After taking off from Vancouver enroute to Calgary Trans-Canada Air Lines Flight 810 had to gain altitude to go over the mountains that surround Vancouver and the Fraser Valley. Because of a reported fire combined with the potential of severe weather the plane did not achieve the necessary altitude to clear the mountains or had enough room to perform the necessary emergency maneuvers. It crashed into Mt Sleese at the 7600-foot level.
According to the Aviation Safety Network website, the ICAO Accident Digest No.9, Circular 56-AN/51 (16-25) reported:
"The cause for the aircraft being at an altitude low enough to strike Mount Slesse is undetermined, but there is a high probability that the aircraft, while flying on 3 engines, encountered either severe icing, turbulence, subsidence, or a combination of all three, or suffered some other difficulty of such a sudden or dire nature that the crew were unable to communicate with any agency or control the aircraft.
For undetermined reasons the aircraft was not on Green Airway No. 1 to which it had been cleared by Air Traffic Control. The following factors contributed to the accident:
a) Loss of engine power No. 2 engine shut-down, fire suspected.;
b) Existence in the area of known subsidence, severe turbulence, and moderate to severe icing probably in the lower levels."