REPORTEDLY, THE PLT TOOK OFF & LANDED IN A DAIRY PASTURE WITH NO APPARENT DIFFICULTIES. HE THEN TOOK OFF AGAIN ON AN EASTERLY HEADING. AFTER TAKEOFF, HE TURNED TO A MODIFIED CROSSWIND/DOWNWIND PATTERN & WAS PROCEEDING IN A NORTHWESTERLY DIRECTION. REPORTEDLY, AFTER REACHING AN ALT OF APRX 125 FT AGL, THE VEHICLE ENTERED A NOSE DOWN, WINGS LEVEL ATTITUDE & BEGAN DESCENDING. THE VEHICLE CONTINUED DESCENDING UNTIL COLLIDING WITH THE GROUND. NO EVASIVE ACTION WAS OBSERVED NO PREIMPACT/MECHANICAL FAILURE WAS VERIFIED; HOWEVER, THE PLT'S SEAT WAS FOUND SEPARATED FROM ITS FORWARD & REAR ATTACH FITTINGS. SPEED TAPE HAD BEEN USED TO ATTACH THE SEAT SUPPORT TUBES TO THE MAIN FRAME & WAS FOUND BROKEN. AN INVESTIGATION REVEALED THAT APRX 10 DAYS BEFORE THE ACCIDENT, THE PLT WAS HAVING DIFFICULTY IN GETTING THE VEHICLE TOFLY. HE HAD CONFERRED WITH A ROTEC REP & WAS TOLD THE CG WAS TOO FAR FORWARD & TO MOVE THE SEAT APRX 2 INCHES AFT. THE PLT REMOVED THE BOLTS THAT CONNECTED THE SEAT, BUT DID NOT REINSTALL THEM, INSTEAD, HE USED 'SPEED TAPE.'