DURING A NIGHT OP; VISUAL APCH 7 PT 5 NM NW OF BWI; THE APCH CTLR TURNED THE FLT BACK TO ITS ORIGINAL 90 DEG INTERCEPT HDG FOR RWY 10; BELIEVING THAT A TURN TO THE L WAS TO LINE THE FLT UP WITH RWY 15 AT BWI; MD.

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WE WERE FLYING INTO BWI. WHILE BEING VECTORED W OF THE FIELD AT 3000 FT; BWI APCH ASKED US TO RPT THE FIELD IN SIGHT (APPROX 15 MI AT 10 O'CLOCK POS). THIS WAS MY FIRST; MAYBE SECOND; TIME INTO BWI AT NIGHT. CAPT SAID HE HAD THE FIELD -- I; HOWEVER; DID NOT. THE CAPT SAID TO WAIT TO CALL THE FIELD. AT ABOUT 10 MI FROM BWI; I STILL DID NOT HAVE IT IN SIGHT. THE CAPT SAID THAT HE DID; AND TO CALL THE FIELD. WE WERE ON A SOUTHERLY HDG AND WITHIN 10 MI AND THE ARPT WAS ON THE CAPT'S SIDE. I TOLD APCH THAT WE HAD THE FIELD. APCH CTL CLRED US FOR THE VISUAL TO RWY 10. WE BOTH HAD THE LOC TO RWY 10 TUNED AND THE APCH COURSE SET. AT ABOUT 7.5 MI; I STILL COULDN'T FIND THE FIELD; BUT I WAS MONITORING THE LOC COURSE. WE WERE ON A 90 DEG INTERCEPT. AT THIS POINT; I REITERATED TO THE CAPT THAT I DID NOT HAVE THE FIELD. AT ABOUT THE SAME TIME; APCH TOLD US TO CONTACT THE TWR. I ANSWERED HIM; BUT DID NOT SWITCH FREQS IMMEDIATELY BECAUSE WE WERE CONFIGURING AND RUNNING CHKLISTS. THE CAPT STARTED A L TURN; I ASSUME TO REDUCE OUR INTERCEPT ANGLE. DURING THIS MANEUVERING; APCH CTL CALLED US AND ASKED IF WE WERE STILL THERE. I RESPONDED WITH 'YES; WE ARE ABOUT TO SWITCH TO TWR.' HE SAID TO STAY WITH HIM AND GAVE US A VECTOR BACK TOWARD A 90 DEG INTERCEPT ANGLE. THE CTLR SAID THAT IT LOOKED LIKE WE WERE LINING UP WITH RWY 15. DURING THIS RADIO EXCHANGE AND VECTOR; I FINALLY SAW THE ARPT. WE COMPLIED WITH HIS INSTRUCTION AND I TOLD HIM THAT WE DID HAVE RWY 10 IN SIGHT. HE SAID THAT WE WERE 'STILL CLRED FOR THE VISUAL' AND TO CONTACT THE TWR. WE LANDED ON RWY 10 WITHOUT INCIDENT.