What happened
On September 29, 1982, a Heliswiss SA 315 B "Lama" helicopter, registered as HB-XNA, was conducting external load operations in the Lungern area of Obwalden. The pilot had previously completed concrete deliveries in the Val Milà region and returned to the Erstfeld base before departing for Hüttstett at approximately 11:15 local time. The mission involved transporting construction materials from an altitude of 1,660 meters to a building site located on a steep, forested western slope at 2,043 meters.
During the eleventh rotation, the pilot adjusted the external load sling from 10 meters to 14 meters to accommodate the height of the trees at the pickup location. As the helicopter approached the designated loading zone, the extended nylon sling hooked onto a fir tree in the canopy. The sudden release of the snagged sling caused it to whip upward into the main rotor disk. This impact severed one of the blade limit cables and damaged the rotor blade near the trim tab area. The resulting imbalance generated severe vibrations, shattering the front windshield and causing the aircraft to yaw clockwise. Despite losing effective control authority, the pilot managed to execute a hard emergency landing in a terrain depression nearby.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the wreckage, focusing on the main rotor system, the external load mechanism, and the pickup site. Inspections revealed that the helicopter held valid airworthiness certificates and had undergone recent conversion from an SA 318 C model. The pilot possessed a valid commercial license with over 1,400 total flight hours.
Forensic analysis of the main rotor blade showed clear impact marks approximately 3.5 meters from the hub, consistent with a collision with the load sling. A shackle connecting the blade arm to the limit cable had failed due to excessive force. At the accident site, investigators identified a fresh break in a fir tree branch at the height where the sling would have passed. The weather conditions were reported as good and played no role in the incident.
Findings
The primary cause of the accident was an approach conducted at an altitude too low for the extended load configuration. The pilot either failed to account for the increased 14-meter sling length or misjudged the distance to the tree line, resulting in the sling snagging on an obstacle. The subsequent violent release of the load into the main rotor disk caused critical mechanical failure and loss of control.
Contributing factors included:
- Aircraft: SA 315 B "Lama" HB-XNA
- Operator: Heliswiss, Schweizerische Helikopter AG
- Fatalities: 0
- Injuries: None reported among crew or third parties
- Causal Fact: Main rotor strike caused by external load sling snagging on a tree due to low-altitude approach.
Safety action
The investigation highlighted the risks associated with changing external load configurations during flight operations. While no specific regulatory changes were mandated in this report, the findings underscored the necessity for pilots to rigorously verify sling lengths and obstacle clearance when transitioning between different pickup environments.