Amateur-built SILENT 2 ELECTRO accident near Wallis, Texas, July 22, 2023

The NTSB's narrative final · quoted from the NTSB record

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.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.