Accident Sequence
During cruise flight over mountainous terrain, a loss of engine power occurred. The pilot radioed air traffic control that the engine was losing power and would not sustain flight, and was attempting to reach an airport when the airplane impacted trees in mountainous terrain.
Airframe and Engine Examination
An airframe inspection revealed no discrepancies that would have precluded normal operation. The engine was equipped with a single-drive dual magneto that remained secure at its mounting pad. During manual rotation of the crankshaft, investigators heard a "snapping" sound from the coupling, but no spark was observed at any of the leads. Magneto-to-engine timing could not be established because the contacts were not breaking electrical continuity.
A functional check by the magneto manufacturer revealed that both magneto primary circuits had short-circuited to ground in all magnet positions, consistent with the breaker points not opening. Disassembly showed that the plastic cam followers were melted and smeared in the direction of normal cam rotation. The breaker cams, made of sintered iron material, are supposed to be impregnated with oil per the service manual or production instructions for lubrication at each overhaul, as the oil lubricates the cam surface over its service life.
Maintenance History
Four months prior to the accident, the single-drive dual magneto had been removed. A mechanic at a local fixed base operator had internally timed the magneto, then completed a ground run and full operational check. The accident flight occurred 38.2 hours after this maintenance operation. According to the manufacturer, to internally time the magneto, maintenance personnel must partially disassemble the magneto and physically touch the cam and breaker points. In accordance with the Service Support Manual instructions, during this process the breaker cams and contact points should be inspected for lubrication, wear, burning, or melting.
Additional Findings
On-scene examination revealed that the pilot was not wearing his shoulder harness, which remained stowed in the stowage sheath above the pilot's cabin door. The airframe manufacturer indicated that the airplane had been delivered new in 1975 with shoulder harnesses as standard equipment for the pilot and co-pilot. The coroner listed the cause of death as blunt force craniocerebral trauma due to an airplane crash.