A B757 PIC DELAYS HIS RA GOVERNED DSCNT FROM 3000 FT AND HAS A POTENTIAL CONFLICT WITH TFC 20 MI SE OF BWI; MD.

Narrative

ON NORMAL VECTORS FOR APCH TO RWY 33L BALTIMORE (BWI). AT 3000 FT MSL HEARD SUPVR GET ON AND GIVE EMER VECTORS TO ANOTHER ACFT. AT SAME TIME WE GOT TA ON TCASII. LOOKED DOWN AND SAW YELLOW TFC CO-ALT AT EDGE OF SCOPE GO QUICKLY FROM 3 O'CLOCK POS TO 6 O'CLOCK POS AND WENT OFF SCOPE AT 6 O'CLOCK POS. SHORTLY AFTER WE GOT A DSND; DSND NOW RA. CAPT (PF) CLICKED OFF AUTOPLT AND DELAYED DSCNT SAYING 'HE'S NOT ON SCOPE -- I CAN'T SEE ANYTHING.' I SAID TO 'DSND.' SHORTLY AFTER TCASII GAVE US AN 'INCREASE RATE OF DSCNT.' CAPT STARTED DSCNT. I NOTIFIED ATC AND WE WERE TOLD TFC HAS YOU IN SIGHT. WE GOT A CLR OF CONFLICT RPT AFTER THAT. MY CONCERN: CAPT (PF) DELAYED DSCNT IN RESPONSE TO AN RA BECAUSE HE 'DIDN'T HAVE CONFLICT IN SIGHT NOR WAS CONFLICT APPARENT ON SCOPE.' MY RESPONSE IS THAT THERE MIGHT BE A POP UP TFC YOU ARE GETTING THE ADVISORY FOR -- NOT THE TFC THAT JUST WENT OFF SCOPE. DELAYING DSCNT UNTIL YOU HAVE TFC IN SIGHT MAY CREATE A WORSE CONFLICT. SUPPLEMENTAL INFO FROM ACN 477429: ATC WAS TOLD WE WERE LEAVING ASSIGNED ALT (3000 FT) BECAUSE OF TCASII ADVISORY.