While flying at a cruise altitude of 600 feet, the pilot lost control of the airplane for undetermined reasons. He had not reported experiencing a mechanical problem, nor were any discovered during the examination of the wreckage at the accident site. There was no significant weather observed at the time of the accident. The post mortem examination of the pilot failed to disclose a malady which would have prevented the pilot from performing his normal duties. However, the airplane descended from the assigned altitude and was last observed at 5600 feet before disappearing off radar. Wreckage was scattered over an area about one mile long in the opposite direction of the intended flight path. The left wing flap assembly and about a two foot section of the right horizontal stabilator trim tab were not recovered for examination. The left wing flap assembly separated from the airframe and was located about 1/2 mile west of the main wreckage. Components from the empennage section were found about one mile from the main wreckage.