Incident Overview
An instructor pilot was flying when a partial loss of engine power occurred. The instructor took control of the aircraft and applied carburetor heat, then turned back toward the airport. As the power continued to decrease gradually, the instructor decided to land in an open field. The field was small, and in some areas the mud was up to one foot deep.
Landing and Rollover
During the landing, the aircraft rolled approximately 75 to 100 feet before flipping over. The aircraft came to rest in the muddy field.
Engine Examination
After the incident, an examination of the engine was conducted. The inspection revealed that the number one exhaust valve adjusting screw nut had come off. This nut jammed the intake valve spring, resulting in a bent intake valve push rod. Additionally, the exhaust valve adjusting screw was found to be 1 1/4 turns from completely backing off the rocker arm. When checked, the exhaust valve would not open. The intake valve would barely open due to the large clearance caused by the bent push rod.
Maintenance and Operational Findings
The findings indicate that the loss of engine power was due to mechanical issues related to the valve train. The loose adjusting screw nut and the resulting bent push rod prevented normal valve operation. The instructor's decision to land in a field was a response to the gradual power loss. The small, muddy field contributed to the aircraft flipping over after touchdown.