Accident Description
The airplane experienced a ground loop during landing, resulting in the fracture and separation of the left main landing gear strut. The pilot informed a Federal Aviation Administration (FAA) inspector that the landing gear strut collapsed unexpectedly and without warning during the landing roll, causing the airplane to ground loop to the right. The airplane was relatively light at the time, with no chemical load aboard and approximately 1,300 pounds of fuel. The inspector noted that the landing was not subjected to an abnormal load.
Damage and Examination
The left landing gear strut, a single spring steel leaf, separated in the shoulder area where the strut enters the saddle at the fuselage skin line. The left landing gear wheel was broken in the outboard flange area, and the tire was deflated, exhibiting deep radial scratch marks on the outboard side wall. The left wing sustained structural damage: the aileron hinge bracket at the wing tip was bent inboard, and striations on the left wing lower surface were oriented spanwise. The outer half-span of the left wing was bent upward, accompanied by compression wrinkling of the upper wing skin.
Metallurgical Analysis
The FAA inspector reported that the left landing gear leaf broke inside the saddle, with rust and beach marks in the fracture face indicative of a pre-existing crack. A metallurgical laboratory examined the fracture and determined that the strut met the manufacturer's chemical, hardness, and materials specifications. The fracture exhibited two distinct modes: a fatigue crack measuring 0.08 inches in depth and 0.37 inches in width, with multiple initiation sites along the top surface in an area that had fretting damage. The remainder of the fracture face displayed gross overload features. The laboratory report concluded that the transverse failure of the strut resulted from an overload condition, with high side and drag loads well in excess of the design ultimate load for the component.