On June 26, 1996, an Air Tractor AT-301 (registration N8883S) was involved in an aviation accident near Ness City, KS. Investigators recorded the probable cause as: The pilot's decision to conduct a forced landing due to engine vibration caused by a broken bolt in the propeller counterweight; the source of the bolt failure was not determined. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A pilot experienced engine vibration during a crop dusting run, forcing a landing on wet terrain that resulted in a nose-over after a propeller counterweight bolt broke.
What happened
The pilot reported that while conducting a swath run, the aircraft engine began to vibrate significantly. In response, the pilot reduced engine power but found that applying power again did not resolve the issue as the vibration persisted. Consequently, the pilot elected to conduct a forced landing. During the landing phase, the airplane encountered wet terrain which caused it to nosed over.
The investigation
A subsequent examination of the aircraft revealed that a bolt located in the propeller counterweight had broken. Investigators were unable to determine the specific source or cause of the break in the bolt.
Findings
The primary mechanical failure identified was the broken bolt within the propeller counterweight assembly. The inability to identify the origin of this fracture left the root cause undetermined, though it directly contributed to the engine vibration that necessitated the emergency landing.
Probable cause
The pilot's decision to conduct a forced landing due to engine vibration caused by a broken bolt in the propeller counterweight; the source of the bolt failure was not determined.