What happened
The aircraft was operating under instrument flight rules when it leveled off at an altitude of 13,000 feet. The pilot set the power settings to 18 inches of manifold pressure and 2,300 RPM. Approximately five minutes later, the right engine began to surge, with the RPM climbing toward 3,000 and nearly pegging the gauge. The pilot attempted to regain control of the right propeller, eventually stabilizing the engine at 2,300 RPM.
Following this incident, the pilot notified air traffic control and initiated an immediate descent toward the nearest airport. As the aircraft descended through approximately 7,000 feet, the left engine began running roughly, with the condition progressively worsening. When the left engine RPM dropped to about 1,500, the pilot feathered the left propeller. At nearly the same time, the right engine surged again, prompting the pilot to feather the right propeller as well.
After descending below the overcast layer, the pilot executed a forced landing in a blackberry patch. The impact resulted in damage to the right main gear.
The investigation
A post-accident examination revealed that the nut securing the right propeller spinner baffle had backed off. This disengagement had worn grooves into the air valve, which allowed the pre-load air pressure within the propeller hub to release. Additionally, investigators found that the pre-load air pressure on the left propeller was at 45 PSI, significantly lower than the required 70 PSI.
Findings
The primary mechanical failure involved the loss of pre-load air pressure in the right propeller hub due to a disengaged spinner baffle nut. Contributing factors included the low pre-load air pressure on the left propeller and the subsequent dual engine failure during descent.