TSB (Transportation Safety Board) Chair Kathy Fox releases details of an investigation report for a 2016 Citation jet crash near Kelowna, B.C. that killed four, including former Alberta premier Jim Prentice, in Calgary, Alta. on Thursday, April 26, 2018. THE CANADIAN PRESS/Larry MacDougal

CALGARY - The Transportation Safety Board expressed frustration Thursday that without flight recorders it was unable to pinpoint a definitive cause for the plane crash that killed former Alberta premier Jim Prentice and three others.

The Cessna Citation jet went down shortly after its 9:30 p.m. takeoff from Kelowna, B.C., on its way to the Springbank airport west of Calgary in October 2016. After reaching a peak altitude of 2,620 metres above sea level, the aircraft took a steep descending turn to the right, consistent with a spiral dive.

The board said the most plausible scenario was that pilot Jim Kruk had his hands full flying the aircraft alone and became spatially disoriented.

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 investigation determined that Kruk did not have enough recent experience flying in the dark to meet Transport Canada requirements for carrying passengers at night. He was supposed to have done five takeoffs and landings in the previous six months, but had only completed two.

Senior investigator Beverley Harvey said there's a greater risk of pilots becoming disoriented when they don't have enough proficiency flying at night. They may react based on strong but erroneous physical sensations — like the feeling of tumbling backward — and doubt correct information provided by their instrumentation.

Investigators ruled out medical and environmental issues. Both engines were producing substantial power and no parts fell off during flight. The wreckage was too badly damaged to say for sure if there was a mechanical or electrical problem.

"It's extremely frustrating when at the end of 18 months, we've come up with a likely scenario of spatial disorientation, but it is just that — a likely scenario," said Harvey.

The absence of a cockpit voice recorder and flight data recorder meant the TSB had no detailed sequence of what went on.

"All we have is a hypothesis — a scenario that doesn't have enough facts to be definitive," said TSB chair Kathy Fox.

"And that simply isn't good enough, which is why today we are recommending that Transport Canada require the mandatory installation of lightweight flight recording systems by commercial operators and private business operators not currently required to carry them."

The board has recommended recorders on small planes as far back as 1991.

It also raised concern over how Transport Canada oversees private business jets. There was no record of the operator of the jet involved in the fatal Kelowna crash, Norjet Inc., ever having been inspected by Transport Canada.

Transport Minister Marc Garneau said departmental officials are reviewing the report and will provide a formal response within 90 days.

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. He had previously been a Conservative federal cabinet minister.

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."

News from © The Canadian Press, 2018