What happened
The pilot was conducting flight training in a Cessna 172 (type not specified in source, but context implies single-engine training) for approximately 45 minutes, practicing turns and instrument approaches under visual flight rules (VFR). After completing a practice instrument approach to runway 1, the pilot was following the published missed approach procedure.
At an altitude of approximately 500 to 600 feet, the pilot reported hearing a loud pop, after which the engine stopped immediately without warning. The pilot declared an emergency and was cleared to land. While attempting to turn left onto a short base leg for runway 1, the aircraft was unable to maintain altitude. Unable to reach the runway, the pilot elected to land in a field, where the aircraft impacted trees short of the runway threshold. There were no fatalities reported.
The investigation
An examination of the engine disassembly revealed that the engine failure was caused by a broken crankshaft gear bolt and the subsequent separation of the crankshaft gear from the rear of the crankshaft.
Laboratory analysis of the failed crankshaft gear bolt (part number STD-2209) determined that the component had failed due to hydrogen embrittlement. The NTSB Materials Laboratory concluded that the manufacturing cycle, specifically involving the plating and bake processes, was the most likely source of the embrittlement.