Flying colours.

The design of a new museum in East Anglia, sheltering a collection of historical American aircraft, pays powerful tribute to all US airmen, past and present.

Of all proposals to Norman Foster in recent years, the one by the Imperial War Museum to design the American Air Museum in Britain must have caught his imagination with particular force. A pilot himself, his passion for flying and aircraft is well-known.

The American Air Museum is part of the aviation museum, run under the auspices of the Imperial War Museum at Duxford, about eight miles from Cambridge. As part of one of the best collections of historic aircraft in the world, the museum also has what is thought to be the finest collection of American military aircraft outside the USA.

This featureless stretch of land is an emotive place. The airfield has been in existence as an RAF station since 1918, when British pilots training for the Great War were joined by an American squadron. In the Second World War, Duxford was a Battle of Britain fighter station and occupied between 1943 and 1945 by the US Eighth Air Force. Closed down in 1961, it was taken over by the Imperial War Museum and opened to the public about 15 years later.

Today, standing beside the airfield, which is still active, you can watch the more agile of the old planes - Spitfires, Mustangs and Lightnings - climb into the sky. Relics of the Great War are the hangars which, now listed, serve as exhibition halls for the various collections.

The Foster design of a new American hangar took shape over 10 years ago when it became clear that aircraft displayed outside, rotting and corroding in the English climate,(1) had to be sheltered. Recession and lack of funds delayed construction until the arrival of a grant from the Heritage Lottery Fund. It was supplemented by money from fundraising on both sides of the Atlantic - by such glamorous former pilots as Charlton Heston and the late James Stewart - and from Saudi Arabia, grateful for British and US efforts in the Gulf War.

The client wanted a landmark but also a building that would be a neutral backdrop for the planes; and Foster's museum is both an elaboration of the basic hangar and in substance a monumental tribute to the machinery it shelters.

Set at one extremity of the airfield, the building is a huge shell-like structure emerging from a grassy mound (it is not the same, but you cannot help recalling the Sainsbury Centre extension). This toroidally generated shell - like the sliced half of an odd bi-valve - is slashed around the base by a crescent of glass and enclosed where sliced through by a glass wall. Abstract form and the building's tendency to dematerialize (it is sheathed in a silvery membrane that at times disappears into the skies), dissipate size and make accord with the ephemeral nature of the wide Cambridgeshire landscape.

Inherent in the design of the structure is a nice opposition, for if aircraft design is to do with detailing, the architects of this starkly dynamic shed have eliminated it, leaving the thrust of the structure to speak for itself. At its greatest, the free-spanning arch of the roof measures 90m across and 18m in height, and elliptical on plan, the building is 100m long. Its design was driven mainly by the need to house the sinister B-52 Stratofortress, the long-range bomber famously used in Vietnam and the Gulf with a wingspan of 61m and a tailfin 16m high; but also by the desire to suspend some of the smaller aircraft, weighing up to 10 tonnes, from the roof. The building had also to be economic to build, run and maintain, for the only source of Duxford's income is admission charges. While heating costs were felt to be prohibitive, humidity control was considered essential. As much daylight as possible was to be admitted to reduce lighting bills.

Comparative analyses showed that a concrete building could keep the temperature above dew-point, so that condensation did not occur and dehumidifying plant could be kept to a minimum; and in effect, the Foster and Arup engineers have devised a structure that is self-regulating - within limits. Thermal mass helps delay the change in temperatures, ironing out peaks and troughs and taking 2 to 3 degrees Celsius off external extremes.(2) Daylight flooding through the glass facade is augmented by light bounced through the inclined glass crescent off surfaces at the rear of the hall.

The roof is composed of two shells, each built up from precast concrete panels and fitted together like a Roman vault. Because a torus is defined by only two constant radii, using toroidal geometry simplified and cheapened construction. There are only five panel types for the entire roof. Those of the lower shell are T-shaped in cross section, the stems of the Ts separating the shells and forming the ribs to which the upper panels are fixed. The roof was erected, much as Roman vaults were, by means of false towers; and when completed, sockets of the lower panel were cleaned out and converted into suspension points, each having a capacity of 12 tonnes. At the front of the building the structure behaves mainly as an arch, and at the back, where it is very flat, as a beam. Membrane action of the shells permits load sharing in two directions under the weight of the suspended aircraft.

Forces from the roof shells are collected into an in-situ upper ring beam, passed across the glazed crescent through steel arms to a lower in-situ ring beam, and finally to the abutments in an arrangement borrowed from ancient habits of military defence. As well as stabilizing the structure, the embankments help to insulate it. Massive concrete foundations resist the thrust of the arch into local chalk.

The glass facade is demountable so that aircraft can be taken in and out. Here too, the architects were able to rationalize the structural system. The wall is stabilized out-of-vertical by the roof but is otherwise self-supporting.

You approach the museum from the northwest. Passing a Renato Niemis glass sculpture commemorating the dead, you enter a tunnel in the earthworks, resembling the reinforced shelters in airfields. Entrance is dramatic, for at once you find yourself on a platform looking straight at the wicked nose of the B-52. Overhead, Gary Powers' U2 spy plane hovers like a black shark. To right and left a ramp following the edge of the roof curves down to the ground, the curve dynamically opposed to angled buttress-like walls enclosing the rear of the main hall. Behind them, and buried in the abutments, are small exhibition rooms, services and the dehumidifying plant.

On the ground, you are free to wander around at will, to crawl under wheel arches and touch. An extraordinary slice of recent history is embodied here. For enthusiasts there is a logical progression around the 20 or so planes on display; but even the averagely imaginative visitor must be stirred by the human ingenuity, good or bad, that is present on all sides. Visions of destruction exert a horrid fascination. There is too a magical relationship between the defunct aircraft in the museum, and the airfield outside that is buzzing with resurrections and very much alive.

1 Boeing's B-29 Superfortress, for example, the long-range heavy bomber used for atomic raids on Japan and a gift from the Americans, arrived from dry storage in the Nevada desert. Once in Britain, dust and sand in the structure trapped moisture causing rampant internal corrosion. Unique in Europe, the B-29 has been restored, as have many other aircraft, by Duxford's experts and teams of enthusiastic volunteers.

2 Last year's exceptionally hot summer caused problems, for the structure is calculated to cope with a maximum of 29 degrees, and summer heat exceeded 33 degrees. Venting the structure would have negated the effects of dehumidifying plant.