The Transportation Safety Board of Canada is investigating after a helicopter toppled down the side of a mountain near Tulita, N.W.T., last month, leaving the pilot with serious injuries.

It happened on Feb. 15 in a helicopter operated by Great Slave Helicopters, for Sahtu Helicopters.

The pilot was performing a ground engine run to warm up the machine on a helipad when the helicopter began to vibrate, "bounce and yaw to the left," Transport Canada's incident report states.

The pilot attempted to lift the machine off the pad but lost control, and the helicopter tumbled roughly 91 metres down the mountain. 

Tulita, N.W.T.

Tulita, N.W.T., lies on the Mackenzie River southeast of Norman Wells. The incident happened just outside the community. (CBC)

While the pilot was wearing a lap and shoulder harness, they were not wearing a helmet — something Blair Elliott, the director of operations for Great Slave Helicopters, said they are not required to do for this type of activity.

Elliott said the pilot was medevaced to Edmonton for a severely broken arm.

It was -30 C at the time of the incident, he said, adding it happened near Bear Rock.

The aircraft has yet to be recovered and there's "no smoking gun" pointing to the cause of the incident, Elliott said.  

With files from Kate Kyle