FLC OF A WDB ACR ACFT STRUCK THE UNDERSIDE OF THE ACFT TAIL DURING LNDG DUE TO A MISCALCULATION OF THE ACTUAL LNDG WT OF THE ACFT BY 100000 LBS.

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WE WERE MAKING AN ILS 33R APCH INTO RIYADH. THE APCH WAS PROCEEDING NORMALLY; AND AT APPROX 100 FT I ADVISED THE CAPT MINUS 3 KTS ON THE RED SPD BUG. AT 40 FT I ADVISED THE CAPT MINUS 5 KTS (THIS PUT OUR SPD AT V-REF WHITE BUG). AT APPROX 5 FT THE STICK SHAKER ACTIVATED AND APPROX 1 SECOND LATER WE TOUCHED DOWN. THE ATTITUDE OF THE ACFT DID NOT SEEM EXCESSIVELY HIGH AND IT SEEMED TO BE A NORMAL LNDG. ON SHUTDOWN THE FLC REVIEWED THE RELEVANT DATA TO DETERMINE WHY THE STICK SHAKER ACTIVATED AND DISCOVERED V-REF WAS BASED ON A WT OF APPROX 100000 LBS LESS THAN ACTUAL. ON THE POST-FLT INSPECTION THE FLT MECH NOTICED THE REAR DRAIN MAST HEATER BROKEN; AND ON CLOSER INSPECTION; NOTICED SCRAPE MARKS ON THE TAIL. IN CONFERENCE BTWN THE 3 FLC MEMBERS AND THE FLT MECH; IT WAS AGREED THERE WAS NO STRUCTURAL DAMAGE TO THE ACFT; AND THAT THE FLT COULD CONTINUE. THIS SIT OCCURRED BECAUSE ONLY 1 CREW MEMBER DETERMINED THE V-REF; AND THERE IS NO PUBLISHED XCHK PROC. THIS SIT COULD BE REMEDIED BY MAKING THE APPROPRIATE CHARTS/GRAPHS READILY AVAILABLE TO ALL CREW MEMBERS TO REVIEW THE APPROPRIATE DATA.