A CARJ CREW ON A VISUAL DOWNWIND TO BOI; OVERSHOT FINAL; PUTTING THEM IN HARM'S WAY WITH TFC ON THE PARALLEL APCH.

Narrative

WE WERE CLRED FOR A VISUAL APCH; R DOWNWIND; FOR RWY 28R AT BOI. IT WAS THE FO'S LEG. WE WERE ADVISED BY ATC THAT A CESSNA WAS ON FINAL FOR RWY 28L. WE DID NOT HAVE THE CESSNA IN SIGHT; BUT DID HAVE A LIKELY TARGET ON TCASII. AS WE ROLLED OUT ON BASE; I SIGHTED THE CESSNA AT 11 O'CLOCK POS; 1 - 1 1/2 MI. AT THIS TIME I ALSO SIGHTED THE RWYS; AND IT APPEARED THAT WE WERE ABOUT TO OVERSHOOT FINAL FOR RWY 28R. WE ALSO RECEIVED A TA ON TCASII. (SINCE WE WERE IN R TFC; I HAD LOST SIGHT OF THE RWYS FROM A POINT ON DOWNWIND.) THE FO NOTICED ALL OF THIS ABOUT THE SAME TIME AND PUT THE ACFT IN A STEEP (40-45 DEG BANK) TO MINIMIZE THE OVERSHOOT. I CALLED 'FLAPS' TO INDICATE WE NEEDED ALL THE FLAPS; AS I WAS CONCERNED ABOUT MANEUVERING SPD AND MINIMIZING OUR TURN RADIUS. SINCE WE WERE IN A R BANK; WE LOST SIGHT OF THE CESSNA. WE AND THE CESSNA LANDED WITHOUT FURTHER INCIDENT. I THOUGHT THAT WE HAD OVERSHOT FINAL FOR BOTH RWYS; ALTHOUGH THE FO FELT WE OVERSHOT ONLY RWY 28R BY A LITTLE. NOTHING WAS SAID ABOUT THE INCIDENT BY THE CESSNA PLT OR ATC. I AM NOT SURE IF THE CESSNA HAD TO TAKE ANY EVASIVE ACTION. THIS INCIDENT WAS CAUSED BY A POORLY PLANNED AND FLOWN VISUAL APCH; BUT MOSTLY BY ME NOT EXERCISING MY COMMAND AUTH. I WAS CONCERNED ABOUT NOT HAVING THE CESSNA IN SIGHT; BUT SAID NOTHING TO THE FO. I THOUGHT ABOUT EXTENDING OUR FINAL UNTIL WELL PAST THE CESSNA; BUT DID NOT INSTRUCT THE FO TO DO SO. IF I HAD SAID SOMETHING AS SIMPLE AS 'LET'S MAKE SURE WE DON'T OVERSHOOT FINAL;' THE INCIDENT MAY HAVE BEEN AVOIDED.