On August 22, 2002, at 0730 Pacific daylight time, an Air Tractor AT-301 restricted category airplane, N8793S, came to rest inverted in a newly disked open field during a forced landing that was precipitated by a loss of a propeller blade while in cruise flight. The airplane, operated by Spain Air under the provisions of 14 CFR Part 137 as a aerial application operation, sustained substantial damage. The commercial pilot, the sole occupant, was not injured. Visual metrological conditions prevailed for the local area flight and no flight plan had been filed. The flight departed a private airstrip about 0650. According to the pilot, he had been crop dusting since midnight. There were no mechanical anomalies noted during any of the flights. On the accident flight part of the propeller blade came off. The pilot stated that the engine started to vibrate and he made an emergency landing in a cotton field. On the landing rollout the landing gear caught on the soft dirt and the airplane nosed over, coming to rest inverted. AIRCRAFT INFORMATION Airframe records indicated that the propeller had been overhauled on October 31, 2001. During the overhaul, the propeller blades, manufactured by Pacific Propeller, Kent, Washington, were converted to the Hydromatic Blade 6533A-12. The overhaul records show that the customer supplied the hub assembly, and that there was pitting on the blade shanks and bearings. According to Pacific Propeller the appropriate inspection document for the propeller blades is the Hamilton Standard 130B Blade Overhaul Manual. The manual contains information for a dye penetrant inspection to detect cracks or other discontinuities in damaged areas, as well as dimensional checks for dressing out the propeller blade during overhaul. The manual has a section that deals with a local (field) repair to grind or file corrosion out of the blade. Once the grinding is complete a dye penetrant inspection should be done to detect any damage. Airworthiness Directive 81-13-06 R2 (Amendment 39-4133, revised by Amendment 39- 4409, and further revised by Amendment 39-6271) applies to Hamilton Standard propeller blades and was issued to prevent propeller blade failure due to corrosion and fatigue. The AD calls for an inspection of the propeller blades to be accomplished within 90 days after July 1, 1982, or within 18 months since the last inspection. If corrosion is found the propeller blades are required to be reinspected at intervals not to exceed 18 months since the last inspection. The 18 month interval can be extended up to 60 months if no corrosion is found. According to the manufacturer's Federal Aviation Administration approved airplane flight manual preflight visual walk-around inspection, the pilot is to inspect the "prop blades for nicks or cracks." The propeller blade was examined by the Safety Board Metallurgical Laboratory in Washington, D.C. The blade had a chordwise fracture about 40 inches from the butt end. The blade fracture surface displayed shiny, faceted fracture features that spanned 60 percent of the chord's width. This region had crack arrest marks and a smoothly curving boundary, features consistent with fatigue. The fatigue features emanated from multiple origins from a nick on the flat side of the blade's leading edge. The nick measured 0.2 inch wide chordwise, 0.15 inch spanwise and 0.06 inch deep. Adjacent to the nick were several straight, parallel grooves. The grooves were uniform, and consistent with tool marks.