WPR18LA233 · Blue Oak LLC. CCK-1865

After completing routine maintenance on the experimental, amateur-built airplane, the private pilot decided to conduct a test flight near the departure airport. He completed a preflight inspection, a ground engine run, and taxi checks, which were normal; he noted that about 12 gallons of fuel was onboard the airplane. About 10 minutes after departure, he chose to extend the flight, as he did not observe any anomalies with the airplane. After an uneventful period of flight, the airplane was about 500 ft above ground level over mountain ridge tops when the engine lost partial power. The pilot maintained level flight and maneuvered the airplane toward more favorable terrain before the engine lost total power. Subsequently, the pilot performed a forced landing in a small clearing within heavily wooded mountainous terrain. During the landing roll, the airplane struck a tree and came to rest upright.

Postaccident examination of the airplane revealed no evidence of preexisting mechanical malfunctions or failures with the engine or fuel system, including the fuel quantity sight gauges, that would have precluded normal operation. The pilot reported that, after the accident, he noticed that the left-wing fuel cap was not fully secured. Wreckage recovery company personnel indicated that the right-wing fuel tank was void of fuel, and the left-wing fuel tank contained about 1/2 gallon of fuel. Review of a photograph provided by the wreckage recovery company taken 10 days after the accident revealed a small amount of residue and discoloration directly aft of the left-wing fuel filler neck, extending to the trailing edge of the fuel tank cover. It is likely that fuel vented through the loose fuel cap in flight, which reduced fuel quantity and led to the subsequent loss of engine power. Further, the pilot failed to adequately monitor the fuel quantity sight gauges while in flight.

Verbatim NTSB analysis and probable cause from the NTSB dataset