On February 7, 2013, about 1600 central standard time, an Air Tractor AT-502 airplane, N198LA, collided with terrain during a landing near Forrest City, Arkansas. The commercial rated pilot was injured and the airplane was substantially damaged. The airplane was registered to and operated by South Delta Aviation, LLC, West Helena, Arkansas, under the provisions of 14 Code of Federal Regulations Part 91 as a maintenance test flight. Visual meteorological conditions prevailed for the flight that operated without a flight plan. The pilot reported that he departed with 150 gallons of water to calibrate the spray system on the airplane. After making a spray pass, he started a climbing turn to the left; the airplane then began vibrating and shaking. The pilot added that he managed to get the airplane straight and level, but still rocked left and right. He further added that the left aileron was going up and down; stop to stop. The pilot found that as he slowed the airplane to 90 mph, he would lose aileron control and the airplane would start a slow left turn while rocking back and forth. The pilot then performed a long circle and selected a hay field, next to the airport for the emergency off-field landing. According to the responding Federal Aviation Administration (FAA) inspector, the airplane was substantially damaged during the accident. Additionally, inspection of the airplane revealed that the two bolts in the aileron push-pull rod were missing. The bolts and associated hardware were not located among the wreckage. The inspector also noted that the airplane had undergone extensive maintenance, about five months before the accident. A review of the airplane's maintenance manual revealed that the nuts installed in the airplane control system are self-locking nuts.