Accident Overview
On 23 September 1976, at approximately 1340 local time, an SA 315 B Lama helicopter registered HB-XFX operated by Air Grischa AG was engaged in a material transport and type-conversion training flight in the Val Cardena region near Augio, Grisons. The helicopter was transporting concrete in a suspended bucket to a construction site for avalanche barriers at the foot of Fil de Dragiva, 2000 m above sea level. The flight instructor was also conducting conversion training for a newly hired pilot on the SA 315 B Lama type.
Flight Sequence
The crew had performed 36 rotations piloted by the instructor, followed by three rotations by the trainee from the right seat. For the return leg of the third rotation, a different flight path was chosen due to increasing valley wind. The downwind leg was flown up-valley in descent, mostly in autorotation, on the left side of the valley, intending to turn left into the final approach into the wind. Shortly before turning onto final approach, at about 50–60 m above ground, the pilots felt a sudden jolt followed by vibrations throughout the airframe, particularly in the rudder pedals. The instructor concluded that something had struck the tail rotor, took over controls, and jettisoned the empty concrete bucket. As the bucket was released, the helicopter yawed sharply to the left; the pilot saw parts flying off the tail section, and the helicopter began a rapid counter-clockwise rotation around the vertical axis, confirming complete tail rotor loss. By pushing the cyclic forward and left, the instructor slowed the rotation and, while sliding to the right, maneuvered toward a suitable emergency landing site. Just above the ground, he instructed the trainee to shut down the engine using the fuel shutoff valve. This eliminated the turbine gyroscopic moment, halting the rotation, and the pilot set the helicopter down. After landing, the helicopter slid backward against a stream bed reinforcement, coming to rest heavily damaged.
Findings and Cause
The accident investigation determined that the empty concrete bucket, suspended beneath the helicopter, collided with a 500-meter-long transport cable spanning from coordinates 730'020/135'870 to 730'260/136'260. The collision occurred approximately 35 meters above ground at coordinates 729'930/135'950. The cable was listed in NOTAM C No. 1126/73 and obstacle notification No. 287/73, and was shown on the 10th edition of the obstacle chart dated 26 August 1976. However, the cable was unmarked and located within a hatched red area on the obstacle chart indicating a cable-infested region. The flight instructor had reconnoitered the area from the ground multiple times for cables but had missed this particular one.
The impact threw the bucket into the tail rotor, damaging it and causing an imbalance that tore off the tail rotor gearbox and rotor assembly. The investigation noted that the approach for the accident flight was conducted down-valley due to changed wind conditions, bringing the helicopter into hazardous proximity to the slope in a cable-infested area.
Both pilots held valid licenses and were not impaired. The helicopter was properly certified, and weight and balance were within limits. Weather at the time was a transition from high pressure to a south foehn situation with scattered stratocumulus and good visibility.
The probable cause of the accident was the tail rotor failure resulting from a collision with a transport cable.