Incident Overview
On August 18, 2017, at approximately 1145 central daylight time, a Piper PA-18-150 airplane, registration N82912, sustained substantial damage while landing at Avenger Field Airport (SWW) in Sweetwater, Texas. The commercial pilot and his passenger were not injured. The aircraft was registered to and operated by the United States Department of Agriculture (USDA) as a public aircraft.
Pilot Actions and Environmental Conditions
The pilot reported choosing to land on runway 17 because the automated weather observing system indicated a surface wind of 9 knots from the south-southwest. The pilot intended to perform a wheel landing with the flaps fully extended.
Sequence of Events
Shortly after the airplane touched down on the right main landing gear (MLG), the pilot felt a "little bump" and the airplane began to swerve left. Despite applying full right rudder and right brake inputs, the pilot was unable to regain directional control. The airplane continued swerving left and departed the left runway edge, where the right MLG collapsed after entering soft terrain. The airplane came to rest on a north heading.
Damage Inspection
The airplane sustained substantial damage to the right wing, both main wing spars, and the right elevator. A Federal Aviation Administration inspector examined the wreckage and identified several fractured MLG components, which were subsequently sent to the National Transportation Safety Board Materials Laboratory for further examination.
Component Examination Findings
The left-side MLG hydrasorb shock unit was intact, with its external shock cord rings wrapped around its tee flanges. The left MLG brace tube was bent and fractured near its attachment point to the left hydrasorb shock unit. Adjacent to the fracture, the brace tube exhibited local thinning or necking. The fracture surface showed a cup-shaped morphology or a flat orientation angled at about 45° relative to the tube direction. These features and tube damage were consistent with tensile overstress. No evidence of preexisting cracks or damage was found on the left MLG brace tube or its hydrasorb shock unit.
The right-side MLG brace tube had buckled and was partially fractured about 5.75 inches from the fitting hole. The fracture surface had a rough texture with a dull luster, consistent with tensile overstress. The opposite end of the right MLG brace tube had buckled inward toward the partial fracture. This damage was consistent with bending overstress.
The right-side MLG hydrasorb shock unit exhibited a fracture of its internal piston shaft. The piston shaft had bent downward adjacent to the fracture surface. One side of the fracture showed a generally flat surface on one half before transitioning to a 45° shear lip. Scanning electron microscope examination revealed dimple rupture in a slanted orientation. These features and damage were consistent with bending overstress. No evidence of preexisting cracks or damage was found on the internal piston shaft. The chemical composition, microstructure, and measured material hardness were consistent with a martensitic precipitation-hardened stainless-steel alloy.