The lack of flight recorders prevented the TSB from definitively concluding what caused a plane crash that killed former Alberta premier Jim Prentice and three others, but the pilot was probably disoriented, investigators say.

The Transportation Safety Board called on Transport Canada to make flight recording systems mandatory for all commercial and private business operators, as it released its report from an 18-month investigation into the crash.

Read the TSB report

The Cessna Citation jet disappeared from radar shortly after takeoff from Kelowna, B.C., on its way to the Springbank airport west of Calgary on Oct. 13, 2016.

Killed in the crash with Prentice were the pilot Jim Kruk, a retired RCMP officer; optometrist Ken Gellatly, the father-in-law of one of Prentice's three daughters; and Calgary businessman Sheldon Reid.

Investigators couldn't pinpoint the exact cause of the crash because the plane, which was built in 1974, was not equipped with a cockpit voice recorder or a flight data recorder, the TSB said. Neither is a requirement for that type of aircraft.

An overview shows crash wreckage of the Cessna Citation. (Transportation Safety Board)

"All we have is a hypothesis," TSB chair Kathy Fox said as she released the report in Calgary.

"And that simply isn't good enough."

Pilot likely had 'spatial disorientation'

The safety board says the "most plausible scenario" for the cause of the crash was that Kruk became disoriented due to a heavy workload at the controls.

There were no emergency or distress calls made, according to the TSB.

The investigation revealed that the plane's climb rate changed rapidly several times within 30 seconds just before it went down, lead investigator Beverley Harvey said.

It also deviated to the right by 20 degrees, she said.

TSB chair Kathy Fox says Transport Canada should make flight recording systems mandatory for all commercial and private business operators. (CBC)

"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 determined the pilot didn't have enough recent 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 Harvey.

"Pilots who do not have sufficient night proficiency are at a greater risk of experiencing what's known as spatial disorientation."

Fox called on Transport Canada to make flight recording systems mandatory.

The board also said it has concerns about the way Transport Canada conducts oversight of private business aviation in the country.

"During the course of its investigation, the TSB found no record that the operator of this aircraft had ever been inspected by TC," the board said in a release.

"As such, TC was unaware of safety deficiencies in its flight operations, such as the failure to obtain approval for single-pilot operation of the aircraft and the pilot's lack of recent night flying experience required to carry passengers at night."

Prentice family thanks TSB

Prentice, 60, was also a former federal cabinet minister.

He became premier of Alberta in September 2014 after winning the leadership of the province's governing Progressive Conservative Party. He quit politics in May 2015 after the NDP swept the PCs from power.

The Prentice family thanked the TSB for its work on the investigation.

"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," the family said in a written statement.

"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. Our thoughts and prayers are with the other families impacted by this tragedy."

In anticipation of Wednesday's TSB report, Greg McConnell, national chairman of the Canadian Federal Pilots Association, said one change he would like to see is regular Transport Canada inspections for small aircraft such as the business jet Prentice died in.