Incident Summary
During an aerial application operation at approximately 100 feet above ground level, one of the airplane's two propeller blades separated from the propeller hub. Subsequently, the remaining propeller blade and hub also separated from the airplane, leading to a forced landing in an open field. The pilot executed the landing without further incident.
Propeller and Aircraft Configuration
The aircraft involved was a Cessna 188A equipped with a Teledyne Continental Motors (TCM) IO-520 engine. Examination of the propeller assembly revealed that the propeller blade model was correct for the propeller hub, but the propeller model itself was not approved for installation on the Cessna 188A with the TCM IO-520 engine.
Post-Accident Examination
Inspection of the propeller components showed no evidence of red dyed oil on the surface of either propeller blade or the propeller hub. Red dyed oil is used as a crack detection method; its presence would indicate the application of this inspection technique. The separated propeller blade failed within the outboard blade retention threads, as determined by further examination of the fracture surface.
Findings
The investigation found that the propeller model installed was not approved for the aircraft-engine combination. Additionally, the absence of red dyed oil suggested that crack detection measures had not been performed or were not evident at the time of examination.