What happened
On June 26, 2016, at Sion Airport (LSGS), a Schempp-Hirth Nimbus-3 glider, registered HB-1638, was involved in a takeoff accident during a glider tow operation. The aircraft was positioned to the right of the grass runway to maintain a required 75-meter clearance from the paved runway centerline. As the towplane, a Robin DR400, began the takeoff roll, a wing walker released the right wingtip. Immediately following this release, the glider's right wing dipped and struck the ground.
The pilot attempted to correct the deviation with rudder inputs, causing the glider to weave in an S-shaped pattern behind the towplane. During this period of instability, the left wingtip also made contact with the ground. This contact triggered a sudden rotation and pitch-up maneuver at an angle of approximately 45 degrees. The tension became so great that the tow cable snapped. The glider subsequently overturned onto its back on the paved runway. The pilot sustained one serious injury, including a deep forehead laceration and anterograde amnesia, but survived due to being properly restrained.
The investigation
Investigators examined the aircraft's technical condition, the environmental factors, and the operational procedures in place at Sion Airport. The investigation utilized video footage from the airport and statements from the pilot and the towplane pilot. The inquiry focused on the stability of the glider during the initial ground roll and the specific takeoff alignment procedures used at the aerodrome.
Findings
Several contributing factors led to the loss of control. The investigation established that the water ballast in the wing tanks was not distributed evenly, which compromised the glider's longitudinal stability. Additionally, the pilot was holding the control stick slightly to the right, which, combined with a 15-knot wind with 25-knot gusts, caused the wing to drop immediately upon the wing walker's release.
Because the glider was being towed via a center-of-gravity (CG) hook rather than a nose hook, the pilot lacked the stabilizing aerodynamic moment provided by a nose-tow configuration, making attitude control significantly more difficult. Furthermore, the investigation identified that the standard takeoff procedure at Sion Airport, which requires the glider to be offset from the grass runway centerline, created a systemic risk by forcing the towplane to converge toward the centerline, complicating the initial alignment.
Safety action
Following the investigation, the SUST issued a safety notice stating that aerodrome operational procedures for glider operations should be updated. The goal is to allow gliders to position themselves directly on the runway centerline for takeoff to avoid the risks associated with the previous offset alignment procedure.