The Transportation Safety Board says the pilot of a plane that crashed, killing former Alberta premier Jim Prentice, was probably disoriented while flying in the dark, but investigators will never know for sure because the aircraft didn't have flight recorders.
The Cessna Citation jet went down shortly after takeoff from Kelowna, B.C., on its way to the Springbank airport west of Calgary in October 2016.
The plane took off about 9:30 p.m. and shortly after took a steep descending right turn and hit the ground from 2,580 metres above sea level.
The safety board says the "most plausible scenario" is that pilot Jim Kruk became spatially disoriented because he had a lot to do at the controls.
Kruk, a retired RCMP officer, optometrist Ken Gellatly, the father-in-law of one of Prentice's three daughters and Calgary businessman Sheldon Reid all died with Prentice.
"The most plausible scenario is that the pilot, who was likely dealing with a high workload associated with flying the aircraft alone, experienced spatial disorientation and departed from controlled flight shortly after takeoff," the TSB said in a release issued ahead of a news conference in Calgary.
The investigation also determined that the pilot did not have enough experience flying after dark.
"The pilot, although experienced, had very little recent experience flying at night with just two night takeoffs in the past six months. This did not meet Transport Canada's requirements to carry passengers at night," said senior investigator Beverley Harvey.
"Pilots who do not have sufficient night proficiency are at a greater risk of experiencing what's known as spatial disorientation."
Prentice was Alberta's premier from October 2014 until his election loss the following spring when the Progressive Conservatives were kicked out by the NDP after more than 40 years in power.
His family issued a statement thanking the board for its work.
"While this report cannot restore what has been lost, it is our hope the learnings from this tragic event can be used to prevent similar accidents in the future," it said.
"We are proud of Jim's contributions to Alberta, to Canada and to public service, but he was first and foremost a loving husband, father, grandfather and sibling. We will always miss him."
Harvey said the "physical illusions" that accompany disorientation while flying can occur during prolonged acceleration such as during an initial climb after takeoff.
"Even though they are erroneous, these sensations can be intense causing pilots to doubt their instruments, to incorrectly adjust controls or even put the aircraft into an accidental spiral dive."
The lack of concrete proof as to what happened was extremely frustrating for investigators, she said.
Kathy Fox, the safety board's chairwoman, said the plane involved in the crash was not required to have a flight data recorder. The Transportation Safety Board is recommending such recorders, as well as cockpit recorders, be required on all commercial and private business aircraft.
"We don't like having to say we don't know when asked what caused an accident or why," Fox said. "We want to provide definitive answers to the victims' families, to Canada's aviation industry and to the Canadian public."
