On August 9, 1984, an Aeronca 7AC (registration N83233) was involved in an aviation accident near Prineville, OR. Investigators recorded the probable cause as: The improper grinding of the crankshaft journal, which resulted in a fracture near the number two crankthrow. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A pilot experienced a sudden loss of power and heard engine chatter while maneuvering, forcing a landing in a field. Investigation found the crankshaft broke due to improper grinding.
What happened
While the aircraft was being maneuvered, the pilot reported hearing a loud chattering noise originating from the engine, which was immediately followed by a significant loss of power. In response to the mechanical failure, the pilot reduced the remaining available power and initiated an emergency forced landing in a rough barley field.
The investigation
A post-accident examination of the aircraft revealed that the engine crankshaft had fractured near the number two crankthrow. Further technical analysis determined that the journal surface was improperly ground, which likely contributed to the structural failure.
Probable cause
The improper grinding of the crankshaft journal, which resulted in a fracture near the number two crankthrow.