What happened
On July 6, 1979, an SA 315 B Lama helicopter, registration HB-XDN, was performing a positioning flight from San Carlo to Vogorno. Approximately ten minutes after takeoff, the pilot experienced a loud metallic bang followed by intense vibrations and a total loss of engine power.
At an altitude of roughly 150 meters and about two kilometers from the destination, the pilot immediately initiated an autorotation. In an attempt to reach the only available landing spot in the area, the pilot adjusted the collective lever. This action caused the rotor RPM to decrease slightly, leading to a hard impact with the ground. During the landing, the main rotor struck a tree approximately 5 cm in diameter. The impact caused damage to the landing gear and the front of the cabin, though the aircraft remained upright. The three occupants—the pilot and two assistants—sustained no injuries.
The investigation
Investigators examined the Artouste III B/SN 693 engine to determine the cause of the power loss. The disassembly revealed that the failure was caused by the fracture of an axial compressor blade. A visual inspection of the fracture surface showed a semi-lunar fatigue crack extending across the entire width of the blade, originating near the top surface, approximately 5 mm above the blade root.
Laboratory analysis of the titanium alloy (Ti A16 V4) components by EMPA found no structural defects or material anomalies. While the manufacturer advises against using trichloroethane (chlorothen) for cleaning titanium alloys due to the risk of stress corrosion cracking, experimental testing showed no significant difference in fatigue strength between samples cleaned with alcohol versus those exposed to chlorothen. The compressor had completed 4,155 hours of operation, well within the manufacturer's 6,000-hour limit, and had undergone a non-destructive inspection 396 hours prior to the accident.
Findings
- The engine failure was caused by the fatigue fracture of an axial compressor blade.
- The primary origin of the fatigue crack could not be definitively determined, though a surface nick from a foreign object could not be ruled out.
- The pilot's emergency procedures and response to the engine failure were appropriate.
- The landing site was characterized by terrain that was unsuitable for a safe emergency landing.