DURING HIS WX BRIEFING, THE PLT WAS ADVISED OF AN AREA OF RAIN SHOWERS & SCATTERED THUNDERSTORMS ALONG HIS ROUTE OF FLT, AS WELL AS FLT PRECAUTIONS FOR OCCASIONAL MODERATE TURBULENCE BELOW 8000 FT WHICH WAS FORCAST. HE DEPARTED NORFOLK AT 1500. AT ABOUT 1630, THE PLT REPORTED TO ARTCC THAT 'WE'RE IN SOME HEAVY TURBULENCE NOW.' THIS WAS THE LAST RADIO TRANSMISSION. THE ACFT SUBSEQUENTLY WAS OBSERVED SPINNING & TUMBLING BEFORE IMPACT. THE OUTER PANELS OF BOTH WINGSHAD SEPARATED. MODERATE TO HEAVY RAIN SHOWERS EXISTED IN THE VICINITY WHERE RADAR CONTACT WAS LOST. THE PIC WAS SEATED IN THE RIGHT FRONT SEAT. THE PAX/PLT SEATED IN THE LEFT FRONT SEAT HELD A PRIVATE PLT CERTIFICATE WITH AN AIRPLANE SINGLE-ENG LAND RATING & WAS NOT INSTRUMENT RATED. HE HAD RECEIVED HIS INITIAL FLT INSTRUCTION FROM THE PIC. THE LAST ENTRY IN THE PAX/PLT'S LOG BOOK WAS 5/2/81.