Primary finding
Probable cause
fatigue failure of the right main landing gear strut.
Investigator assessment
Analysis narrative
The certificated private pilot was landing on a gravel area of beach next to a river. During the landing roll, the right main wheel assembly broke. The landing gear strut began dragging on the ground and the airplane nosed over. Examination of the gear strut revealed that it was broken along a horizontal plane, perpendicular to the outside vertical surface of the strut. A fracture had progressed through the upper mounting bolt holes that attach the axle and brake mounting plate to the strut. The surface of the fracture exhibited a dark brown/red appearance and ridges or stop marks radiating from the lower inside edge of the aft mounting bolt hole. These stop marks were similar in appearance to progressive beach marks, indicative of fatigue. The remaining fracture surface, at the inboard edge of the strut and between the bolt holes, exhibited a dull gray appearance and a shear lip oriented on about a 45 degree plane from the lower surface. The remaining fracture surface areas on both sides (forward and aft) of the bolt holes were also gray in appearance with small areas of red rust and corrosion.
Source record
Factual narrative
On April 30, 1996, about 1250 Alaska daylight time, a wheel equipped Cessna 170B, N2760D, crashed during landing in a remote area, about 4 miles southwest of Togiak, Alaska. The airplane was being operated as a visual flight rules (VFR) business flight under Title 14 CFR Part 91 when the accident occurred. The airplane, registered to and operated by the pilot, sustained substantial damage. The certificated private pilot and the sole passenger received minor injuries. Visual meteorological conditions prevailed. The flight originated at the Dillingham airport, Dillingham, Alaska, about 1135. A Federal Aviation Administration (FAA) airworthiness inspector, Anchorage Flight Standards District Office (FSDO), reported that the airplane was being utilized in support of the fishing vessel "Ultimate", also owned by the pilot. The pilot was landing on a gravel area of beach next to the Quigmy River. During the landing roll, the right main wheel assembly broke. The landing gear strut began dragging on the ground and the airplane nosed over. The lower end of the right main gear and axle assembly was examined by the National Transportation Safety Board (NTSB) investigator-in-charge (IIC). The lower end of the gear strut was broken along a horizontal plane, perpendicular to the outside vertical surface of the strut. It progressed through the upper mounting bolt holes that attach the axle and brake mounting plate to the strut. Examination of the lower fracture surface with a 10X and 25X magnifier revealed a dark brown/red surface about 13/16 wide and 9/16 high located between the upper mounting bolt holes. The surface also exhibited areas of rust and corrosion. The surface of the fracture exhibited ridges or stop marks radiating from the lower inside edge of the aft mounting bolt hole. These stop marks were similar in appearance to progressive beach marks. The remaining fracture surface, about 2/16 wide at the inboard edge of the strut and between the bolt holes, exhibited a dull gray appearance and a shear lip oriented on about a 45 degree plane from the lower surface. The remaining fracture surface areas on both sides (forward and aft) of the bolt holes were also gray in appearance with small areas of red rust and corrosion. -