What happened
On January 15, 1961, a pilot was conducting a five-hour soaring flight in a single-seat S-19-II glider, registration HB-544, near Holderbank, Switzerland. The flight, which began at Birrfeld airfield, was intended to fulfill requirements for a silver soaring badge. While performing a right-hand turn at an altitude of approximately 250 meters above the Chestenberg ridge, the pilot heard a distinct cracking sound coming from the base of the control column.
Immediately following this sound, the control column lost all resistance and became disconnected from the control cables, leaving the aircraft unsteerable. The aircraft entered a pitch-up maneuver, followed by a dive that rapidly steepened into a vertical descent. The pilot successfully exited the aircraft via parachute and landed safely in a limestone quarry. The S-19-II continued through two loops before impacting the ground approximately 200 meters from the pilot's landing site, resulting in the total destruction of the aircraft.
The investigation
The investigation focused on the mechanical integrity of the control system. Investigators examined the torsion tube and the control column support, which are difficult to inspect without removing the control column from the fuselage. The inspection revealed that the internal components of the torsion tube contained significant amounts of rust, metal debris, and dust, suggesting that the area had not been cleaned or inspected for a considerable period.
Investigators also reviewed the maintenance history and the manufacturing process of the critical components. They noted that while the design itself was dated, the specific failure was linked to how the components were joined during assembly. Furthermore, the investigation looked into the oversight provided by the aircraft operator and the regulatory authorities regarding periodic inspections.
Findings
The investigation established that the primary cause of the accident was a fatigue fracture in the control column support. This failure was driven by a welding defect where the connection between the support bracket and the torsion tube did not comply with the original engineering drawings. Instead of a single, integrated weld, the manufacturer had performed two separate welds that failed to achieve proper fusion, creating a significant stress concentration point.
Several contributing factors were identified:
- The improper welding technique created a notch effect, making the joint highly susceptible to fatigue under normal control loads.
- The structural design of the support bracket created abrupt changes in stiffness, which exacerbated stress concentrations.
- A lack of rigorous maintenance and inspection by the operator meant that the developing cracks went undetected.
- The difficulty of inspecting the internal control components meant that cracks could only be identified through manual checks of control surface resistance, which were not performed.
- The investigation noted that the operator relied heavily on official inspections rather than maintaining a dedicated internal maintenance system.