B737 FLC ATTEMPTS TO CAPTURE THE 255 DEG RADIAL INTO DCA VOR INSTEAD OF THE ASSIGNED 255 DEG RADIAL OUT OF BAL VOR; 10 MI SW OF BAL; MD.

Narrative

APCHING BAL VOR SPD 210 KTS. AT 10000 FT MSL; WE WERE INSTRUCTED BY ATC: 'AFTER BALTIMORE; FLY THE BALTIMORE 255 DEG RADIAL.' FO WAS OFF FREQ RPTING AN INFLT MECHANICAL (APU DETECTOR INOP LIGHT ILLUMINATED) TO OPS. ALREADY NAVING USING LNAV; I INPUT THE DCA INTERCEPT RADIAL AND UTILIZED A DIRECT INTERCEPT TO TRACK THE DCA 255 DEG RADIAL INBOUND. THE FO AND I HAD ALREADY BRIEFED THE ROSSLYN LDA RWY 19 INTO DCA. BECAUSE OUR RADIOS WERE SET UP FOR THIS APCH AND BECAUSE I WAS NAVING LNAV; I INITIALLY THOUGHT MY INPUT OF THE ABOVE-MENTIONED INTERCEPT WAS APPROPRIATE AND ALSO RELIEVED ME OF HAVING TO REMOVE THE APCH INFO. AFTER PASSING BAL; AS THE ACFT TURNED SWBOUND TO INTERCEPT; I REALIZED THAT I SHOULD HAVE UTILIZED RAW DATA FOR BAL INSTEAD OF LNAV. AS I WAS SETTING THIS UP; ATC CONTACTED US (FO WAS STILL TALKING TO OPS) AND SAID WE WENT THROUGH THE RADIAL AND TURN IMMEDIATELY TO A 290 DEG HDG AND DSND TO 8000 FT. HE SAID THERE WAS EITHER AN F16 (OR F4) OFF OUR L AND REPEATED HIS INSTRUCTION. I IMMEDIATELY REMOVED THE AUTOPLT AND AUTOTHROTTLE AND FOLLOWED THE ATC INSTRUCTION. I GLANCED AT THE RADAR/TCASII AND DID NOT SEE YELLOW (TA) OR RED (RA). WE HAD THE WX RADAR ON (SHOWING GREEN) SO I COULD NOT CLRLY SEE THE ALT INFO OF THE CLOSEST ACFT (NON CONFLICTING). THERE WAS NO AURAL TCASII WARNING (TA OR RA). NOTHING ELSE WAS SAID BY ATC AND WE WERE VECTORED FOR OUR APCH. AT THIS POINT; FO WAS BACK ON FREQ AND I EXPLAINED TO HIM THE LOGIC I USED AND WE DISCUSSED HOW IT MIGHT NOT BE ACCURATE. FOR THE LAST SEVERAL MONTHS OF ONLY FLYING THE DCA SHUTTLE EVEN THOUGH I AM VERY AWARE OF THE COMPLACENCY ISSUE; I AM IN THE 'LNAV' MODE 99% OF THE TIME. I THINK THAT CONTRIBUTED TO MY VERY SLOW TRANSITION TO THE 'RAW DATA' MODE. ALSO; BECAUSE WE ALREADY BRIEFED THE APCH; WHICH IS VERY NONSTANDARD; I DID NOT WANT TO CHANGE MY RADIO NAV FREQ OR COURSE. I BELIEVE MY WANTING TO 'KEEP IT SIMPLE' ALSO CONTRIBUTED TO MY 'CHOICE.' THE FO BEING 'OUT OF THE LOOP' WAS ALSO A FACTOR. VIGILANCE AND AWARENESS OF THE 'LITERAL' NATURE OF ATC INSTRUCTIONS IS KEY.