On July 7, 2014, about 1615 central daylight time, an Air Tractor AT-502 airplane, N9185G, collided with a pond at the end of a private airstrip during takeoff in Carthage, Illinois. The pilot received minor injuries, and the airplane was substantially damaged. The airplane was registered to and operated by Air Assault LLC as a 14 Code of Federal Regulations Part 137 aerial application flight. Visual meteorological conditions prevailed for the flight, which not operated on a flight plan. The local flight was originating at the time of the accident. The pilot reported the airplane was loaded with 364 gallons of chemical and both fuel tanks were ¾ full. He stated the wind was calm and the outside temperature was 88 degrees Fahrenheit. The takeoff was being made with 10 to 15 degrees of flaps extended. The pilot taxied to the end of the runway and applied the brakes while advancing the engine power. He released the brakes and the torque increased to 1,550 foot-pounds within 3 seconds. He noted the propeller rpm was above 2,200 and the first third of the takeoff was normal. The tail lifted off the airstrip about 2/3 the way down the runway and not at the half-way point as the pilot expected, nor did the aircraft accelerate as the pilot expected. The pilot stated that at this point he was committed to the takeoff so he dumped the load and attempted to fly the airplane. The airplane became airborne and cleared a pond, but contacted a bank on the far side of the pond, and the airplane flipped inverted. The pilot stated the engine did not lose power during the takeoff and given the conditions of the day, it should have flown. A postaccident examination of the aircraft by a Federal Aviation Administration inspector revealed that the Pratt & Whitney PT6-27 engine had been replaced with a higher powered -34 engine. On March 21, 2014, a new torque gauge was installed in the aircraft. The torque gauge installed was for the lower powered -27 engine which resulted in the redline on the gauge being lower than it should have been.