HDG TRACK DEV WHEN ACFT'S AUTOPLT CAPTURES THE WRONG LOC DURING RADAR VECTORING FOR IAP ILS APCH.

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ON APCH TO IAD; WE WERE GIVEN A SW VECTOR (APPROX 240-250 DEGS) TO INTERCEPT THE 19R LOC AT IAD. FROM OUR POS; THIS MEANT FLYING DIAGONALLY ACROSS THE 19L LOC; ALSO IN USE. I WAS THE PF; AND AT THE TIME; THE AUTOPLT DIGITAL AFCS WITH EFIS WAS ENGAGED. WHEN CLRED ON THE SW VECTOR TO INTERCEPT THE 19R LOC; I ARMED THE SYS FOR CAPTURE. THE CAPT CONFIRMED THE CORRECT FREQS BOTH TUNED AND IDENTED. THE SYS CAPTURED THE BEAM; AND INITIATED A 40-50 DEG L TURN TO THE INBOUND COURSE (190 DEGS). THE CDI NEEDLE SHOWED A FLY R INDICATION; BUT I DID NOT THINK THIS UNUSUAL; GIVEN OUR DISTANCE OUT. SEVERAL SECONDS LATER; THE APCH CTLR SAID SHE SHOWED US 'INTERCEPTING THE 19L LOC.' THE CAPT CONFIRMED THAT WE HAD THE CORRECT FREQS TUNED FOR THE R SIDE; BUT THE CTLR WENT AHEAD AND CLRED US ON THE 19L APCH. WE RE-TUNED THE RADIOS AND CONTINUED WITHOUT INCIDENT. AFTERWARD; I RECALLED BEING TOLD OF SIMILAR INSTANCES BY OTHER COMPANY PLTS OF THE AUTOPLT SYS CAPTURING THE BEAM OF A PARALLEL RWY WHEN FLYING ACROSS IT; EVEN THOUGH THE CORRECT NAV FREQ WAS USED. JUDGING FROM THE FEW ACFT ON THE FREQ; I DO NOT BELIEVE THIS CAUSED A CONFLICT. IN THE FUTURE; I WILL BE SURE TO MONITOR THE AUTOPLT'S BEHAVIOR MORE CLOSELY.