On August 20, 1993, a Piper PA-25-235 (registration N4693Y) was involved in an aviation accident near Washburn, ND. Investigators recorded the probable cause as: The pilot's inability to maintain control during a forced landing due to a loss of engine power caused by a jammed impulse coupling flyweight in the left magneto, which seized the magneto and stripped the magneto drive gear and camshaft gear. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
An aircraft lost engine power during a spray run, forcing a landing in a field where it struck fence posts. Investigation found a seized magneto caused the failure.
What happened
The pilot was conducting an aerial application flight when the airplane experienced a complete loss of engine power while maneuvering for a spray run. Unable to maintain altitude or reach a suitable runway, the pilot executed a forced landing in an adjacent field. During the landing roll, the aircraft struck two fence posts, resulting in damage to the airframe.
The investigation
Following the accident, investigators examined the engine and its components to determine the cause of the power loss. The examination revealed that the impulse coupling flyweight in the left magneto had jammed. This jamming caused the left magneto to seize completely. The seizure of the magneto resulted in mechanical failure of the drive system, specifically stripping both the magneto drive gear and the camshaft gear.
Findings
The loss of engine power was directly caused by the mechanical failure of the ignition system. The impulse coupling flyweight in the left magneto jammed, which seized the magneto and led to the stripping of the magneto drive gear and the camshaft gear.
Probable cause
The pilot's inability to maintain control during a forced landing due to a loss of engine power caused by a jammed impulse coupling flyweight in the left magneto, which seized the magneto and stripped the magneto drive gear and camshaft gear.