WRONG APCH SPDS USED FOR APCH AND LNDG. A HARD LNDG RESULTS.

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OUR FLT WAS FROM ROA TO GSO; A VERY SHORT FLT OF ONLY 70 NM. AT OUR COMPANY; WE USE A LAPTOP COMPUTER TO FIGURE OUR LNDG SPDS. TO DO THIS; WE MUST INPUT OUR ESTIMATED LNDG WT INTO THE COMPUTER. THE ESTIMATED LNDG WT IS FOUND BY SUBTRACTING OUR PROJECTED FUEL BURN FROM OUR TKOF GROSS WT. WHILE ENROUTE; WE WERE IN A HURRY AND SUBTRACTED THE PROJECTED FUEL BURN FROM THE ZERO FUEL WT INSTEAD OF THE TKOF GROSS WT. AS A RESULT OF THIS MISTAKE; THE COMPUTER GAVE US LNDG SPDS THAT WERE 10 TO 12 KTS SLOWER THAN WHAT WE SHOULD HAVE HAD. OUR ACTUAL LNDG WT WAS AROUND 140000 LBS BUT I INPUT 123000 LBS IN THE LAPTOP (WHICH GAVE US THE SLOWER SPDS). AS A RESULT OF THE SLOWER SPDS; THE CAPT MADE A HARD LNDG. AFTER THE HARD BOUNCE; HE ELECTED TO DO A GAR. HE WAS PUZZLED AS TO WHY THE LNDG WAS SO HARD. ON THE GAR; I FIGURED OUT WHAT HAPPENED AND RE-FIGURED THE CORRECT SPDS FOR AN UNEVENTFUL SECOND LNDG. FACTORS CAUSING MISTAKE. SHORT FLT; IN TOO BIG OF A HURRY. DID NOT DOUBLE CHK SPD. NEW CREW; NONE OF US HAD FLOWN IN A FEW DAYS.