On 3 June 1994 a memo from the military-aircraft testing establishment at Boscombe Down in Wiltshire to the Ministry of Defence criticised the computer system controlling the Chinook's two engines. The software was "unverifiable and therefore unsuitable for its intended purpose". A former MoD computer engineer comments: "That meant you shouldn't really have been flying this software at all."
But that did not prevent an RAF board of inquiry from finding the two pilots, Flight Lieutenant Jonathan Tapper and Flight Lieutenant Rick Cook, guilty of "gross negligence" for causing the crash. They had selected "an inappropriate rate of climb", failed to observe minimum care in bad weather, and flown themselves and their valuable cargo into a hillside.
But the board of inquiry made no mention of the Boscombe Down memo of 3 June. That emerged only much later as a result of inquiries by the pilots' families and Channel 4 News, adding strength to the arguments for reopening the case.
The RAF stands accused of failing to observe its own strict rules of accident investigation, that "only in cases in which there is absolutely no doubt whatsoever should deceased air-crew be found negligent". The other worry is that the RAF brought the Chinook into service before it was ready, yet the MoD consistently opposes demands for a new inquiry.
When a Scottish civil judge concluded that there was no proof that the pilots were to blame, the ministry dismissed his judgment. The new Labour government has adopted the same line, insisting that the verdict of "gross negligence" should stand. But a study of the known facts - and others now emerging - suggests that the doubts surrounding the story are so grave that it will not go away.
The RAF has a fleet of 30 Chinooks. In the early Nineties they were being sent back to the manufacturers, Boeing, for a "mid-life update". A key addition was a new computer which both controlled the engines and monitored the flying systems. It was known as Full Authority Digital Engine Control, or FADEC.
FADEC had a number of teething problems - warning lights would come on for no apparent reason, engines would cut out or suddenly run up out of control. A feature of the incidents is that they often left no trace. In fact FADEC problems were a good deal worse than the MoD has ever admitted.
Just how much worse was not clear until Malcolm Perks, a former Rolls- Royce helicopter computer engineer, decided to speak out. He had been recruited by the Ministry of Defence in 1994 to look into a serious incident in January 1989. During a ground test at Boeing on the first Chinook to be fitted with FADEC an engine ran out of control. The helicopter was badly damaged.
The MoD was furious. The entire programme for the update of its Chinooks was at risk. The RAF's competence to shift troops and equipment in difficult terrain in places such as Bosnia, the Falklands and Northern Ireland was at stake.
The MoD sued Boeing and the engine manufacturers, Textron Lycoming. Boeing settled out of court, but Lycoming went to arbitration. Malcolm Perks helped present the MoD's case.
He demonstrated that Lycoming had failed to supervise the development of the FADEC project; the malfunction was the result of faulty design of the computer software: "Textron Lycoming hadn't been doing the job they'd been paid to do and had been developing the system in a slapdash manner," he said.
The MoD won the case and was awarded $3m damages. But it was still not known whether the design problem in the software had been corrected. "If you don't do it right in the first place, you can't put Band-Aids on later," says Mr Perks.
In 1993 the MoD commissioned a British firm, EDS SCICON, to make an independent assessment of the improvements to the FADEC software. In its findings dated 23 July 1993 - but never published - it said that it could gain access to only 17 per cent of the software codes but it had still found a large number of "anomalies". To Mr Perks that meant "unexpected behaviour under certain circumstances ... It confirmed what I would have imagined; the basic problems I could see in 1989 had not been fixed."
The first Chinook Mk 2 with its new FADEC had returned to the RAF from the Boeing factory in May 1993, going straight for trials at Boscombe Down. In November that year the Mk 2 was given its first equivalent of a civil air-worthiness certificate and released for squadron service. But Boscombe Down was still so concerned about possible FADEC failure that it allowed the Mk 2 to fly with limited loads only, so that if FADEC shut down one engine the helicopter could still fly on the other.
Ian Kingston, then a Navy pilot flying the Mk 2 with the RAF in Northern Ireland, described the dilemma facing pilots operating under these restrictions: "If you flew to a rendezvous in South Armagh expecting to pick up 20 troops and you found there were 25, what were you supposed to do? With the Mk 1, carrying extra fuel, you just picked them all up. With the Mk 2 - carrying limited fuel because of the weight restrictions - you either left the extra five on the ground, or disobeyed the restrictions."
Shortly before the crash Mr Kingston recalled that his flight commander, Flight Lt Jonathan Tapper, had queried whether, in an emergency, pilots could break the restrictions imposed by Boscombe Down. The pilots were told that if they did so they would be held personally liable. "That made me feel angry," says Mr Kingston.
The test pilots were also having FADEC problems. In March 1994 an unexplained engine shut-down held up test flying for six weeks. After five more incidents "due to FADEC malfunction", according to an official letter, the test pilots suspended flight trials again. That was on 1 June, the day before the crash.
But the operational squadrons were simply ordered to go on flying. The pilots were astonished. Ian Kingston recalls: "It was quite a concern to us ... [but] we were just told to get on with it."
At 1742 hrs on 2 June Jonathan Tapper and Rick Cook took off from RAF Aldegrove with their cargo of 25 top intelligence officers, bound for a security conference in Inverness.
Several Chinook Mk 2s had already suffered a number of FADEC-related incidents, including ZD576, the newly upgraded machine which had arrived in Northern Ireland two days before. Both pilots had complained to their fathers that they were uneasy.
The flight reference cards for the Mk 2 - the pilot's manual in any emergency - weren't yet ready. Jonathan Tapper had asked to have a Mk 1 Chinook standing by as a replacement but his request was refused.
Eighteen minutes into the flight the helicopter crashed into the Mull of Kintyre. There were no survivors. The Chinook carried no accident or cockpit data recorder - the "black box" - so no one will ever know for certain what happened.
The board of inquiry seemed ready to acknowledge that "an unforeseen malfunction of the type being experienced on the Chinook Mk 2, which would not necessarily have left any physical evidence, remained a possibility, and could not be discounted".
But it still condemned the pilots for not "exercising appropriate care and judgement": the pilots were "negligent to a gross degree".
No Reference at all was made to the fact that the day after the crash, 3 June 1994, Boscombe Down's memo suggested that the electronic assessment section certainly had continuing doubts about the FADEC software. Mr Perks is clear what the writer meant: "You shouldn't really be flying this software in a flight-safety critical application."
Tony Collins, executive editor of Computer Weekly and an expert on aviation software, says: "The fact that this report refers to software used by an aircraft which was actually in service is quite frightening."
Evidence given to the Scottish Sheriff's civil inquiry revealed that the chief air accident investigator from Farnborough, Mr AN Cable, had not heard of the memo either. It seems inconceivable that he would not have looked into the functioning of the FADEC software had he known about its history; and had he known that the MoD had called in Mr Perks as an independent expert witness precisely because of doubts about the manufacturers' capabilities.
Mr Cable was not the only one uninformed of FADEC's past. During questions in parliament, official written answers and a debate in the House of Lords, no one mentioned the FADEC history, nor the SCICON report, nor the Boscombe Down memo.
When these issues were raised for the first time, in Channel 4 News on 24 November, John Reid, Minister for the Armed Forces, insisted: "All this information was available to ministers, the board of inquiry and others investigating the incident."
The next day, James Arbuthnot, Minister of Defence Procurement in the last Conservative government, said on Channel 4 News that he had not known about the history of FADEC and called for a new inquiry.
Then Sir Malcolm Rifkind, Secretary of State for Defence at the time of the crash, stated: "I have no recollection of being briefed on these problems with FADEC. If these matters are now in dispute with the Ministry of Defence, we need a reinvestigation." A third former Conservative defence minister, Sir Jeremy Hanley, has also endorsed a new inquiry, unlike the House of Commons defence committee which last week rejected the call for one.
It Is Hard to avoid the conclusion that the RAF - under pressure to meet the demands put upon them - brought the Chinook Mk 2 into service before it was ready. In the process 25 senior intelligence officers and four crew were killed. But, once the Ministry of Defence had blamed the pilots, diverting attention from its own failings, it had to do everything to maintain its own version of events.
There is no proof that FADEC caused the crash; nor is there any proof that it did not. But there is persuasive evidence that the computer software problems had not been resolved before ZD576 flew into the Mull of Kintyre. This raises doubt that all responsibility for the crash belongs to the pilots, and the RAF's own rules state that if there is a single established doubt then the verdict of negligence cannot stand.
As a former defence minister put it last week: "This is a matter of justice." Five-year-old Christopher Tapper summed it up while watching Channel 4 News: "That's my Daddy they're talking about."
David Harrison produced Channel 4's 'Cutting Edge' documentary on the Chinook crash, and recent reports for 'Channel 4 News'.Reuse content