What happened
On March 22, 1967, a SA 321 Super Frelon, registration F-WJUX, was engaged in a specialized heavy-lift operation to deliver a metal dome component to the Jungfraujole observatory atop the Sphinx. The mission, operated by Sud Aviation, required the helicopter to transport the heavy load from Rarogne to the high-altitude site.
During the initial approach to the observatory platform, the pilot encountered significant difficulties. Despite utilizing full power from all three turbines, thermal turbulence and wind conditions prevented the aircraft from maintaining a stable hover over the target. After two failed attempts to position the load, the pilot determined that the aircraft lacked sufficient fuel reserves to continue the operation and decided to return to the starting point in Rarogne.
While en route back to the departure point, approximately five minutes before reaching the destination, the external load suddenly detached from the helicopter. The cargo fell onto a snowy slope of the Grisighorn, south of Brigue. There were no injuries to the crew or individuals on the ground, though the cargo, valued at approximately 40,000 francs, was destroyed upon impact.
The investigation
The investigation focused on why the release mechanism, a specialized load-release device (délesteur), opened during flight. Investigators examined the electrical circuits and the mechanical components of the release system. The crew, consisting of a pilot, co-pilot, and flight mechanic, had all necessary professional licenses and authorizations for the mission.
Technical examinations of the electrical system revealed two specific vulnerabilities: the presence of blueing and welding on two relay contacts—likely caused by vibration or inductive effects—and electrical wiring with excessive lengths of exposed conductor, which created a risk of short-circuiting.
Findings
Several factors contributed to the loss of the load. The investigation could not definitively determine the exact trigger for the release, but identified three possible scenarios:
- Technical malfunction: The electrical inspection confirmed that the release mechanism was susceptible to unintended activation due to electrical faults or vibration-induced contact failure.
- Inadvertent pilot action: The pilot noted the possibility that a thick flight glove might have accidentally struck the electrical release button on the cyclic stick.
- Emergency release: The investigation considered whether the pilot may have deliberately released the load to avoid a catastrophic engine failure or fuel exhaustion over populated areas, given the extremely tight fuel margins.
Ultimately, the investigation concluded that the release of the load was caused by the opening of the release hooks, though whether this was due to a technical defect or pilot input remained unproven.