The airplane was being used to haul skydivers. According to the pilot, after about the third or fourth flight, there was a partial loss of power, due to a fuel tank vent that had been blocked by a mud dauber nest. The pilot noted that when a fuel cap was removed, he heard a ‘hiss’ as differential air pressure was relieved. Maintenance was performed to clear the vent, then the pilot continued hauling skydivers.

Read the NTSB report…