Garry Malley, from Dundee, died instantly and four other children with special needs were injured, when the helicopter - arranged by the television celebrity Noel Edmonds' charity, Airborne, crashed in a field during an emergency landing.
Earlier this year the pilot, Robert Hobson, was cleared of negligence relating to the crash at Glamis Castle, Angus, in July last year, when he appeared before a Crown Court.
Yesterday an inquiry at Forfar Sheriff's Court, was told the pilot's visibility was hampered because the helicopter - a Bell Jet Ranger - misted up in wet and humid weather.
Another pilot, Paul Smith, told the inquiry that moments before the crash Mr Hobson told him on the radio he had visibility problems.
``He said he had a problem and he couldn't see through the windscreen,'' Mr Smith said. He said he had experienced problems with that make of helicopter. ``I have flown all different types and all seem to have a very inadequate de-mister,'' he said.
``When conditions are like those in July last year, it just doesn't demist and it can get suddenly worse in the air.''
Earlier the inquiry was told that in the run-up to the crash, there seemed to be confusion between the organisers and those responsible for the flying operation, Forth and Clyde Helicopters, based at Edinburgh Airport. An organiser for the Airborne charity, Sarah Joyce, admitted there seemed to be "confusion" surrounding boarding arrangements for children on the three helicopters operating that day. A list found in the Bell Jet Ranger that crashed contained details of passengers who should have been on another helicopter.
The inquiry continues.