What aircraft was involved and where did it happen?

The accident on 1967-06-16 involved a SCHEMPP-HIRTH KG SHK-1, registration HB-864, at Moudon, VD, CH.

What was the probable cause of the accident?

The accident was caused by a sudden, aggressive pilot input on the elevator and rudder during the landing flare, which triggered an aerodynamic roll and inversion due to the inherent stability limitations of the SHK-1's V-tail configuration.

Glider inversion during landing approach near Moudon

A Schempp-Hirth SHK-1 glider inverted and struck the ground during a landing approach after the pilot deployed spoilers and a braking parachute.

What happened

On June 16, 1967, a SHK-1 glider, registration HB-864, was participating in a national gliding championship when the pilot was forced to terminate a cross-country flight near Moudoden, Switzerland. Due to fading thermals, the pilot opted to land in a mown field near the Broye river.

During the final approach, the pilot deployed the spoilers and the braking parachute, maintaining a speed of approximately 120 km/h at a 20-degree glide angle. As the aircraft reached roughly 10 meters above the ground, the pilot attempted to flare for landing. During this maneuver, the pilot experienced a significant reduction in elevator effectiveness and responded with a sudden pull on the control stick. This action, combined with rudder input, caused the aircraft to yaw left and enter a left-hand half-roll. The left wing struck the ground, and the glider ultimately came to rest on its back.

The investigation

The investigation examined the aircraft's configuration, the pilot's experience, and the aerodynamic characteristics of the SHK-1 design. The aircraft was found to be in good airworthiness condition, with the center of gravity within prescribed limits. Flight tests conducted by the Federal Office of Civil Aviation (OFA) were performed to replicate the accident conditions.

These tests confirmed that while slow control inputs resulted in stable flight, a rapid, full pull on the control stick—when accompanied by even slight rudder deflection—triggered a sudden roll and pitch sequence, potentially leading to an inversion. The investigation also analyzed the aerodynamic impact of the V-tail configuration and the lack of fixed surfaces on the tail unit.

Findings

The primary cause of the accident was a rapid control input on the stick combined with rudder pressure, a maneuver that systematically induced an inversion or half-roll in this aircraft type.
The reduction in control effectiveness during the flare was exacerbated by the aircraft entering the boundary layer near the ground, which decreased relative airspeed.
The SHK-1's V-tail design, featuring fully movable surfaces without fixed stabilizers, contributed to reduced dynamic stability during aggressive maneuvers.
Asymmetric lift caused by sideslip (exacerbated by the V-dihedral effect) and asymmetric lift during yaw created significant rolling moments, which were not sufficiently damped by the tail assembly.