Three inquiries have failed to determine the cause of the crash
New evidence has emerged that computer software faults may have caused a RAF Chinook helicopter crash in which 29 people died.
An official RAF enquiry into the incident concluded that the aircraft was airworthy and found the pilots guilty of gross negligence.
But the BBC has been passed internal MoD documents - written nine months before the crash - which cast doubt on the helicopter's airworthiness, and describe the Chinook engine control computer software as "positively dangerous".
The MoD maintain that since the incident, the Chinook Mk 2 has a "remarkably safe and successful service history" and that the documents in question were available to the crash inquiry and so cannot be classed as new evidence.
The density of deficiencies is so high that the software is unintelligible
Chinook Mk 2 helicopter ZD576 crashed in the Mull of Kintyre in June 1994 killing all those on board, including 25 top Northern Ireland security experts.
The families of the pilots Flt Lts Jonathan Tapper and Richard Cook have been fighting for the last 15 years to try and clear their names.
According to his father, Flt Lt Tapper raised concerns over the helicopter's airworthiness in their final telephone conversation, just days before he died.
If you blame the pilots you sweep the whole thing under the carpet
Squadron Leader Robert Burke
"He said the new Mk 2 was coming in tomorrow and he'd have to test it very carefully - and the timbre in his voice completely altered - he really hit the roof about the computer technology inside the Chinook," says Mike Tapper.
"He was very concerned: I think the whole squadron was very concerned indeed."
Mr Tapper claims his son was effectively forced to fly the aircraft in spite of these concerns. A few days later, he was informed of his son's death.
The official RAF report - that the pilots and not the helicopter were to blame - shocked Mr Tapper.
"I couldn't understand it," he says. "When the penny dropped, grief was subsumed by anger."
He says the findings were "clearly false" and that the board lacked the evidence to come to the conclusion of negligence.
Three subsequent inquiries found that the cause of the crash was inconclusive.
Inquiries in the House of Commons and the House of Lords found that the verdicts of gross negligence were unsustainable and unjustified.
Despite this, the MoD and the RAF have refused to reopen the Inquiry or annul its findings.
The internal MoD document passed to the BBC proves there were serious concerns and warnings over the engine control computer system, known as the FADEC.
The report was written nine months before the crash by a senior engineering officer at the MoD Aircraft Testing Centre at Boscombe Down.
Pilots Jonathan Tapper and Richard Cook were blamed for the crash
"The hazard analysis of Chinook Mk 2 identifies the software in the engine FADEC as safety critical and states that 'any malfunctions or design errors could have catastrophic effects'," the report states.
"Twenty-one category one and 153 category two anomalies have been revealed. One of these is considered to be positively dangerous.
"The density of deficiencies is so high that the software is unintelligible Pilot's control of the engine(s) through FADEC cannot be assured."
The report concludes that the release of the Chinook Mk2 into service "cannot be recommended". It calls for a software rewrite, which did not happen.
The report was not included in the report from the original Board of Inquiry.
Nine months later, on the day of the crash, the Officer Commanding of the Rotary Wing Test Squadron (RWTS) at Boscombe Down issued another stark warning about the FADEC engine control computer software.
"The recommendations with respect to FADEC have, to date, been ignored," it states.
"Until RWTS is provided with clear, unequivocal and realistic explanation of the faults with corrective action, further Chinook [Mark] 2 flying shall not be authorized."
The warning came too late for the 29 crew and passengers of Chinook ZD576.
The handling of the crash has come under severe criticism from the chief test pilot for the Chinook Mk 2, Squadron Leader Robert Burke.
They cannot get away with this. We're not after money. Just for his name to be cleared - purely a question of honour
Pilot's father Mike Tapper
On hearing of the incident he thought the software could have been to blame, and was the first port of call for the Civilian Air Accident Investigator at the scene of the crash.
But his involvement was stopped by a senior RAF officer, who told him not to talk about the accident to anyone - officially or unofficially.
Despite his expertise in the aircraft he was not asked at any point to take part in the official RAF inquiry into the crash.
He believes the RAF rushed the Chinook into service knowing it to be dangerous - and then blamed the pilots to save face.
"The Chinook Mark 2 was very, very late coming into service," he explains.
"The Army were complaining bitterly about this - they couldn't get their training done. Indeed the RAF couldn't get their training done.
"This was a showcase flight by the RAF to show the Army that the Chinook Mark 2 was finally fit for service - and as a result of the RAF's assurances, the Northern Ireland Office and the Army put on board the most valuable possible group of passengers it could - which was the heads of the security services in Northern Ireland. And it all went wrong.
"If you blame the pilots, you sweep the whole thing under the carpet."
The reason for the loss of Chinook ZD576 in the Mull of Kintyre will never be known - the bulk of the aircraft was destroyed in the crash.
The Ministry of Defence has persistently refused to review or annul the decision to blame the pilots - saying there is no new evidence.
But in light of the recent Nimrod review - which exposed deep rooted failings across the military airworthiness system - and the documents passed to this programme - the families of the pilots believe this position is now untenable.
"There's too much evidence against the MoD, there's too many people who know the truth now," says Mike Tapper.
"They cannot get away with this. We're not after money. Just for his name to be cleared - [it is] purely a question of honour."
The MoD issued the following statement:
"Our thoughts remain with the families of those who tragically lost their lives when ZD576 crashed in poor visibility on the Mull of Kintyre in June 1994.
"Since this tragic incident, the Chinook Mk2 has had a remarkably safe and successful service history.
"Ministers have repeatedly stated that they would reopen the Board of Inquiry if any new evidence is raised.
"Despite numerous representations over the years, nothing has been presented to successive Secretaries of State that would justify reopening the Inquiry.
"This latest information is from an RAF document; it was available to the Inquiry team and cannot be classed as new evidence."
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