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This wow effect is intended to begin at the beginning, which for many visitors will be on the roof of the multistorey car park, T5's adjacent building. The car park is set 30 m away from the terminal building to lessen the blast from a car bomb. Between this building and the main terminal is what Forster terms a "canyon", which is a landscaped public plaza overshadowed by the towering bulks of the main terminal and car park. Spanning this canyon will be a series of glass-enclosed sky-bridges carrying passengers from the drop-off point on the roof of the multistorey car park onto the top floor of the main terminal, where they will have a close-up view of its dramatic arched roof.

Passengers then progress to check-in, through passport control and then cascade through to the departures lounge and departure gates on the opposite side of the terminal, which has views out over the airport.

The experience for travellers coming in the other direction is more straightforward. They will cross the terminal in the opposite direction, through the immigration and baggage halls, which are stowed on the level beneath the departure lounges. They will then emerge onto the arrivals concourse, next to the car park and transport interchange.

The interiors design team was limited in what it could do to the terminal's external appearance because the design submitted for planning in February 1993 was what the public inquiry had approved. The building's colour, cladding materials, height and dimensions, the transparency of the facade and its 10% rooflight area were all fixed.

There was more flexibility in designing the interior. Forster says designing that is all about reflecting the airport's "character". It is also about reconciling the interests of 43 types of user, such as airport security and retail tenants, some of whom are have conflicting preferences. This process is mitigated somewhat by BA's decision last year that it would be the sole tenant for T5 and that it would be housed in the terminal from the outset, so at least Forster has only one airline to deal with.

So, what is the aim of the wow factor? Forster says it is about defining a sense of place. "What kind of experience is a Heathrow experience?" he asks rhetorically. "It is the world's airport to a world city". In other words, "Heathrow is the gateway to London – it's about capturing the London buzz," he says, verging on the cryptic. What is the London buzz when applied to an airport? Well, it is "vibrant … international … contemporary" and it is not "sterile … calm … or peaceful". According to Forster, London is also "responsible" and "alert" – neither of which are adjectives that immediately spring to mind when defining the capital, but hey, they are good adjectives to chose if you're defining your ideal airport in this day and age.

To set the benchmark for the interior of the new terminal, Forster along with Tony Douglas, BAA's T5 managing director, and Andrew Wolstenholme, T5's construction director, sought enlightenment in the Far East. The three visited five new airports in a series of stopovers that lasted a mere four-and-a-half days (see below). He says they came back with a common understanding of what the new terminal should look like.

In April, Forster signed off the floorplans for the interior of the main terminal building, finally fixing the layout of the terminal's seven floors. The next step is to develop the design as far as the specification of fine details and finishes. Fortunately for the design teams charged with turning BAA's aspirations into tenders – "nailing the product" – Forster has produced a character book full of images and words. This is handy because, by the end of June, Forster says the key interior elements such as the floor finish will need to have been picked. And by early next year, installation work will begin on the first major fittings, such as the escalators.

What's still to be done on site
Twin rivers
In August, excavation of the twin rivers that run through the site will begin in earnest.

In their current location, the streams slice through both sets of rail tunnels, the track transit tunnels, the services tunnel and the airside road tunnel. "There are 10 major structures cut off by the rivers," says Richard Rook, a construction manager at Laing O'Rourke. The excavation and removal of these will be a "major milestone" in the project.

Raising the roof
The first section of the main terminal's roof has been lifted into position, but there are still four more three-bay sections to be lifted and one single-bay section. The roofing team expects to have the whole of the roof in place, insulated and waterproofed, by the end of the year.

Fit-out of terminal
The main terminal is being constructed from south to north. Once the roof is up over the south of the building, the construction of the internal superstructure can begin, along with installation of the curtain walling. Once construction of the superstructure is under way, fit-out work can begin.

Station box construction
One item on the roof team's critical path is the box for the rail station that is under construction in the centre of the basement of the main terminal. Once the box is done, the terminal's basement can to be finished, which will in turn allow the ground-floor slab to be constructed – whereupon the final sections of the roof can be jacked into position.

Construction of Terminal T5B
This satellite building is being put up from south to north. The most critical area of this construction is the basement and the track-transit system that links the satellite to the main terminal. The southern end of the building needs to be complete by October to allow the aircraft stands to the south of the building to be handed over to the airport operator.

Car park construction
Piling work has begun on the 4000-space car park. This is situated next to the main terminal building. Its basement will house the terminal's railway station; its roof will form the drop-off area for car-borne departing passengers. When complete, the car park will be linked to the terminal by sky-bridges.

Live from the satellite
Were not for its big brother, the £350m satellite terminal would be the star of T5. On its own, it is bigger than Terminal 4. It will also provide the only aircraft stands on T5 capable of receiving the Airbus A380 "superjumbo", shown to the press at Toulouse earlier this month. And it is the main baggage hub for the whole of BA.

The glass-clad satellite is integrated into T5. The extensions to the Piccadilly Line and Heathrow Express run beneath the building, and the track-transit system will transfer passengers between the terminals.

Construction of the 45 × 432 m terminal is critical to the overall T5 programme: the aircraft stands to the south of it are scheduled for handover in November, so its basement and the envelope in this area must be complete by then.

As with the main terminal building, the satellite is being constructed from south to north. At the south, four superstructure levels are nearing completion and the "trees" to support the roof have been craned into position.

The waveform roof structure is similar to that of the main terminal. It is a single-span construction, consisting of a row of curved box-section steel rafters set at 18 m centres and supported on the abutment trees. The trees are much smaller than those used on the main terminal, and are fabricated on site in specially constructed jigs close to the satellite.

The public inquiry imposed a height restriction of 19.86 m on the building. So, to keep floor thickness to a minimum, post-tensioned concrete floor slabs are being used, supported on precast concrete columns.

As with the main terminal, the building services will be modular.

The most critical part of the project now is to complete the construction of the three-storeys of substructure. The lowest point of this is the "walkway" level, which is largely taken up with circulation and plant. Next is the track-transit platform level, and above that, the "baggage basement" where the passengers' luggage, arriving and departing, will be sorted.

A second, similar satellite is planned for construction under phase two of the project, however this is not due to start until 2006.