Heathrow Airport is so busy that it could only spare a two-hour window in which to move a 330-tonne link-bridge two miles across runways and into its intended slot. Andy Pearson joined a night-time convoy with one question on its mind: will it fit?
It is 8pm on a dark, chill October evening at London's Heathrow Airport. Huddled together on a stark concrete apron close to the airport's cargo handling area a team of construction specialists is talking nervously among themselves. Voices have to be raised every couple of minutes to be heard above the aircraft thundering down the adjacent runway and off into the night sky.

Tonight is crunch time. Over the past 35 weeks, all their efforts have been focused on the design and construction of a 330-tonne link-bridge for airport operator BAA. The bridge, which weighs almost as much as a jumbo jet, has been carefully assembled away from the main passenger terminals on a quiet section of the cargo apron. Tonight it is due to be moved into position. Its journey will take it two miles across the airport, crossing one of Heathrow's busy main runways en route, before reaching its final destination at pier 5, which it will link to a newly constructed departure lounge. It is the first time that such a feat has been attempted at the airport. "We've set a precedent," says production team leader Nigel Cole of Mace.

The logistics of transporting a 52 m long, 15 m high, and 6 m wide structure across one of the world's busiest airports are far from simple. The move is only possible with the runway closed – hence the team's nocturnal gathering. Originally, it had been planned to move the structure in two phases, over two consecutive nights. On the first night the bridge was going to be transported across the runway from its point of assembly to a holding bay adjacent to pier 5; the following night it would be eased into position. A date for closing the runway had been negotiated that allowed a two-hour window for the contractor to get the bridge across the runway. "We had to agree the date six months ago," Cole says.

However, on the first day of the planned operation, severe gales disrupt flights in and out of the airport, ruining the team's careful planning. Under pressure from airline operators desperate to clear their backlog of flights, the first night's two-hour slot is withdrawn to allow the runway to be kept open into the early hours. With six months' notice required for a runway closure and a handover date already agreed for the new departures lounge, the contractor is left no option but to attempt the entire operation in one night. "We've got to go for it," says Cole simply.

The new departure lounge at pier 5 forms part of BAA's programme of continuous improvement at the airport as the operator fights to maintain Heathrow's status as Europe's principal air travel hub. The extension has also been designed to meet strict new requirements issued by the Department for Transport following the events of 11 September 2001.

"These requirements call for arriving and departing passengers to be kept segregated," explains Cath Shuttleworth, BAA's project E E leader. To comply with this edict, a corridor will soon be added to the roof of pier 5, directly above the existing departure corridor, to funnel arriving passengers directly to passport control. This elevated corridor will be constructed under a later phase of the project; however, the link-bridge has been designed as a double-height structure in readiness for the corridor's installation.

The airport's insistence that the service road be kept open during construction of the bridge was a key factor in the decision to build it elsewhere on the site. Another factor was cost: if the £2.5m bridge had been built in situ, construction work could have taken place only during a four-hour slot each night when road closure was possible. However, night working would have pushed up labour costs "by at least £250,000", says Cole. Quality would have been another potential issue if operatives had been assembling components at night.

The bridge was assembled on three trestles using BAA's standard kit of components where possible. Two Pratt trusses form the primary steel structure, allowing the 52 m span to be achieved. Crossbeams tie the two trusses together and act as supports for the roof and the aluminium floor sections. The structure has been clad using a standard Schmidlin system. Simon Penny, an associate at project structural engineer Buro Happold, says: "The bridge's two-storey height meant we could design a deep, stiff structure, which was essential to avoid damage to the glass and cladding while it was being transported."

Now, on the big night, in preparation for the move, the trestles have disappeared and the bridge is balanced on two giant motorised bogies provided by heavy-lifting specialist Brambles. Each bogie has 24 pairs of steerable wheels, driven by an electronically controlled power unit. A dedicated operator controls each bogie. A cord links the bogie to the operator's control unit, which the men wear on a strap around their neck, giving them the appearance of ice-cream sellers at a cinema. A computer link connecting the front and back power units ensures the bogies move simultaneously.

The structure's route across the airport has been planned in fine detail. Computer modelling has been used to ensure that the enormous vehicle can be manoeuvred around the various obstructions on its route. The most critical requirement of all is that, whatever happens, the bridge must be clear of the runway when flights resume in the morning.

Every contingency has been catered for, with three aircraft stands closed along the route to allow the team to park the bridge if necessary and a host of support vehicles set to accompany the move. These include a truck carrying a spare power pack, a crane to lift the power pack into place, a maintenance crew should the bogies need immediate attention and two vehicles with banks of floodlights.

The bridge is scheduled to begin its journey at 8pm. Travelling at a top speed of 1mph, the two-mile route should be completed in two hours. The plan is to drive the bridge to a holding point close to the runway, then to wait for clearance from the airport operator that the runway is closed before it continues on its way.

But as the team starts to arrive and forms a huddle on the airport's concrete apron, it is clear that there are some last-minute problems. Their nervous conversation concerns two stray aircraft parked on the cargo apron, jeopardising their well-orchestrated plans. The aircraft must be moved. One frantic radio conversation later and the problems have been resolved: the aircraft closest to the bridge will be moved immediately, but the team will have to wait until 11.30pm for the second to have its cargo loaded before it too can move clear of their route.

With time of the essence, a decision is made to start the move. The bridge will be transported as far as the second aircraft. Then, after the plane has departed, it will continue its journey.

Later than planned, but still within schedule, the move begins. A nifty bit of manoeuvring by Brambles' operatives lets the bridge edge out of its assembly site and then turn through 180° so that it faces its direction of travel end-on like a giant railway carriage.

The move commences. It is a strange procession that makes its way slowly across the airport, led by a runway control car adorned with orange flashing lights. Following on foot is the construction team. Next in the convoy comes the bogie-mounted bridge accompanied by its attendant nursemaids, one with each bogie and one to keep a watchful eye on the operation. Behind this is the truck carrying the spare power pack, then a mobile crane, then the maintenance vehicle. The floodlight vehicles bring up the rear.

A few hours later and, much to everyone's relief, the manoeuvre has gone as planned. The cargo plane left on schedule, allowing the bridge to resume its passage across the airport; the runway was traversed without a hitch; and the tricky manoeuvres through the airport stands near to the passenger pier all went smoothly.

The only remaining task is to slot the bridge into place between the new lounge building and the existing passenger pier. Before the move, the bridge's dimensions had been checked and rechecked by a nervous contractor to ensure that it would fit neatly. However, just to make sure, a 1.5 m infill section had been constructed separately for installation once the structure was in place.

The dimensions are correct and the bridge is finally eased into position in the early hours of Tuesday morning. Later that day, work is to begin on the bridge's fit-out. The team now have until 14 February to complete the works if they are to meet the project's handover date. Says Mace's Cole: "People may not realise it now, but this operation has instigated a major change in the way construction implementation will be viewed on future projects."