Former world rally champion Colin McRae was to blame for the 2007 helicopter crash that killed him and three passengers, a judicial investigation in Scotland ruled.

McRae, 39, his five-year-old son Johnny, the boy's six-year-old friend and a 36-year-old man died when the Eurocopter Squirrel helicopter they were in crashed into trees in Lanark four years ago.

A fatal accident inquiry has found that McRae - who did not hold a valid licence to pilot the chopper - had been "imprudent" by attempting tricky manoeuvres in difficult terrain "contrary to principles of good airmanship".

Lanark Sheriff Nikola Stewart ruled the fatal crash could have been avoided and that McRae should not have been flying the aircraft at the time.

The former rally driver had engaged in difficult low-level flying "when it was unnecessary and unsafe for him to do so," Stewart ruled.

McRae had been flying home from a trip to see a friend when the aircraft crashed at Mouse Valley near his home in Lanark.

The parents of the six-year-old boy on board the helicopter were unaware their son was on the aircraft and had not been asked for their consent.

The sheriff said in a written ruling there was no "operational or logistical reason" for McRae to have descended into the valley at speed.

"It is difficult to avoid the conclusion that his intention was to conclude the flight as he had started it, with a spectacular and dramatic fly-past of the valley and its vegetation, followed by a steep climb out prior to coming into land, all for the benefit of his passengers," she said.

"The deaths and the accident resulting in the deaths might have been avoided had Mr McRae not flown his helicopter into the Mouse Valley.

"Such a precaution would have been entirely reasonable. There was no necessity to enter the Mouse Valley. There were no operational or logistical reasons to enter the Mouse Valley.

"Mr McRae chose to fly the helicopter into the valley. For a private pilot such as Mr McRae, lacking the necessary training, experience or requirement to do so, embarking upon such demanding, low-level flying in such difficult terrain, was imprudent, unreasonable and contrary to the principles of good airmanship."

AFP