The airplane was flying in a remote area. A witness observed the airplane flying at a very low level near, or possibly below, the tops of a river bank. He said he saw the airplane climb rapidly, slow down, and begin a turn to the left. He lost sight of the airplane, but, within a few seconds, heard a loud noise and saw smoke in the vicinity where he had last seen the airplane. When he arrived at the accident site, the airplane was on fire. Postaccident inspection of the airplane disclosed no evidence of any preimpact mechanical anomalies.