PLT OF B767 RPTS THE EXISTENCE OF A FALSE LOC TO THE N OF ON COURSE; ON THE DSCNT C1 ILS; RWY 15 AT SBGL. FEARS THAT FLT AND DSCNT ON THE FALSE COURSE COULD RESULT IN DSNDING FLT OVER HIGH TERRAIN TO THE N.

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ON THE C1 ILS RWY 15 APCH; THERE IS A LOC INTERCEPT ICON JUST INSIDE 41 NM. WE COULD NOT FIGURE OUT WHAT THIS REPRESENTS. THE CONCERN IS THAT IN RIO; THERE IS A HIGH LIKELIHOOD OF A FALSE INTERCEPT THAT FAR OUT AND IF FOLLOWED; COULD DSND YOU INTO A MOUNTAIN TO THE N. THE FALSE LOCK ON HAPPENED TO US THAT DAY AND HAS HAPPENED MANY OTHER TIMES. FURTHERMORE; THE PIRAI RWY 15 ARR LOST COMS INSTRUCTIONS TELL YOU TO EXECUTE THE C1 APCH AT LOC INTERCEPT; WHICH ALSO COULD BE DANGEROUS DUE TO FALSE LOC CAPTURE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR ADVISED THAT THE FALSE LOC SIGNAL IS A KNOWN ANOMALY AMONG FREQUENT USERS OF THIS ARR TO SBGL. IT HAPPENS WHEN WELL TO THE N OF THE INBOUND COURSE AND MIGHT WELL BE THE RESULT OF TERRAIN EFFECTS ON THE SIGNAL. THE EFFECTS SHOW UP ON THE RAW DATA ILS DISPLAY (WHICH TUNES AND DISPLAYS AUTOMATICALLY ON THE NAV DISPLAY WHEN THE APCH IS INSTALLED VIA THE CDU AND THE ACFT IS WITHIN A CERTAIN DISTANCE OF THE DEST). RPTR WAS UNABLE TO CONFIRM WHETHER THE ACFT AUTOFLT SYS RECOGNIZED THE FALSE CAPTURE -- WHICH WOULD HAVE BEEN ANNUNCIATED ON THE COMMAND BARS; FLT PATH MODE ANNUNCIATOR CHANGING TO 'LOC' AND; IF ENGAGED; THE AUTOPLT ATTEMPTING TO INTERCEPT -- INASMUCH AS HE WAS HAND-FLYING THE ACFT AT THE TIME THE EVENT OCCURRED. DUE TO THE CONTINUING DISPLAY OF THE ACFT'S POS ON THE MOVING MAP DISPLAY AND THE MAGENTA COURSE LINE; HE WAS NOT TEMPTED TO FOLLOW THE ERRONEOUS SIGNAL. RPTR WENT ON TO SAY THAT SUBSEQUENT TO THIS EVENT; WHEN THE ACTUAL LOC INTERCEPT TOOK PLACE; ALL INDICATIONS WERE NORMAL.