On September 23, 1983, a Piper PA-12 (registration N4008M) was involved in an aviation accident near Wasilla, AK. Investigators recorded the probable cause as: The pilot's inability to prevent an engine shutdown due to water contamination in the fuel system, which resulted from the fuel cap design allowing water to collect at the level of the filler neck. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A pilot experienced intermittent engine operation followed by a complete shutdown shortly after takeoff. A forced landing was executed safely, and subsequent inspection revealed water contamination in the fuel system.
What happened
The aircraft departed from an airport runway, climbing over trees at the departure end. Shortly after clearing the obstacles, the engine began running intermittently. The power loss progressed until the engine stopped completely. The pilot established a glide and executed a forced landing in the vicinity of the airport.
The investigation
Following the accident, the pilot and an aviation mechanic inspected the aircraft's fuel system. They discovered the presence of water within the fuel lines and tanks. The inspection focused on the fuel cap design, which was found to be nearly flush with the wing surface. This configuration allowed water to accumulate at the same level as the filler neck, facilitating entry into the fuel system.
Findings
The engine failure was caused by fuel contamination due to water ingress. The aircraft's fuel cap design contributed to the accumulation of water, which then entered the fuel supply during flight operations.
Probable cause
The pilot's inability to prevent an engine shutdown due to water contamination in the fuel system, which resulted from the fuel cap design allowing water to collect at the level of the filler neck.