On September 21, 2012, about 1700 Pacific daylight time, an Air Tractor, Inc., AT-502, N4556S, sustained substantial damage when it struck transmission wires and impacted terrain during takeoff initial climb near Firebaugh, California. The airline transport pilot was not injured. Thiel Air Care, Inc., was operating the airplane under the provisions of 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed for the aerial application flight, which was originating at the time of the accident. A flight plan had not been filed. The pilot reported that he ferried the airplane about 25 nautical miles to the private ag-operating airstrip at an elevation of 236 feet. He went into the office to receive the details of his mission, while the airplane was being fully fueled and 400 gallons of defoliant loaded in the hopper. On takeoff roll, the pilot noticed that the engine was not producing its "fully rated torque." About halfway down the runway, because the airplane's tail was not coming up, he added more flaps to help raise it. The pilot said that, at the end of the runway, the airplane "struggled into ground effect and would not climb." As the airplane approached a power transmission line about 0.5 miles from the airstrip, the pilot began dumping the defoliant load in order to climb over the wires. Dumping the load resulted in the airplane gaining some altitude but simultaneously the nose came up, and the airplane reached a "very high" pitch attitude. Subsequently, the airplane impacted the wires and landed hard in an open field several hundred feet beyond the transmission line. The impact bent both wings and damaged the landing gear and engine compartment. On October 25, 2012, a field representative for the engine manufacturer examined the engine and airplane. The examination revealed no evidence of any preimpact mechanical malfunctions or failures. The fuel control unit, fuel pump, fuel heater, and propeller governor were sent to the manufacturer's facility in Quebec, Canada, for further examination. On November 21, 2012, these components were examined and tested under the supervision of an investigator from the Transportation Safety Board of Canada. The testing found no indications of any pre-impact mechanical anomalies or dysfunctions to the components that would have precluded normal engine operation. At 1653, the reported weather conditions at Madera, California, elevation 255 feet, located about 28 nautical miles east of the accident site, were wind from 290 degrees at 7 knots; sky clear, visibility 10 miles; temperature 32 degrees Celsius; dew point 7 degrees Celsius; and altimeter 29.85 inches of mercury. The calculated density altitude was 2,400 feet.