On August 27, 1987, an Air Tractor AT-301 (registration N3159X) operated by Robert R. Jacobs. was involved in an aviation accident near Tillar, AR. Investigators recorded the probable cause as: Failure of all dowel pins securing the dome to the hub assembly, resulting from barrel shims being partially installed along the edge of the dome instead of flat against the dome shelf in the hub, leading to disengagement of the blades from the gearing. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
Aircraft experiences erratic propeller RPM on approach, resulting in uncontrolled descent and hard landing in plowed field. Inspection reveals failed dowel pins due to improper shim installation.
Incident Overview
During approach, the propeller RPM became erratic and then ran completely out of control. Consequently, the aircraft nosed down into a plowed field just short of the intended landing area. The aircraft contacted the ground on the main landing gear, which broke off, followed by the aircraft nosing over.
Post-Inspection Findings
Examination of the propeller assembly revealed that all dowel pins securing the dome stationary to the hub assembly had failed. The failure was determined to result from incorrect installation: the barrel shims were partially installed along the edge of the dome instead of being placed flat against the dome shelf within the hub. This misalignment caused the blades to become disengaged from the gearing mechanism, leading to loss of propeller control.
Conclusion
The investigation identified that the propeller control failure was directly attributable to the improper installation of the barrel shims, which compromised the integrity of the dowel pins and ultimately caused the propeller blades to disengage from their gearing.
Probable cause
Failure of all dowel pins securing the dome to the hub assembly, resulting from barrel shims being partially installed along the edge of the dome instead of flat against the dome shelf in the hub, leading to disengagement of the blades from the gearing.