A DC9-30 FO OVERSHOOTS THE TURN ON FINAL TO RWY 1L AT IAD WHILE LOOKING FOR VISUAL TFC ON APCH TO RWY 1R.

Narrative

WHILE BEING VECTORED ON A DOWNWIND LEG TO RWY 1L AT DULLES; TWR ASKED IF WE HAD THE FIELD IN SIGHT; WHICH WE DID. AT THAT TIME WE WERE CLRED FOR A VISUAL APCH TO RWY 1L AND A L TURN BACK TO THE FIELD WAS INITIATED TO RESULT IN A FINAL OF APPROX 6 MI. WHEN APPROX 60 DEGS FROM THE RWY HDG; TWR RPTED TFC (B757) JOINING A FINAL FOR RWY 1R. WHILE LOOKING FOR THE TFC; THE FO (WHO WAS FLYING THE ACFT) TOOK HIS EYES OFF THE FIELD AND SHALLOWED HIS BANK WHILE LOOKING FOR THE B757. WHEN I REALIZED HE WAS NOT JUST SQUARING OFF HIS FINAL BUT WAS GOING TO OVERSHOOT THE RWY I TOLD HIM HE WAS GOING TO OVERSHOOT AND ORDERED A TURN BACK TO OUR RWY. HE SEEMED DISORIENTED AND WAS SLOW IN RESPONDING RESULTING IN A SIGNIFICANT OVERSHOOT APCHING THE APCH CORRIDOR FOR RWY 1R. A TCASII RA RESULTED WITH A 'MONITOR VERT SPD' COMMAND WHICH WAS COMPLIED WITH. TWR QUESTIONED IF WE HAD THE TFC IN SIGHT WHICH WE ANSWERED IN THE AFFIRMATIVE. WE CORRECTED BACK TO RWY 1L CTRLINE AND LANDED WITH NO FURTHER INCIDENT. IN TALKING TO THE FO AFTER THE LNDG HE INDICATED THAT HE LOST SIGHT OF THE RWY IN THE L TURN; ALSO THAT HE NEVER ACTUALLY SAW THE B757. ALTHOUGH I INDICATED THAT I SAW THE TFC AND POINTED IT OUT; THE FO DID NOT SEE IT BUT I ASSUMED HE DID. I ALSO ASSUMED THAT HE HAD THE RWY IN SIGHT; SO I WAS UNAWARE THAT HE HAD LOST SITUATIONAL AWARENESS. THE LESSON TO ME IS TO NEVER ASSUME ANOTHER CREW MEMBER IS SEEING THE SAME THING I AM AND TO WORK TO COMMUNICATE WHAT I AM SEEING EVEN WHEN WX IS GOOD AND 'EASY' VISUAL APCHS ARE BEING CONDUCTED.