What happened
On August 12, 1967, a Rhönlerche II glider, registration HB-631, was being towed from Saanen to Birrfeld by a De Havilland Chipmunk HB-TUC. During the tow flight, at an altitude of approximately 1,600 meters, the glider disconnected from the tow rope near Boltigen. Witnesses observed significant fluttering at the wingtips both during the release and throughout the subsequent dive.
Following the disconnection, the glider entered a steep dive and began to accelerate. During this phase, the cockpit canopy blew off with a loud bang, and pieces of the right aileron broke away from the aircraft. The glider then veered toward the terrain, eventually striking a mountainside at an altitude of approximately 1,340 meters. The pilot, who had apparently attempted to deploy a parachute, was killed upon impact.
The investigation
Investigators examined the wreckage of the HB-631 and analyzed the structural integrity of the control surfaces. The investigation focused on the cause of the wing flutter and the subsequent structural failure of the right aileron. The inquiry also reviewed the maintenance history of the aircraft, noting a recent total overhaul that included the replacement of all control cables.
Findings
- The primary cause of the accident was wing and aileron flutter, which induced a loss of control and led to the aircraft colliding with the terrain.
- The flutter phenomenon began during the tow flight and intensified during the dive, likely triggered by local wind turbulence.
- The structural failure of the right aileron was exacerbated by improper fabric application; the wing covering was not correctly routed around the spars or properly glued to the ribs.
- The investigation noted that the aircraft lacked mass balancing on the ailerons, a known issue for this model that had previously led to similar accidents in other countries.
- The pilot's decision to disconnect from the tow rope was likely prompted by the onset of the visible and audible flutter.
Safety action
Following the investigation, the manufacturer, in agreement with German authorities, issued a requirement for static mass balancing of the ailerons on the Rhönlerche II to prevent similar flutter-related failures.