On July 27, 1995, a Grumman G-164 (registration N949X) was involved in an aviation accident near Park River, ND. Investigators recorded the probable cause as: Improper magneto breaker point gap (0.003 inch instead of 0.009 inch) and a grounded primary lead in the left magneto. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A pilot reported engine roughness after reducing power to 2050 RPM; full throttle did not restore power. Emergency landing in a reservoir. Post-accident inspection found magneto breaker point gaps set at 0.003 inch (spec 0.009 inch) and a grounded primary lead in the left magneto.
Sequence of Events
The pilot reported that the engine run-up, magneto check, and takeoff were normal. During the flight, when engine power was reduced to 2050 RPM, the engine began to pop and shake. The pilot applied full throttle, but the engine failed to produce additional power. The airplane subsequently lost altitude and airspeed, and the pilot performed an emergency landing into a reservoir.
Post-Accident Findings
A post-accident engine run was unsuccessful. During inspection, the left and right magneto breaker point gaps were found set at 0.003 of an inch instead of the required 0.009 of an inch. Additionally, the primary lead in the left magneto was grounded against the magneto case and distributor cover. After correcting these two discrepancies, the engine started and ran to 2050 RPM.
Probable cause
Improper magneto breaker point gap (0.003 inch instead of 0.009 inch) and a grounded primary lead in the left magneto.