As the House of Lords announces the results of an inquiry in to the tragedy, BBC News Online looks at why the causes of the crash have caused controversy for almost eight years.

Who was on the flight?

The passengers on board the Chinook included senior army officers and members of the Royal Ulster Constabulary's elite special branch along with Northern Ireland Office officials and members of MI5.

All were heading for a conference on Northern Ireland security which was due to take place in an army base at Fort George, Inverness.

Why was the crash significant?

It was the worst peacetime accident involving a helicopter to befall the RAF.

The loss of the cream of Northern Ireland intelligence officers was a blow to the Conservative Government of the time, temporarily confounding the anti-IRA campaign.

The crash posed some embarrassing questions - such as why the UK's top anti-terrorist personnel flew together.

Why has the House of Lords investigated the crash?

Lord Chalfont, a former Labour defence minister, persuaded members of the Lords to set up an inquiry to challenge the verdict of the two senior RAF marshals.

Air Vice Marshal John Day and Air Chief Marshal Sir William Wratten blamed the two dead pilots for the crash.

They said Flight Lieutenant Jonathan Tapper, 28, and Flight Lieutenant Rick Cook, 30 - both Special Forces pilots with exemplary records - were guilty of "gross negligence".

Why was this verdict questioned?

There were serious doubts about the aircraft.

The RAF board, which carried out an investigation after the crash, said it was impossible to establish the exact cause of the crash.

The board had agreed that "a major technical malfunction could not be ruled out" and that technical problems could have distracted the crew.

But this was ignored by the two air marshals who reviewed the findings.

The Ministry of Defence (MoD) has always stood by these findings.

What action was taken to clear the names of the pilots?

The families of the pilots, as well as many of the passengers, were not prepared to accept the RAF judgement.

They received backing from a Scottish fatal accident inquiry that also pointed out the possibility of technical problems and decided that no clear-cut cause could be ascertained.

The families also received backing from many senior figures in aeronautics.

Finally, an all-party coalition of politicians demanded an investigation into the RAF findings.

If not human error, what caused the crash?

The Lords committee set out to examine again whether human error caused flight ZD576 to crash.

The Chinook's engines were fitted with FADEC (Full Authority Digital Engine Control) software to control fuel flow and thus power output.

Concerns over the faulty design of the FADEC software have been denied by a succession of Defence Secretaries, despite the MoD suing the manufacturers of the system after a near-fatal Chinook crash in 1989.

Have any other claims emerged?

It was claimed that just hours before the fatal flight from Belfast, the pilots had warned their superiors in a memo that the helicopter was not fit to fly.

After a test flight the pilots asked engineers to check the controls and the FADEC system.

The memo noted serious "uncommanded flying control movements (UFCM)" and said: "It is our considered opinion that, in the light of these and previous UFCM, this aircraft should not be used for this special flight."

The memo was never seen and the MoD doubted its veracity.