IT HAS been likened to something on the set of a science fiction film or a James Bond epic as it towers more than 100ft above the North Yorkshire countryside.

The true explanation behind the gigantic steel frame which has sprung up on Dalton industrial estate, near Thirsk, is just as spectacular because it represents a trial run for the building of the roof at the planned new Terminal 5 building at Heathrow Airport.

Airport operator BAA is working with a number of companies including the Dalton-based Severfield-Rowen, the UK's leading structural steel specialist, to prove and streamline the roof assembly method before building work on the new terminal begins at the end of the year.

The principal contractor is Severfield-Reeve DesignBuild, also based at Dalton, and Hathaway Roofing of Bishop Auckland is responsible for the specialised aluminium and steel roof. About 80pc of the roof will be prefabricated at Bishop Auckland and transported to Heathrow.

About 40,000 tonnes of steel from regional suppliers, including Corus at Redcar, have been fashioned at Dalton into beams, columns and bracings for the new terminal, which will be a quarter of a mile long and one of the biggest construction projects in the country.

The £2.4m roof trial project at Dalton began in April in an attempt to identify in advance any costly construction problems which might have been encountered at the Heathrow site.

It is scheduled to end in September, when the structure will be dismantled and taken to Heathrow for later assembly as part of the £3.7bn terminal, the roof of which will have 22 similar steel frames. The project also includes a £1.8m, full-height glazed facade.

BAA project leader Mark Cowieson said: "The roof trial has aroused considerable interest among locals and passing motorists as they have struggled to work out what it is for.

"As far as possible, the exact conditions the project team will face during construction at the Terminal 5 site have been recreated at the Dalton yard.

"Constraints at the terminal site, just yards from one of the world's busiest airfields and between the two runways, mean that, when we start assembly, we have to get it absolutely right.

"The main objective was to practise constructing one of the most difficult sections of the roof and support structure before repeating the process at Heathrow. The engineering involved is extremely complex, in a similar way to bridge building.

"Thanks to the expertise of the companies involved, this trial has exceeded our expectations. It has revealed 130 issues which would otherwise not have been known about until the critical time of erecting the structure at the airport.

"We calculate that we have saved about three months' work and a possible £4m which would otherwise have been lost through down time."

Peter Emerson, a director of Severfield-Rowen and managing director of its subsidiary, Watson Steel Structures, said that at its peak the terminal project would involve 6,000 workers, many from the Thirsk area. Most of the 120 steel erectors would come from the North.

Mr Emerson said that, as well as Corus at Redcar, which is facing an uncertain future, the project was maintaining other jobs in North Yorkshire and the North-East by employing other local suppliers, as well as local hauliers to transport materials.

He said the project was worth more than £100m over the next two years to Severfield-Rowen and added: "We are absolutely delighted. It is the culmination of a lot of hard work by a lot of people. It reinforces our position as the UK's leading structural steelwork specialist."

Terminal 5, approved by the Government two years ago after a 46-day public inquiry, has been designed to handle the new generation of airliners and is due to open in 2008. It will handle 30m passengers a year on top of the 63m which already pass through the four existing terminals