Incident
During descent to Toledo Airport at an altitude of 7,000 feet, all four propeller blades on the right engine separated simultaneously. The number two blade penetrated the fuselage, resulting in a sudden cabin decompression. The pilot lost control, and the airplane entered a dive. It subsequently crashed in an open field located two miles southeast of Marseilles, Ohio. The aircraft was totally destroyed, and none of the 38 occupants survived the crash.
Investigation
The official investigation determined that the probable cause was the failure of the right propeller due to the omission of the torque piston nitriding process during manufacture, and the failure of manufacturing quality control to detect the omission. Additional findings include: loads on the torque cylinder caused by the failed torque piston of blade number three exceeded the finite fatigue of the cylinder, leading to fatigue failure; loss of oil pressure in the right propeller due to the failed torque cylinder caused the propeller pitch to decrease at a rate exceeding the propeller pitch lock capability; the right propeller oversped, causing blade separation in overstress; blade number two penetrated the fuselage, destroying structural integrity such that, together with the force of a right yaw from the propeller separation, the fuselage failed along the line of the penetrations. The torque piston for blade number three had not been nitrided for surface hardening of the helical splines during manufacture. The omission was not detected by inspection and was associated with the movement of ten torque pistons from normal production flow to the Allison laboratory and back. The Allison quality control system lacked the accountability necessary to assure individual part quality. The metal contamination oil check intended to isolate defective torque pistons did not serve its purpose, and Allison underestimated the seriousness of the defective torque piston problem.