Accident Details
Probable Cause and Findings
The failure of the helicopter’s center frame tube due to a fatigue crack and corrosion originating from a welded surface. Contributing to the accident was maintenance personnel’s inadequate inspection of the helicopter during its most recent 100-hour inspection.
Aircraft Information
Registered Owner (Current)
Analysis
On September 11, 2013 about 0715 Pacific daylight time, a Bell 47G5 helicopter, N7836S, sustained substantial damage during a precautionary landing about 6 miles southeast of King City, California. The pilot, the sole occupant, was not injured. The helicopter's lower center frame and tailboom sustained substantial damage. The helicopter was registered to and operated by Wilbur-Ellis Corporation as an agricultural spray flight under the provisions of 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed for the flight and no flight plan was filed.
The pilot reported that he was spraying a field about 3-4 feet above the ground when he heard a loud bang. The pilot elected to make a precautionary landing; during the landing flare, the helicopter started to spin. The pilot lowered the collective and the helicopter landed hard onto a road; subsequently, the helicopter's skids spread, and the lower center frame and tailboom were bent.
Postaccident examination by a Federal Aviation Administration (FAA) inspector revealed that one of the helicopter's center frame tubes, located just aft of the cabin, had fracture separated. A senior metallurgist at the National Transportation Safety Board (NTSB) materials laboratory reviewed photographs of the fractured surfaces and reported that a fatigue fracture initiated in the tube at the root of a fillet weld associated with a reinforcing filet welded axially along the tube. The fatigue crack propagated through the thickness of the tube and circumferentially around the tube from both sides of the weld root. Darker corrosion product on the fracture face is indicative of a pre-existing fatigue crack that had been exposed to atmospheric elements. The fracture in the tube occurred in single-sided bending consistent with inflight airframe loads.
On August 8, 2013 the helicopter underwent a 100 hour inspection. According to manufacturer's guidance, the 100 hour inspection requires in part: to "Inspect all structural tubing and fittings for cracks, cuts, bends, corrosion, distortion and damage."
Data Source
Data provided by the National Transportation Safety Board (NTSB). For more information on this event, visit the NTSB Records Search website. NTSB# WPR13LA404