A helicopter pilot's decision not to refuel led to a crash in northern Alberta last year that killed a surveyor studying mountain pine beetles, an investigation has found.

The investigation by the Transportation Safety Board of Canada also found that the flight company's practice to fly helicopters at low fuel levels likely influenced the pilot's decision to continue the flight when it was unsafe to do so.

The Bell 206B Jet Ranger helicopter, operated by Ridge Rotors, was flying two surveyors on Sept. 5, 2016, when it fell 160 feet and collided with trees, said in the report released Monday.

The helicopter was conducting aerial surveys in the boreal forest near Fox Creek as part of a contract with Alberta Agriculture and Forestry.

"The helicopter suddenly lost engine power and, within seconds, descended and collided with trees," the report said

"The surveyor (a 44-year-old woman) sitting in the front was fatally injured when trees penetrated the cockpit, while the other surveyor (a man) seated in the back sustained minor injuries."

The female pilot suffered serious injuries, including a broken leg.

The report said that during a short rest break on a sand bar before the crash, the pilot decided not to refuel at a nearby fuel cache, even though the chopper was close to the minimum recommended fuel level.

The pilot's decision was likely influenced by the practice of Ridge Rotors, based in Blue Ridge, Alta., to fly with low fuel levels, the report concluded.

The low fuel levels allowed air to enter the fuel pumps as the chopper banked into a turn, leading to engine power loss, the report said.

An automatic ignition system could have prevented the sudden power loss, but it was turned off as the company only used the backup system in winter conditions, the report said.

"It is important for operators to understand the limitations of the Bell 206B helicopter fuel system and the risks associated with flights conducted with less than 20 US gallons of fuel," it said.

"If operators do not observe the minimum fuel quantities recommended in the flight manual, there is a risk that the helicopter will be operated at fuel levels conducive to engine power loss."

Since the crash, Ridge Rotors has implemented changes in its standard operating procedures and trained pilots accordingly, the report said.

Fox Creek is 260 kilometres northwest of Edmonton.