If ever a reminder of the life-or-death importance of health and safety legislation was needed, Audrey Wood provided it yesterday.

Mrs Wood's son Stuart was one of the 16 men killed when a Bond Super Puma helicopter crashed into the North Sea in 2009 and yesterday she spoke of her frustration that there would be no criminal prosecution. There had been multiple breaches of health and safety, she told a press conference, and yet there would be no criminal action. That decision, she said, would haunt her and the other relatives of the men who died.

The reaction of Mrs Wood is entirely understandable after five frustrating, upsetting years for her and the other relatives. The accident happened in April 2009 but it was only yesterday that the conclusions of the fatal accident inquiry before Sheriff Principal Derek Pyle were revealed. Mrs Wood said the length of wait for the report had been intolerable, but the families at least had one hope: that the FAI's conclusion that helicopter maintenance was not always done by the book at Bond might change the Crown Office's decision not to prosecute.

However, it quickly became clear yesterday that the Crown Office was sticking to its decision, first made a year ago, not to bring criminal charges. The evidence had been fully assessed, it said, and it was insufficient for prosecution.

That is hard for the families to accept in the face of Sheriff Pyle's conclusion that the correct procedures were not always followed at Bond. "Everyone in the company well knew that maintenance must be done by the book," he said. "On one occasion, that fundamental rule was broken."

Crucially, though, what the sheriff did not say was that this breach of the rules caused the accident. It resulted in the failure to detect a significant fault in the helicopter's gearbox, he said, and it was possible, but only possible, that this had caused the crash.

This has left the families to deal with a gap between the failures of Bond and what the Crown says it needs to prosecute. The sheriff says there were mistakes by Bond - and Bond has admitted this - but the Crown Office has said there is a lack of evidence that these mistakes constituted breaches of health and safety for which Bond could be prosecuted. While that may be the correct legal decision, it leaves a worrying impression that health and safety rules can be broken without any sanction.

What is not known is whether Bond's failures caused the crash or not - and that may never be known - but what is beyond doubt is how important health and safety procedures are in the oil and gas industry and how vital it is that companies are brought to account when they break them. As Mrs Wood said yesterday, everything has to be done by the book in the North Sea and there can be no excuse for not doing so.

Despite the fact that Bond did not meet this standard, it seems likely that it will escape any form of punishment. Whether the decision to prosecute was correct or not, that leaves relatives - and oil workers - with nothing more than Bond's assurance that it has learned lessons from its mistakes. Perhaps it has but that will never be enough for the families.