On April 5, 1991, a Cessna 172N (registration N733MQ) operated by West Valley Flying Club Inc. was involved in an aviation accident near Stockton, CA. Investigators recorded the probable cause as: An improper gasket was installed under the magneto housing, preventing the magneto from being properly tightened to the required torque setting, and gasket material under the nut provided a cushion effect that allowed the nut to vibrate loose. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive; 2 related events involving the same aircraft type or operator are linked below.
During an instrument training flight in VFR conditions, the engine quit while in a holding pattern. The CFI performed a forced landing in an open field, but the airplane nosed over after encountering soft terrain. Investigation revealed an improper gasket caused the magneto assembly to fall off.
Incident Overview
A certificated commercial pilot/certified flight instructor (CFI) and a certificated private pilot/instrument student were conducting an instrument training flight. While in a holding pattern under visual flight rules (VFR) conditions, the engine suddenly failed. The CFI executed a forced landing into an open field. During the landing roll, the airplane encountered soft terrain and nosed over.
Examination Findings
Post-incident examination of the engine revealed that the magneto assembly had fallen off the engine. The cause was traced to an improper gasket installed under the magneto housing. This gasket prevented the magneto from being properly tightened to the required torque setting. Additionally, gasket material under the nut created a cushion effect, which allowed the nut to vibrate loose over time.
Conclusions
The investigation determined the engine failure was due to the loss of the magneto assembly as a result of improper installation.
Probable cause
An improper gasket was installed under the magneto housing, preventing the magneto from being properly tightened to the required torque setting, and gasket material under the nut provided a cushion effect that allowed the nut to vibrate loose.
What happened in the 5 Apr 1991 Cessna 172N accident near Stockton, CA?
During an instrument training flight in VFR conditions, the engine quit while in a holding pattern. The CFI performed a forced landing in an open field, but the airplane nosed over after encountering soft terrain. Investigation revealed an improper gasket caused the magneto assembly to fall off.
What aircraft was involved and where did it happen?
The accident on 5 Apr 1991 involved a Cessna 172N, registration N733MQ, operated by West Valley Flying Club Inc., at Stockton, CA.
What was the probable cause of the accident?
An improper gasket was installed under the magneto housing, preventing the magneto from being properly tightened to the required torque setting, and gasket material under the nut provided a cushion effect that allowed the nut to vibrate loose.
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