On July 17, 2012, approximately 2000 central daylight time, N4638, a Grumman G-164A airplane was substantially damaged when it made a forced landing to a corn field after a loss of engine power near Fergus Falls, Minnesota. The commercial pilot was not injured. The airplane was registered to and operated by West Central AG-AIR Incorporated, of Fergus Falls, Minnesota. Visual meteorological conditions prevailed for the agricultural aircraft operation flight conducted under 14 Code of Federal Regulations Part 137. The flight originated at the Wheaton Municipal Airport (ETH), Wheaton, Minnesota, approximately 1945. The pilot said he was on his fifth spraying pass when he saw a puff of white smoke come from the engine. He turned off the chemical sprayer and turned toward a road. About half way through the turn, the pilot observed more smoke followed by a total loss of engine power. The pilot jettisoned the chemical load and landed in the corn field. The airplane nosed-over after landing, which resulted in substantial damage to the upper wings, rudder and vertical stabilizer. A Federal Aviation Administration (FAA) Airworthiness inspector examined the Pratt & Whitney R1340 radial engine and found that the No. 2 cylinder head was cracked. A review of the maintenance logbooks revealed that FAA Airworthiness Directive 99-11-02, which required a 100-hour visual inspection of cylinder heads for cracking, was complied with at the last annual inspection. No entires were made that cracks were observed during this inspection. Further review of the logbooks revealed that the AD had not been complied with for 253 hours prior to the annual inspection.