In its report on the incident released Wednesday, the TSB said it also determined that lives could likely have been saved if the aircraft had been equipped with shoulder harnesses to complement the available lap belts, which the pilot and three passengers were all wearing.
A shoulder-harness kit was available from and recommended by the aircraft manufacturer but, according to the TSB, the plane's owners were not aware of this before the crash.
Two passengers died
The PA 30 Piper Twin Comanche airplane had veered off course when it crashed 30 kilometres west of Kelowna, near Brenda Mines, in August 2012, after departing Penticton for Boundary Bay Airport, near Vancouver.
The plane was torn apart in the crash, killing passenger Jayson Dallas Wesley Smith, 30, of Vancouver, instantly. Fellow passenger Lauren Patricia Sewell, 24, of Surrey, died from her injuries two days later.
In its report, the TSB said the privately-operated plane veered off course due to a lower-than-expected rate of climb.
The report attributed the reduced rate of climb to atmospheric conditions, a high aircraft weight, a reduced power output in the right engine, and the pilot's failure to use available turbochargers.
According to the TSB's findings, the pilot, who was relatively inexperienced, continued into an area of higher terrain but the plane was unable to climb quickly enough to clear the terrain, leading the aircraft to crash.
Since the incident, the Penticton Regional Airport has installed new signs advising pilots to climb to 5,000 feet before turning west or east to depart the Okanagan Valley.
NAV CANADA, which operates Canada's civil air navigation system, has issued the same warning, amending its information relating to the airports in Penticton, Oliver and Osoyoos.
Read the TSB report online