Incident Overview
A pilot had been airborne for approximately 45 minutes practicing turns and instrument approaches under visual flight rules (VFR). The flight had just completed a practice instrument approach to runway 1 and was following the published missed approach procedure. When the aircraft reached an altitude of about 500 to 600 feet, the pilot reported hearing a loud pop, followed by an immediate engine stoppage with no prior warning.
Emergency Response
The pilot declared an emergency and was cleared to land. He turned left onto a short base leg for runway 1 but could not maintain altitude and determined he would not reach the runway. He elected to land in a field, and the aircraft impacted trees short of the runway threshold.
Engine Examination
A disassembly of the engine revealed that the failure was due to a broken crankshaft gear bolt, which resulted in the separation of the crankshaft gear from the rear of the crankshaft. The failed bolt, part number STD-2209, was sent to a laboratory for examination.
Laboratory Analysis
Laboratory examination determined that the crankshaft gear bolt failed due to hydrogen embrittlement. The NTSB Materials Laboratory's analysis indicated that the preponderance of evidence and reference information pointed to the manufacturing cycle—specifically, the combination of plating, bake delay, and the bake itself—as the most likely source of the hydrogen embrittlement.