The crash scene on the Mull of Kintyre in 1994

An RAF inquiry into the crash, which killed 29 people, found the dead pilots guilty of "gross negligence".

But the two helicopter experts told Channel 4 News that there were flaws in the inquiry and called for the pilots to be exonerated.

Air Commodore John Blakeley said the inquiry's findings were "biased".

Peacetime disaster

He said that engineering evidence "was not correctly analysed or followed up".

He added that the RAF's board of inquiry had either been directed to blame the pilots or was not competent to do its job.

His findings have been backed by Rear Admiral Ron Holley, former director of helicopter projects at the Ministry of Defence (MoD).

The MoD said that in the 10 years since the accident it has examined all the complex technical, legal and airmanship issues raised by those opposed to the board of inquiry finding but has found nothing that undermines it.

Air Commodore Blakeley described the aircraft's defect history as "not a happy one".

He has called for the pilots to be exonerated, the first former MoD insider to do so.

The Board of Inquiry's findings were heavily biased in favour of the verdict that the RAF themselves were not questioned on their decision and shortfalls in their process Air Commodore Blakeley

The crash, which killed 25 of Northern Ireland's senior intelligence officers and four air crew, was the RAF's worst peacetime helicopter disaster.

An inquiry blamed the pilots, Flight Lieutenants Jonathan Tapper and Rick Cook.

Subsequent investigations raised concerns about the airworthiness of the Chinook - sparking a long-running campaign by the pilots' families, the Church of Scotland, and politicians, including former prime minister John Major, to clear the men of any blame.

Air Commodore Blakeley, who was the RAF's most senior engineering officer during the Falklands conflict, has compiled a detailed report.

He said there was "a significant amount of doubt that the fleet of Chinooks was airworthy", and that the helicopter that crashed had carried "a potentially significant number of defects".

'Hugely important'

The retired officer said: "The Board of Inquiry's findings were heavily biased in favour of the verdict that the RAF themselves were not questioned on their decision and shortfalls in their process."

The report has been backed by Rear Admiral Holley, former director of helicopter projects at the MoD.

He said: "Air Commodore Blakeley's report is certainly a credible piece of work. The MoD should take it seriously because he has drawn his research from its own aircraft documentation - and that documentation is hugely important.

"The

Jonathan passed his concerns up through the chain of command line loud and clear, and they were ignored or overruled Michael Tapper

Mr Tapper said: "It has to be remembered that Jonathan was the Flight Commander of the unit.

"He requested that the Mark 1 (Chinook) be retained because he did not feel it was right to fly people around in a Mark 2.

"He said to his widow 'how am I expected to fly this thing around Scotland'.

"So he passed his concerns up through the chain of command line loud and clear, and they were ignored or overruled."

Mr Tapper added: "So far, the best that the air marshals can come up with is a series of speculations but absolutely no concrete proof."