AN F100 ACFT FLYING INTO GDL; FO; SETS UP THE INSTS FOR AN APCH WITH THE ILS. THE ILS CAPTURES AND INTERCEPTS A FALSE LOC. ARPT IS 30 DEGS OFF TO THE SIDE.

Narrative

ATC FACILITY: GDL APCH CTL. LOCATION: GDL/070 DEG RADIAL/10 NM. WHILE BEING RADAR VECTORED TO THE ILS RWY 28; GDL; THE F100 AFS CAPTURED A FALSE LOC AND GS. ACFT HDG WAS 190 DEGS AT 9500 FT MSL WITH THE AUTOPLT ENGAGED AND LAND ARMED; WE WERE CLRED TO INTERCEPT THE LOC AND CLRED THE ILS RWY 28 APCH. THE AUTOPLT CAPTURED A FALSE LOC AND GS APPROX 5 NM N OF THE ACTUAL ILS COURSE. BOTH LOC AND GS CAPTURE ANNUNCIATED ON THE PFD AND THE ACFT BEGAN A R TURN. XCHK OF RAW DATA (LOC AND VOR) AND THE ND MAP (HAD BUILT THE ILS MANUALLY SINCE IT IS NOT IN THE DATABASE) CONFIRMED ACFT N OF THE LOC CTRLINE. DISCONNECTED THE AUTOPLT AND INITIATED A TURN BACK TO HDG 190 DEGS. GDL APCH CTL CONFIRMED OUR POS; GAVE US HDG 180 DEGS AND 7500 FT MSL AND RECONFIRMED APCH CLRNC. COMPLETED THE APCH MANUALLY IN VMC. (AFTER REARMING LAND; NORMAL LOC AND GS CAPTURE OCCURRED.) CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR HAD USED OTHER MEANS OF NAV TO FLY INTO GDL BESIDES THE ILS. THE WX WAS CLR. THE FLC TUNED IN THE ILS; BUT USED A VOR RADIAL AS REF; AND ALSO MANUALLY BUILT AN APCH INTO THE FMC. AS THE ACFT WAS ON A 190 DEG HDG; IT TURNED TO AN ILS HEADING FOR THE LOC INTERCEPT. THE ILS COMMAND SHOWED LOC CAPTURE. FLC RECOGNIZED EARLY THAT THE ACFT HAD TURNED TO AN ILS HEADING AND CAPTURE; BUT THE LOC WAS 30 DEGS OFF FROM THE ARPT. FLC TURNED OFF ALL AUTOPLT FUNCTIONS AND FLT INST COMMANDS AND RESET THE FLT INSTS AFTER TAKING UP A WESTERLY HEADING. THEN; EVERYTHING WORKED PERFECTLY. RPTR FEELS IT WAS JUST AN ANOMALY THAT CAN HAPPEN. THIS IS THE ONLY TIME IT HAS HAPPENED TO HIM AND HAS NOT HEARD OF ANY OTHER INCIDENTS OF THIS TYPE.