An air accident expert yesterday said that the pilot of a helicopter, which crashed killing nine-year-old Garry Malley, was ''unwise'' to take off when his windscreen was misted up.

Mr Robin Tydeman, an experienced pilot and an inspector of air accidents at the Air Accident Investigation Branch at Farnborough, watched a home video of the helicopter as it took off for the flight which was to end in tragedy. It crashed in a field beside Glamis Castle during an event involving the Noel Edmonds charity Airborne.

Giving evidence at the trial of the helicopter pilot, Captain Robert Hobson, Mr Tydeman told how pilots rely solely on visual references during take-off and landing.

Having seen the video footage, Mr Tydeman was asked by Procurator-fiscal Ian Walker if visual references could be seen by the pilot at the point of take-off. He replied: ''In that video I could not see, distinctly, shapes inside that helicopter. It is improbable that someone sitting inside could see clearly objects outside the helicopter.'' He later added that ''in his opinion'' the windscreens were misted up at take-off.

Mr Walker asked the witness, ''Was it safe to take off?'' Mr Tydeman answered: ''It is difficult to answer that question, because when making a decision in flying a helicopter, actually as in the same way as in driving a car, it is your perception of the risk.''

Captain Hobson, 56, Nether Moss, Howgate, Penicuik, faces two charges of contravening Air Navigation Orders on July 13 last year at Glamis Castle and Newhouse of Glamis Farm.

One alleges he endangered the helicopter by taking off when visibility was restricted by the misted windows. The second alleges he had six passengers on board when he was only licensed for five.

Forfar Sheriff Court heard that Mr Tydeman interviewed Captain Hobson after the accident at his home along with police officers.

Mr Tydeman agreed that during the interview Captain Hobson had told him that there had been misting problems but steps had been taken to wipe the windscreen before take off. Captain Hobson had said that after 30 seconds the screen again became misted and after two or three minutes it became a problem and he radioed control to say he was back to land at the base.

Mr Tydeman said Captain Hobson made a sensible move when he decided to land elsewhere when he heard over the radio that another helicopter was lifting off.

The court heard that the aircraft crashed after it was trapped by a wire fence in a field by Newhouse of Glamis.

The trial continues.