What happened
While in flight, the aircraft's left engine began running rough and experiencing severe vibration. In response to the instability, the pilot shut down the left engine. Following this action, the pilot was unable to maintain altitude using only the right engine, resulting in a forced landing in an alfalfa field.
The investigation
Mechanical examinations of both engines revealed distinct failures. Investigators found that the right magneto of the left engine had an accumulation of oil inside its case. Additionally, the spark plug lead from the left magneto for the #1 cylinder was found to have internal damage.
An examination of the right engine identified that the intake duct, located between the air filter and the carburetor, had deteriorated due to exposure to hot exhaust pipes. A piece of the duct, measuring 3 feet by 1/2 foot, had separated from the assembly and was discovered lodged in the carburetor venturi.