ANC18LA018

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.

On January 1, 2018, about 1725 eastern standard time, a Costruzioni Aeronautiche Tecnam P2006T airplane, N594MA, sustained substantial damage when it was involved in an accident near Lynchburg, Virginia. The flight instructor and the pilot receiving instruction were not injured. The airplane was being operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The instructor reported that the flight was an instructional, cross-country flight with the pilot receiving instruction as the pilot flying. At the conclusion of the flight, the pilot receiving instruction, maintained about a 500-ft per minute descent on final, and verified with the instructor that the airspeed was 70 knots. Immediately after touchdown, the left main landing gear (MLG) assembly separated from the airplane. Subsequently, the airplane skidded for about 100 ft, departed the runway to the left, and then came to rest on grass as shown below in figure 1. The airplane sustained substantial damage to the fuselage. Figure 1. A photograph showing the airplane on grass left of the runway after it came to rest (courtesy of the Federal Aviation Administration) The instructor reported that no wind gusts existed during the approach and landing, that no side loading occurred at touchdown, and that, given his experience, he did not feel the landing would have caused any damage. The Airplane Maintenance Manual (AMM) required that the MLG be replaced every 4,000 landings. The review of the records revealed that several of the previous airplane owners and operators had not tracked the number of the airplane’s landings. After the owner bought the airplane, he worked with the FAA to determine its estimated number of landings and whether it was in compliance with the MLG AMM replacement requirements. At the time of the accident, the estimated number of the airplane’s landings had still not been determined; maintenance records revealed that the MLG had not been replaced at any time. The separated left MLG assembly and the intact right MLG assembly were recovered and transported to the NTSB Materials Laboratory for examination. Figure 2 The outer area of the arm’s fracture surface exhibited a slanted morphology and rough surface texture, consistent with a fracture through the weld filler. The inner area of the arm’s fracture surface was shinier and flatter, consistent with the area not having been welded. Most of the flange’s exterior had not been welded. The flange’s upper area exhibited fracture features consistent with progressive cracking that propagated from the interior unwelded area outward through the weld filler. A complete copy of the NTSB materials laboratory report is included in the public docket for this accident.