Background
The pilot conducted a pre-takeoff run-up check of both the right magneto and left coil ignition system, which was satisfactory. He then departed for his destination.
Sequence of Events
Approximately 30 minutes into the flight, the engine began to run rough. The pilot determined that the right magneto was not operating and selected the left ignition. Although the engine initially continued to run roughly, eventually all power was lost.
The pilot selected an open field used by a farmer to operate his Cessna 177 in and out of due to the non-availability of any other adequate landing site. As he maneuvered through a 180-degree turn toward the landing area, he noted a 1,000-foot-per-minute rate of descent, which was greater than he had anticipated. The landing gear had been extended, and as the aircraft was maneuvered onto a short final, the right main landing gear struck a cow. The aircraft yawed into a left skid, contacted the ground, and skidded through a barbwire fence.
Post-Crash Examination
Post-crash examination revealed that the right magneto had grounded internally, resulting in its failure to operate. Additional testing revealed the left coil ignition system had also grounded internally, as evidenced by arcing, and it too had failed.
Probable Cause
The probable cause of the accident was the failure of both the right magneto and left coil ignition system due to internal grounding.