On board were the elite of British military intelligence

Some of the 25 intelligence officers who turned up at RAF Aldergrove on 3 June 1994 brought their golf clubs, intending to mix business with pleasure at their anti-terrorist conference in Inverness.

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Lords clear pilots of blame for Chinook crash

Leading article: The RAF was wrong to blame these pilots

Some of the 25 intelligence officers who turned up at RAF Aldergrove on 3 June 1994 brought their golf clubs, intending to mix business with pleasure at their anti-terrorist conference in Inverness.

They were the cream of the intelligence community in Northern Ireland, representing the Royal Ulster Constabulary Special Branch, military intelligence and MI5: and they were all killed when their Chinook helicopter ploughed into the Mull of Kintyre. Although many people were later shocked at the idea of of so many key personnel being packed into a single helicopter, this was very much routine at the time.

The Inverness conference was an annual event, with intelligence chiefs regularly flying to it in a single craft. It was viewed then as a fairly routine short hop which passed nowhere near any potential republican trouble spots.

After the crash, the authorities emphasised that the Chinook was a safe means of transport, rejecting criticisms that it was imprudent for so many senior personnel to travel in one craft.

Before taking off, the crew of Chinook HC2 ZD576, call-sign F4J40, studied a Met Office weather report which forecast "borderline" weather over the Mull of Kintyre, mentioning the possibility of fog.

The crew was driven to the Chinook at 5pm. Twenty minutes later the intelligence people boarded, and at 5.42 pm they took off. Flying at more than 100mph, they crossed the coastline heading for Scotland. The crash happened at approximately 6pm in circumstances which several inquiries have been unable to clear up.

The loss of so many senior people sent shock waves through the system, partly because each intelligence department lost vital expertise. Many of those killed had built relationships with people from other agencies, and coordination among them seemed to be relatively free of friction. These valuable personal relationships were lost at a single stroke.

The crash came at a crucial point in the peace process, but it seemed to have no major immediate effect. The IRA was then moving towards the ceasefire which it declared three months later, and did not deviate from this after the crash.

One of the most significant figures on board the Chinook was John Deverell who, as head of MI5 in Northern Ireland, had in 1990 persuaded the Northern Ireland Secretary, Peter Brooke, to sanction secret talks with the IRA. Mr Deverell and the other intelligence people did not live to see the new phase.

The loss of their experience and expertise may have contributed to the uncertainty displayed by John Major's government when the IRA ended its ceasefire in late August 1995. The government's expectation was that republicans would call a temporary three-month stoppage rather than a complete halt, as they did.

The RAF has always insisted that the crash was caused by pilot error, and continued to insist that yesterday. But for years, computer experts have said this overlooked the possibility of inherent faults in the Fadec, the computer software used to control the engines.

Proving that "safety-critical" software is safe is almost impossible, industry experts say. Les Hatton, a professor of software reliability at the University of Kent, said yesterday: "You can never be without doubt about electronic control systems, because when they fail they don't leave a footprint. There's nothing physical left behind. The Ministry of Defence left out the possibility that software failure could have caused the crash."

Chinooks have not suffered a similar crash since 1994, but that is probably because the software is continually changed, Professor Hatton said.

The House of Lords agreed in its report that there were unresolved problems with the Fadec in the Chinook, and that Boeing, which had supplied it, used assumptions in its simulations "which are impossible in performance" and parameters "some of which do not fit with what was found by the Air Accident Investigation Board".

The uncertainty boils down to a single, rhetorical statement in the report: "Can it in these circumstances be said that there is absolutely no doubt whatsoever that it was the voluntary action of the air crew – including not only both pilots, but also MALM [Master Air Loadmaster] Forbes who in our view was probably assisting with the navigation – which caused the aircraft to fly into the hill?"