What happened
On June 18, 1970, an Agusta-Bell 47G-3B-1 helicopter, registration HB-XCI, was performing a heavy-lift operation in the Riemenstalden region of Switzerland. The mission involved transporting approximately seven tons of timber for avalanche protection from Riemenstal_stalden to a site located roughly 430 meters higher in a steep forest clearing.
After completing the 34th load delivery, the pilot descended to waist height to allow two ground workers to attach a bundle of ropes to the right skid. Immediately after the ropes were secured, the helicopter experienced a sudden tilt toward the slope. The pilot attempted to compensate by applying full aft cyclic control, but the main rotor blades struck the terrain. The impact caused the aircraft to capsize onto its back near an avalanche barrier.
The investigation
Investigators examined the aircraft's maintenance records and the pilot's qualifications. The Agusta-Bell 47G-3B-1 was found to be well-maintained, with all airworthiness directives completed and the weight and center of gravity within legal limits. The pilot was highly experienced in operating this specific model and held valid commercial mountain flying authorizations.
Witness statements from the ground crew indicated that the movement toward the slope occurred precisely as the first worker attached the load, while the second worker was still holding the rope bundle to prevent it from snagging. The investigation also noted that while wind conditions had been steady earlier in the day, thermal turbulence had increased by the time of the accident.
Findings
- The aircraft was operating in a confined area with a clearance of only approximately three meters between the rotor disc and the slope.
- The pilot was managing a complex maneuver involving a tailwind and the simultaneous addition of a new external load.
- The rotor blade strike was the direct cause of the accident, precipitated by a sudden movement of the aircraft toward the terrain that could not be corrected in time.