Accident Description
The pilot of an Agcat cropduster was conducting the first spray run over a field after takeoff when the Pratt & Whitney R-1340 engine began to run rough. The pilot initiated a climb and executed a right turn away from obstacles while crossing the Snake River. During the turn, the engine abruptly stopped. The pilot rolled the aircraft level and performed a forced landing in a beet field. During the landing, the aircraft crossed an elevated dirt/gravel roadway bounded on its north and south edges by a shallow ditch. The landing gear separated from the airframe, and the aircraft nosed over, coming to a stop.
Engine Examination
The engine, which had been overhauled 81 hours prior and inspected 31 hours prior, was examined post-accident. The examination revealed that the number 3 cylinder exhaust valve steel pushrod was broken near the rocker arm end of the rod. The rod displayed very slight bending deformation, and the separation surfaces of the broken end were mechanically battered. The report notes that removal or separation of the exhaust pushrod prevents the exhaust valve from opening during the exhaust phase cycle for that cylinder, which can result in hot exhaust from the cylinder back-flowing out the intake valve when it opens during the intake cycle.