What happened
The pilot reported that the engine had run roughly during a previous flight. In response, he removed, cleaned, and replaced the spark plugs. After completing this maintenance, he took off again. Approximately seven miles from the departure airport, the engine began to run rough and intermittently once more.
An emergency landing became necessary due to the loss of power. During the landing attempt in rough terrain, the nose gear broke. This failure resulted in significant damage to the fuselage.
The investigation
An FAA inspector examined the wreckage to determine the cause of the reported power loss. The inspection did not find any evidence to explain the reported power loss.
In his accident report, the pilot stated that rubber particles, possibly from fuel lines, were found in the fuel. The aircraft was being operated on auto fuel.
Findings
The primary finding is that rubber particles were present in the fuel system, potentially originating from degraded fuel lines. Despite the pilot's maintenance efforts, the engine continued to perform poorly, leading to the emergency landing and subsequent structural damage.