Fly away home

A week before the Queen opens Sir Norman Foster's new building - the American Air Museum at Duxford, near Cambridge - the architect himself discovers a problem. The museum leaks. Sir Norman is taking a photocall on site when his hawk eye spots the workmen putting the finishing touches to the building, which is five storeys high in order to house the B52 bomber. Drills whining, they bolt slim, customised panels into the gap between the glazed facade and the plastic membrane that keeps the curvaceous, pre-cast concrete roof dry.

Linford Christie couldn't have sprinted faster to stop them in their tracks. Two architects from Foster Associates are summoned to find out why this delicate, shadowy, inverted gap where two surfaces join - a stylistic trick of modernists world-wide - is being panelled. The gap, it turns out, lets in water.

"Get some builders' sand," he commands. To pack into the gap, and hope that it remains desert-like when the facade is sprinkled.

"We have," they reply. "Norman, it leaks."

The pre-cast concrete was chosen for the building because it provides the right humidity inside for veteran aircraft, so this gap must be closed. But Foster spins on his heel and tells them to get a bucket round to the local builders' merchants for some sand, to prove it to him. An pounds 8.4m museum piece, and the press marvelling at the poetry of his building ... this potentially porous joint couldn't have come at a worse time. But Stormin' Norman wants the nicety of that detail. After all, as Mies van der Rohe observed, "God is in the details." And Norman is proud of this economical, workmanlike, handsome museum which shows off the love of his life - planes - to perfection.

Some of Duxford's showpieces have been around since Biggles . Wacky little flying machines such as Tiger Moths still roar up the runway, coaxed into the air by enthusiasts who've outgrown their Airfix phase. United States and British flags flutter alongside the windsocks. Redbrick suburban buildings house the officers' mess, and a workmanlike hangar built by the PSA (Property Services Agency, now defunct) reminds you just why calling Norman's building an aircraft hangar is like calling Quaglino's a canteen. But the collection of vintage American aircraft from the First World War to the Gulf war attracts 400,000 people every year, which is why the Heritage Lottery Fund came up with pounds 6.8m in 1995 for the Imperial War Museum to build the new museum that they commissioned from Foster 11 years ago.

Foster was a happy choice. This is the architect who pilots himself to his spectacular buildings - in Frankfurt, Berlin, Bordeaux, Hong Kong, Scotland, Wales, Bilbao, Valencia. His schedule makes dizzy reading. At 61, he's fitter than men 20 years his junior. Tanned, with grizzled hair, no glasses and an athletic figure, he's always testing himself. For years, for relaxation, he performed aerobatics at intermediate level, spinning, looping and somersaulting - though he admits to not doing that so much any more. He can name every one of the bomber and fighter planes at Duxford. One of the exhibits is a blue Schweitzer glider with which Foster himself caught a few thermals in the States 20 years ago.

The American Air Museum is both the simplest and the most complicated of Foster's buildings to date. Simple, because it is a hangar, with an emphasis on clarity, natural light, and an economical system of atmospheric control. Complicated, because its geometry is based on an arched geometric shape called the torus, and it is elliptical in plan.

This torus, explains David Nelson, of Foster Associates, is like a doughnut with a bite taken out of it, dunked into a grassy knoll. In real life it is shaped like a thumbnail, with the base of the nail low in the ground, and with a tunnel-like entrance for visitors. It soars into a gigantic arch over the B52 to end at the nail-tip, 90 metres away, in a manicured glass facade overlooking the operational runway where Norman parked his helicopter.

It's odds on that visitors will want to approach the museum this way, lured by all those flying machines glimpsed through the glass, but to do so would be to miss the suspenseful approach to this awesomely scaled, beautifully lit building past Renato Niemis's glass sculpture, Counting the Cost, and straight on to the nose of the B52 bomber looming above.

Inside, an encircling ramp allows you to walk - or take a wheelchair, since there are no level changes - under or around these monstrous machines. Bathed in light from what is really only a narrow strip of window encircling the building, the silvery-grey cement ramp is angled to bounce back daylight. It is not ash-grey, like so many cement surfaces, but burnished and silky smooth, with a reflective quality.

"There is a certain kind of spirit, around aircraft in hangars and service bays for helicopters, that is to do with cleanliness and immaculate supervision. I wanted this museum to have that kind of feeling," Norman Foster explains.

Eleven years ago, when he first sketched out the concept (which never changed, despite funding problems dragging it out until the Saudi Arabians post-Gulf war, and American veterans, chipped in) Foster Associates went into consultation with Ove Arup and the contractors to realise the building. It had to cover the B52 and allow enough space for the rest of the collection, as well as possess the structural capability for planes to be hung from the roof. At first they worked it out in steel, but the complications of the roof and the humidity levels required to house aluminium aircraft meant cement had to be used, pre-cast in panels.

For a pre-fab, it is fabulously calculated: 300 pre-cast concrete ribs that form the walls and roof, and 700 outer panels, each weighing 11 tons, were towed to the site. Spans were determined as much by the lorries that transported each panel to Duxbury as they were by the 61-metre wing span of the B52 bomber. Sockets in each panel were bolted together and cemented, because the pre-cast panels couldn't be allowed to flex: the roof works as a monolithic piece only because it is a true arch. Inside, those sockets became the rigging for planes suspended from the roof. Each socket can support 12 tons in any one section.

Less than pounds 8.5m for a building that size isn't much. When a project is tightly constrained by economic forces, designers are stretched creatively, says Sir Norman: "There's no room for frills." If more money had been available, what would have been on his shopping list? "A metal roof and a wall that opens up with a plunge door, so that you can push a button in the cockpit of a plane and fly out." n