On November 10, 2005, a Bell 47D1 (registration N16356) was involved in an aviation accident near Rancho Murieta, CA. Investigators recorded the probable cause as: The pitch change link bolt failed due to reverse bending fatigue caused by improper preloading. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
During cruise, a pilot experienced vibration in anti-torque pedals and attempted a precautionary landing. The tail rotor assembly separated, causing the helicopter to rotate and land hard, with subsequent bounce and secondary impact.
Incident Details
A helicopter experienced an in-flight separation of its tail rotor assembly during cruise flight. The pilot reported feeling a vibration in the anti-torque pedals and decided to perform a precautionary landing in an open field. Before reaching the intended landing spot, the tail rotor assembly separated from the helicopter. The aircraft then began to rotate and subsequently landed hard. Upon initial ground contact, the helicopter bounced back into the air, struck the ground a second time, and the transmission was bent over the cockpit area.
Examination Findings
Post-accident examination of the tail rotor assembly revealed that one of the pitch change link bolts had separated. The shank portion of the bolt was broken from its threaded portion. A metallurgical examination of the bolt showed that the fracture surface of the shank had been subjected to reverse bending fatigue. The fracture exhibited symmetrical arced lines on each side of the shank, culminating in a thin, dark, hourglass-shaped region at the center of the two arced lines. Burnishing observed on the fracture surface is typical of failures in bolts that have not been properly preloaded.
Probable cause
The pitch change link bolt failed due to reverse bending fatigue caused by improper preloading.