WITNESSES OBSERVED PARTS FALL FROM THE AIRPLANE DURING THE INITIAL SWATH RUN. AS THE AIRPLANE ENTERED A RIGHT CLIMBING TURN TO NEGOTIATE 30-FT HIGH TELEPHONE WIRES, IT ENTERED INTO A LEFT WING-OVER AND NOSED DOWN INTO THE GROUND. THE LEFT WING SPAR HAD SEPARATED, AND THE FRACTURED SURFACES DISCLOSED EVIDENCE OF CORROSION. THE 16-FT LONG LOWER STEEL SPAR CAP HAD SEPARATED FROM THE ALUMINUM WING SPAR WEB ALONG THE ENTIRE LENGTH. FRACTURE SURFACES IN THE HUCK LOCKBOLT TYPE FASTENERS ON THE SPAR DISCLOSED EVIDENCE OF CORROSION. THE AREA OF THE HUCK LOCKBOLT CORROSION IS NOT NOTICEABLE BY VISUAL INSPECTION. THE CORROSION IS MASKED BY THE STEEL SPAR CAP. NONE OF THE HUCK LOCKBOLT COLLARS WERE LOOSE OR MISSING. THE AIRPLANE HAD COMPLETED AN 100-HR INSPECTION 71 HRS BEFORE THE ACCIDENT. AT THE TIME OF THE INSPECTION THERE WERE NO DISCREPANCIES NOTED WITH THE WING SPAR.