What happened
The pilot reported that while performing a pull-up during a crop-dusting operation, the propeller suddenly moved to a low pitch angle and the engine ceased developing full power. The tachometer indicated RPM near the red line limit. In response, the pilot retarded both the propeller pitch control and the throttle. At that time, the oil pressure gauge appeared to show normal readings.
Believing he retained sufficient thrust to reach the edge of the field, the pilot attempted to dump his load to minimize potential crop damage. However, he determined there was not enough power to clear the surrounding trees. Consequently, a forced landing was executed into the tree line, resulting in damage to the aircraft.
The investigation
A post-accident examination revealed that the number 6 connecting rod had failed approximately one inch from the rod cap. Evidence of localized overheating was present on the component. The number 6 rod typically receives splash and spray oil from an oil port located in the main journal. Investigators found that the main journal bearing appeared to have spun, effectively closing off this oil port.
This obstruction reduced the flow of cooling oil to the number 6 connecting rod. Additionally, the propeller governor receives its hydraulic oil from the same source. The blockage of oil supply to the governor would allow the propeller to drive into a low pitch and high RPM position, consistent with the pilot's observations.
Findings
The primary mechanical failure was traced to the main journal bearing spinning, which cut off oil flow to both the connecting rod and the propeller governor. This led to localized overheating and subsequent failure of the number 6 connecting rod. The loss of hydraulic pressure to the governor caused the propeller to feather or go into a low pitch/high RPM state, resulting in a complete loss of engine power.
The pilot's decision to continue the flight path after the initial power loss was based on the assumption that sufficient thrust remained, but the mechanical failure was total. The forced landing in the trees was the direct result of this sudden power loss during a critical phase of low-altitude operations.