The pilot and one passenger were killed when the Air Tindi plane went down near a community on the east arm of Great Slave Lake in October 2011.
A Transportation Safety Board investigation found the weather that day was rainy and overcast with poor visibility.
"The aircraft was flown at low altitude into an area of low forward visibility, which prevented the pilot from seeing and avoiding terrain," the investigation concludes. "Weather during the accident flight was marginal for (visual flight rules) flight."
The Cessna 208B Caravan did not have electronic aids such as a terrain awareness and warning system or terrain-warning GPS. But it was fully equipped for instrument flying and the pilot and the company were qualified in such navigation.
"Flying under (instrument flight rules) would have provided a margin of safety given the weather conditions," the board wrote. "It could not be determined why the pilot chose to fly under (visual flight rules)."
The report also found the pilot was flying over Great Slave Lake beyond the gliding distance of his airplane.
There was another issue as well.
"Toxicology testing revealed that concentrations of cannabinoids found in the pilot's bloodstream were sufficient to have impaired pilot performance and decision-making during the flight."
Those concentrations were "considerably greater" than levels that impaired pilot performance in flight simulator tests, the report says.
"The quantity of psychoactive components in the pilot's system is considered to have been sufficient to have resulted in impairment of cognitive processes."
The flight did not have a co-pilot.
The board says Air Tindi has since instituted random drug tests for all employees in safety-sensitive jobs.
In a prepared statement, Air Tindi said: "The (board) report indicates that this accident was a function of various factors, including poor weather conditions and some of the decisions made by the pilot. The accident was a tragedy by any measure and we remain deeply saddened by the tragic consequences of that accident."
The statement also says that Air Tindi has "always had ... stringent monitoring for all team members who work in safety-sensitive positions."
"We would like to again express our condolences to the friends and family affected," said Air Tindi president Sean Loutitt.
The company did not answer questions about the board's report.
Since 1991, the Transportation Safety Board has found four air, marine or rail accidents in which operators either tested positive for marijuana or were known to have used it while responsible for their vehicles.
Mandatory drug testing for employees in federally regulated transportation industries does not exist in Canada, although it does in the U.S.
The regularly scheduled flight went down about 25 kilometres from the community of Lutsel K'e, narrowly missing a massive cliff but crashing onto a narrow peninsula. Rescuers responded immediately.
A privately owned Twin Otter landed on Great Slave Lake near the crash site and rescuers beat their way through the bush to the site.
A helicopter owned by Great Slave Helicopters also flew in, as did a Royal Canadian Air Force Hercules search-and-rescue aircraft. RCMP also choppered to the site.
Two of the four passengers, both from Lutsel K'e, were injured. Rescuers were unable to save the pilot, Matthew Bromely, 28, and passenger Timothy Harris, 54.
Since the accident, the board has required all planes with more than six passenger seats to have terrain awareness and warning systems installed by July 2014.