CEN23LA319

After completing a cross-country flight, the pilot set up his final glide at 79 knots (kts), the top of the normal operating airspeed range, to descend from 6,000 ft. About 4,000 ft, the pilot experienced a severe vibration and shaking of the glider. The pilot stated that the glider did not sustain a bird strike or encounter severe turbulence. The pilot deployed the spoilers to slow the glider for possible elevator flutter and the vibration stopped when the glider reached about 60 kts. The pilot verified glider control and slowly increased the airspeed, at which time the vibration returned. The pilot slowed the glider again and proceeded to land. The straight-in landing was uneventful, and the glider rolled to a stop. After exiting the glider, the pilot noticed the composite tail boom structure was cracked in multiple locations. A materials examination of the tail boom structure revealed a damage pattern in the aft portion of the tail boom that contained multiple cracks orientated at ±45° relative to the transverse plane, consistent with torsion loading in both clockwise and counterclockwise rotation directions. Thus, a flutter event likely produced high alternating torsion loads on the fuselage and subsequent damage. The examination found no clear evidence of preexisting damage or weakness that would have led to reduced stiffness of the fuselage structure. The glider sustained a vibration event consistent with flutter while operating below the design maneuvering airspeed. The vibrations dissipated as the glider slowed, which also was consistent with flutter. Flutter could occur due to operation outside of the design flight envelope, loose flight controls, changes in control surface mass, or reduced stiffness in the control panel, wing, stabilizer, or fuselage. Based on the available evidence, the reason for the flutter event could not be determined.

On July 22, 2023, about 1600 central daylight time, an Alisport Srl Silent 2 Electro glider, N64JG, sustained substantial damage when it was involved in an accident near Wallis, Texas. The pilot was not injured. The glider was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. According to the pilot, after completing a cross-country flight, he set up his final glide at 79 kts to descend from 6,000 ft. About 4,000 ft while in straight-and-level flight, he experienced a severe vibration and shaking of the glider. The pilot stated that the glider did not sustain a bird strike or encounter severe turbulence. The pilot slowed the glider, fearing elevator flutter, and deployed the spoilers to slow down. Upon reaching an airspeed around 60 kts, the vibration stopped. The pilot verified glider control and slowly increased the airspeed, at which time the vibration returned. About 2 miles from his gliderport, the pilot again slowed the glider and proceeded for a straight-in landing. The landing was uneventful, and the gilder rolled to a stop. After exiting the glider, the pilot noticed the composite tail boom structure was cracked in multiple locations (see Figure 1). Figure 1. Tail boom structure cracks The damaged tail boom structure was sectioned from the glider and sent to the National Transportation Safety Board (NTSB) Materials Laboratory for examination. According to the NTSB Materials Laboratory, the fuselage skin was made from fiber-reinforced polymer composite materials with both glass fiber and carbon fiber fabric reinforcement. The fuselage skin was constructed of two halves (left and right) that were spliced together at the upper and lower sides. At each splice location, the skin layers butted together, and strips of glass-fiber reinforced fabric were bonded to the interior surfaces to form single-strap butt joints. Skin cracks were located about 20 inches forward of the vertical stabilizer and were oriented about 45° relative to the transverse plane on both the left and right side. Additionally, longitudinal cracks were observed on the upper and lower sides of the skin. The 45° cracks intersected the longitudinal crack at the upper side of the skin, and on the lower side the 45° cracks were separated from the longitudinal crack. A portion of the upper skin containing the longitudinal crack, and a section of the upper splice was cut from the remainder of the fuselage skin. On the interior side, a portion of the strap had a lighter appearance, and the change in color was consistent with a disbond between the strap and the skin. Scanning electron microscope images showed all fibers were observed in bundles, consistent with fabric construction. No evidence of glass fiber roving was observed, including the resin pocket between the strap and the skin. According to the glider flight manual, the normal operating airspeed range is between 40 – 81 kts, the design maneuvering airspeed is 81 kts, and the never exceed airspeed instill air is 118 kts. Availability of information about the construction and various test and maintenance procedures was limited due to the glider manufacturer and subsequent companies no longer being in operation.