AS THE PNF; SIC; HANDLING RADIO COMS. PRIOR TO THE INCIDENT; SOCAL TRACON HAD CLRED US FOR THE ILS APCH TO RWY 26L. AFTER THE SOCAL HDOF; I CONTACTED ONT TWR. IT WAS OBVIOUS FROM THE COMS THAT THE TWR CTLR WAS HANDLING COMS FOR BOTH RWYS. WHEN WE WERE ON A 2 OR 3 MI FINAL; I HEARD THE CTLR ADVISE A FALCON THAT A CHALLENGER (OUR ACFT) WAS ON FINAL APCH FOR RWY 26L AND TO RPT THE CHALLENGER IN SIGHT. THE CTLR THEN ADVISED US THE FALCON WAS ON DOWNWIND (N SIDE) FOR RWY 26R. I RPTED TO THE TWR WE HAD A TARGET IDENTED ON TCASII AND WERE LOOKING (VISUALLY) FOR THE FALCON. AT NO TIME DID I HEAR THE FALCON PLT RPT US IN SIGHT. THE TWR CTLR SUBSEQUENTLY CLRED THE FALCON FOR A VISUAL APCH TO RWY 26R. I FIRST VISUALLY IDENTED THE FALCON ON OUR R SIDE; ABEAM OUR POS; WING UP; IN A R BASE TO FINAL TURN. I WOULD ESTIMATE THE INITIAL SEPARATION TO BE 500-700 FT. THE FALCON WAS PERHAPS 100 FT ABOVE US; AND DSNDING INTO OUR ALT. OUR ACFT HAD BEEN WELL ESTABLISHED ON THE ILS SINCE THE OM. I BELIEVE I RPTED THE FALCON IN SIGHT TO THE TWR. AT THAT POINT; BASED ON THE FALCON'S FLT PATH AND CLOSURE RATE; I ALSO WARNED MY CAPT TO PREPARE FOR A 'BREAKOUT.' BY THAT I MEANT A HAND-FLOWN EVASIVE MANEUVER TO PREVENT A MIDAIR COLLISION. WITHIN 5-10 SECONDS; THE SEPARATION DETERIORATED TO WHAT I CONSIDERED TO BE MINIMUM SAFE SEPARATION (LESS THAN 100 FT LATERALLY AND PERHAPS 20 FT ABOVE OUR ALT) AND I THEN ADVISED THE CAPT THAT I WOULD 'BREAK OUT' NOW. AFTER I STATED THIS; I LEANED BACK IN MY SEAT SO THAT THE CAPT COULD LOOK OUT MY R WINDOW TO VIEW THE FALCON'S PROX TO OUR ACFT. I SAW HIM GLANCE OVER AND; AT THAT POINT; THE CAPT SAID 'THAT'S IT; TELL 'EM WE'RE GOING AROUND.' I ADVISED THE TWR WE WERE MISSED APCH AND RECEIVED INSTRUCTIONS. I SUBSEQUENTLY REQUESTED AND OBTAINED CLRNC TO PERFORM A L BREAK OVER RWY 26L TO ENTER DOWNWIND S. WE MADE L TFC AND PROCEEDED TO MAKE AN UNEVENTFUL APCH AND LNDG TO RWY 26L. CONTRIBUTING FACTORS TO THIS INCIDENT INCLUDE THE TWR CTLR'S DECISION TO CLR THE FALCON FOR THE VISUAL APCH WHEN THE FALCON DID NOT HAVE THE CHALLENGER TFC FOR THE PARALLEL RWY IN SIGHT. THE FALCON PLT'S DECISION TO TURN R BASE BEFORE HE HAD US IN SIGHT. THE FALCON PLT'S DECISION NOT TO EXTEND DOWNWIND UNTIL HE IDENTED THE CHALLENGER VISUALLY. WITH THE FALCON BEING WING UP IN THE TURN; A BLIND SPOT IS CREATED; AND I DOUBT THE FALCON PLT COULD SEE US AT ALL. THUS; THE FALCON PLT WOULD HAVE NO WAY OF MAINTAINING VISUAL SEPARATION FROM OUR ACFT. SUPPLEMENTAL INFO FROM ACN 555651: I LATER CALCULATED THAT I HAD LESS THAN 4 SECONDS TO DETERMINE IF THE FALCON WAS GOING TO STAY ON HIS SIDE OF THE APCH OR NOT. THINGS WOULD HAVE BEEN ENTIRELY DIFFERENT AND CERTAINLY MUCH SAFER HAD THE FALCON WAITED UNTIL HE HAD US IN SIGHT OR CONFIRMED ABEAM BEFORE HE MADE HIS BASE TURN.