The Terminal Five model project will end in September

The 35-metre high steel structure at a steelwork contractor's site in Dalton, near Thirsk, is a trial run for the roof of the building.

The �3.5bn new terminal has to be constructed between two runways at one of the world's busiest airports and the developers say they need to get the assembly exactly right.

Airport operator BAA says they have already identified 130 construction problems they would otherwise not have known about.

It estimates that the trial has saved about three months work and a possible �4m as well as significantly reducing safety risks by practising the construction.

'Wow factor'

Peter Emmerson, the managing director of Watson Steel, the company constructing the roof, told BBC London the finished product would be stunning.

"The term 'wow factor' has been used when talking about the roof of Terminal Five and that describes it very well," he said.

"There will be an intake of breath when people see it. It is iconic.

"It is the kind of striking structure that people will see, will understand it's out of the ordinary and I think will be very excited about."

Open in 2008

The exact conditions the project team will face at the Heathrow site have been recreated as far as is possible at the Dalton yard.

The construction of the main 396-metre long terminal building is due to start in November.

In 2001 Terminal Five was given the go-ahead after a four-year public inquiry - the longest in the UK's history.

The new terminal will be capable of handling 30m passengers a year and is due to open in 2008.