NTSB Identification: LAX95LA331.
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Accident occurred Sunday, September 10, 1995 in YUMA, AZ
Probable Cause Approval Date: 07/03/1996
Aircraft: Bell 47G-3B2, registration: N7942J
Injuries: 1 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

The pilot said he was en route to spray a field at 150 feet agl when he heard a loud bang, and a fluttering vibration began through the airframe. The helicopter did not respond to anti-torque control inputs and began to spin. The pilot lowered the collective and reduced throttle to control the spin and the helicopter landed hard. The aft left engine mount frame was found separated from the fuselage frame attachment clevis ears. The clevis ears were found fractured. The engine was displaced forward and to the right and was impinging on the collective servo. The tail rotor short shaft was pulled out and disconnected. The components were sent for metallurgical examination. The clevis ear fractures revealed features indicative of fatigue cracking emanating from multiple origins on the forward and aft faces. The fractures exhibited severe oxidation damage with rust colored deposits evident externally. The metal in the frame and clevis ears was correct for the material specification in the manufacturing drawings. During assembly a washer is welded circumferentially around the outside diameter to the inboard side of each clevis ear. Microhardness testing of the welds, heat affected zones (HAZ), and the base metal of the clevis ears and corresponding washers revealed that the tensile strength of the HAZ adjacent to the fracture of the forward clevis ear was significantly higher than the strength of the weld and the base metal. An annual inspection was accomplished on the helicopter 77 hours prior to the accident and the area of the cracks are the subject of a daily inspection requirement.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

the failure of the pilot and company maintenance personnel to detect a crack in the clevis ears during the required daily inspection and the recent annual inspection. A factor in the accident was the improper welding and stress relief procedure used to assemble the components, which induced high residual stresses in the clevis ears and led to fatigue cracking of the part.

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