YUMA, AZ, USA
N7942J
Bell 47G-3B2
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.
On September 10, 1995, at 0920 hours mountain standard time, a Bell 47G-3B2 helicopter, N7942J, rolled over during a hard forced landing near Yuma, Arizona. The forced landing was precipitated by a pilot reported loss of tail rotor authority and drive train power. The helicopter was owned and operated by Sundown Helicopters, Inc., of Yuma, Arizona, and was on an aerial application flight under 14 CFR Part 137 of the Federal Aviation Regulations. Visual meteorological conditions prevailed and included calm wind conditions. The helicopter sustained substantial damage. The certificated commercial pilot, the sole occupant, was not injured. The flight originated at Yuma on the day of the accident about 0850. In a telephone interview, the pilot said he was en route to a field to be sprayed at 150 feet above ground level 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. Postaccident examination of the helicopter by an FAA airworthiness inspector from the Scottsdale, Arizona, Flight Standards District Office revealed that the aft left engine mount frame was 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 removed from the helicopter and sent to the Safety Board's Materials Laboratory for metallurgical examination. The complete report of the examination is appended to this report. Detailed examination of 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. Stereo microscope examination of paint chips adjacent to the fracture disclosed at least six layers of paint of various colors to a thickness about 0.1 inches. EDS analysis of the metal in the frame and clevis ears yielded results typical for the material specification called out in the manufacturing drawings. According to Bell Helicopter, during assembly a washer is welded circumferentially around the outside diameter to the inboard side of each clevis ear. During the metallurgical examination, microhardness testing was performed on the welds, heat affected zones (HAZ), and the base metal of the clevis ears and corresponding washers. The tensile strength of the HAZ adjacent to the fracture of the forward clevis ear was 100 KSI higher than the strength of the weld, and 121 KSI higher than the tensile strength of the base metal. The reporting FAA inspector stated that an annual inspection was accomplished on the helicopter June 20, 1995, about 77 hours prior to the accident. The fractured frame assembly has about 1,700 hours total time in service. Documentation was provided by Bell Helicopter which states that the "crack tendency is a well known occurrence most common to wide cabin model 47G series helicopters, and specific to the left side engine mount area of the [fuselage] centerframe." The company further reported that "any crack that develops should be quickly detected as a function of the daily inspection...[and]...the existence of a crack is normally indicated by local area discoloration from the resultant iron oxide stain."
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.
Source: NTSB Aviation Accident Database
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