FLT CREW OF AN A300 DSNDED AND TURNED INBOUND PREMATURELY ON APCH TRANSITION TO RWY 5R AT MEX.

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WE WERE ON THE 160 DEG RADIAL SMO; APPROX 5 DME; AND CLRED FOR THE APCH ILS DME 2; RWY 5R; MEX. I WAS THE PF. THE AUTOPLT CLICKED OFF JUST AS THE FO WAS CHANGING FREQS FROM APCH TO TWR. I REACHED UP AND REENGAGED THE #1 AUTOPLT. WE HAD ALREADY CHKED RADIOS AND EFIS. THE ACFT STARTED A L TURN. I THOUGHT IT WAS TO INTERCEPT THE LOC; SO STARTED THE DSCNT TO 8800 FT MSL. I LOOKED AT THE DISPLAY TO SEE IF 'LOC' APPEARED. I DID NOT SEE IT. I COULD SEE BUILDINGS DOWN OFF MY L AND THEN LOOKED AT MY NAV DISPLAY TO SEE WE WERE OFF COURSE TO THE L. I STOPPED THE DSCNT AT AROUND 9300 FT MSL AND STARTED BACK TO THE R WHEN TWR CTLR ASKED ABOUT OUR POS. HE SAID WE WERE L OF COURSE. WE ASKED FOR A VECTOR TO INTERCEPT THE LOC FOR RWY 5R. HE ASKED IF WE HAD GND CONTACT AND ALSO IF WE HAD THE ARPT IN SIGHT. WE SAID WE HAD GND CONTACT; BUT NOT THE ARPT AT THAT TIME; SO HE GAVE US A HEADING TO INTERCEPT THE LOC AND WAS OK WITH THE ALT WE WERE HOLDING. WE COMPLETED THE APCH AND LANDED. I COULD HAVE DETECTED THIS PREMATURE TURN EARLIER IF I HAD BRIEFED THE TURN RADIAL 248 DEGS AND HAD THE FO ON RAW DATA. NEXT TIME; I WILL DEFINITELY BRIEF THAT AND BE READY FOR THE UNEXPECTED. I DON'T KNOW WHY THE AUTOPLT CLICKED OFF.