HARD LNDG IN AN MD80 AFTER A HIGH FLARE. FLC CITES WRONG LNDG WT WITH AN 8 KT SPD DEFICIT.

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I WAS THE CAPT ON MD80 FLT FROM BUF TO LGA ON SATURDAY MORNING JAN/SAT/98. I FLEW THE APCH AND LNDG IN VISUAL CONDITIONS TO RWY 31. THE APCH AND LNDG WERE MADE PER PLT HANDBOOK PROCS. THE LNDG WT USED TO DETERMINE THE APCH SPD WAS TAKEN FROM THE GROSS WT WINDOW ON THE FUEL TOTALIZER. THE APCH WAS STABLE AND SEEMED NORMAL UNTIL THE FLARE. THE FLARE WAS INITIATED AT APPROX 50 FT; BUT THE ACFT RESPONDED SLUGGISHLY. WE LANDED FIRMLY; BUT IT DID NOT SEEM HARD. A POSTFLT WALKAROUND BY MAINT INDICATED A TAILSKID STRIKE AND SOME SKIN DAMAGE. I WAS SURPRISED BY THE DAMAGE GIVEN THAT THE LNDG WAS MADE FROM A STABLE APCH WITH A SINK RATE OF 700 FPM. IF THE FLARE DID ANYTHING; IT HAD TO HAVE DECREASED THE SINK RATE BELOW THAT. ACFT CERTIFICATION REQUIRES TRANSPORT CATEGORY ACFT TO BE ABLE TO WITHSTAND -- WITHOUT DAMAGE - - AN IMPACT OF 600 FPM AT FULL TKOF GROSS WT. WE WERE NEARLY 20000 LBS UNDER THAT. THERE WAS NO SHARP BUMP OR STRUT BOTTOMING FEELING TO THE LNDG. THE FLT ATTENDANTS ON THE AFT JUMP SEAT DIDN'T FEEL ANY TAIL STRIKE. HOWEVER; SINCE THE FO STATED THAT HE DID NOT NOTICE THE DAMAGE ON HIS PREFLT WALKAROUND; I ASSUMED IT WAS DONE ON THIS LNDG; AND WROTE THE ACFT UP AS HAVING A POSSIBLE HARD LNDG. ON REVIEW OF MY PART OF THE FLT; I REALIZED THAT I HAD USED A WRONG GROSS WT FOR LNDG. ALTHOUGH I HAD ESTIMATED THE LNDG WT BASED UPON THE GROSS WT READ-OUT ON THE FUEL TOTALIZER; THE READ-OUT WAS WRONG. THE READ-OUT IS A WINDOW THAT DISPLAYS THE SUM OF THE ZERO FUEL WT AND THE REMAINING FUEL. IF AN ERRONEOUS ZERO FUEL WT IS SET INTO THE TOTALIZER; AN ERRONEOUS GROSS WT WILL BE DISPLAYED. EVIDENCE SUGGESTS THIS IS EXACTLY WHAT HAPPENED. ONE OF THE STEPS IN THE BEFORE TKOF CHKLIST CONTAINS 2 IMPORTANT ITEMS. THE FIRST ITEM IS GERMANE TO THE TKOF; THE OTHER TO THE LNDG. THE STEP IS TITLED 'TKOF DATA AND ZERO FUEL WT...SET.' DURING THIS STEP THE V-SPDS ARE SET ON THE AIRSPD INDICATOR AND THE PROPER ZERO FUEL WT IS SET INTO THE FUEL TOTALIZER. THE V-SPDS ARE ESSENTIAL; BUT; AT THIS POINT IN THE FLT; A DISPLAY SHOWING TOTAL GROSS WT IS NOT. PLTS ARE TRAINED FROM DAY ONE TO CONTINUOUSLY SORT AND ATTACH AN APPROPRIATE PRIORITY TO INCOMING INFO. CONSEQUENTLY; WHEN ONE IS SORTING WHAT IS NEEDED TO MAKE A TKOF; THE IMPORTANCE OF SETTING A ZERO FUEL WT MAY DIMINISH TO NEARLY ZERO AND A MISSED STEP IS THE RESULT. I BELIEVE; THIS MAY HAVE BEEN WHAT HAPPENED IN THE PROCESS OF LAUNCHING THIS EARLY MORNING FLT. WE MUST HAVE SATISFIED; AT LEAST IN OUR MINDS; THIS STEP BY SETTING THE V-SPDS ONLY. AS A RESULT; THE ZERO FUEL WT WAS NEVER SET PROPERLY. TWO EXPERIENCED PLTS MADE THE SAME ERROR SIMULTANEOUSLY. THEREFORE; I BELIEVE; THIS STEP SHOULD BE SPLIT; FOR EMPHASIS; INTO 2 STEPS. ONE FOR EACH ITEM ACCOMPLISHED; EACH WITH ITS OWN RESPONSE. BUT THAT IS ONLY PART OF THE STORY. THE SPDS ACTUALLY USED AND THE SPDS THAT SHOULD HAVE BEEN USED WERE DIFFERENT BY 8 KTS. GIVEN THE EXTRA SPD ADDITIVES USED; THAT SPD DIFFERENCE SHOULD NOT HAVE RESULTED IN A HARD LNDG AND DAMAGE.