What happened
While cruising at 6,500 feet with a power setting of 2500 RPM and 21 inches MP, the pilot experienced difficulty synchronizing the left propeller. The pilot reported that the left propeller speed decreased to 1500 RPM, surged up to 3000 RPM, and then dropped back down to 1500 RPM.
In an attempt to manage the engine, the pilot tried to feather the propeller but was unable to do so. Because the aircraft could not maintain altitude using power from the right engine alone, the pilot performed a forced landing on an interstate highway. During the landing roll, the left wing struck an embankment, causing the aircraft to veer off the side of the road. A fire subsequently started in the right engine, resulting in the destruction of the right wing.
The investigation
Post-accident inspections of the left propeller did not identify a specific reason for the RPM fluctuations or the failure of the propeller to feather. However, investigators discovered water throughout the left engine fuel system.
While the propellers had been overhauled prior to the flight, a bench test of the left propeller governor revealed that although it was capable of operation, it was operating slightly outside of its specified operating range.