Incident Overview
A bi-wing agricultural airplane, operated by a private pilot with 2,697 flight hours, encountered an engine anomaly while completing a spray pass over a field. The engine lost power and exhibited unusual vibrations, prompting the pilot to initiate a return to the departure airport.
Sequence of Events
The pilot turned the airplane southward in an effort to fly back to the originating airport. However, realizing the engine was not producing sufficient power to reach the airport, the pilot executed an approximate 180-degree turn toward a road that was considered a suitable landing area. Before reaching the road, the engine began emitting popping noises, trailing smoke, and experienced a further reduction in power. The pilot then elected to land in a cotton field.
During the landing roll, the left main landing gear collapsed, causing the airplane to nose over and come to rest inverted.
Post-Accident Examination
An examination of the engine revealed a fracture of the number five-cylinder exhaust valve rocker arm mounting boss, with the fracture extending approximately 180 degrees around its circumference. This structural failure prevented the exhaust valve from opening, remaining in the closed position.