THE PLT HAD ONLY FLOWN 1 FLT SINCE HE HAD CRASHED THE SAME ULTRALIGHT VEHICLE NEARLY 1 YR PREVIOUSLY. AN EYE WITNESS, WHO WAS PRESIDENT OF THE LOCAL ULTRALIGHT ASSOCIATION, HAD TEST FLOWN THE REPAIRED VEHICLE BEFORE THE PLT FLEW IT. THE PLT, THE EYE WITNESS & A 3RD PLT THEN FLEW THEIR RESPECTIVE VEHICLES TO THE JENKO AIRSTRIP & RETURNED SUCCESSFULLY. NOTING THAT THE WIND HAD INCREASED, THE EYE WITNESS DEPARTED ALONE TO TEST THE WX. HE STATED HE ENCOUNTERED THE WORST TURBULENCE HE HAD EVER FLOWN IN. BEFORE HE COULD TELL THE OTHERS NOT TO TAKEOFF, THEY HAD ELECTED TO DEPART. SUBSEQUENTLY, THE EYE WITNESS SAW THE ACCIDENT VEHICLE IN AN INVERTED DIVE TO THE RIGHT FROM APRX 600 FT AGL. HE REPORTED THAT THE VEHICLE APPEARED TO RECOVER AT ABOUT 150 FT AGL, BUT THEN ROLLED TO THE RIGHT AGAIN & HIT THE GROUND. THE 3RD PLT SAID THE WIND WAS 'VERY SEVERE' & HE HAD A HARD TIME CONTROLLING HIS VEHICLE. THE PLT WAS WEARING HIS HELMET & HAD HIS SEAT BELT FASTENED, BUT THE VEHICLE WAS NOT EQUIPPED WITH A SHOULDER HARNESS.