On April 25, 2012, about 1052 central daylight time, a Thrush Aircraft Inc. S2R-H80, N510KW, descended and impacted terrain near Bryan, Texas. The pilot was trying to retrieve a dropped item in the cockpit when the airplane began the descent. The commercial pilot sustained minor injuries. The airplane sustained substantial damage to the fuselage and wings. The airplane was registered to and operated by Thrush Aircraft Inc. under the provisions of 14 Code of Federal Regulations Part 91 as a test flight. Visual meteorological conditions prevailed and a flight plan had not been filed for the local flight that originated from Coulter Field Airport (CFD), Bryan, Texas, at time unknown. The pilot, who was employed by the operator, stated that he departed CFD for the field site where agricultural spray pattern testing was to be performed. Upon arrival at site, the wind had shifted so equipment had to be rearranged. He started flying orbits to wait for the equipment to be set up while maintaining a low altitude to stay beneath overlying airspace. A coffee cup came loose and started rolling around floor. He leaned over to try to secure the cup and then felt aircraft roll to the right. By the time he sat back up, the airplane had entered a nose-down, right roll. He leveled the wings and attempted to pull the nose up, but he did not have enough altitude to recover. The airplane impacted the ground in a slight nose-up, wings-level attitude and airplane slid to a stop. The pilot reported that there were no mechanical malfunctions with the airplane. A witness reported that he saw the airplane making a low pass about 10-20 feet above the ground. The airplane then went into a steep pull up and entered a 70-90 degree bank, followed by the wings leveling. The witness then saw the airplane at an altitude of about 300 feet above ground level when it pitched down about 10 degrees and then pitch up and bank. The airplane continued to roll until it returned to level flight. The nose of the airplane struck the ground in a 10 degree nose down attitude with the wings level. The airplane slid along the ground for about 100 feet and turned 90 degrees before stopping.