What happened
During an aerial application operation, a Cessna 188A was flying at approximately 100 feet above the ground when one of the aircraft's two propeller blades separated from the propeller hub. Following this initial failure, the remaining propeller blade and the hub itself also separated from the aircraft. The pilot was forced to perform an emergency landing in an open field.
The investigation
An examination of the propeller assembly determined that while the specific propeller blade model was correct for the unit, the propeller model itself was not approved for use on a Cessna 188A equipped with a TCM IO-520 engine. Investigators also found that the propeller should have been filled with red dyed oil to facilitate crack detection. However, a visual inspection of the aircraft showed no evidence of red dyed oil on the surface of the propeller hub or either blade. A detailed inspection of the separated blade revealed that the failure originated within the outboard blade retention threads.