Accident Overview
Witnesses observed the airplane suddenly turning or rolling left until its wings became perpendicular to the ground, followed by a nose-down descent. The impact completely destroyed the aircraft and resulted in the deaths of all 23 individuals on board, among them U.S. Senator John Goodwin Tower of Texas and astronaut Manley "Sonny" Carter.
Propeller System Examination
Post-accident examination of the left propeller components revealed a blade angle of approximately 3°, whereas the left propeller control unit (PCU) ballscrew position corresponded to a commanded blade angle of 79.2°. Investigators found extreme wear on the PCU quill spline teeth, which normally engage the titanium-nitrided splines of the propeller transfer tube. The titanium-nitrided surface was significantly harder and rougher than the nitrided surface of the quill. Consequently, the transfer tube splines acted like a file, causing abnormal wear on the quill gear teeth. This wear pattern had not been evaluated during the certification of the propeller system.
Probable Cause and Contributing Factors
The official probable cause was determined to be a loss of control in flight resulting from a malfunction of the left engine propeller control unit, which allowed the propeller blade angles to go below the flight idle position. Contributing to the accident were the deficient design of the PCU by Hamilton Standard and the approval of that design by the Federal Aviation Administration. The design failed to correctly assess the failure mode that occurred during this flight, leading to an uncommanded and uncorrectable movement of the left propeller blades below the flight idle position.