What happened
On December 29, 1966, an Agusta Bell 47 G 3 B helicopter, registration HB-XCD, was performing field landing training near Mittelhäusern, Switzerland. The flight proceeded normally, and the pilot successfully completed a landing at approximately 16:15 local time. During the period following the landing, while the engine was running at idle, a loud noise was heard accompanied by a violent jolt that caused both cabin doors to fly open. The pilot immediately observed that the tail rotor had detached from the aircraft.
The investigation
Investigators examined the aircraft's maintenance history and identified a previous incident on April 13, 1966. During a mountain rescue mission at the Rottal glacier, the tail rotor had struck hard snow. While the tail rotor blades were inspected and the strike indicators were replaced, the blades themselves remained in service. Laboratory analysis of the broken blade revealed that the fracture was caused by material fatigue, a condition typical of blades that have previously impacted water, snow, or similar substances.
Findings
- The primary cause of the accident was the fatigue failure of a tail rotor blade.
- A previous strike against snow in April 1966 had introduced structural damage to the blades.
- There was a conflict between manufacturer instructions regarding blade replacement: Agusta technical bulletin No. 97 mandated the replacement of both blades if strike indicators showed any bending, whereas Bell Helicopter Service Bulletin No. 1435 B allowed for continued use if no visible damage was found on skid-equipped helicopters.
- Although the blades were inspected by technical staff, non-visible deformations persisted, leading to abnormal stresses that eventually caused the fatigue fracture.