Event Sequence
While conducting an IFR flight, the aircraft was leveled at 13,000 feet with power set to 18 inches of manifold pressure and 2300 RPM. Approximately five minutes later, the right engine began to surge, with RPM increasing to 3000 and almost pegging. The pilot attempted to control the right propeller. Eventually, the right engine stabilized at 2300 RPM. The pilot informed ATC and initiated an immediate descent toward the nearest airport.
At about 7,000 feet, the left engine began running rough, progressively worsening. As the left engine RPM decreased to approximately 1500, the left propeller was feathered. Around the same time, the right engine surged again, and subsequently the right propeller was also feathered. After descending below the overcast, a forced landing was made in a blackberry patch. During the landing, the right main landing gear was damaged.
Examination Findings
Post-incident examination revealed that the nut securing the right propeller spinner baffle had backed off. This had worn grooves in the air valve, allowing the pre-load air pressure in the hub to release. Additionally, the pre-load air pressure on the left propeller was found to be 45 PSI, whereas it should have been 70 PSI.