AN ACR MD88 MADE AN APCH TO MXF. THEIR DEST WAS MGM. TRACK DEV. FLT ASSIST.

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CLRED FOR VISUAL APCH TO RWY 28 AT MGM IN CAVU CONDITIONS. IDENTED ARPT (INCORRECTLY) ABOUT 20 NM OUT. APCHING FROM N ON A DIRECT ENTRY TO A R BASE; TURNED FINAL AND DSNDED ON VISUAL APCH TO MXF. I HAD THE MGM RWY 28 ILS TUNED AND NOTED A MAJOR GS DISCREPANCY WHICH I MENTIONED ALOUD TO CAPT. BTWN 1000 FT AND 500 FT AGL; CAPT AND MGM TWR ABOUT SIMULTANEOUSLY QUESTIONED OUR POS. MGM TWR ISSUED GAR VECTORS; FROM WHICH WE COMPLETED A VISUAL TO RWY 28 AT MGM. WE DID NOT ACTUALLY LAND AT MXF; PROBABLY 600 FT AGL MINIMUM. MITIGATING FACTORS: 1) NEITHER PLT HAD EVER FLOWN INTO MGM IN DAYLIGHT. AT NIGHT MXF IS USUALLY DARK AND MGM EASILY SEEN; 2) MXF NOT SHOWN ON MGM ILS RWY 28 COMMERCIAL PAGE; WHICH WAS USED TO BRIEF A VISUAL WITH ILS BACKUP. MXF IS SHOWN ON THE MGM ILS 10 PAGE!; 3) MGM VOR WAS OTS; SO CLOSEST VOR WAS JYU; ABOUT 30 NM AWAY. WHEN ILS WAS TUNED; AND MAYBE EVEN BEFORE; OUR NON-IRS FMS WENT INTO DEAD RECKONING AND BEGAN TO DRIFT. THE MAP WAS SHOWING SOMETHING ROUGHLY CONSISTENT WITH LNDG AT MGM; WHICH IS PAR FOR A NON-IRS AHRS EQUIPPED ACFT; AND 4) SIMILAR RWY ALIGNMENT AT MXF AND MGM. MISTAKES: 1) NEITHER PLT CHKED RWY HDG AFTER ROLLING OUT ON FINAL; 2) ALTHOUGH THE PNF TUNED AND IDENTED THE MGM ILS RWY 28; NEITHER PLT SELECTED A RAW DATA DISPLAY TO CONFIRM OUR POS. THIS WOULD HAVE PREVENTED THE INCIDENT. LESSON: IF ARPTS ARE CLOSE TOGETHER; YOU CAN EASILY RECEIVE IDENT; LOC AND GS INFO WHILE LNDG AT THE WRONG PLACE; AND 3) I BRIEFED THAT MGM RWY 28 WOULD HAVE A VASI ON THE R; BUT I SAW A PLASI ON THE L AT MXF AND I DIDN'T GET IT EVEN THEN! RECOMMEND: 1) PUT MXF ON MGM RWY 28 APCH PAGE; AND 2) ADD NOTE TO OUR AIRLINE'S MGM DESCRIPTION PAGE WARNING ABOUT MXF. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATES THAT THIS WAS ENTIRELY THE CREW'S FAULT. MGM TWR LCL CTLR APOLOGIZED TO THE CREW FOR NOT WARNING THEM OF THEIR ERROR. THE TWR WAS 'HEADS DOWN.' WHEN THE RPTRS DID NOT SHOW UP ON FINAL; THE TWR WARNED THEM OF THEIR ERROR. TWR SAYS; 'IT HAPPENS ALL THE TIME.' THE RPTR BELIEVES THAT A WORKING MGM VOR OR A VOR ON THE FIELD WOULD PREVENT THIS. THE FMC IN THE AHRS EQUIPPED MD88 REQUIRES A VOR TO KEEP IT UPDATED ALL OF THE TIME. THE FMS WILL 'DRIFT VERY RAPIDLY' IF IT IS NOT RECEIVING ANY VOR SIGNAL. THE NEAREST VOR WAS SELMA; OUT OF RANGE; AND MGM WAS OTS. THIS WAS THE FIRST TIME INTO MGM BY DAY FOR THIS RPTR. A PAX CALLED THE FAA 'SNITCH LINE;' SO THE ACR HAS GIVEN EACH PLT A 'PERMANENT BAD LETTER' IN AN ATTEMPT TO PREVENT THE FAA FROM TAKING ENFORCEMENT ACTION. THE RPTR HAS NOT HEARD FROM THE FAA ON THIS. THE RPTR BELIEVES THAT SHOWING MXF ON THE MGM IDLV APCH; AS IT IS ON THE IMGM APCH; MIGHT HAVE HELPED.