What happened
On June 27, 1968, an Agusta Bell 47 G-3B-1 helicopter, registration HB-XBW, was performing a commercial supply flight near the Gauli Hut in the Bernese Alps. The aircraft, operated by Heliswiss, was transporting a passenger and 200 kg of firewood.
While hovering approximately 50 cm above the landing site to drop off the cargo and passenger, the pilot heard a sudden metallic noise. This was immediately followed by an uncontrolled rotation of the fuselage. In an attempt to stabilize the aircraft, the pilot applied left pedal input, but the movement persisted. The pilot then lowered the collective pitch to force the aircraft toward the ground. However, the helicopter continued to spin and drifted laterally off the landing plateau, descending several meters into a hollow. During this descent, the main rotor blades struck the slope, causing the rotor to shatter. The aircraft came to rest in a ravine and caught fire. The pilot escaped without injury, while the passenger sustained a slight head injury.
The investigation
Investigators examined the wreckage and focused on the tail rotor drive shaft, which had snapped at the rear. The shaft, a 5.6-meter component consisting of three tubular sections, was analyzed by the Federal Laboratory for Testing of Materials. The investigation centered on the area near the articulated coupling, where the break occurred on a reinforced steel sleeve.
Findings
Technical analysis revealed that the failure was caused by fatigue due to internal corrosion. The investigation identified a circumferential groove running around the entire internal diameter of the tube at a critical point near the reinforcement sleeve. This corrosion had progressively weakened the section of the shaft until it failed under normal operational loads.
Furthermore, the examination of the protective coatings—specifically the zinc chromate and aluminized lacquer—showed significant blistering. This indicated that the protective layers had been applied over a surface that was either insufficiently cleaned or already experiencing active corrosion. The component had completed 1,038 hours of service since its last overhaul.
Safety action
Following similar failures in other aircraft of the same type, the manufacturer issued Service Bulletin No. 47.145 in May 1970. This directive required all operators to replace the tail rotor drive shaft tubular elements with new parts designed with improved protection against corrosion.