Incident Overview
On August 7, 2014, at approximately 0946 central daylight time, an Enstrom F-28F helicopter, registration N213BL, was substantially damaged during a precautionary landing in a cornfield near Tomah, Wisconsin. The pilot, who was the sole occupant, sustained serious injuries. The helicopter was registered to and operated by Paul Peterson Aviation LLC under 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed, and the flight was not operated on a flight plan.
Flight and Landing Sequence
The flight originated from a loading site approximately half a mile from the accident location. The pilot was conducting the third application flight of the morning. After departure, the flight proceeded without incident until, about half a mile from the planned application field, the pilot felt a vibration in the cyclic control and noted an "irregular" running engine. He immediately decided to perform a precautionary landing in a cornfield, where the corn was about 10 feet tall.
During the landing, the landing skids collapsed, and a main rotor blade severed the tailboom. A portion of the tail rotor drive shaft separated from the airframe, consistent with a main rotor strike. The tail rotor transmission separated into two sections, with fracture surfaces indicative of overstress failure.
Postaccident Examination
Examination of the helicopter revealed no preimpact anomalies in the flight control system or rotor drive system. The engine, a 225-horsepower Lycoming HIO-360-F1AD fuel-injected reciprocating engine, was inspected. The No. 4 cylinder lacked compression during crankshaft rotation. Further inspection showed that the intake valve was not seating properly, with a gap of approximately 0.015 inch when fully seated. Linear scoring was observed on the valve stem, and debris consistent with valve guide material was found upon valve removal. No other engine anomalies were noted.
Maintenance and Ownership History
The accident helicopter was purchased by the operator on June 4, 2014, and received a restricted category special airworthiness certificate for agricultural operations on July 25, 2014. The total airframe time at the time of the accident was 2,385.1 hours. The most recent annual inspection occurred in December 2013 at 2,327.4 hours. The engine was overhauled in March 2006 and had accumulated approximately 831.2 hours since overhaul, with 57.7 hours since the annual inspection. The latest maintenance entry, dated August 4, 2014, involved replacement of the spark plugs. No unresolved maintenance issues were recorded.