Accident Description
A pilot reported that while in cruise flight and looking at birds out the left side cockpit window, he observed peripherally a 'white flash' fill the right side of the forward windscreen, followed immediately by an audible 'thump'. The airplane began an uncontrollable descent and yaw to the right. The engine continued to run smoothly. Advancing the engine power controls had no noticeable effect on the pilot's ability to arrest the descent.
Investigation
The NTSB investigator in charge visited the accident site. No evidence was found of a bird strike. The accident site was surrounded by tall (white) birch trees. The pilot stated that he was not sure what he struck and that it could have been a tree.
Probable Cause
The probable cause of the accident was the failure of the pilot in command to maintain the proper altitude. The trees were a factor in the accident.