What happened
On June 10, 1978, an Agusta Bell 204 B helicopter, registration HB-XCQ, was preparing for a transport flight from Val Medel to Geneva. The aircraft, operated by Heliswiss, was loaded with five passengers and various equipment, including two barrels of kerosene.
During the takeoff phase at approximately 08:45, the pilot performed a power test in a stationary hover. Finding insufficient power reserves to climb against the wind toward the rising terrain, the pilot decided to accelerate downhill with a tailwind. During this acceleration phase, the aircraft struck a 16 kV high-voltage power line. The collision damaged the tail rotor and severed several wires, causing the helicopter to crash into the bed of the Froda stream. While the pilot sustained fatal injuries, the five passengers survived the impact.
The investigation
Investigators examined the aircraft's condition, the pilot's flight history, and the environmental factors at the time of the accident. The investigation established that the Agusta Bell 204 B was approximately 56 kg over its maximum takeoff weight, weighing roughly 3,916 kg against a limit of 3,860 kg.
Technical analysis of the wreckage showed that the tail rotor was damaged by the cables and the main gearbox was separated from the engine, causing the cabin to compress onto the cargo. The investigation also noted that the pilot had exceeded the maximum monthly flight time regulations by approximately five hours. Furthermore, the high-voltage line was not recorded on the obstacle charts, as those charts only include lines with a ground clearance of more than 25 meters.
Findings
- The primary cause of the accident was the collision with high-voltage power lines due to insufficient reconnaissance of obstacles in the chosen takeoff direction.
- The pilot failed to adequately verify the path for a downhill takeoff, likely due to the dark background of the wires making them difficult to spot until the last moment.
- The aircraft was operating in an overweight condition.
- The pilot's attempt to climb over the wires failed because the available power reserve was insufficient to clear the obstacle.
- The survival of the passengers was aided by the presence of kerosene barrels in the rear of the cabin, which prevented the structural collapse of the cabin from reaching the passenger area.