CHI96LA281

The pilot reported he was at an altitude of 41 feet, entering his eighth swath run, when the number nine cylinder 'exploded.' The windscreen became covered with oil. He made a shallow turn to avoid a rotary irrigation system and made a forced landing in a corn field. During the forced landing, the left wing impacted the corn and the landing gear 'started sinking into the mud.' The airplane 'spun around' sustaining substantial damage to the lower wings, fuselage, and left wing support struts. Examination of the airplane revealed the number nine cylinder was fractured between the barrel and the head. Evidence of fatigue cracking was evident around the entire circumference of the head. The major fatigue zones accounted for approximately 25 percent of the fracture surface.

On August 7, 1996, at 0919 mountain standard time, a Grumman-Schweizer G164A, N8842H, operated by Ag-Land, Inc., sustained substantial damage when the right wing impacted corn during a forced landing near Grant, Nebraska. The commercial pilot reported a loss of engine power while he was spraying. He reported no injury. The local aerial application flight originated at 0859 and was conducted under 14 CFR Part 137. Visual meteorological conditions prevailed and no flight plan was filed.The pilot reported he was at an altitude of 41 feet, entering his eighth swath run, when the number nine cylinder "exploded." The windscreen was covered with oil. He made a shallow turn to avoid a rotary irrigation system and conducted a forced landing in a corn field. During the forced landing, the left wing impacted the corn and the landing gear "started sinking into the mud." The airplane "spun around" sustaining substantial damage to the lower wings, fuselage, and left wing support struts.

The operator reported that the engine had 894 hours since overhaul.

The airplane was examined by a Federal Aviation Administration (FAA) Inspector who reported that the number nine cylinder was fractured between the barrel and the head. He reported no other evidence of preimpact malfunction.

The cylinder was examined by a metallurgist at the NTSB laboratory. He reported that "fatigue cracking was evident around the entire circumference of the head... . Two large fatigue zones were visible in the area of the fracture adjacent to the exhaust port... . Both major fatigue zones initiated at multiple origins in a thread root on the inner diameter of the head... . Smaller areas of fatigue progression were visible around the remainder of the cylinder circumference." The major fatigue zones accounted for approximately 25 percent of the fracture surface. Both valves were in place and "appeared undamaged." Combustion deposits were noted on the outboard end face and bevel of the barrel.