LTSS ON THE FINAL APCH COURSE.

Narrative

WE WERE ABOUT 9 MI NE OF DEN AND BEING VECTORED FOR A VISUAL APCH TO RWY 17L. AFTER A SERIES OF HDG; ALT; AND SPD CHANGES; WE WERE ON A L BASE FOR RWY 17L WITH THE ARPT IN SIGHT. ATC POINTED OUT TFC AT OUR 12 O'CLOCK POS AND ASKED IF WE HAD THE ACFT IN SIGHT. THE CAPT SAID HE DID AND I REPLIED IN THE AFFIRMATIVE. ATC THEN CLRED US FOR A VISUAL APCH TO FOLLOW THAT ACFT TO RWY 17L. WE WERE ABOUT 3-4 MI FROM THE FINAL APCH COURSE. I LOOKED DOWN TO IDENT THE LOC FOR RWY 17L. AFTER I HAD COMPLETED THE IDENT; I LOOKED UP AT THE EHSI AND NOTICED THAT WE WERE LESS THAN A MI FROM THE FINAL APCH COURSE AND THE CAPT HAD NOT BEGUN A TURN TO FINAL. I IMMEDIATELY SAID; 'YOU'RE GOING TO OVERSHOOT FINAL' AND THE CAPT BEGAN A L TURN TO INTERCEPT. DUE TO OUR SPD AND THE LATE TURN; WE OVERSHOT FINAL BY MORE THAN A MI AND ATC SAID SOMETHING LIKE 'ACR; ARE YOU GOING TO MAKE THAT TURN?' THE CAPT ROLLED INTO A HARD R TURN. SURPRISED AT THIS UNEXPECTED TURN; I ASKED WHAT HE WAS DOING. HE RESPONDED; 'TELL ATC I'M GOING TO MAKE A R 270 DEG TURN TO REINTERCEPT THE FINAL.' EVEN THOUGH I DIDN'T THINK THIS WAS A WISE MANEUVER WITH PARALLEL TFC FOR RWY 17R NEARBY; I DECIDED IT WAS BETTER TO INFORM ATC OF OUR ACTION RATHER THAN DEBATE IT WITH THE CAPT. WHEN I INFORMED ATC THAT WE WERE NOW IN A R 270 DEG TURN THEY RESPONDED; 'NO; NO; NO; TURN L IMMEDIATELY!' ATC WAS PROBABLY UPSET DUE TO THE PARALLEL TFC FOR RWY 17R BEING IN CLOSE PROX. WE COMPLIED WITH THE ATC INSTRUCTION; REINTERCEPTED THE RWY 17L FINAL AND LANDED WITHOUT FURTHER INCIDENT. THE CAPT LATER EXPLAINED THAT HE SUDDENLY THOUGHT THAT A R 270 DEG TURN WOULD HAVE BEEN MORE EFFICIENT TO REINTERCEPT THE FINAL; BUT HE FORGOT TO CONSIDER THE PARALLEL TFC. SUPPLEMENTAL INFO FROM ACN 311970: MY UNDERSTANDING WAS THAT THE TFC WAS FOR RWY 17L; AND WE WERE TO FOLLOW IT TO THE ARPT. THE SPACING SEEMED SOMEWHAT 'TIGHT;' SO I STARTED ADJUSTING FOR THIS CONTINGENCY; IE; SLOWING AND SQUARING OFF THE PATTERN. THE MORE IMPORTANT TASK OF LINING UP WITH THE CORRECT RWY BECAME SECONDARY. I WILL TURN OFF MY FLT DIRECTOR ON A VISUAL APCH; BUT DID NOT DO SO ON THIS MORNING. EXTENDED RWY CTRLINES HAVE BEEN 'SACRED' TO ME ON VISUAL APCHS AND I FAVOR MY SIDE ON ANY PARALLEL VISUAL APCH.