UPDATE: 11:40 p.m.

The Transportation Safety Board of Canada is recommending Transport Canada require all commercial and private business aircraft carry flight recording systems, after an 18-month investigation into the fatal 2016 plane crash near Kelowna netted no definitive answers.

At a press conference in Calgary Thursday morning, Beverley Harvey, TSB lead investigator, said they don't like saying “we don't know” after 18 months of investigation, but without the assistance of a flight data recorder, they were unable to find a conclusive reason for the 2016 crash that left four people dead, including former Alberta premier Jim Prentice.

Harvey said the “likely reason” for the crash was due “spatial disorientation,” which can occur when flying at night, and involves feelings of falling backwards and vertigo.

While pilot Jim Kruk was required to have five nighttime takeoffs and landings in the past six months to fly at night with passengers, he had only had two.

Additionally, the aircraft was required to have Transport Canada approval to fly with a single pilot, but it did not.

Harvey said lack of recent experience with nighttime flying can lead to spatial disorientation, and a second pilot can intervene in those situations.

The operator of the aircraft had also never been inspected by Transport Canada.

“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,” the TSB said in a press release. “Since this occurrence, TC has said that it will conduct targeted inspections of private business operators starting in April 2018.”

The investigation also found there was a "weight-balance issue" with the way the pilot had entered data prior to takeoff, but the TSB determined this didn't contribute to the crash. Additionally, while the plane was barred from flying through icy conditions due to a "windshield-heat anomaly," they determined there was no ice on the evening in question. 

Harvey said Transport Canada has 90 days to respond to its recommendation regarding mandatory flight recorders, but the independent agency has been making the same recommendation for decades.

Kathy Fox, chair of the TSB, says in the past five years, the recorders have come down in price and now weigh less, making their implementation on smaller planes more feasible.

ORIGINAL: 10 a.m.

Castanet's Nich Johansen is speaking live with Transportation Safety Board investigator Beverley Harvey.

We'll update this story with more reports from the TSB press conference in Calgary into the 2016 plane crash that killed former Alberta premier Jim Prentice and three others.

Recapping the TSB's live streamed findings, the board says spatial disorientation is the most plausible explanation for the crash.

The aircraft did not lose any parts in flight, and there were no emergency radio communications prior to the crash east of Wood Lake.

However, the company that owned the aircraft did not have operational approval to fly it with a single pilot, and the pilot did not have adequate nighttime flying experience to be carrying passengers.

Harvey said spatial disorientation is a "plausible, likely scenario."

The TSB does assign blame or criminal responsibility, however.