LOSS OF AIRCRAFT CONTROL FOR AN UNKNOWN REASON. FACTORS RELATED TO THE ACCIDENT WERE: THE WEATHER CONDITIONS (CLOUDS WITH LOW CEILING), CONTINUED VFR FLIGHT BY THE PILOT INTO INSTRUMENT METEOROLOGICAL CONDITIONS (IMC), THE PILOT'S LACK OF INSTRUMENT EXPERIENCE, AND FAILURE OF MAINTENANCE PERSONNEL TO INSTALL THE REAR WING-TO-FUSELAGE ATTACH BOLTS.