A phenomenon that affects pilots' perceptions of the horizon when flying in the mountains is being considered a likely factor in the crash of a Convair 580 plane fighting a wildfire near Lytton in July of 2010.

A report released by Transportation Safety Board of Canada Friday said the chopper, which was working as a tanker, was approaching a ravine near the Fraser River canyon and hit some trees. The retardant was unexpectedly dropped and seconds later the plane went into a tailspin and crashed to the ground.

The two men aboard, Tim Whiting, 58, of Langley and Brian Tilley, 36, of Edmonton died in the crash.

Rick Pederson, senior vice-president of the aviation company Conair, said the visual illusion is well known among experienced pilots.

"It's something that affects even the most experienced pilots. The captain on the aircraft was one of our most experienced pilots," he said.

"It's something that even though you're experienced, can happen to you."

The illusion occurs in mountainous or hilly terrain and pilots train to deal with it.

"You train for how to deal with it when you know it's going to be there," he said.

Although Pederson isn't a pilot, he tried to describe the effect.

"It's a visual event that happens where your eyesight when you're approaching a fire," he said.

"What typically happens is a horizon is optically, from a visual point of view, seen as being higher than it is. It actually disappears from the perspective when you're in mountains. You're in a valley and looking at mountain top horizons.

"Your eye sees mountain ridges as where they are but your position is lower than that. You can get trapped, to a certain degree, to being below a certain altitude relative to the terrain."

The accident prompted the company to double its efforts to ensure pilots are aware of the problem, he said.

"The first part is awareness, it's making sure the most experienced pilots are aware they're exposed to this environment. Our roster is experienced pilots, they know that can be expected," he said.

The training part also emphasizes using the right kind of tools, instruments in the plane as well as being able to take a look what's around.

"It's an environment we work in every day. We train crews for that, we do have a good record of being able to operate in this environment."

While Conair is based in Abbotsford, it does have a bird dog plane and a couple of other aircraft based in Kamloops, he said.