Incident Summary
A certified flight instructor (CFI) was conducting an instructional flight when the engine began to run rough during airwork. He diverted to a nearby airport and performed an uneventful precautionary landing. On the ground, he conducted an engine run-up and noticed a discrepancy with the left magneto. He operated the engine at a high rpm with the fuel/air ratio leaned, attempting to clear what he believed were lead-fouled spark plugs. After the run-up, the left magneto drop was within limits (150 rpm). The CFI did not contact the operator for assistance regarding the precautionary landing and decided to return to the original departure airport.
Sequence of Events
During climbout, approximately 6-7 miles from the destination airport, the engine vibrations began, and the rpm fluctuated by 200 rpm, then decreased to 2,000 rpm, and further decreased to 300-400 rpm. The CFI maneuvered the airplane toward a known grass airstrip, but due to low light and visibility restrictions, he flew past the airstrip. Because of the low altitude, he elected to land in a field perpendicular to the airstrip. While approaching the field, the airplane collided with powerlines, then struck the ground and nosed over.
Post-Accident Examination
Postaccident examination of the engine revealed no evidence of a failure or malfunction of the power section. Examination of the fuel system showed no preimpact failure or malfunction. The spark plugs were satisfactory but were wet with oil, consistent with an inverted engine. The left magneto operated intermittently during postaccident bench testing. Disassembly of the left magneto revealed a worn point cam and eroded points. Additionally, the coil tested out of limits; the condenser tested within specification.