ANC18LA018 · Costruzioni Aeronautiche Tecna P2006 T

The flight instructor reported that he and the pilot receiving instruction were conducting an instructional, cross-county flight with the pilot receiving instruction as the pilot flying. Immediately after touchdown at the conclusion of the flight, the left main landing gear (MLG) assembly separated from the airplane. Subsequently, the airplane skidded about 100 ft, departed the runway, and came to rest on grass. The airplane sustained substantial damage to the fuselage. Examination of the left MLG assembly revealed that it had failed due to a fatigue fracture along the inside circumferential weld on the trailing arm’s attachment flange. The fatigue cracking had progressed toward the trailing arm tube’s outer surface from the inside corner of the weld, where it had likely initiated. Only half of the tube section exhibited weld features, indicating that the weld only occurred on the outside. The remaining unwelded area left a gap, creating a stress concentration that likely initiated multiple fatigue cracks along the inside corner of the weld. If both sides of the tube had been welded, the tube assembly would have had a thicker cross-section and no gap, which would have reduced local stress concentrations and the likelihood of fatigue cracks initiating and increased the propagation time if any fatigue cracks had initiated. The fatigue cracking features were consistent with low-stress, high-cycle fatigue crack propagation. The fatigue cracking exhibited multiple crack initiation sites. All the fracture surfaces exhibited rust, consistent with postfracture oxidation due to exposure to water and moist air; therefore, it is likely that the fatigue cracking had been present for a long time The fatigue cracks on the interior of the trailing arm assembly would have made seeing the crack during inspection of the fillet weld difficult, likely requiring complete disassembly of the MLG. Further, the gap from the unwelded side of the joint could have been easily mistaken for a false crack indication, complicating part inspection. The Airplane Maintenance Manual required that the MLG be replaced every 4,000 landings. However, the Airplane Flight Manual did not contain guidance for owners or operators to track the number of landings. A review of the airplane's maintenance records revealed that several previous airplane owners and operators had not tracked the number of the airplane’s landings.

Verbatim NTSB analysis and probable cause from the NTSB dataset