THE AIRPLANE WAS BEING OPERATED AS A PART 135 AIR TAXI FLIGHT TO AN ISLAND. THE PILOT WAS LANDING ON A DIRT AIRSTRIP THAT WAS ABOUT 2,600 FEET LONG AND 30 FEET WIDE. WHILE ON FINAL APPROACH, THE PILOT REPORTED EXPERIENCING A DOWNDRAFT AND THE AIRCRAFT TOUCHED DOWN HARD ABOUT 10 FEET SHORT OF THE APPROACH END OF THE RUNWAY. SUBSEQUENT INSPECTION REVEALED THAT THE WING, TO WHICH THE LANDING GEAR IS ATTACHED, RECEIVED DAMAGE REQUIRING REPLACEMENT OF ABOUT 2 1/2 FEET OF THE REAR SPAR.