What happened
While operating in cruise flight at 5,500 feet MSL, the pilot of a light twin-engine airplane noticed that the propellers were out of sync. Attempts to synchronize the engines using the propeller levers were unsuccessful. The right propeller subsequently experienced an overspeed of approximately 1,000 RPM, and further adjustments to the right propeller lever yielded no results.
In an attempt to manage the situation, the pilot retarded the throttle; however, proper engine RPM could not be maintained. The pilot decided to divert to the nearest airport, located roughly 12 miles away. During the diversion, the pilot observed that the number two engine was beginning to overheat, prompting a second reduction in throttle and an increase in airspeed to assist with cooling. As the aircraft began to shake, the pilot observed the propeller wobbling. The pilot then executed a forced landing into a muddy wheat field.
The investigation
The aircraft had undergone an annual inspection on December 12, 2003, and was determined to be in an unairworthy condition. A special flight permit for maintenance had been issued on April 20, 2004, with an expiration date of April 30, 2004.
Investigators tested the propeller governors on a test stand. While they functioned normally, they were found to be out of specification. The maximum RPM recorded on the left governor was 2,394 and the maximum RPM on the right governor was 2,310, whereas the specified RPM is 2,435. Additionally, while the specified feather RPM is 1,700, the left governor achieved 1,671 RPM and the right governor achieved 1,634 RPM. Upon disassembly, both governors were found to be old and worn, exhibiting similar wear patterns, though they remained functional.