The verdict would dismay safety experts and leave a question mark over an aircraft with an otherwise excellent safety record. Until the cause is known, no action can be taken to prevent a similar accident occurring again.
The United Airlines 737-200 flipped over as it approached Colorado Springs airport, Colorado, on 3 March 1991, and hit the ground with such force that it ploughed up a huge crater. Five crew members and twenty passengers died.
In the past 16 months the US National Transportation Safety Board (NTSB), United Airlines and Boeing have spent millions reconstructing every system of the plane, but although there are many theories, they still do not know the cause. Their inquiry has been hampered by the limitations of an old-style black box flight recorder in use on the plane.
'It's a tragedy beyond a tragedy that the root cause of the accident has not been uncovered,' said John Galipault, the head of the private Air Safety Federation, which was involved in the investigation.
John McCarthy of the National Centre for Atmospheric Research, who was also involved, says that it is the longest running investigation ever by the NTSB. 'The Board wants to have a cause,' he said, explaining the latest delay in the publication of its findings, 'they just don't have any accident investigations without conclusions.'
More than a thousand 737- 200s - built between 1967 and 1988 - are in use worldwide, mainly for short-haul flights, as well as many more later model 737s - the 300, 400 and 500.
A spokesman for the NTSB said an 'exhaustive and difficult' investigation was under way and the final report would not be issued until an explanation was found, or the investigators had gone as far as possible.
He speculated that the disaster could have been caused by a combination of a rudder damper failure and the effects of a horizontal tornado or 'wind rotor' coming off the Rocky Mountains. 'No one has been able to measure (the wind rotor) effect on that day.
'It remains a mystery,' he said.
Full report, page 17
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