On May 8, 2013, about 1600 central daylight time, an Air Tractor AT-301 airplane, N23257, impacted terrain near Lyon, Kansas. The commercial pilot was not injured and the airplane sustained substantial damage to the fuselage. The airplane was registered to R&B Flying and operated by Ag Air Service under the provisions of 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed for the flight, which operated without a flight plan.In a telephone interview with the pilot, he stated that while leveling off the airplane at the start of an aerial application run, he heard a loud bang. A portion of the right door's acrylic glass broke and struck the pilot's shoulder. The pilot was momentarily distracted from being struck and the airplane descended into the crops. The impact with the crops slowed the airplane down even after the pilot applied full power. He could not get the airplane to climb and it impacted terrain in an adjacent field. An examination of the airplane by the responding Federal Aviation Administration inspector revealed that the aft hinge of the right door was cracked at the front and rear attach points, and the right door had opened in flight.