What happened
On January 26, 1965, a Bell 47G-3B-1 helicopter, registration HB-XBU, was performing a commercial cargo transport near the Lukmanier Pass in Switzerland. The pilot was delivering approximately 75 kg of material to a drilling site at Alp Gana Bubaira.
Upon arriving at a temporary landing platform, the pilot attempted to release the underslung load using the electrical release mechanism. When the load failed to detach, the pilot attempted a second release using the mechanical emergency lever. During this second attempt, the pilot had to momentarily release the pitch and throttle controls. As the load finally released, the helicopter experienced a sudden pitch-up motion. In an instinctive attempt to correct the movement, the pilot pushed the control stick forward, causing the aircraft to transition into a forward descent. The helicopter struck the edge of the landing platform and overturned. A geology student, who was standing nearby watching the maneuver, was struck by the main rotor blades and killed. The pilot sustained minor injuries, while the passenger suffered serious injuries.
The investigation
The investigation examined the mechanical and electrical components of the cargo release system. Investigators discovered that one of the three electrical connection points used to power the release mechanism had a faulty connection; not all individual wires were properly inserted into the terminal, creating an intermittent contact.
Furthermore, the commission reviewed the pilot's flight experience and the regulatory requirements for mountain operations. While the pilot had not yet reached the 100-hour flight time requirement for mountain landings established in June 1964, the investigation determined that this regulatory oversight was not a causal factor in the accident.
Findings
- The primary cause of the accident was the loss of control of the helicopter during a difficult cargo release procedure necessitated by a technical malfunction.
- An electrical defect in the release mechanism, caused by an improperly crimped cable connection, prevented the automatic or electrical detachment of the load.
- The pilot's decision to perform an emergency release at a low altitude (approximately 5 meters) while having to momentarily release essential flight controls contributed to the loss of stability.
- The sudden pitch-up of the aircraft was triggered by the pilot's physical movement of the control stick while reaching for the emergency lever.