N8033BCessna 172 · 1957

On April 1, 2013, at 1220 central daylight time, a Cessna model 172 airplane, N8033B, was substantially damaged during a forced landing following a loss of engine power shortly after takeoff from Three Castles Airpark (4D1), Wonewoc, Wisconsin. The commercial pilot was not injured. The airplane was registered to and operated by the pilot, under the provisions of 14 Code of Federal Regulations Part 91, without a flight plan. Day visual meteorological conditions prevailed for the personal flight. The flight originally departed Park Falls Municipal Airport (KPKF), Park River, Wisconsin, at 1030, and was destined for St Louis Metro-East Airport (3K6), St. Jacob, Illinois.The pilot reported that he planned on making one fuel stop during the cross-country trip, although he had not selected the actual airport because the airplane still had ample fuel. The engine began running rough during cruise flight while operating on the right wing fuel tank. After attempting to resolve the issue with an application of carburetor heat, he made an uneventful precautionary landing at a nearby grass airstrip (4D1). After landing, the pilot reported that he couldn't find anything wrong with engine during a visual check of the engine. However, he heard a "whoosh" sound and heard the metal fuel tank "crinkle" as he loosened the right fuel tank cap. The pilot reported that he attributed these sounds to an accumulated pressure differential that occurred during his rapid descent while operating on a single fuel tank. The left wing fuel tank contained about 2 gallons of fuel. The right tank contained about 10 gallons of fuel. The pilot stated that he thought he had resolved the fuel tank venting issue when he removed the right fuel tank cap. The pilot subsequently restarted the engine, with the fuel selector drawing fuel from both fuel tanks, and performed an uneventful engine run-up before departing again. Shortly after takeoff, at an altitude of about 300 feet, the engine began running rough. The pilot reported that the engine lost power completely at an altitude of about 200 feet as he attempted to return to the airport. The airplane's left wing collided with a tree during the forced landing into a seasonal marsh.

A postaccident examination was performed by a Federal Aviation Administration Airworthiness Inspector. The inspector reported that the external fuel tank vent line, located on the left wing strut, was obstructed by foreign material likely deposited by insects. Additionally, he noted that the cross-tank vent line was also obstructed. The engine responded to cockpit controls during a postaccident operational test run. Following the engine test run, the inspector heard a "whoosh" sound as he loosened the right fuel tank cap. Testing of the right fuel cap revealed a restriction to the flapper-valve assembly, which prevented air to pass freely through the vented cap as designed. Although there was no foreign object debris or material noted around the valve seat, the rubber-flap material would remain engaged to its corresponding metal seat when pressure was applied via plastic tubing. The air restriction became progressively less apparent after several applications of pressure and suction by the FAA Inspector.