The Piper PA-30 Twin Comanche left the local airport with the pilot and three passengers aboard on Aug. 12, 2012, and crashed shortly afterwards in a treed area near the Brenda Mines site.
Two passengers, 30-year-old Dallas Smith of Vancouver, and his girlfriend, 24-year-old Lauren Sewell, died and the pilot and third passenger were seriously injured.
The report says the aircraft was about 70 kilograms overweight and couldn't gain enough altitude to make it out of the steep terrain.
The TSB also says the pilot failed to use available turbochargers to gain altitude and that reduced power to the right engine contributed to the slow rate of climb.
The safety board found the pilot and passengers were at increased risk of injury because the small plane wasn't equipped with shoulder harnesses.
Also on HuffPost