B767; WITH AUTOFLT ARMED; HAS FALSE CAPTURE OF LOC AT BFI; WA.

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WE WERE ASSIGNED HDG 280 DEGS; MAINTAIN AT OR ABOVE 3000 FT UNTIL ESTABLISHED ON FINAL APCH COURSE; CLRED FOR ILS. WE WERE IN VMC AT DAWN. BFI VICINITY IN SIGHT; BUT NOT RWY. AT APPROX 3 MI E OF LOC; HDG BUG SWUNG TO 310 DEGS; FLT DIRECTOR AND AUTOPLT TURNED TOWARD 310 DEGS. LOC NOT CAPTURED AND EFIS MAP INDICATED WE WERE NOT ON LOC. AUTOPLT DISENGAGED PRIOR TO 10 DEG TURN AND HDG RETURNED TO 280 DEGS. FO SELECTED RAW DATA. THIS CONFIRMED POS E OF LOC. (THIS HAPPENED WITH LOC ARMED.) APCH ARMED AT THIS TIME. HDG BUG AGAIN TURNED TO 310 DEGS AND FLT DIRECTOR INDICATED R TURN. AIRPLANE HAND FLOWN TO INTERCEPT APCH. FLT DIRECTOR CORRECTED AT THIS TIME AND FLEW NORMALLY TO LNDG. I WAS TOLD ON GND THAT THIS WAS AN ANOMALY WITH ICHJ LOC AND HAD HAPPENED BEFORE. LATER THAT DAY; BFI CTLRS WERE ASKED ABOUT THIS AND THEY SAID THEY DID NOT KNOW ABOUT IT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR SAID THAT THERE WAS NO 'CASE BREAK' OF THE LOC AND YET THE AUTOFLT PERFORMED AS IF THERE WAS A CAPTURE OF THE LOC BY SLEWING THE HDG BUG TO THE LOC TRACK AND BEGINNING A TURN WITH THE AUTOPLT. A MANUAL DISCONNECT OF THE SYS WAS MADE AND HAND FLOWN TO LOC AND FOR THE REST OF THE APCH. DISCUSSION ON THE GND FOUND THAT TRACON WAS NOT AWARE OF ANY PROBS. IT WAS LEARNED THAT OTHER COMPANY PLTS HAVE NOTICED THIS ANOMALY AT BFI ON THE RWY 31L LOC. THE AIRLINE COMPANY IS INVESTIGATING ALONG WITH BOEING. IT IS NOT KNOWN IF THERE WAS VEHICLE OR OTHER TFC IN THE AREA OF THE LOC XMITTER.