Cessna 172 N8033B accident near Wonewoc, Wisconsin

Probable cause

The restricted vented fuel cap and obstructed fuel vent lines, which resulted in a total loss of engine power due to fuel starvation.

NTSB analysis

The pilot reported that, during the flight before the accident flight, the engine began running roughly during cruise flight with the right wing fuel tank selected. After attempting to resolve the issue by applying carburetor heat, he made a precautionary landing at a nearby grass airstrip. After landing, the pilot loosened the right fuel tank cap and heard a "whoosh" sound and the metal fuel tank "crinkle." The pilot stated that he thought he had resolved the fuel tank venting issue when he removed the right fuel tank cap. After spending 30 minutes on the ground, the pilot restarted the engine with both fuel tanks selected and performed an uneventful engine run-up before departing. Shortly after takeoff, the engine experienced a total loss of power, and the pilot executed a forced landing into a marsh. A postaccident examination revealed that the external and cross-tank vent lines were obstructed by foreign material likely deposited by insects. In this condition, the engine responded to cockpit controls during an operational test run. Following the engine test run, the right fuel tank cap was loosened, and a "whoosh" sound was heard. Testing of the right fuel cap revealed a restriction to the flapper-valve assembly, which prevented air from passing freely through the vented cap as designed. The combination of a restricted vented fuel cap and the obstructed vent lines likely prevented the fuel from flowing to the engine and resulted in fuel starvation and the total loss of engine power.

Text from the NTSB. Open the NTSB docket for CEN13LA216 for the full record.