What happened
On May 1, 1966, a Rhönlerche II glider, registration HB-696, was conducting a solo training flight near Thun, Switzerland. The student pilot, who had recently resumed training after a seven-month hiatus, departed via aero-tow at approximately 14:23 local time. After releasing from the tow cable near Goldiwil, the aircraft initially climbed slightly before entering a steady descent.
While attempting to find thermals over the city center, the pilot realized the aircraft was descending below the instructor's prescribed safety minimums. Upon checking the altimeter, the pilot believed the aircraft was still at 400 meters; however, the actual altitude was significantly lower. Realizing the airfield was unreachable, the pilot attempted to reach a field on the outskirts of the city but lacked sufficient altitude to clear the residential buildings. Consequently, the pilot executed an emergency landing in a grain field. During a steep right-hand turn into the wind, the right wing struck the ground, causing the aircraft to skid and impact heavily on its skid.
The investigation
Investigators examined the aircraft's instruments and the environmental conditions at the time of the accident. The investigation focused on the discrepancy between the pilot's perceived altitude and the actual terrain elevation. The investigation also reviewed the pilot's recent training history and the instructor's flight assignments.
Findings
- The altimeter had been incorrectly set to a pressure of 983 mb, whereas the actual local pressure (QFE) was approximately 958 mb. This error resulted in an altitude reading that was roughly 25 mb too high.
- The altimeter exhibited excessive sluggishness in its readings, likely due to heavy internal contamination, which prevented timely responses to altitude changes.
- The pilot failed to maintain the required altitude margin for a safe return to the airfield due to the reliance on the faulty instrument.
- The instructor's decision to assign a solo flight following a long training break was noted as overly optimistic given the student's limited recent solo experience.