What happened
On July 30, 1967, a student pilot launched a solo training flight from Grenchen in a Rhönlerche II, registration HB-589. After being towed to altitude, the pilot identified a strong thermal updraft near the Stierenberg area and began circling to gain height. While focused on maintaining altitude, the pilot inadvertently entered a cumulus cloud.
Inside the cloud, the aircraft encountered intense turbulence, causing the airspeed to accelerate rapidly toward the upper limits of the instrument scale. The pilot experienced significant vibrations and fluttering. During this period of instrument flight, the aircraft suffered structural failures, including the detachment of parts of the ailerons and wing fabric. Upon exiting the cloud and regaining visual contact with the ground, the pilot discovered that the flight controls were no longer functioning correctly. Due to the compromised state of the aircraft, the pilot successfully deployed a parachute and jumped from the glider.
The investigation
Investigators examined the wreckage at the crash site near the Stierenberg mountain house. The analysis revealed that the left aileron, along with its attachment fittings, had separated from the wing, and significant portions of the left wing's underside fabric had torn away. The right aileron also suffered partial structural failure. The investigation focused on the aerodynamic forces acting on the aircraft during the period of high-speed flight within the cloud.
Findings
- The primary cause of the accident was the pilot's unintended entry into a cloud, which resulted in a loss of aircraft control.
- The structural damage was caused by exceeding the permitted flight speed limits, which triggered severe wing and aileron flutter.
- The Rhönlerche II model, which lacked mass-balanced ailerons, was noted to be susceptible to such flutter phenomena.
- The pilot's lack of experience contributed to a failure to properly judge cloud clearance and a lack of effective corrective actions during the uncontrolled flight phase.
- The aircraft's instrumentation was insufficient for performing instrument flight (blind flight) during the encounter.