What happened
On September 14, 1965, a pilot operating for Heliswiss was performing a cargo transport mission near Bernina Suot, Switzerland. The mission involved transporting wooden beams from the Bernina Suot area to a construction site at Piz Lanyard. After completing six successful flights earlier in the day, the pilot commenced the seventh flight at approximately 14:09 CEST.
During the initial climb, the pilot hovered to allow a mechanic to secure the heavy load. Once the load was attached, the helicopter transitioned into forward flight at an altitude of approximately 10 meters. Shortly after reaching a distance of 30 meters from the takeoff point, the pilot heard a loud bang from the rear right and felt a violent vibration. The pilot immediately released the external load. Despite attempting to maintain control, the helicopter's altitude dropped rapidly, causing the aircraft to strike the uneven ground approximately 150 meters from the departure point. The impact caused the main rotor to strike the tail, resulting in significant damage to the airframe and landing gear.
The investigation
Investigators examined the Agusta Bell 47G-3B-1, registration HB-XBW, and its Lycoming TV0-435-B1 engine. While the engine itself showed no signs of mechanical failure or carburetor issues, the investigation focused on the drive components.
Post-accident analysis of the cooling fan drive belts revealed that one belt had snapped, while the second belt was insufficiently tensioned. Fragments of the broken belt, showing signs of singeing, were found among the engine's cooling fins. The investigation also considered the state of the centrifugal dry clutch used to drive the main rotor.
Findings
- The primary cause of the accident was a sudden engine-related malfunction which led to an immediate loss of airflow (stall) over the main rotor blades.
- The sudden loss of rotor RPM was likely triggered by a momentary slipping of the clutch between the engine and the main rotor.
- This clutch slip caused a sudden shift in torque, placing an extreme overload on the cooling fan drive belts.
- The physical impact heard by the pilot may have been the broken belt striking the firewall behind the cabin.
- The pilot was unable to recover the altitude due to the low flight height at the time of the malfunction.