Colin McRae's father says he saw his son's helicopter was off its normal flightpath before it crashed, killing the former world rally champion.
The bodies were removed from the helicopter on Sunday
Colin, 39, his son Johnny, five, and two family friends died in a helicopter crash in Lanarkshire on Saturday.
Jimmy McRae, himself a former rally champion, said the helicopter was off its normal line near Lanark.
"He's had a problem further back and he's obviously been trying to clear the trees to get to the field," he said.
"He would normally be a mile, half a mile up in the top of the valley.
"If there's been a problem with the helicopter, or whatever, we've got to find out what it is and stop this happening to another family."
Police said the aircraft came down on Saturday at about 1600 BST in Jerviswood, roughly a mile from Lanark, and caught fire.
Johnny's friend, six-year-old Ben Porcelli, and a family friend, Graeme Duncan, 37, also died.
They had been flying from Hamilton to Lanark.
A team from the Air Accidents Investigation Branch (AAIB) began work at the scene yesterday morning.
Jimmy McRae said he thought his son was trying to clear the trees
A spokesman for the Department of Transport said investigators were studying the crash site for signs of mechanical failure.
The team of experts will consider whether they need to take any of the wreckage to their headquarters in Farnborough, Hampshire, for further examination.
The flight log, air traffic control records, weather conditions and other outside factors will be studied, the spokesman added.
Mr McRae said an online condolence page had 25,000 hits.
He said: "That's how popular he was. He was world champion and to be champion of the world you've got to be champion of the world."
Mr McRae said his grandson Johnny "was beginning to show signs" that he wanted to take up rally driving "even at his early age".
He said no words could describe the impact their deaths has had on family and friends.