THE ACFT DPTD DCA WITH 4508,70GAL FUEL,AND 43LBS EQUIPMT ON BOARD WHICH COMPUTED OUT TO BE 26LBS OVER MAX GROSS. IT WAS FLOWN DIRECTLY TO LEESBURG,VA WHERE THE WINDS WERE GENERALLY OUT OF THE WEST AND THERE WAS THE POTENTIAL FOR WINDSHEAR W/I 2000AGL. BOTH PLT AND CO-PLT REMEMBER CROSSING OVR THE RWY #'S BETWEEN 20 AND 30 FT AGL BETWEEN 65 AND 68KTS. APPROX 500-700FT DOWN THE RWY THE NOSE OF THE ACFT WAS OVSVD BY A WITNESS TO ASCEND SLIGHTLY AND THE ACFT LOST FLYING SPEED AND FELL ONTO THE RWY. IT THEN BOUNCED BACK UP INTO THE AIR,STILL IN A NOSE HIGH ATTITUDE, AND SANK AGAIN BACK ONTO THE RWY. IT WAS THEN OBSVD TO VEER OFF TO THE LEFT AND NOSE OVER BEFORE COMING TO REST IN FRONT OF THE PARKING APRON JUST SOUTH OF THE FSS. A WITNESS SAW THE WINDSOCK INDICATING A 90 DEGREE X-WIND & STANDING APRX'STRAIGHT OUT.' ANOTHE R WITNESS SAW A'HEAVY EXTENSION OF FLAPS' & A NOSE HIGH ATTITUDE. BOTH OBSERVATIONS OCCURRED AS THE ACFT FLEW LEVEL FOR APRX 500-700 FT ABOUT 20FT AGL. A ROAR OF PWR WAS HEARD AT TIME OF IMPACT.