What happened
On September 23, 1976, an SA 315 B Lama, registration HB-XFX, was engaged in a cargo transport and training mission in the Val Calanca region of Switzerland. The crew was transporting concrete buckets to an avalanche protection construction site. During the flight, the aircraft was performing a series of rotations to transport materials from Augio.
While executing a descent on the valley side to approach for a landing, the aircraft encountered a sudden impact. A suspended aluminum concrete bucket struck an unmarked transport cable. The force of the impact sent the bucket into the tail rotor, which subsequently tore the tail rotor and its associated transmission gears away from the airframe.
Following the loss of the tail rotor, the helicopter began a rapid counter-clockwise yaw. The pilot took immediate control, jettisoned the empty bucket, and utilized forward and right cyclic inputs to manage the rotation. By shutting down the engine to eliminate gyroscopic effects, the pilot stabilized the aircraft and executed a successful emergency landing in a nearby meadow. However, during the landing sequence, the HB-XFX slid backward into a stream bed structure, resulting in significant damage to the tail boom, landing gear, and fuselage.
The investigation
The investigation established that the transport cable, which spanned approximately 500 meters, was an active material cableway. While the cable had been officially reported in NOTAMs and obstacle charts, it was not marked with any visual indicators. The investigation found that the flight instructor had previously scouted the area from the ground but had failed to spot the specific cable in question.
Technical analysis confirmed that the collision with the cable caused the concrete bucket to strike the tail rotor assembly with enough force to rip the entire tail rotor and gearbox from the aircraft. The investigation also noted that the change in wind conditions had prompted the crew to fly a different approach path, bringing the aircraft closer to the terrain and the cable-laden area.
Findings
- The primary cause of the accident was the loss of the tail rotor due to a collision with a transport cable.
- The approach path was altered due to increasing valley winds, which placed the aircraft in closer proximity to the slope and the cable hazard.
- The transport cable was not visually marked, making it difficult to detect during flight or ground reconnaissance.
- The crew's actions following the mechanical failure were exemplary, directly enabling a successful emergency landing and preventing fatalities.