On January 31, 1990, a Douglas DC-9-31 (registration N8978E) operated by Eastern Air Lines, Inc was involved in an aviation accident near Louisville, KY. Investigators recorded the probable cause as: The Safety Board believed that the nut stripped off when the loose torque links allowed excessive vibration, and the undersize apex bolt threads allowed the nut to shear away the bolt pin designed to retain the nut. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive; 1 related events involving the same aircraft type or operator are linked below.
Aircraft registered N8978E. Photo: Cory W. Watts from Madison, Wisconsin, United States of America / CC BY-SA 2.0, via Wikimedia Commons
Following a landing with noted vibration, investigation found the left main gear torque link apex bolt had backed out, allowing the truck to rotate 360 degrees. Laboratory tests revealed excessive play in attachment pins and undersized bolt threads.
Incident Description
After landing, the flight crew experienced a vibration. Post-flight inspection revealed that the left main gear torque link apex bolt had vibrated out of the torque link. This allowed the main gear truck to rotate 360 degrees during the landing roll. The nut had stripped off the bolt.
Investigation Findings
Laboratory examination of the torque link assembly found that the attaching pins had excessive play. Both new and used apex bolts were measured and found to fail the most stringent quality control check for correct thread size. The safety board determined that the nut stripped off when the loose torque links allowed excessive vibration. Additionally, the undersize threads on the apex bolt allowed the nut to shear away the bolt pin designed to retain the nut.
Probable cause
The Safety Board believed that the nut stripped off when the loose torque links allowed excessive vibration, and the undersize apex bolt threads allowed the nut to shear away the bolt pin designed to retain the nut.
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