Incident Description
Approximately 30 minutes after takeoff, while performing airwork at an altitude of 2,000 feet and returning to the airport, a certified flight instructor (CFI) noted that the oil pressure was below the lower end of the green arc. The CFI questioned the aircraft owner about this condition, who confirmed it was abnormal. Shortly thereafter, the engine began to run rough and subsequently lost power. Attempts to restore engine power were unsuccessful.
The CFI initiated a forced landing to a field. During the final approach, while the throttle was still applied, engine power momentarily returned. The CFI then abandoned the intended landing field and aimed for a freshly plowed field, landing perpendicular to the plowed rows. Upon touchdown, the nose landing gear collapsed, and the engine partially separated from the airframe.
Post-Accident Examination
Disassembly of the engine revealed no evidence of preimpact failure or malfunction. The oil cooler bypass valve was tested and found to not meet the minimum extension at the midpoint of the test range. The engine had accumulated approximately 2,091 hours since manufacture, with no record of the bypass valve being replaced.