What happened
On July 24, 2018, a Robinson R44 II helicopter, registration LN-OAL, was engaged in reindeer herding operations in the Skäckerfjällen mountains of Jämtland, Sweden. The flight, operated by Midtnorsk Helikopterservice AS, was conducting low-altitude maneuvers on the leeward side of Sandfjället. During the mission, the pilot encountered increasing winds and moderate to severe turbulence.
As the aircraft flew at an altitude of approximately 100 feet, a low rotor RPM warning triggered. The pilot attempted to lower the collective to increase engine RPM and sought to climb to safety, but the warning persisted. Realizing that the aircraft's performance was insufficient to reach a pre-planned escape route through a nearby ravine, the pilot decided to perform an immediate emergency landing in a nearby boulder field. During the landing, a wind gust struck the aircraft, causing significant damage to the rotor blades, tail boom, and landing gear. There were no fatalities and no injuries to the two occupants.
The investigation
The investigation, conducted by the Norwegian Accident Investigation Board (delegated by the Swedish authority), examined the aircraft's mechanical condition and the environmental factors present during the flight. While the pilot initially reported that the incident was purely weather-related, he later suggested that a reduction in engine power might have contributed to the loss of altitude.
Technical examinations of the Lycoming engine and the drive train were performed. Investigators found no evidence of mechanical failure, such as metal debris in the oil filter or fuel strainer, or internal damage to the pistons or valves that could explain a sudden loss of power. The engine had been properly maintained according to regulations. The investigation focused on whether the aircraft's performance was compromised by the combination of high ambient temperature, high altitude, and vertical wind components.
Findings
- The primary cause of the accident was the helicopter encountering turbulent air and downdrafts on the leeward side of the mountain that exceeded the aircraft's climb capability.
- Environmental factors, including high temperatures and the aircraft's operating altitude, likely reduced the available engine performance.
- The pilot's attempt to use a nearby ravine as an escape route was unsuccessful because the aircraft was already too far from suitable low-altitude terrain.
- The investigation noted that the pilot's delayed reporting of suspected engine issues hindered the technical examination, as the aircraft was stored in an unprotected environment, leading to surface corrosion.