Incident Overview
During a banner-towing operation over a fairground, the pilot observed a complete loss of oil pressure. Attempting to return to the airport, the engine subsequently lost power. The pilot executed a forced landing in a freshly mown hay field. During the landing roll, the airplane encountered a drop off, resulting in structural damage.
Postaccident Examination
A postaccident inspection revealed that the oil filter adapter had separated from a boss on the crankcase cover. An FAA inspector noted that approximately 80% of the threads in the crankcase cover boss were stripped. This condition likely allowed the adapter to detach, leading to the loss of oil pressure and eventual engine failure.
Pilot Actions
The pilot's immediate response to the oil pressure loss was to attempt a return to the departure airport. When engine power ceased, the pilot selected a suitable field for landing. The forced landing was performed without injury to the pilot, though the aircraft sustained damage from the terrain irregularity.
Conclusion
The investigation identified the separation of the oil filter adapter due to thread stripping as the mechanical failure that precipitated the loss of oil pressure and subsequent power loss. No other contributing factors were detailed in the report.