What happened
On Good Friday, April 16, 1965, a Cessna 170 A, registration HB-CAU, departed Thun for a private VFR flight to Strasbourg, with a planned stop in Basel. The flight was initially interrupted when the pilot encountered poor visibility near the Jura mountains, leading to an unplanned landing at Bern airport. After refueling and waiting for weather improvements, the pilot attempted the flight again at 15:40.
Approximately 35 minutes into the second leg, the aircraft entered an area of heavy cloud cover and intense weather activity. Witnesses near the Raimeux mountain observed the aircraft flying through thick clouds and heard irregular engine noises. During a maneuver, the aircraft's left wing detached, causing the plane to plummet into a pasture at Plain Fayen. All four occupants perished in the crash, and the aircraft was destroyed.
The investigation
Investigators examined the wreckage and debris, which had been scattered over several kilometers. Laboratory analysis of the structural components, including the wing attachment hardware and the fuselage, ruled out material fatigue or manufacturing defects as the cause. Instead, the examination of the left wing revealed significant deformations consistent with extreme structural stress.
The investigation also reviewed the pilot's credentials, noting that while the pilot had significant flight experience, his formal transition training on the Cessna 170 A had not been officially completed, meaning he was not technically authorized to fly the type solo at the time of the accident.
Findings
- The primary cause of the accident was that the pilot continued flight operations into weather conditions that made maintaining visual flight rules impossible.
- The aircraft was subjected to aerodynamic loads exceeding its structural design limits.
- The pilot likely encountered severe turbulence and heavy precipitation while flying in clouds without the necessary instrument flight training or equipment.
- The structural failure was triggered by a negative load factor that caused the wing spar to buckle, leading to the detachment of the left wing.
- The pilot had underestimated the dangers of the approaching weather front, despite warnings from air traffic services and other pilots.