Too much weight, a hot day and a well-known optical illusion may have all been factors in a small plane crash that killed four men on British Columbia's Apex Mountain.

The Piper Comanche's mysterious disappearance in August set off an intense air search and even prompted a cash reward in hopes the men would be found alive.

The crash site was found seven days later and all aboard, including two brothers, were dead.

Transportation Safety Board spokesman Bill Yearwood said all four men were large and the plane was leaving with a full fuel tank.

The hot summer day may also have diminished the lift the plane needed to power itself over the western mountains from the Penticton, B.C., airport, Yearwood said.

Pilots leaving that airport are warned they should try to reach 1,500 metres before heading west because of the mountains.

Yearwood, who was once an instructor at the same airport, said he trained pilots about an optical illusion that's created as they head towards the western mountains.

"You can easily be lured into thinking that the terrain is not as high as it actually is," he said. "You reach a point where you can't turn around."

Yearwood said other pilots have flown too low to clear the mountains, and the signatures of the August crash are similar.

"There have been cases where pilots have got in there and ... (they) ran into trouble."

The plane carrying the four men hit a heavily treed area at slow speed and wrapped itself around several trees before hitting the ground, Yearwood said.

"That combination of temperature and altitude and weight is not good for performance," he said. "The damage and the accident-site scars etcetera are consistent with the lack of performance in colliding with the trees."

Killed in the crash were Rama Tello, 36, his brother Maya Tello-Wrigley, 21, Salem Dedovic, 30 and 35-year-old Jasson Minor. All the men were from the Okanagan area.

The Transportation Safety Board is not doing a full report on the crash and instead will be handing over its findings to the BC Coroners Service for its investigation.