Introduction
On 24 May 1991, a SA 315 B "Lama" helicopter, registration HB-XII, operated by Air Zermatt AG, was involved in an accident on the "Alp Nussböüm" near Betten, Canton Valais, Switzerland. The aircraft was destroyed and the pilot sustained serious injuries.
Flight History
The helicopter departed from its base in Raron at approximately 0800 local time to perform commercial underslung load operations in the Betten-Bettmeralp area. The crew consisted of the pilot and three flight assistants. Initially, the crew transported material to a reservoir above Betten until about 0945. Subsequently, they worked on concreting irrigation hydrants at various locations. Some hydrants were located on either side of the cables of the Betten 1. Section cable car line. The flight assistants recommended using a 16-meter steel cable to extend the concrete bucket; the pilot complied. The first rotations proceeded without incident, though some hydrants were near obstacles such as barns, trees, and the cable routing.
Accident Event
Around 1035, the pilot departed from the load pickup point (the sports field) for the last rotation. Two hydrants (Nos. 13 and 14) remained to be concreted. The terrain was steep and generally free of ground obstacles. The cable car cables crossed the area at heights between 23 and 33 meters above ground. A flight assistant advised the pilot via radio to fly over the cables, but the pilot did not hear this recommendation (he was monitoring three radio frequencies simultaneously). The pilot estimated a clearance of about 3 meters between the main rotor and the cables and decided to fly under them. While looking down through a bubble window to monitor the underslung load, the pilot concentrated on avoiding ground contact. When he looked up, the main rotor blades struck a support cable. The helicopter fell uncontrolled and impacted the ground.
Findings
The investigation determined that the pilot held the necessary licenses and was fit for duty; no health issues were identified. The helicopter was in a technically sound condition and properly maintained. Contact between the rotor blades and the cable was confirmed. The cable car was in operation at the time, and its operator had not been informed of the helicopter activities. The Federal Office for Civil Aviation (BAZL) had previously issued guidelines for operations near cable lines. Weather conditions were sunny, windless, with visibility exceeding 20 km and a temperature of about 14°C at the time of the accident.
Cause
The accident was attributed to the erroneous decision to fly under the cables and an insufficient safety margin between the cables and the ground.