Accident Summary
An airplane was en route at 9,500 feet mean sea level, approximately 12 miles east of its destination, when the pilot reported a loss of engine power. At 2,200 feet msl, the pilot stated that he would not reach the runway. During the subsequent off-airport landing, the aircraft struck a tree, resulting in the separation of the right wing from the airframe. The airplane then descended into water and came to rest inverted in water 7 feet deep.
Investigation Findings
Examination of the wreckage revealed engine oil present in the single drive dual magneto. One of the rear case bearing retaining plate screws was found broken. The magneto was removed and disassembled for further analysis. A metallurgist determined that the screw failure was caused by delayed fracture due to hydrogen embrittlement, with the hydrogen most likely introduced during cadmium plating for corrosion resistance at the time of the screw's original manufacture.
The magneto had accumulated 775.58 hours since its last overhaul in 1993. The most recent annual inspection occurred in November 1998, during which the magneto points and timing were checked. Only 3.27 hours had been logged since that inspection.
Magneto Service Bulletin
Service Bulletin (SB) 643, dated February 8, 1994, recommended that magnetos be overhauled or replaced at the expiration of four years, regardless of accumulated operating hours since new or last overhaul. No evidence was found that this service bulletin had been accomplished on the accident magneto. However, under 14 CFR Part 91 operations, compliance with the service bulletin is not mandatory.