What happened
On September 3, 1961, a Sokol MID aircraft, registration HB-TAH, was conducting a paid sightseeing flight over the Sempacher, Baldegger, and Hallwilersee lakes as part of a flying day organized by the Olten section of the Ae.C.S. At approximately 14:30, while the pilot was on the return leg at an altitude of roughly 1,050 meters, the engine suddenly seized.
Faced with a total loss of power, the pilot initiated an emergency landing in a meadow near Mosen. During the landing roll, the pilot steered the aircraft toward a utility pole to avoid hitting a roadside embankment. The left wing struck the pole, causing the aircraft to swerve and come to a halt. Despite the impact, there were no injuries to the pilot or the two passengers.
The investigation
The investigation focused on the mechanical failure of the Walter Minor 4/III engine and the circumstances of the landing. Technical examinations were supported by findings from the Swiss Federal Institute of Materials Testing. Investigators analyzed the engine components to determine if the seizure was caused by external factors, such as engine overstress, or internal mechanical failure.
Findings
Technical analysis revealed that the engine failure was caused by a fatigue fracture of connecting rod number 3. The investigation found no evidence of engine overstress or defects in the material or heat treatment processes. While the pilot's actions were deemed appropriate for the emergency, the investigation noted that the touchdown speed was somewhat high.
Safety action
During the investigation, it was noted that fatigue-related connecting rod failures had been observed in later production series of this aircraft model.