Coroner Brigitte Windley opened her inquest into the death of William Bruce Andrews (49), on December 15, 2013, in Queenstown yesterday.
Mr Andrews, a pilot with more than 15,000 hours' experience, left Rat Point, near Queenstown, at 4.49pm to fly via Dumpling Hut to pick up an injured Department of Conservation worker and fly him to Milford.
His last contact was about 4.55pm. He did not make it to Dumpling Hut and his locator beacon was not activated.
The destroyed, smouldering helicopter, which had recently been overhauled, was discovered from the air by colleagues Sarah de Reeper and Sean Mullally about 9.15 that night.
Miss de Reeper said when they first spotted the crashed helicopter it was "obvious no-one could survive that''.
The flight path showed Mr Andrews had flown into Glade Burn, over a saddle and into the Glade Valley.
There, he took an inexplicable right turn, before turning left and crashing moments later.
The weather had been variable that day, but improved in the afternoon.
The CAA found it likely Mr Andrews had "flown into cloud, became spatially disorientated and had a controlled flight into terrain''.
However, Sir Richard Hayes, who was part of the search team on December 15, said that was "purely speculation''.
"I wasn't there. Nor was anyone else.
"Did he fly into cloud? [Was there] a mechanical failure? [A] medical problem? "I don't know.
"I can't comment on that and I struggle how anyone else can speculate on what happened, because no-one else in this room was there.''
The CAA found "ample evidence'' there was cloud cover in the valley. One of the reasons was because Mr Andrews' helicopter slowed to a "crawl''.
Sir Richard said while that was possible, there were multiple reasons a helicopter would slow, or stop to hover, during a flight, including mechanical failure, a medical event, or allowing the pilot to assess conditions.
"The first thing that happens when something goes wrong ... is to slow the aircraft.''
Sir Richard had known Mr Andrews since about 1984 and described him as "meticulous'' and a "bloody good pilot'' who displayed the highest level of professionalism and had an exemplary attitude towards safety.
The coroner heard Mr Andrews, a father of two, recently passed a medical check by a "flying doctor'' based in Palmerston North.
He had returned from Bangkok on November 26 and the next contracted days he worked for Milford Helicopters were December 14 and 15.
On the day of the fatal flight, Mr Andrews was questioned about seven times by Mr Mullally and Miss de Reeper about leg pain he was experiencing, which they believed stemmed from an old ankle injury.
That day he told Miss de Reeper he had not experienced pain like it before and it was "up the side'' of his leg.
During the day he began limping. It became more pronounced, but, when questioned, he insisted he was fine to fly.
Neither of his colleagues, nor Milford Helicopters owner Jeff Shanks, were aware of any issues he had with his vision. However, friend and fellow pilot Russell Knight said a few days before the crash Mr Andrews told him his "eyes would go hazy and fuzzy from the top down'' sometimes when he was flying.
"About five minutes later he said `It's happening now','' Mr Knight said.
"He didn't say how long it lasted. Presumably after about a minute it was OK.''
Mr Knight said Mr Andrews had intended to visit a doctor about his vision issues, but was not sure if he'd done so before the fatal flight.
A total of 14 witnesses are to be called during the inquest, which is expected to continue until Friday.
