On May 14, 1999, at 1209 central daylight time, an Air Tractor AT-301, agricultural airplane, N4329S, impacted trees during the takeoff/initial climb near Brownwood, Texas. The aircraft was owned by Ag-Air Equipment, Inc., of Wichita Falls, Texas, and operated by Flying V Air Service Inc., of Wichita Falls, Texas. The commercial pilot, sole occupant, received minor injuries. The airplane was destroyed by impact forces and a fire. Visual meteorological conditions prevailed for the 14 Code of Federal Regulations Part 137 aerial application flight, and a flight plan was not filed. The local flight was originating from a private grass airstrip at the time of the accident. According to the 2,000-hour pilot, during the 3 weeks prior to the accident, he had averaged 15-hour work days, which consisted of flight and ground duty. On the day of the accident, the pilot's first flight, which was a ferry flight to the private airstrip, commenced at 0600. At 0722, the airplane began spraying local fields with a mixture of water and zinc sulfate solution. At 1203, the airplane returned to the airstrip from its 20th application flight and was refueled. During the refueling process, the pilot retracted the flaps to "improve the footing" of the person, who was fueling the left main wing fuel tank. The 320-gallon hopper tank was then loaded with 300 gallons of spray solution in preparation for the airplane's 21st application flight of the day. The pilot initiated the takeoff roll to the southeast from the 3,000 foot, wet, soft, airstrip. The pilot reported in a written statement that he "failed to extend the flaps for takeoff." The pilot stated that it "felt like [the airplane] wouldn't climb." Subsequently, the airplane contacted a tree, which was located at the departure end of the airstrip, "dead center on the engine." The right wing contacted another tree and separated at midspan, and the left wing contacted a tree and separated 3 feet inboard from the wing-tip. The airplane contacted the ground and came to rest inverted approximately 150 feet beyond the departure end of the airstrip. A postcrash fire erupted and consumed the airplane. The pilot exited the airplane from the left door. The pilot reported that at the time of the accident, the visibility was greater than 10 miles, there were broken clouds at 10,000 feet, and the winds were from 180 degrees at 10 knots, gusting to 15 knots. At the request of the NTSB investigator-in-charge, a representative from Air Tractor, Inc., examined the airplane at the accident site. He identified the flap actuator jack screw and ball nut assembly in the wreckage. The ball nut was located within 1/4 inch of the large gear-box ball bearing on the jack screw. According to the representative, this finding is "consistent with the flaps being in the retracted position." The operations section of the AT-301 flight manual requires that the flaps be extended 10 degrees for takeoff when operating with a full hopper load or from a "short" airstrip. The pilot added that the Air Tractor owner's manual states that "10 degrees of flaps are required during takeoff to ensure clearing obstacles." The pilot stated that he did not use a checklist. The pilot stated in the Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2) that this accident could have been prevented if the following actions had been taken: 1. Development and use of a checklist for Part 137 Operations. 2. Development and use of a company policy restricting "on-duty" and flight times for Part 137 operations.