Joy Robson, 51, from Skye, died after a rally car crashed at the Snowman Rally near Inverness in February 2013.
The car struck a large rock at the side of the track, causing the driver to lose control and the Honda Civic to somersault for about 40m (131.2ft).
Investigator George Lemmon said what happened next was beyond the driver.
The car was driven by 31-year-old Graeme Schoneville, who earlier told the inquiry that it had "just somersaulted in the air" during the crash.
The inquiry heard that the car came to rest 10m (32ft) off the track, uprooting a tree which fell on Ms Robson.
Traffic officer Mr Lemmon told the fatal accident inquiry (FAI) in Edinburgh that it was not possible to say how fast the car had been travelling, but said the average target time would have been 60 to 65mph.
Asked if the driver could have "done anything to take control" after the rear wheel was broken by the stone, Mr Lemmon said: "It was completely out of his control. It had launched itself after hitting the stone.
"He's been launched by this large stone and it's simply a case of where the car comes to rest."
Mr Lemmon said he and a colleague had driven along the track, at between 20 and 30mph. He said: "I would not like to have gone much faster. Forest tracks are a different kettle of fish. Nice area for walking dogs, but not for traffic."
John Clayton, 72, was one of two marshals at the hairpin bend where the crash happened. He told the inquiry that he had contacted managers to ask that the rally be stopped as he was concerned about the "wall" of spectators gathered in the area.
Cars that had not started the race were stopped immediately, but there were already six vehicles heading for the bend. The fourth car was the one which crashed, with Mr Clayton describing a "bang" and the car "spiralling up in the air".
He said that since the crash the Snowman Rally had a much better spectator area and methods of handling spectators.
A joint FAI is being held into the death of Mrs Robson as well as the deaths of three people - Iain Provan, Elizabeth Allan and Len Stern - at the Jim Clark Rally near Coldstream in the Borders in 2014.
The inquiry continues.
A driver involved in a fatal accident at a rally would not have been able to regain control of his car after it struck a rock, an inquiry has heard.