For the 48-year-old Russian engineer it was the last act in a tragedy that began on a clear night over Lake Constance on the Swiss-German border just over three years ago.

Before entering the Swiss courtroom yesterday he was given a bitter consolation: a promise of €80,000 (£54,000) compensation for the loss of his wife, Svetlana, his ten-year-old son, Konstantin, and his four-year-old daughter, Diana. It will mean little to a man who has told the authorities: "What do I need money for — to buy rope to hang myself?"

The courtroom was filled with grieving, angry families from North Ossetia, in the Caucasus, determined to show support for their countryman. The prosecution has demanded a 12-year sentence. But for the Russians, the stabbing of Peter Nielsen, the air traffic controller, was a justified act of revenge. "I went to Nielsen as a father who loves his children, so he could see the photos of my dead children and next to them his kids, who were alive," Mr Kaloyev told a hushed courtroom. "Everyone can make mistakes but these are my children."

Mr Kaloyev’s world collapsed in July 2002 when a Bashkirian Airlines Tupolev 154 crashed into a Boeing cargo aircraft operated by the DHL courier service. Forty-five children died in the crash. Diana’s body was recovered virtually unscathed since her fall had been broken by trees. Mr Kaloyev said: "She was carried to earth by an angel."

Konstantin and Svetlana fell 36,000ft. The boy landed on asphalt in front of a bus shelter in the small German township of Überlingen. Mrs Kaloyev’s body was found in a cornfield.

The engineer who was waiting for his family in Barcelona — he had a building contract in Spain — took the first available flight and was the first relative to see the crash site on the banks of Lake Constance. Since then he has been inconsolable.

The nub of the defence case presented yesterday by his counsel Markus Hug was that a man so distraught could not possibly be judged by normal judicial standards.

The state prosecutor wants Mr Kaloyev jailed for "killing with intent" — which though less than murder nonetheless carries a maximum sentence of 20 years.

But Herr Hug argues that Mr Kaloyev was mentally deranged at the time of the killing and was therefore guilty only of manslaughter, with a maximum sentence of ten years. Mr Kaloyev says that he remembers nothing of the killing but accepts that he did it.

Eighteen months after the aircrash Mr Kaloyev tracked down the air traffic controller, a Dane working for the company Skyguid. Mr Kaloyev found the house of Mr Nielsen with the help of a Moscow private detective agency. He went on to the traffic controller’s balcony and called out his name.

Mr Nielsen went out and closed the door behind him. There was a short argument and then Mr Kaloyev repeatedly stabbed him with a knife. "It was a crime of passion, of impulse," Herr Hug said.

Prosecutors in Germany and Switzerland are still conducting investigations against members of the Skyguid staff to establish whether they could be charged for negligent killing.

For all the Russian families this investigation has been proceeding too slowly. When Mr Kaloyev attended a memorial service in Überlingen in 2003 he exploded with anger that Skyguid was accepting only partial responsibility for the crash. According to the prosecutor in yesterday’s trial, that was the moment when Mr Kaloyev formed an intention to kill the air traffic controller. The engineer has been in prison and psychiatric clinics since the stabbing in 2004.

Psychiatrists say that he is fit to stand trial but various expert testimonials to be presented to the court paint a picture of a man still crippled by grief and barely able to make sense of the world. His home in northern Ossetia has become a shrine: all the beds are scattered with dolls, toys and photographs and surrounded by candles. The engineer’s life savings have gone towards building a large gravestone and tomb for the family as well as a plaque at the spot where his wife fell to earth.

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