On June 20, 2005, approximately 0930 mountain daylight time, a Cessna 188A, N2133U, sustained substantial damage following a forced landing after experiencing a propeller failure about 27 nautical miles west of Fairview, Montana. The certificated commercial pilot, the sole occupant, received minor injuries. Visual meteorological conditions prevailed for the aerial application flight, which was being operated in accordance with 14 CFR Part 137, and a flight plan was not filed. The flight had departed the Fairview Airport at approximately 0915. According to a Federal Aviation Administration (FAA) airworthiness inspector, who traveled to the accident site and conducted the preliminary investigation, the pilot was en route to spray a field at an altitude of about 100 feet when one of the two propeller blades separated from the propeller hub. After dumping the load the pilot reported that the remaining propeller blade and propeller hub assembly separated from the airplane after the engine's crankshaft failed. The pilot subsequently made a forced landing to an open field, during which the right main landing gear separated, followed by the airplane spinning around 180 degrees to the right before coming to rest in an upright position. The FAA inspector reported to the NTSB investigator-in-charge (IIC) that further examination of the aircraft revealed the aircraft was equipped with a McCauley propeller, model D2A34C49-N, blade model 90A-2. According to a representative from McCauley Propellers, the model 90A-2 blade is the correct blade for the D2A34C49-N propeller; however, it is not an approved model propeller (or blade) for the Cessna 188A with a TCM IO-520 engine. It is only approved on O-470 or IO-470 engines with four 6th order dampers. Additionally, the propeller should have been filled with red dyed oil for crack detection. The FAA inspector confirmed that there was no indication that the propeller had been filled with the red dyed oil, and that there was no visible evidence of red dyed oil on the surface of either propeller blade or the propeller hub. A further inspection of the separated propeller blade indicated that the failure occurred within the outboard blade retention threads. The inspector also reported to the IIC that the registered owner refused to provide the FAA with all pertinent aircraft records and logbooks as requested.