What happened
On November 9, 1972, a Bell 47G-3B-1 helicopter, registration HB-XBT, was performing a transport flight near Piz Muttler in Switzerland. The aircraft was carrying a pilot and two passengers to a high-altitude landing site to assist with a telecommunications installation.
As the helicopter approached the landing platform on the mountain ridge, witnesses on the ground observed a loud bang and saw metal fragments flying away from the aircraft. Immediately following this event, the helicopter performed a sharp maneuver and entered a descent along the mountain slope. Approximately fifteen minutes after the initial disturbance, the aircraft struck a rocky outcrop roughly 100 meters below the summit. The impact caused the helicopter to strike a ledge with its left skid, leading to a series of rolls down the steep terrain. All 3 occupants of the aircraft perished in the accident.
The investigation
Investigators examined the wreckage and the engine components to determine the cause of the mechanical failure. The analysis focused on the engine's cooling fan, which is a ring-shaped propeller made of aluminum-copper-magnesium alloy. Laboratory testing revealed that the failure was caused by material fatigue originating at a sharp edge on an underlying washer.
The investigation also traced the history of the cooling fan, noting it had been salvaged from a different helicopter that had crashed in 1970. While the component had undergone a general overhaul, the inspection was limited to a visual check and did not include a specialized crack detection procedure. Additionally, investigators reviewed the pilot's recent activity, noting a very short period of rest prior to the flight, and analyzed the pilot's blood alcohol content, which was measured at 0.19/0.22 ‰.
Findings
- The primary trigger for the accident was the fatigue-induced breakage of the engine cooling fan blades.
- The failure of the fan caused secondary damage, including the loss of several adjacent blades and damage to the ignition cables, though these did not significantly impair the engine's power output.
- The pilot attempted to abort the landing and initiate an emergency maneuver (likely an autorotation) due to the sudden noise and visible debris, but the maneuver resulted in terrain collision.
- The pilot's attention was likely diverted to monitoring instruments following the engine disturbance, which may have led to the failure to clear the rocky ledge.
- The pilot's recent rest period was insufficient, and the recorded blood alcohol level was near the recognized danger threshold of 0.2 ‰.
Safety action
- The commission recommended a review of operational rules regarding minimum rest periods, suggesting that the current requirements should be increased to at least 8 hours of rest, including meal times.