On August 18, 2007, at 0752 mountain daylight time, an Air Tractor AT-301, N69WN, operated by Rocky Mountain Ag, Inc., was substantially damaged during a forced landing near Alamosa, Colorado. Visual meteorological conditions prevailed at the time of the accident. The aerial application flight was being conducted under the provisions of Title 14 Code of Federal Regulations Part 137 without a flight plan. The commercial pilot sustained minor injuries. The local flight departed Monte Vista Municipal Airport (MVI), approximately 0730. According to the owner of Rocky Mountain Ag, Inc., the airplane was carrying a load of MH-30, an anti-sprouting chemical, when the number one cylinder head separated. as a result, the windscreen was covered in oil, obscuring the pilot's view. The pilot dropped his chemical load and performed a forced landing on a dirt road. During the landing, the left wing impacted a sand berm and the airplane nosed over. The outboard three feet of the left wing was separated, the rudder and vertical stabilizer were crushed down, and the skid plate on the fuselage canopy separated and came aft into the cabin. Aside from the separation of the number one cylinder, the pilot reported no additional anomalies with the airplane's systems. The number one cylinder was sent to the National Transportation Safety Board's Materials Laboratory in Washington, D.C., for further examination. Examination of the fracture face revealed a rough grainy surface typical of overstress, with the exception of an area between the intake and exhaust ports. The area between the ports displayed a smoother surface that was oriented perpendicular to the cylinder wall, typical of fatigue cracking, and covered with a black deposit consistent with combustion products. Combustion products were also evident on the adjacent cooling fin surface and oriented radially outwards. Closer examination of the fracture face revealed that the fatigue cracking initiated from multiple locations in a root of the head threads and propagated in an arc to the outer surface of the combustion chamber wall. The fatigue zone contained shallow depressions consistent with the presence of porosity in the cylinder head material. According to the engine maintenance records, the number one cylinder, part number R 1340 (223), was overhauled on June 22, 2005, by Aircraft Cylinders of America, Inc. The last 100-hour inspection was performed on the engine on April 24, 2007. In addition, the number three cylinder was removed and replaced due to a "blown head" on July 18, 2007. Following this accident, Air Tractor revised their service letter #97 "Cockpit Overturn Skid Plate" to incorporate a "new wider clamp" and larger attaching bolts to "strengthen both the front and rear connections to the airplane frame structure."