Nigel Gresham denies causing the deaths by dangerous driving
A man who crashed his Land Rover into a river, killing four of his seven children, carried out "incompetent" changes to the car, a court heard.
Nigel Gresham, 37, from Chapel Hill in Lincolnshire, has pleaded not guilty at Lincoln Crown Court to causing the deaths by dangerous driving.
His children, Willow, two, Angel, four, Thor, six, and Keavy, eight, died when the car careered into the River Witham.
It happened on 16 September 2007 at Tattershall Bridge near Coningsby.
Prosecutor Timothy Spencer QC told the jury that the Land Rover had parts from four different vehicles.
He said the accident was caused by the manner of Mr Gresham's driving and because the Land Rover was "dangerous and unroadworthy".
Mr Gresham, he said, had undertaken a string of modifications to the vehicle for his off-roading hobby which had been "ill-judged" and "incompetently carried out".
He also accused him of failing to carry out a number of repairs.
The jury was shown a DVD of road tests carried out on the Land Rover at RAF Scampton.
Keavy, Thor, Angel and Willow Gresham died in the crash
The tests found that the calliper on the offside front brake was bigger than the one on the nearside, while the brake pedal was also worn.
Other drivers had seen the Land Rover speed past them before the fatal crash.
It was estimated to have been travelling at between 50mph and 60mph (80 to 96km/h) when the incident happened.
Mr Spencer said: "Of course this was a tragedy.
"No-one suggests that Mr Gresham set out in order to have a road traffic accident and no one suggests that he deliberately intended to endanger the lives of his family.
"But it was an avoidable tragedy. He must have known, despite what he said to police, that this was a defective vehicle. He must have known it was in a dangerous condition and it should not have been on the road in the state it was in."
Mr Spencer told the court how, having passed a Clio in front, Mr Gresham's car then swerved towards its offside towards the river and rolled over, tumbling down the bank into the Witham.
The jury heard that the Land Rover had passed an MoT in February - seven months before the crash.
The trial continues.