On June 20, 2007, at approximately 0800 central daylight time, an Air Tractor AT-301, N23667, sustained substantial damage during a forced landing following a severe engine vibration during cruise flight near Fergus Falls, Minnesota. The 14 Code of Federal Regulations Part 137 flight departed Fergus Falls Municipal Airport-Einar Mickelson Field (FFM), Fergus Falls, Minnesota, at approximately 0750 for a local aerial application flight. The commercial pilot reported minor injuries. Visual meteorological conditions prevailed at the time of the accident. No flight plan was filed.

The pilot reported that he was in cruise flight about 500 feet above ground level (agl) when the engine and airframe started to vibrate violently. The windshield became covered with fluid, and because the door had come open, fluid was getting into the cockpit making it difficult for the pilot to see. He executed a forced landing to a field. The main landing gear broke off and the airplane flipped over. The pilot released his harness and evacuated the airplane.

The inspection of the airplane revealed that one of the propeller blades was missing about nine inches of the propeller blade tip. The propeller blade was cut and the piece with the fracture surface was sent to the National Transportation Safety Board's (NTSB) Materials Laboratory for examination. The inspection of the fracture surface revealed that the central and leading edge portions of the fracture face were relatively flat and perpendicular to the exterior surfaces of the blade, gross features normally associated with fatigue. The trailing edge portion of the fracture was on a slant plane and displayed a grainy surface texture, consistent with a final overstress fracture region. The central portion of the fracture displayed an impacted surface consistent with intermittent contact with its mating fracture face; another gross feature consistent with fatigue. The inspection revealed a black indentation on the flat surface of the blade and that the crack arrest marks would be indicative of a crack originating in that vicinity. The indentation was oriented chordwise and penetrated progressively deeper into the surface toward the trailing edge, features typical of impact of a rotating propeller blade with a foreign object.

Aircraft maintenance records indicated that the Hamilton Standard propeller, model 22D40, type 6533A-12, was repaired and overhauled on October 21, 2003. The overhaul documents indicated that no discrepancies were found when the propeller blades and hub underwent visual and non-destructive testing. The propeller hub was checked using magnaflux and eddy current non-destructive testing procedures. The propeller blades were checked using the zyglo non-destructive testing procedure.

The aircraft operator reported that he had purchased the propeller blade in the winter of 2005/2006, and that the propeller was installed on the airplane in the spring of 2006. The propeller was purchased from another operator as a "0" time blade since major overhaul (SMOH). The operator reported that the propeller blade appeared to be "freshly overhauled" and it looked fine to him. Since installing the propeller on the accident airplane, it had been flown for a total of about 30 hours. The accident airplane was used as a backup airplane, so it did not fly regularly. The operator reported that the accident airplane did not have a propeller strike with a foreign object during the 30 hours of operation prior to the accident flight.

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