Incident
A sightseeing flight operating under 14 CFR Part 135 was returning from Grand Canyon, Arizona, to Scottsdale, Arizona. The pilot reported that the aircraft experienced a power loss while all other instrument indications remained normal. The pilot enriched the mixture and changed fuel tanks, but these actions did not restore power.
The pilot elected to divert to Pulliam Airport (FLG) in Flagstaff, Arizona. After turning the aircraft towards Flagstaff, smoke began entering the cabin area from beneath the instrument panel. The pilot secured all electrical power, and the smoke diminished momentarily. Shortly thereafter, the engine began to lose more power, and the cabin began to fill with smoke. The pilot then chose to land in an open field.
During the landing roll, the aircraft struck three fence lines and a dirt embankment before coming to a stop.
Examination
Examination of the engine revealed that the No. 1 cylinder aft attachment exhaust studs had sheared off, and the forward studs' attachment nuts were loose.