What happened
On July 24, 2018, a Robinson R44 II helicopter, registration LN-OAL, was engaged in a commercial reindeer herding operation in the Skäckerfjällen mountains in Sweden. While flying at a low altitude on the leeward side of Sandfjället, the aircraft encountered moderate to severe turbulence and shifting wind directions.
During the flight, the pilot received a low rotor RPM warning. Despite attempts to lower the collective to increase engine RPM, the aircraft's performance continued to degrade. The pilot, unable to reach a pre-planned escape route through a nearby ravine, determined that an immediate emergency landing in a nearby boulder field was necessary to maintain control. During the landing, a sudden wind gust struck the aircraft, causing it to impact the ground on its landing gear. While the two occupants escaped without injury, the aircraft suffered extensive damage, including broken rotor blades, a broken tail boom, and a fractured landing skid.
The investigation
The Norwegian Safety Investigation Authority (NSIA) conducted the inquiry, focusing on whether technical failures contributed to the loss of performance. Although the pilot initially suggested that a reduction in engine power might have been a factor, technical examinations of the Lycoming engine found no evidence of mechanical failure or power loss. Investigators found that the engine and airframe had been properly maintained according to regulations.
Technical inspections of the engine revealed some corrosion and marks on the cylinder walls, but these were attributed to unprotected storage in an unheated barn following the accident rather than a cause of the incident. The investigation also noted that the pilot's delay in reporting his suspicion of engine issues hindered the ability to distinguish between accident-related damage and post-accident degradation.
Findings
- The primary cause of the accident was the helicopter encountering downdrafts and turbulence that exceeded the aircraft's performance capabilities while flying on the leeward side of the mountain.
- Environmental factors, including high ambient temperatures and high density altitude, likely contributed to the reduced performance of the aircraft.
- The accident was a result of functional resonance, where several non-critical variations—such as increasing winds, shifting directions, and reduced engine performance due to environmental conditions—combined to create an uncontrollable situation.
- The pilot's inability to reach lower terrain prevented a controlled recovery of rotor RPM.