1979 SA 315 B Lama Accident at Corippo (HB-XDN)

Overview and Flight Details On 6 July 1979, an SA 315 B Lama helicopter, registration HB-XDN, operated by Eliticino SA, was conducting a commercial ferry flight from San Carlo (Valle di Peccia) to Vogorno (Valle Verzasca). The aircraft was in cruise flight when the incident occurred. ## Sequence of Events Approximately ten minutes after takeoff, the pilot heard a loud metallic bang, followed by severe vibrations and a total loss of engine power. At the time, the helicopter was about 2 km from its destination and at an altitude of approximately 150 m above ground. The pilot immediately entered autorotation. To reach the only available, though limited, landing area, he slightly pulled the collective pitch lever. This caused a slight decrease in rotor speed, and the helicopter landed hard. The main rotor struck a tree approximately 5 cm in diameter. The landing gear and front of the cabin were damaged, but the helicopter remained upright. ## Casualties and Damage The occupants, consisting of the pilot and two flight assistants, were not injured. The aircraft was damaged. ## Investigation Findings Disassembly of the Artouste III B engine (serial number 693) revealed that the failure was caused by the fracture of an axial compressor blade (part number 206 15 137). The resulting damage led to the extensive destruction of the engine. A visual examination of the fracture surface indicated a semi-lens-shaped fatigue crack extending across the full width of the blade. The fatigue fracture originated from a point on the blade's upper surface, approximately 5 mm above the blade root. The axial compressor operated at approximately 35,500 rpm, with operating temperatures below 100°C. The eleven blades, made of titanium alloy (Ti Al6 V4), were secured to a three-disc hub wheel with two bolts each at their roots. The manufacturer's specified service life for the axial compressor was 6,000 operating hours. The total operating time of the compressor was 4,155 hours. The last overhaul, which included non-destructive testing, was performed 396 operating hours, or about nine months, before the incident. During this overhaul, the compressor was cleaned with petrol and then with chloroethane, but no mechanical rework, particularly of the blade leading edge, was carried out. Laboratory investigations by EMPA found no structural defects or anomalies that could have favored crack formation. The determined mechanical properties and material analysis results were satisfactory. An examination for deformations and dimensional deviations, particularly of the mounting bolts, found no significant deviations in the bolt dimensions of the damaged blade compared to the intact blades. No acceleration of crack initiation or propagation due to corrosive effects could be proven. According to the manufacturer, the use of trichloroethane (chloroethane) for cleaning titanium alloy parts is not permitted due to the risk of stress corrosion cracking or fatigue crack corrosion. In many cases, no corrosion products are detectable in these types of corrosion. To determine the immediate effect of short-term contact with trichloroethane on the fatigue properties of Ti Al6 V4, tests were conducted. The results showed essentially no significant difference in fatigue strength under alternating bending stress between samples cleaned once with alcohol and samples repeatedly wetted with chloroethane. ## Weather Conditions The weather in Switzerland was characterized by a high-pressure system. In the accident area, the sky was overcast with a cloud base at approximately 2,600 m, visibility was more than 20 km, wind was light and variable, air temperature was 24°C, dew point was 14°C, and the air pressure was 1,020 mbar QNH. ## Assessment The engine failure was attributed to the fatigue fracture of an axial compressor blade. The primary cause of the fatigue crack formation could not be determined. Because the fracture origin zone was secondarily damaged, no precise information about the condition of the fracture origin could be provided. The possibility that a surface injury, for example from a foreign object, was present there could not be excluded. The pilot's actions during the failure were deemed appropriate.