On April 20, 2005, at 1815 central daylight time, a Weatherly 620B, N9026F, sustained substantial damage during a forced landing to a farm field about seven miles north of Downs, Kansas, after a loss of engine power. The commercial pilot received minor injuries. The 14 CFR Part 137 aerial application flight departed from a local field and was applying chemicals when the loss of engine power occurred. Visual meteorological conditions prevailed at the time of the accident. No flight plan was filed. The pilot reported that he was "pulling the airplane up into a right hand turn" at the end of a spray run when he heard a loud "bang" and the engine started to run "really rough." He executed a forced landing to a field. The pilot reported that the landing gear broke off due to the soft field conditions. The airplane was a Weatherly 620B, serial number 1639. The engine was a 450 horsepower Pratt & Whitney R-985-14B engine, serial number 20590. The engine was overhauled 43 hours prior to the accident on March 4, 2005. At overhaul, the engine had accumulated a total time of 7,027.1 hours. The engine logbook noted that AD 78-0807 was complied with. (AD 78-0807 was effective May 2, 1978 for ultrasonic and visual inspections every 150 hours time in service to detect cracking and "to prevent cylinder head separation from the barrel") The logbook did not contain specific information on the previous histories of the installed cylinders. The inspection of the airplane revealed that the number 2 cylinder was broken at the third large cooling vent up from the bottom. The upper portion on the head assembly had separated from the sleeve. The cylinder was sent to the National Transportation Safety Board's (NTSB) Material Laboratory for further examination. The NTSB Materials Laboratory report noted that the fracture faces were covered by extensive and heavy combustion products and required extensive cleanings to reveal fracture details. Examinations of the fracture faces revealed features and topography indicative of a large area of fatigue progression. The fatigue area was on the aft side of the cylinder, biased toward the exhaust valve. Fracture face markings indicative of multiple fatigue initiation at the inner diameter threads along a wide arc of approximately 140 degrees. Progression was radial through the head with complete penetration over and arc of about 60 degrees centered in the fatigue region. The cylinder head was permanently marked in various locations. They markings UT05, UT97, UT91, UT89, and UT89 were stamped in the head near the intake port. The "UT" markings on the head are consistent with the markings required by AD 78-0807 and they denote five different ultrasonic inspections since 1989.