WPR23LA270

The accident occurred during takeoff on an instructional flight. The instructor reported that before reaching rotation speed, the airplane felt as if it were sinking to the left as if they had a flat tire. The flight instructor took over control of the airplane, reduced engine power to idle, and applied the hand brake. The airplane then became uncontrollable and veered to the right. He applied corrective control inputs, but the airplane departed the runway, struck an embankment, and sustained substantial damage. The instructor reported that the inbound landing was uneventful, and the data recorded by the airplane’s primary flight display corroborated this. Examination revealed that the left main landing gear leg, which was comprised of composite glass fiber layups, was cracked in multiple areas along its length and had delaminated. Laboratory examination revealed the cracks occurred within the center bulk of fibers parallel to the lamina. Shiny areas consistent with voids and disbonded material, indicative of a manufacturing anomaly, were spread throughout the fracture surfaces. The voids would have affected the structural integrity and allowed cracking to be initiated more easily within the strut. Entrapped contaminants were also found within the strut matrix and in several locations on the fracture surfaces, which was further evidence of cleanliness issues during manufacturing. Flattened features were also present on the fracture surfaces and were likely caused by repeated contact between the two halves of the strut as the crack progressed, indicating the crack had been present for some time. Similarly, the presence of dirt/debris on the interior surfaces was likely the result of the gap between the fracture surfaces opening during aircraft operation and allowing dirt/debris to enter. Consequently, the flattened features and embedded dirt/debris are consistent with a pre-existing delamination in the strut before the accident flight. Both the instructor and the student performed preflight procedures, which included checking the landing gear legs for cracks. Both reported that they did not see any damage or anomalies. It could not be determined why they did not see the cracks, but it is possible that the fractures were imperceptible during a preflight examination, when the airplane was not loaded to an extent that the cracks would open and be visible. A service bulletin had been issued about 10 years before the accident warning of similar cracks in the landing gear caused by an apparent manufacturing defect. However, this bulletin was not applicable to the accident airplane based on its year of manufacture.

On July 11, 2023, about 1055 Pacific daylight time, a Sling Aircraft, Sling LSA, N294JY, was substantially damaged when it was involved in an accident in Palm Springs, California. The flight instructor and student pilot were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The flight instructor stated that the student pilot performed an uneventful landing at Palm Springs International Airport (PSP), after which both occupants exited the airplane and went to the local fixed base operator (FBO) for a short break. They then both independently performed a preflight check, and with no anomalies or damage noted, proceeded to board the airplane. The taxi to the active runway was uneventful, and the winds were light. After lining up on the runway centerline, the student pilot applied full engine power. Both occupants reported that shortly before the airplane reached rotation speed, the airplane felt as if it was sinking or “buckling” onto its left side. Both stated that the sensation was very unusual, and that the airplane did not have a left turning tendency nor did it feel like a typical wind gust encounter. The flight instructor initially thought that a tire had gone flat, so he took control of the airplane, reduced engine power to idle, and applied the hand-controlled brake lever. He stated that the airplane rapidly became uncontrollable and veered to the right. He applied left rudder, but the airplane continued off the runway surface and over an embankment, sustaining substantial damage to the horizontal stabilizer and left aileron. The airplane was equipped with a Garmin G3X (GDU 460) primary flight display that was configured to record (at one second intervals) aircraft parameters including acceleration. Review of the inbound landing indicated that the approach was stabilized, with the airplane traveling at a true airspeed of 71 knots (KTAS) as it crossed the threshold, and 50 KTAS as it touched down about 1,500 ft down the 4,953 ft runway. About the time of touchdown, the “Normal Acceleration” parameter indicated a change from 0.956g to 1.183g, then to 0.88g over a 3-second period. The airplane continued on a straight track along the runway, while progressively decelerating to about 30 knots. The airplane then exited from the final taxiway and taxied to the FBO. The airplane was a conventional low-wing design, constructed primarily of aluminum. It included tricycle landing gear with a steerable nose wheel and main landing gear that used a single continuous composite spring/strut leg. Postaccident examination revealed multiple longitudinal cracks along the length of the left side of the strut (figure 1). The assembly was removed and examined at the NTSB Materials Laboratory. Figure 1 - Left landing gear leg with cracks The strut was comprised of layers of glass fiber weave, consisting of a center bulk of fibers oriented in the lateral axis, bracketed by cross-biased layers at the outersurfaces. A brake line slot was routed into the aft side of each leg, with holes drilled into each end to accommodate the wheel axle. The cracks on the left leg appeared to occur within the center bulk of the fibers parallel to the lamina. Six well defined cracks were observed on the lower forward face of the leg, with five observed on the rear face. One of the center cracks extended the full length of the leg, resulting in the leg delaminating into two complete sections. A single, less well-defined crack was noted on the lower end of the right leg. The end of the left strut was cut away for examination. Disassembly revealed flattened and shiny void-like features along the primary crack, along with sections of flattened features with dark embedded dirt and debris. Additionally, entrapped contaminants were also observed in several locations within the matrix weave, including a small blue speck and red fibers. The airplane was manufactured in August 2022 and a stamp on the strut revealed that it was manufactured on February 17 of the same year. The airplane was primarily used for flight instruction, and at the time of the accident had accrued 633 hours of flight time. Service Bulletin 1, issued by Sling Aircraft on April 8, 2014, warned of longitudinal cracks appearing in the main landing gear for legs manufactured between August 2013 and April 2014. The service bulletin described the cracks being caused by a deviation in the glass fiber layup. The bulletin urged the replacement of those legs should cracking be identified. Representatives from Sling Aircraft stated that they have had no reports of similar cracks occurring in landing gear since the accident. Additionally, they have since upgraded their composite manufacturing facility due to their expanded use of composites in other aircraft, and as part of this process they have enhanced their quality control procedures. The preflight inspection section of the airplane’s pilot operating handbook, current at the time of the accident, provided an instruction to check the condition of the wheel struts for condition/cracks.