What happened
While performing a 2 to 2.5 mile extended downwind leg for landing on runway 13, the pilot observed the tachometer RPM changing drastically. After reporting this anomaly to the tower and stating a need to land, the controller cleared the pilot to execute a left turn for a straight-in approach to runway 17.
Approximately 30 seconds after completing the turn toward runway 17, the engine began making loud popping and backfiring noises, which was followed by a total loss of power. The pilot informed the controller that the aircraft would be unable to reach the runway.
The pilot chose to land in a nearby field located approximately 1.5 miles north of the runway 17 threshold. During the landing roll in the field, the main gear struck a drainage ditch running alongside a road, causing the airplane to nose over. The occupants were not injured.
The investigation
An examination and teardown of the engine was conducted. The investigation revealed that the crankshaft had failed in a heavy cast area (web) located between the #1 and #2 rod journals.