Accident Overview
On Sunday, 30 July 1967, at 12:39 PM, a student pilot departed from Grenchen airfield in a Rhönlerche II glider, registration HB-589, for a solo soaring flight. The glider was owned by the Segelfluggruppe Grenchen and had a valid airworthiness certificate. The student, born in 1946, held a learner's permit for powered and glider aircraft and had accumulated approximately 12 hours of flight time with 66 landings. His last solo flight was on 6 July 1967. He was preparing for his license exam, needing only a third solo flight.
Sequence of Events
After release from the tow plane at about 1000 meters above the airfield, the student located a thermal with a lift rate of approximately 2 m/s and began circling. He gained altitude rapidly. While looking upward to check cloud clearance, he believed the distance to the clouds was sufficient. Suddenly, without warning, he entered a cumulus cloud sideways. Once inside the cloud, he experienced severe turbulence and quickly lost control. The airspeed indicator pegged at its maximum (200 km/h) twice, and he felt strong accelerations and flutter. He did not deploy the dive brakes or take any corrective action. After approximately one minute in the cloud, parts of both ailerons and fabric covering detached. Around 1258 hours, the student regained visual contact with the ground in a steep attitude and noticed the controls were no longer fully responsive. He immediately bailed out, landing unhurt on a forest path about 1 km west of the Berghaus Stierenberg. The glider descended, struck trees, and slid to the ground, suffering substantial damage. No third-party damage occurred.
Damage and Injuries
The pilot, the sole occupant, was uninjured. The glider sustained about 60% damage, with an estimated cost of 5000 Swiss francs. There was no ground damage.
Investigation Findings
Examination of the wreckage revealed that the left aileron with its fittings and large sections of the lower fabric covering of the left wing had separated in flight. The inner half of the right aileron and the upper fabric of the outer half also detached. All other components, including the tail, remained intact except for impact damage. The investigation concluded that the damage was likely caused by exceeding the glider's speed limits, leading to combined wing/aileron flutter. The Rhönlerche II had a history of flutter incidents due to a lack of mass balancing in the ailerons, and a mandatory modification requiring mass balance was in preparation. The student's limited experience (only 39 minutes flying in the previous 12 months) contributed to his inability to assess cloud distance or recover from the loss of control. He had not practiced emergency procedures. The aircraft's instrumentation was inadequate for blind flying. However, his decision to bail out after exiting the cloud was correct.
Probable Cause
The accident was directly caused by the student pilot unintentionally entering a cloud.