Accident Sequence
During cruise at flight level 240, a propeller hub arm on the left propeller failed, releasing the propeller blade. The released blade struck an adjacent blade, breaking off its tip. This sequence caused a severe engine vibration and forced shutdown of the left engine. The left engine was displaced downward and inboard on its mounts. The cabin depressurized, possibly from blade contact with the fuselage.
The flight crew initiated an emergency descent and received a vector to divert for an instrument landing system (ILS) approach to Dubuque. The aircraft was unable to maintain altitude and descended in instrument meteorological conditions. Subsequently, the airplane collided with a silo and crashed approximately 8 miles south of Dubuque.
Investigation Findings
Examination revealed that the left propeller hub failed due to fatigue cracking. Fatigue originated from multiple initiation sites on the inside diameter surface of the hole for the pilot tube. The fatigue properties of the hub material were diminished by a combination of factors, including machining marks or scratches, mixed microstructure, corrosion, decarburization, and residual stresses. All eight occupants were killed in the accident, among them George Mickelson, 52, Governor of South Dakota.
Probable Cause
The probable cause was determined to be fatigue cracking and fracture of the propeller hub arm. The separation of the hub arm and blade caused damage to the engine, nacelle, wing, and fuselage, resulting in significant degradation of aircraft performance and control that made a successful landing problematic. The hub arm fracture occurred due to a reduction in fatigue strength from manufacturing and time-related factors (decarburization, residual stress, corrosion, mixed microstructure, and machining/scoring marks), likely combined with exposure to higher-than-normal cyclic loads during operation of the propeller at a critical vibration frequency (reactionless mode), which was not adequately addressed during the airplane and propeller certification process.