...Glider’s Encounter With Flutter During Cruise Flight That Resulted In Structural Damage To The Fuselage

Location: Giddings, Texas Accident Number: CEN23LA405
Date & Time: September 7, 2023, 16:25 Local Registration: N321AD
Aircraft: Alisport SRL Silent 2 Electro Aircraft Damage: Substantial
Defining Event: Aircraft structural failure Injuries: 1 None
Flight Conducted Under: Part 91: General aviation - Personal

Analysis: The pilot stated that the glider was in cruise flight at approximately 81 kts and 4,800 ft when the cockpit began to shake violently and noisily side-to-side. When the glider slowed to about 60 kts, the shaking ceased. The pilot landed the glider without further incident. The side-to-side shaking described by the pilot was consistent with a vibration event associated with flutter in straight and level flight. The vibrations dissipated as the airplane slowed, also consistent with flutter. The damage pattern in the aft portion of the fuselage showed multiple cracks oriented at ±45° relative to the horizontal plane, which was consistent with damage from torsion loading in both clockwise and counterclockwise rotation directions. 

Thus, the flutter likely produced high alternating torsion loads on the fuselage. The observed damage likely resulted from these high alternating torsion loads. Flutter could occur due to operation outside the design flight envelope, loose flight controls, changes in control surface mass, or reduced stiffness in control panel, wing, stabilizer, or fuselage structure. As discussed in more detail below, the examination found no clear evidence of preexisting damage or weakness that could have led to reduced stiffness of the fuselage structure.

The glider manufacturer was no longer in business and information received by the investigative staff was limited. The glider had unbalanced horizonal and vertical control surfaces. The ailerons were balanced. A verification of proper flight control cable rigging, elevator trim tab free play, static balance of the control surfaces was not performed due to the lack of available instructions for continued airworthiness.

Probable Cause and Findings: The National Transportation Safety Board determines the probable cause(s) of this accident to be -- The glider’s encounter with flutter during cruise flight that resulted in structural damage to the fuselage.

FMI: www.ntsb.gov