On April 22, 2000, at 1500 central daylight time, an Air Tractor AT-401-B, N6096P, collided with trees while maneuvering for a forced landing near Lawrenceburg, Tennessee. The aerial application flight was operated by Thornton Aerial Application under the provisions of 14 CFR Part 137, with no flight plan filed. Visual weather conditions prevailed at the time of the accident. The airplane sustained substantial damage, and the commercial pilot was not injured. The flight departed Lawrenceburg, Tennessee, at 1450. The pilot stated that he had made an application run and was pulling up when he heard a loud pop and observed black smoke coming from the engine exhaust system. The pilot selected an area for a forced landing, and while maneuvering for the landing, the airplane collided with trees. According to the pilot, "due to the low altitude of this type of operation a suitable landing spot could not be chosen." During a subsequent examination of airframe systems, the carburetor model NAY91E1 S/N 5962942 was installed on another engine and tested for proper operation. The engine was operated at 700, 1200, 1400, 1600, and 2000 rpm for five minutes at each point. The engine was leaned with the mixture control and a normal magneto check was performed. The propeller lever was also cycled to various rpm. All operations of the carburetor were normal. Examination of the engine off of the airplane, S/N ZP10053, revealed that the number 3 cylinder's exhaust valve was stuck in the open position. Weather conditions reported in the area at the time of the accident were favorable for the formation of carburetor ice as indicated by the icing probability curves chart.