Incident Description
A pilot was en route to spray a field at an altitude of 150 feet above ground level when a loud bang was heard, followed by a fluttering vibration throughout 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 manage the spin, resulting in a hard landing.
Examination Findings
Post-accident inspection revealed that the aft left engine mount frame had separated from the fuselage frame attachment clevis ears. The clevis ears were found fractured. The engine had displaced forward and to the right, impinging on the collective servo. The tail rotor short shaft was pulled out and disconnected.
Metallurgical examination of the clevis ear fractures showed 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 conformed to 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 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.
Inspection History
An annual inspection had been accomplished on the helicopter 77 hours prior to the accident. The area where cracks were found is subject to a daily inspection requirement.