FLC DSNDS EARLY GOING INTO FOREIGN ARPT. CONFUSION OVER DISTANCE FROM RWY.

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ON RADAR VECTORS SIGEN 1 ALPA STAR; WE HAD ATIS WHICH ONLY SAID WX AND RWY 7 IN USE (NOT TYPE OF APCH) I ASKED CTLR WHAT APCH TO EXPECT; ILS RWY 7L OR RWY 7R? HE REPLIED WE WOULD BE TOLD LATER -- FO FLYING WAS ON HIS QUALIFYING 1ST TRIP TO EUROPE (VERY EXPERIENCED USAF F16 NATIONAL GUARD) (RELIEF PLT ALSO VERY EXPERIENCED EX UNITED STATES NAVY A-4'S). PF BRIEFED ON ILS TO RWY 7R WHICH WE PUT IN FMC AND SET UP FOR -- FFM VOR WAS NOTAMED OTS AND WE WERE USING TEMPORARY (YELLOW) ACFT CHARTS. ON DOWNWIND; CTLR ANNOUNCED WE WERE GOING TO GET NDB DME RWY 7R APCH -- WHICH WAS SELECTED IN CDU AND FW NDB PUT ON FIX PAGE AND MAPS ON HSI. PF TRIED TO TUNE FFM VOR TEMP FREQ TO 110.95 WHICH COULD NOT BE TUNED ON OUR TYPE OF CTL HEAD AS THE .95 WOULD NOT GO IN AS THE PNF (CAPT) (INSTRUCTOR) I HAD NOT BEEN TO FRA FOR SEVERAL MONTHS BUT DID REMEMBER HOW THEY USE THESE NDB APCH TO RWY 7 AND I WAS CONTENT LOOKING AT MY FMC DISTANCE FROM FW NDB. RELIEF PLT ANNOUNCED WE WERE WITHIN 10 NM OF FW AND GOOD DOWN TO FW XING ALT OF 1720 FT. I CONCURRED AND FO STARTED DOWN -- LEAVING 4000 FT AT ABOUT 3500 FT (EST) CTLR TOLD US SHE SHOWED US A 'LITTLE LOW' 'FOR OUR DISTANCE FROM RWY' -- PF LEVELED OFF AT 2800 FT UNTIL CLOSER TO FW AND DSNDED TO CROSS FW NDB AT 1720 FT. WE ALL 3 PLTS MISTOOK THE FMC DISTANCE FROM THE FW NDB AS GOVERNING WHEN IN FACT THE APCH IS BASED ON DME FROM FFM VOR! FFM VOR WAS NOT TUNEABLE BUT NEXT DAY WE LEARNED THEY HAD A 'GHOST' FREQ WHICH WE COULD HAVE USED! INST APCH SHOULD NOT USE DISTANCES BASED ON VORS AT THE OTHER END OF RWY. THEY SHOULD USE DISTANCE FROM THE FACILITY. I REALIZE THIS IS NOT ALWAYS PRACTICAL BUT IT COULD REALLY MESS UP YOUR DAY. AFTER A 9 HR FLT THIS KIND OF CONFUSION IS EASY TO GET INTO -- AN ILS APCH WOULD HAVE PREVENTED THIS -- WHY DO MAJOR ARPTS USE SUCH ANTIQUATED AND MISTAKE PRONE APCHS IS BEYOND ME.