Accident Overview
On 7 December 2015, at approximately 1120 local time, an Airbus Helicopters AS 350 B3 (registration LN-OXG) was involved in an accident near Brydalsfossen in Masfjorden municipality, Hordaland, Norway. The helicopter was operated by Airlift AS and was being used to lift poles and other materials for power line work.
Flight Details
Prior to the mission, the pilot and the customer had conducted safety job analyses (SJA) using both the customer's and Airlift's SJA forms. On the day of the accident, the helicopter had completed 16 lifts with an 18-meter longline before being called to a new location along the power line to transport return poles.
Accident Sequence
The accident occurred when the helicopter was positioned over a steep slope at an estimated height of 20 meters above the ground. Through his floor window, the pilot observed a tree falling from his left side. The tree first struck the empty longline and then hit a person on the ground. It was later determined that two trees had fallen; the tree that struck the person was approximately 15 meters long and had its root system pulled out.
Injuries and Damage
One person on the ground sustained two fractured ribs and a punctured lung. The helicopter sustained no damage, and no other injuries or damage were reported.
Pilot Report
In his report, the pilot stated that the rotor downwash from the helicopter likely triggered the tree fall.
Company Response
According to Airlift's internal investigation, the company had not experienced a similar incident previously, despite many of their operations involving flying near trees. The company acknowledged that it is extremely difficult for pilots to identify trees with poor root anchorage. Consequently, Airlift decided to update the 2016 version of its safety course to include information about this incident and to emphasize the hazards associated with rotor downwash.
Investigation Board Assessment
The Accident Investigation Board of Norway (SHT) assessed that this accident is more akin to a workplace accident on the ground than an aviation accident, but it marginally meets the definition of an aviation accident because rotor downwash triggered the tree fall. The board noted that experience indicates rotor downwash from helicopters of this size does not normally topple healthy trees. SHT concurred with Airlift that it is unrealistic to expect pilots to identify trees with poor root anchorage, and that this task should primarily fall to ground workers as part of their ongoing accident prevention efforts.