What happened
On April 27, 1975, at approximately 07:45, a Bell 206 B helicopter, registration HB-XEF, was performing a commercial passenger flight near the Rosenegg mountain landing site in Switzerland. The flight was transporting a group of ski tourists and a mountain guide from the Steingletscher Hotel area to the landing site.
During the final approach to the landing area, the pilot encountered significant updrafts. Approximately 20 meters above the ground, the pilot noted an intermittent whistling sound, signaling an engine failure. To avoid immediate impact with a terrain ridge, the pilot initially maintained altitude at the expense of rotor RPM before maneuvering away from the ridge and transitioning into an autorotation. Realizing the engine would not restart and that the aircraft could not clear the mountain pass, the pilot executed an emergency landing on a sloping snow field north of the pass height. Upon touchdown, the right skid slid beneath the snow, causing the helicopter to skid and overturn on its right side.
The investigation
SUST investigators examined the aircraft's engine, specifically the Allison 250-C20, and tested various components on a test bench. While the fuel control unit, fuel pump, and fuel nozzle were found to be functioning correctly, the investigation focused on the fuel system's integrity.
Analysis of fuel samples revealed high levels of evaporation residue, exceeding legal specifications. Furthermore, a dark brown sediment consisting of iron and zinc corrosion products and mineral dust was discovered in the filter housing. Investigators also found that the Fuel Control Unit contained powdery contaminants, including earth, dust, and fibers.
Findings
- The one injured occupant was the pilot; the four passengers remained uninjured.
- The primary cause of the engine failure was contamination within the fuel system.
- A specific vulnerability was identified in the fuel system's overflow line, which allowed unfiltered fuel to bypass the primary filter and reach the fuel pump under certain conditions.
- The presence of dust and organic matter in the Fuel Control Unit likely caused a momentary interruption in fuel flow, leading to the engine flameout.
Safety action
Following the discovery of the contamination path, the manufacturer issued a service bulletin mandating the installation of a filter in the overflow line to prevent contaminants from entering the fuel control and governor units.