Accident Overview
On June 1, 2023, at about 1330 eastern daylight time, a Piper PA-28-181 airplane, registration N4376H, was involved in an accident near Pellston, Michigan. The airplane was being operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The flight instructor and student pilot were not injured. The aircraft sustained substantial damage.
Flight and Incident Sequence
The flight instructor reported that he and the student pilot had completed two touch-and-go landings. During the takeoff after the second landing, they heard an unusual thumping sound or bang. Initially suspecting an engine issue, they planned a precautionary landing back to the runway. While on approach, the airport manager observed that the left main landing gear wheel assembly was hanging by the brake line from the landing gear strut and advised the flight instructor via radio not to land. The flight instructor then diverted to another airport due to the availability of increased airport fire and rescue services.
Landing and Damage
The airplane landed on the runway centerline with the partially separated left main landing gear assembly. The landing resulted in substantial damage to the left wing.
Postaccident Examination
Examination revealed that the landing gear strut housing clevis was fractured at the upper torque link connection. The wheel assembly remained attached via the brake line hose. The airplane and strut housing were retained for further examination.
The National Transportation Safety Board’s Materials Laboratory examined the remnants of the landing gear strut, including a sectioned portion of the cylinder and the torque link assembly arm attached to the broken cylinder clevis via the attachment bolt. The chemical composition of the fractured cylinder was consistent with A356 aluminum-silicon casting alloy. The cylinder side fracture surfaces exhibited crack arrest marks consistent with progressive fracture, and ratchet marks indicative of multiple crack initiation sites. The crack features were consistent with initiation along the interior surface between the clevises, which was machined smoother than the exterior surfaces. Outside of the cracks, the fracture surface exhibited dimpled rupture, consistent with subsequent overstress fracture.
Several initiation sites were examined along the radius with the machined surface. One initiation site exhibited a flat, faceted region from which river patterns and striations emanated. Closer examination revealed layered hackles typical of localized brittle fracture and consistent with a silicon-rich phase, typical in many aluminum-silicon castings.
A smooth surface feature adjacent to the fracture edge exhibited a spherical, dendritic morphology consisting of a solidification void from the casting of the part. Fatigue striations were found propagating from the void rather than from the fracture edge. The features were consistent with crack initiation from the solidification void. Another solidification void, exhibiting smooth, rounded dendritic shapes along the fracture surface edge, contained a secondary crack running through the feature. Fatigue striations were found propagating parallel to the edge, consistent with initiation along the secondary crack from the solidification void.
Strut Failure Mechanism
The strut assembly fractured from fatigue cracks that had initiated along the radius of the clevis arms within the inner machined surface. These cracks initiated at multiple casting solidification voids and silicon-rich phases. The cracks propagated through the clevis cross-sections until the remaining cross-sections fractured from overstress.