What happened
On August 18, 1991, an SA 315B "Lama" helicopter bearing registration HB-XMC, operated by Air Grischa AG, was conducting a commercial passenger and cargo flight in the canton of Graubünden. The aircraft departed from St. Vittore with two passengers and a cabin attendant on board, carrying an internal load to Val Cama.
Upon approaching the landing site at Val Cama, located at an altitude of 1,280 meters, the helicopter touched down on uneven ground. Immediately after touchdown, the aircraft experienced ground resonance. To correct this, the pilot lifted the skids slightly off the ground and attempted to reposition the helicopter approximately one meter forward.
During this low-altitude maneuvering phase, two of the main rotor blades struck the tail boom structure. The impact severed the protective sleeve of the tail rotor drive shaft and bent the shaft itself. The pilot immediately shut down the engine by closing the fuel valve and landed the aircraft safely. All occupants exited the helicopter uninjured, and the damage to the airframe was assessed as minor.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the flight operations, pilot qualifications, and mechanical condition of the aircraft. The pilot held a valid professional helicopter license and had accumulated 19 hours on this specific type in the preceding 90 days. The aircraft was airworthy under VFR conditions, with mass and center of gravity within limits.
Technical inspection revealed that the intact main rotor blade (yellow) had an axial tolerance slightly higher than the other two blades, but no prior technical defects were found. Other pilots who regularly flew this type reported no history of ground resonance issues. The investigation focused on determining whether mechanical failure or pilot error caused the collision between the main rotor and the tail structure.
Findings
The inquiry concluded that the primary cause was the pilot's abrupt cyclic stick input immediately after initial contact with the rough terrain. Because the pilot was relatively inexperienced with this helicopter type, the rapid backward movement of the cyclic disk caused the main rotor blades to strike the tail boom.
Contributing factors included:
- Limited flight experience on the SA 315B type by the pilot.
- The uneven and rugged nature of the landing terrain.
The investigation noted that the pilot did not feel vibrations in the cyclic control prior to the incident, suggesting the input originated from manual control rather than mechanical feedback. The prompt response to shut down the engine was deemed appropriate.
Safety action
No specific safety recommendations were issued by the SUST in this final report regarding regulatory changes or mandatory training modifications for this specific event.