Roger's fifth.

The proposed Terminal 5 for Heathrow, London promises to bring clarity of movement and spatial excitement to a major international airport that is sadly lacking both at the moment.

Terminal 5 at Heathrow promises to take one step further the new approach to airport design witnessed already at Stansted, Stuttgart and Kansai. If the proposals by the Richard Rogers Partnership emerge relatively unaltered following the current public inquiry, the new terminal will confirm the arrival of a fresh generation of airport buildings. Terminal 5 combines two principal technical innovations: new rapid baggage handling and assisted people movement systems.

Space, structure and volume draw upon ecological metaphors. The rationality of movement is tempered by great tranquil spaces where variegations of light and shade are meant to recall natural patterns. The juxtaposition of invisible mechanical systems which move baggage with unprecedented speed and efficiency with sensuous tent-like shelters and floating undulating ceilings supported by branching columns gives Terminal 5 an altogether different character from Heathrow's earlier buildings. The plays of light, structure and diaphanous material are, according to the architects, intended to give passengers a positive memorable experience in order to combat the trend whereby terminals look the same the world over.

Terminal 5 represents a new appreciation by the British Airports Authority (BAA) of the commercial value of good design. The brief instructed Richard Rogers' office to develop a design which was 'unmistakably of the UK' and which would 'act as a front door to the country'. It required that all the principal buildings (terminal, hotel, office, car parks) should share a common architectural style with the Rogers' office being 'the guardian of design principles'. According to BAA, quality of environment is the main means by which customer perceptions are shaped. Large interior spaces beneath an undulating and unfolding roof, plenty of natural light, and a structural system which is reminiscent of trees in a park achieve together that distinctiveness which may help set Terminal 5 apart from other major world airports. The design concept is not without precedent in Rogers' office: there is undeniably a hint of the South Bank envelope and elements of the Shanghai Masterplan, both evolved in 1994 while Rogers was writing his Reith Lectures for the BBC.

The main terminal is rectangular in plan and, like Stansted (AR May 1991), extends a bay of roof outwards to protect the landside approach road and the airside access jetties. Hence, passengers are sheltered at the car and bus drop-off point on one side of the terminal and at the point where planes are boarded on the other. The sheltering roofs are not canopies attached to the side of the building but part of a single undulating roof which rises and falls to mirror the activities inside the terminal. At its highest point, the roof glides over a double-height interior space which contains banks of airline and customs staff offices which effectively define the divide between arrival and departure concourses. The principal public areas are marked by four wide spines of rooflights which run immediately over the main concourses: assembly and check-in, shopping, customs and departures, arrivals and airside aisle. Progression through the building is steered by natural light: successive bands of daylight signal the next stage in the journey through the terminal, Immediately beneath each line of rooflights stand the branched columns which support the roof. Consequently, the columns and their six radiating arms are picked out in light adding a further element to passenger orientation. Two types of natural light are employed: direct light which enters the centre of the building, and a softer, diffused light which filters through the complex roof structure. Since artificial light is the principal element of bought-in energy (accounting for about 40 per cent of building running costs), the design seeks to maximise natural sources. Roof glazing allows daylight penetration into the core of the building where major offices (for airline, customs and immigration staff) and concourses are located. The form of the undulating roof is also intended to reduce light spillage into the night sky: a potential hazard for pilots and a source of community annoyance.

Terminal 5 owes its geometric simplicity and structural elegance to the precedent set by Stansted, yet it takes Stansted's tree-like columns and islands of rooflights a step further. The undulating roof gives interest and focus whereas Stansted's flat ceilings are without a sense of hierarchy or progression. Terminal 5 and Stansted may share similarities in plan and structural arrangement but the cross-sections of the two buildings are quite different. Terminal 5 is a multi-level terminal with departures above arrivals in the traditional arrangement but it is split to allow diagonal daylight penetration. A central bank of elevated offices allows public spaces to flow beneath and gives justification for the roof to rise in the middle. The irregular elevation given to the roof not only enlivens the building from the outside (particularly one anticipates from the air), it also gives meaning to the interior progression. In this respect it is a hybrid between the exuberant, structurally muscular Kansai Airport (AR November 1994) and the neutral yet refined flat-roofed Englishness of Foster's design at Stansted.

The roof is a major defining element in the design. Its wave-like form extends the precedent of Kansai in two important ways. Whereas Renzo Piano's roof has a double asymmetrical shallow and abrupt curve, the design of Terminal 5 consists of five symmetrical waves of varying height. The effect is not one of a single wave but a series of ripples peaked in the centre.

The other significant departure from Kansai concerns the construction of the roof. Kansai is beefy and vigorous in its structure and detail with several layers of roof construction, each individually expressed. The design from Rogers' office seeks what John Young (project director) calls 'a single layer skin' which passengers perceive as a delicate cover that is supple and that shapes the space. The single skin roof is, like Stansted and Stuttgart (AR May 1991), free of services so that the elegance of the structure is not compromised. The height of the roof allows smoke ventilation by natural means.

The masterplan combines a detached satellite terminal arrangement with the idea of a core terminal served directly by aircraft parked on the apron. It dispenses with long elevated piers preferring to use relatively short lengths of underground passageways with travelators. These not only serve the two independent satellite terminals (a third one is planned for the future) but give direct access via lifts and escalators to the extended Piccadilly metro and the Heathrow Express railway system. The arrangement allows aircraft to park close to the buildings in a toast-rack plan, maximising apron and taxiing areas; the compact layout also reduces travel distances for passengers and provides ease of interchange from one mode of transport to another. The satellite buildings (or mini-terminals) are designed as smaller versions of the core terminal.

When it is completed in 2003, Terminal 5 will handle 30 million passengers a year, nearly half of Heathrow's predicted total at that time. This compares with 90-100 million expected at Seoul Airport by 2000 (where the principal terminal is being designed by Terry Farrell & Partners), 25 million at Kansai and 10 million at Stansted.

With such numbers, architecture is the main vehicle available to uplift the spirits and provide a spectacle in the tradition of the great stations of the past. Light is the key to the architectural experience at Terminal 5 - it is taken to the depths of the underground railway station via light shelves and light deflecting walls. Light is also used to help distinguish public from private routes and between arrivals and departures areas. Problems of glare and solar gain have had to be overcome using louvres, angled walls, fabric canopies and eaves overhangs. These provide a measure of complexity and local richness to the design.

Passenger needs of comfort, stress-free travel, legibility and excitement expressed in the brief have been translated into an elegant design. It is not a classical modernist pavilion in a verdant park (like Stansted) but a building shaped by environmental and site planning factors and the need to give passengers a sense of direction. That BAA should value design both as marketing tool and means of promoting passenger orientation represents a welcome departure from earlier practice. For the Rogers office, the design confirms the trend towards a new blend of culture and nature: architectural form which draws upon ecological principles even in the unlikely setting of one of the world's busiest airports.