Introduction
On 18 March 2019, at approximately 1130 local time, a Robinson R44 helicopter (LN-OGT) was involved in an accident at Uvdal alpinsenter in Nore og Uvdal municipality, Viken county. The private flight was intended to return the pilot to Asker. The helicopter sustained damage to the tail rotor, tail rotor drive shaft, and vertical fin. The pilot, the sole occupant, was not injured.
Event Sequence
The pilot had spent the weekend at a cabin near Uvdal alpinsenter. On Sunday, he attempted to start the helicopter but the engine would not start, a recurring issue in cold or humid weather. He placed a heater in the engine compartment overnight. On Monday at 1030, he started the engine normally, performed a full start-up procedure, and ground-ran the helicopter for about five minutes until oil and gearbox temperatures reached normal operating levels. He then shut down the engine, cleared the area, and prepared for departure.
At approximately 1130, after a normal start, the pilot cleared the area and lifted the helicopter into a low hover. He noticed unstable behavior around the yaw axis. Intending to turn the nose left to depart into the wind (from 310–340 degrees), he applied left pedal. The helicopter initially turned left but then began a rapid yaw to the right. The pilot compensated by increasing left pedal input, eventually applying full left pedal. He managed to stop the right yaw but could not turn the helicopter left. With the nose pointing 50 to 60 degrees right of its original heading, he checked the instruments, which indicated normal operation.
Deciding to abort the takeoff, the pilot lowered the collective. As the skids contacted the ground, the helicopter yawed sharply left. He then fully lowered the collective, heard a metallic sound, and shut down the engine. Investigation revealed that the tail rotor and vertical fin had struck the snow-covered slope behind and to the right of the helicopter. The helicopter had drifted approximately 4 meters to the right during the hover.
Weather Conditions
According to the Norwegian Meteorological Institute (Værvarslinga på Vestlandet), observations around Uvdal alpinsenter at the time showed predominantly northwest wind direction with varying strength and recorded gusts. No precipitation or clouds were observed in the area. The air was unstable with turbulence. At Geilo airport Dagali (11 km north), mean wind was 10–15 kt with gusts of 20–25 kt between 1000 and 1200. In a larger area, mean wind was 10–30 kt, with strongest gusts of 34 kt at Geilo and 36 kt at Finsevatn. A station north of Uvdal recorded gusts of 38 kt. All stations were at 800–1200 m elevation; Uvdal alpinsenter lies at 750 m. Based on upper winds, the meteorological institute considered it probable that gusts up to 40 kt could have occurred at Uvdal around the time of the accident.
The pilot had checked weather through METAR from Oslo Gardermoen, a mobile weather app, web cameras, and a conversation with an acquaintance in Oslo. He perceived the wind as weak when he entered the helicopter.
AIB Assessment
The Accident Investigation Board (SHT) noted that the pilot used several available tools to gain weather knowledge but that precise weather information is difficult to obtain, especially in mountain areas. The board found it likely that variable and sudden downdrafts struck the terrain, causing the pilot to lose directional control when the helicopter was in the hover for takeoff. The side wind likely also caused the helicopter to drift about 4 meters to the right. When the pilot aborted and lowered the helicopter, it had drifted far enough for the tail rotor to contact the snow-covered slope.
The investigation could not rule out that wind, due to downdrafts, increased significantly in the minutes after the pilot entered the helicopter. Such an increase in wind from the left side may have led to loss of tail rotor effectiveness (LTE) and subsequent loss of directional control. LTE is a phenomenon known to cause accidents in helicopters, often resulting from strong side wind while hovering or at low speed. On helicopters with a counterclockwise-rotating main rotor (as seen from above), such as the Robinson R44, wind from the left between the 8 and 11 o'clock positions is critical.
Damage
The tail rotor, tail rotor drive shaft, and vertical fin sustained substantial damage. No other damage was reported.