Accident Overview
On August 4, 2011, at approximately 1000 central daylight time, an Air Tractor AT-301, registration N4345S, sustained substantial damage during a forced landing near Nekoosa, Wisconsin. The pilot was not injured. The agricultural airplane was operated by Kafer Stone Applicator Service and was conducting a repositioning flight under 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed for the flight, which was not operated on a flight plan.
Flight Details
According to the pilot, the engine ran for approximately 20 minutes prior to departure to warm up. The run-up and takeoff were normal. However, about 150 feet in the air, the engine backfired and completely lost power. The pilot executed a forced landing to a field approximately 500 feet from the departure end of the runway. Attempts to restore engine power before the landing were unsuccessful. The airplane sustained substantial damage to the fuselage and wings.
Postaccident Inspection
A Federal Aviation Administration inspector conducted an on-scene postaccident inspection. The inspection did not reveal any anomalies consistent with a preimpact loss of engine power. Fluid consistent in appearance and odor with aviation fuel was observed in the carburetor. The spark plugs exhibited signatures consistent with normal operation.
Engine Examination
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, and 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 and 17 hours since the annual inspection.
The NTSB Investigator-in-Charge supervised a full engine teardown examination. The teardown 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 intact and undamaged. No other anomalies were observed. Metallurgical review of the fractured gear teeth noted overload failure signatures on the fracture surfaces, with no indications of fatigue failure.
A representative of the engine overhaul facility stated that the 10:1 gear 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
Weather recorded at Alexander South Wood County Airport (ISW), about 5 miles northeast of the accident site, at 0954 included: 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 conditions.