One of the mysteries of the Erebus crash not understood by many is how Captain Jim Collins and his crew could fly into the side of the mountain in broad daylight with good visibility.

Paul Davison, QC who represented the Collins family and the Pilot's Association at the Royal Commission describes what happened: "He had been insidiously tricked into believing everything was safe by all of the systems he was using and all the resources he was using including
his eyes."

The phenomenon Collins came up against is known as sector whiteout - a situation, says Davison, where you maybe operating in clear air, but your eyes are not able to provide surface and distance definition because the diffuse light can create a convincing ocular illusion.

When the decision was made to descend Davison says Collins, like any pilot, was concerned to use his vision to maintain terrain avoidance and keep the aircraft in a safe location.

This would involve him looking to identify features of landscape and topography of the area.

"Believing that his aircraft was in the centre of McMurdo Sound he would expect to see an expanse of flat sea ice to South."

Which is what he saw, but even so, in making his descent he was ultra cautious by adhering to his Nav track and turning the aircraft first to the right before proceeding back to the north where he had just come from.

In making his decent he undertook a descent pattern which ensured he kept the aircraft in clear air over the flat sea ice and only then did he recapture the Nav track to proceed south.

"What he is doing is covering territory he knows it is safe to descend into."

As Davison points out, for aviators, one's eyes are one's protection and insurance in these circumstances.

"That is the great irony because here his eyes deceived him. The intersection of the overcast with the mountain and the gradation of shade would have given an illusion of an horizon many miles away when he was actually looking at the slopes of Mt Erebus which would have been only several miles away."

At that point he went under the overcast cloud, but remained in clear air.

It didn't help either that Air New Zealand pilots hadn't been briefed on what whiteout conditions were.

"Had he been briefed to understand the risks of proceeding under overcast he wouldn't take that risk without the appreciating the danger."

The insidious nature of these deceptions lead him to believe his understanding of his location was accurate.

His navigation system told him he was on course.