Engine Power Loss During Stall Practice
During a training flight involving stall practice, a flight instructor and a student pilot experienced a sudden and complete loss of engine power. The aircraft was forced to land, and the engine was subsequently examined in a postaccident investigation.
Postaccident Examination Findings
Disassembly of the engine revealed that the left crankcase half idler gear shaft had become dislodged from its mounting boss. The two bolts that retained the idler gear shaft were found broken. The idler gear, which connects the crankshaft and camshaft, remained on the shaft but displayed a U-shaped fracture in the gear ring area that separated two full teeth.
Optical examination of the gear fracture showed fracture face markings consistent with fatigue progression. Arrest lines indicated that fatigue covered more than 95 percent of the fracture surface. Both retaining bolts had separated near their heads and exhibited severe circumferential wear through the threaded shanks, removing the majority of the bolt diameters. High-magnification viewing of the bolt fractures revealed features indicative of multiple origin fatigue propagating through the remaining cross sections.
The lock plate was fractured in several locations and heavily distorted. The bolt holes in the lock plate were severely elongated and enlarged beyond their original diameter. While it is likely that loosening of the bolts precipitated the gear failure, no conclusive evidence was uncovered to confirm that scenario.
Maintenance History
The engine's most recent annual inspection was performed about four months prior to the event. At that time, the engine had accumulated 1,226 hours since its last overhaul in 1985, and 639 hours since it was disassembled and reassembled following a propeller strike in 2001.