Accident Overview
An agricultural pilot was setting up for an aerial application pass when the engine suddenly quit. Unable to reach a cleared area, the pilot performed a forced landing in a wooded area.
Post-Accident Examination
Following the accident, the engine was examined. The investigation revealed that an impulse coupling stop pin from the left magneto had failed. This failure caused the magneto to jam, which subsequently resulted in the shearing of the teeth on the magneto drive gear in the engine.
Consequences
The shearing of the drive gear teeth rendered both magnetos inoperative, leading to a complete stoppage of the engine. The loss of engine power necessitated the forced landing in the wooded terrain.
Technical Details
The impulse coupling is a component of certain magneto ignition systems that aids in engine starting by providing a retarded spark and then advanced timing. The stop pin prevents the coupling from rotating beyond its intended range. Its failure allowed the magneto to over-rotate, causing internal jamming and mechanical damage to the drive gear.
Conclusion
The engine failure was caused by a cascade of mechanical failures originating from a defective impulse coupling stop pin in the left magneto. This event underscores the importance of magneto inspection and maintenance in aircraft ignition systems.