On August 4, 2011, about 1000 central daylight time, an Air Tractor AT-301 airplane, N4345S, was substantially damaged during a forced landing following a loss of engine power after takeoff near Nekoosa, Wisconsin. The pilot was not injured. The agricultural airplane was registered to and operated by Kafer Stone Applicator Service. The repositioning flight was being conducted under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed for the flight, which was not operated on a flight plan. The flight originated from a private airstrip near Nekoosa, Wisconsin. The intended destination was a private airstrip near Oshkosh, Wisconsin.

The pilot stated that the engine ran approximately 20 minutes prior to departure in order to warm up. The run-up and takeoff were normal. However, about 150 feet in the air, the engine backfired and completely lost power. He executed a forced landing to a field approximately 500 feet from the departure end of the runway. The pilot reported that his attempts to restore engine power prior to the forced landing were unsuccessful. The airplane sustained substantial damage to the fuselage and wings during the forced landing.

A Federal Aviation Administration Inspector conducted an on-scene postaccident inspection. The postaccident inspection did not reveal any anomalies consistent with a preimpact loss of engine power. Fluid consistent in appearance and odor to aviation fuel was observed in the carburetor. The spark plugs exhibited signatures consistent with normal operation.

The airplane was powered by a 600-horsepower Pratt & Whitney R-1340-AN-1 radial engine, serial number ZP-101988. An engine overhaul was completed in March 2000. An annual inspection was completed on June 20, 2011. At the time of the accident, the engine had accumulated about 7,382 hours total time, with approximately 566 hours since overhaul. About 17 hours had accumulated since the annual inspection.

The NTSB Investigator-in-Charge (IIC) supervised a full engine teardown examination. The teardown examination revealed the separation of three gear teeth on a 10:1 gear (part number 14951) located in the accessory section of the engine. The remaining gear teeth appeared to be intact and undamaged. No other anomalies were observed during the teardown examination. Metallurgical review of the fractured gear teeth noted the presence of overload failure signatures on the fracture surfaces. No indications of fatigue failure were observed.

A representative of the engine overhaul facility stated that the 10:1 gear (p/n 14951) identified during the teardown examination functioned as an intermediate gear between the engine crankshaft floating gear and the supercharger impeller gear. Separation of several gear teeth is common in the case of a propeller strike event, such as during a forced landing. Failure of the gear in-flight commonly results in damage to the remainder of the gear.

Weather conditions at the Alexander South Wood County Airport (ISW), located about 5 miles northeast of the accident site, at 0954, were recorded as: wind from 130 degrees at 5 knots, clear sky, 10 miles visibility, temperature 25 degrees Celsius, dew point 18 degrees Celsius, and altimeter 30.02 inches of mercury. Federal Aviation Administration guidance indicated a possibility of engine carburetor icing only at glide power based on the recorded weather conditions.

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