Introduction
On Tuesday, 9 June 1964, at 1616 MEZ, a flight instructor departed from Hausen a.A. airfield in a Bücker 131 Jungmann, registration HB-URS, for an alpine familiarization flight with a student pilot as part of a Swiss Air Transport School course.
Aircraft and Occupants
The aircraft was a single-engine, two-seat biplane of mixed construction, powered by a Hirth HM 504-A2 engine. It was owned by the Wallis section of the Aero-Club of Switzerland. At the time of the accident, the aircraft's weight was approximately 610 kg, within the maximum permissible weight of 670 kg. The center of gravity remained within limits throughout the flight. The aircraft had accumulated 1543 hours since new and 265 hours since last overhaul; the engine had 1100 hours since new and 164 hours since overhaul. No technical defects were found during the investigation.
The flight instructor, born in 1932, held a professional pilot's license valid until 19 July 1964, with approximately 1200 total flight hours, including about 150 hours on the accident type. The student pilot, born in 1940, held a glider pilot's license and a student motor pilot license, with about 300 glider hours and 50 motor flight hours (20 dual, 30 solo).
Sequence of Events
The flight proceeded into the central Alps, with the instructor selecting the area south of the Vorderrhein valley as the training zone. Around 1700 MEZ, the student executed several ridge and pass crossings under the instructor's guidance. The instructor then intended to demonstrate lee downwind effects. After crossing Passo Nalps (2750 m) into Val Cadlimo, the aircraft lost altitude quickly. The student then flew back over Passo Vecchio (2760 m) into Val Curnera.
Approximately one kilometer north of the pass, at an altitude between 2700–2750 m, the instructor took control and entered a left turn into a firn basin leading toward Piz Borel (2952 m). Benefiting from slope updrafts, the aircraft began to climb. The instructor expected to clear a ridge at 2860 m, though the aircraft was initially 100–200 m below it. He flew parallel to the slope, about 100 m to his left, heading toward the ridge.
Instructor's Account
The instructor reported that upon nearing ridge height, he noticed a gradual power loss, with no engine malfunctions such as misfires. He did not check the tachometer but believed the engine eventually windmilled. Recognizing that crossing the ridge was impossible, he initiated a right turn away from the slope to return to open air over Val Curnera. Before turning, the airspeed indicator read slightly above the yellow mark (115 km/h). During the turn, with a bank angle of no more than 20°, he estimated his speed at the yellow mark. After completing the 180° turn, he encountered strong, gusty downdrafts. The terrain ahead sloped downward, but he felt altitude was insufficient. He may have reacted instinctively by attempting to level off. He attributed a potential error in height judgment to a rapid change in lighting from bright sunlight to shadow. He recalled the aircraft then entering a slow right-turning motion before impact, with the rotation not exceeding 90°. He emphasized strong turbulence in the area, which could have caused an aerodynamic stall.
Student's Account
The student stated that after the instructor took control, the aircraft transitioned from a climb to level flight. He did not notice anything unusual about the engine. The airspeed remained at approximately 115 km/h (yellow mark), and the variometer indicated a sink rate of about 2 m/s. When it became clear that crossing the ridge would be difficult, the instructor initiated a normal right turn. After about 90° of heading change, the student felt a lateral acceleration pushing him toward the left side. Simultaneously, the nose pitched down steeply. He instinctively covered his head. He recalled the aircraft rotating around its longitudinal axis at least half a turn but no more than one and a half turns before impact. He observed the propeller rotating at low RPM. The weather was clear with scattered small cumulus clouds, and visibility was unlimited.
Accident Location
The accident occurred in a firn basin at approximately 2800 m, about 200 m east and 30 m below the ridge. Coordinates: 696.750/159.550, municipality of Tavetsch, Graubünden.
Injuries and Damage
Both occupants sustained serious injuries. The aircraft was destroyed.