Engine Performance Issues
The pilot reported that the engine started and ran normally before takeoff. During the flight, however, the engine began running rough. The pilot reduced the power setting, which initially eliminated the roughness. After checking each magneto and noting no malfunction, the pilot diverted to a nearby airport. While diverting, the engine again began running rough and a severe loss of power occurred.
According to witnesses, the aircraft struck a palm tree during a left descending turn at low altitude and crashed.
Magneto Examination
The engine was equipped with Bendix model S4LN-200 series magnetos. During a bench check, both magnetos began misfiring after about 15 to 20 minutes of operation. Disassembly of the magnetos revealed that the nonconductive material in the coils was cracked and was beginning to liquefy, which resulted in misdirection of the high voltage spark. The magnetos had accumulated approximately 2,316 hours in service.
Maintenance History
According to Airworthiness Directive (AD) 73-07-04, the coils in the magnetos of this aircraft should have been replaced after accumulating approximately 2,000 hours of operation. The aircraft had flown 11 hours since the last annual inspection.