What happened
On March 2, 2021, a Piper PA-38-112 performing a local flight instruction mission experienced a landing gear failure at the San Lorenzo aerodrome in Santa Fe, Argentina. During the third landing of the flight, the left main landing gear wheel detached from the aircraft upon touchdown. This caused the left spring tine to sink into the ground, forcing the aircraft to veer left and exit the runway on the left side of runway 09. The impact resulted in the sudden stoppage of the engine, damage to the nose gear, and structural deformation of the fuselage near the wing junction. The two occupants, an instructor and a student pilot, were uninjured.
The investigation
The Argentine Transportation Safety Board (JST) examined the wreckage and the components of the landing gear assembly. Investigators focused on the bolts securing the wheel axle to the spring tine. Laboratory analysis of the four bolts used in the assembly revealed significant discrepancies in material composition and structural integrity. While one bolt was identified as high-quality AISI 8740 steel, the other three were composed of a different, inferior steel (AISI 4037) and exhibited various microscopic and surface defects.
Furthermore, the investigation compared these components to a new AN4 bolt taken from the flight school's maintenance workshop. The laboratory confirmed that the new bolt was a counterfeit part, as its chemical composition and microstructure differed from the required standards and contained manufacturing-induced flaws.
Findings
- The detachment of the left main landing gear wheel was caused by the failure of the bolts securing the axle to the spring tine.
- One bolt failed due to fatigue, while the remaining two experienced brittle and ductile fractures.
- The use of unapproved parts was a primary factor, as the bolts used in the aircraft were found to be counterfeit and did not meet the required aeronautical standards.
- The fatigue failure of the first bolt occurred under landing loads, which subsequently created an asymmetrical load distribution that caused the remaining substandard bolts to fail.