The HSE said the death of experienced maintenance engineer Gary Whiting, 51, at the BAE plant at Brough, East Yorkshire, was an "entirely preventable tragedy".
Mr Whiting, from Hull, died on November 10, 2008, when he was part of a team performing a routine service on a large metal press. The HSE said the piece of machinery was the size of a two-bedroom house and used to make components for Hawk jets.
Hull Crown Court heard how Mr Whiting entered the machine to remove a piece of equipment but, at the same time, a colleague at the other end of the press started a full test cycle. He was trapped by the 45 square metre frame as it descended and he died the same day in hospital.
The HSE said its investigation exposed a series of flaws in safety practices during maintenance of the metal press, some of which had existed for many years. Failings included an absence of a suitable assessment of the risks associated with the test process and a lack of engineering control measures to prevent entry by workers to dangerous parts of the machine during testing or to stop the machine if anyone did enter a danger zone.
A statement released on behalf of Mr Whiting's widow, Jackie, two children and brother, said: "We are pleased that, more than four-and-a-half years after Gary was killed, BAE Systems have been brought to justice. The complete absence of any safe system of work for the job Gary was performing beggars belief. There wasn't a suitable risk assessment for the job he was doing.
"The machine he had been working in had been operated with a person inside it before and it is pure luck no-one was killed in the machine before Gary. If BAE Systems had spent even a relatively small amount on safety features for the machine he was working in, Gary would have been alive today."
In a statement, the company said: "BAE Systems deeply regrets the death of its employee and colleague, Gary Whiting, in November 2008. BAE Systems appreciates the anguish this accident has caused and will continue to cause to Gary's family, friends and colleagues and for that BAE Systems sincerely apologises. "
The company said the incident was an "isolated failure at a site that generally has an excellent health and safety record" and said it pleaded guilty after acknowledging there was a flaw in its safety procedures. A spokesman added that since the tragedy a number of improvements have been made at the site to ensure that such an accident never happens again.
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