THE PLT FILED A VFR FLT PLAN AFTER THE WX IMPROVED FROM IFR TO VFR WITH A CHANCE OF THUNDERSTORMS EN ROUTE. OCNL MOD TURBULENCE BELOW 5000 FT WAS FORCAST DUE TO STRONG/GUSTY LOW LVL WNDS. WHILE THE ACFT WAS EN ROUTE, A GND WITNESS HEARD THE ENGS REV UP MORE THAN NORMAL, THEN HE SAW THE ACFT ON A NORTHERLY HEADING. SHORTLY THEREAFTER, HE HEARD A LOUD NOISE& SAW 2 OR MORE PIECES SEPARATE FROM THE ACFT. IT THEN ENTERED AN UNCONTROLLED DSCNT & CRASHED. THERE WAS EVIDENCE THAT THE ACFT SUFFERED AN IN-FLT SEPARATION OF THE FIBERGLASS NOSE ASSEMBLY, THAT PORTIONS OF THE SEPARATED NOSE STRUCK THE STABILATOR & THAT A DAMAGED AREA ON THE RT STABILATOR ACTED AS A TRIM TAB & CAUSED A PITCH-UP OF SUFFICIENT MAGNITUDE TOCAUSE POSITIVE BENDING IN THE WINGS. THE LOADING ON THE STABILATORS CAUSED THEM TO FAIL IN NEG BENDING. THE ACFT THEN ENTERED A DESCENDING COUNTER-CLOCKWISE SPIN WHICH CONTINUED UNTIL GROUND IMPACT OCCURRED. PARTS OF THE ACFT & ITS CONTENTS SEPARATED & DESCENDED IN SCATTERED FRAGMENTS.