The Cessna 172 was in cruise flight near Wonewoc, Wis., drawing fuel from the right tank when the engine began running rough.
Unable to resolve the issue by applying carburetor heat, the commercial pilot made a precautionary landing on a grass airstrip.
When he loosened the right fuel tank cap, he heard a “whoosh” sound followed by the sound of metal crinkling. He stated he thought he had resolved the issue when he removed the right fuel tank cap.
After spending 30 minutes on the ground, he restarted the engine with both fuel tanks selected and performed an uneventful engine run-up, then took off.
Shortly after takeoff, the engine lost power. He made a forced landing in a marsh, resulting in substantial damage to the airplane. The pilot was not injured.
The post-accident examination revealed that the external and cross-tank vent lines were obstructed by foreign material likely deposited by insects. The investigation included an engine test run. When the right fuel tank cap was loosened, the 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 NTSB determined the probable cause of the accident was the restricted vented fuel cap and obstructed fuel vent lines, which resulted in a total loss of engine power due to fuel starvation.
NTSB Identification: CEN13LA216
This April 2013 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.