Thomas Sutcliffe: When architecture is pain relief

In 1896, the American architect Louis Henry Sullivan published an essay titled "The Tall Office Building Artistically Considered" – a hugely influential piece of prose that, among other things, gave the world the phrase "form follows function", one of the founding dogmas of Modernism. Sullivan's essay approached the steel-framed high-rise as a quite new kind of architectural problem, one that couldn't be solved with the inherited language of traditional buildings. And I was reminded of it last week when I visited Terminal Five at Heathrow, British Airways' new home at the airport, which is to be opened by the Queen later this month.

I found myself wondering whether anyone had ever done a Sullivan for the modern airport terminal. If so, I haven't been able to track it down since. And if not, such an exercise is wildly overdue, because in architectural terms there is something decidedly paradoxical about this now ubiquitous kind of building.

One odd thing about terminals is that the outside doesn't really matter. Now, there's one sense in which this is clearly absurd. Architects continue to care a lot about the outsides of terminal buildings and – from Saarinen's justly famous TWA terminal at New York/JFK to Santiago Calatrava's Central Terminal for Bilbao airport – there are plenty of examples of just how expressive they can get when carried away by the idea of flight. But the truth is that it's a very rare airport in which the public ever get an opportunity to admire those exteriors properly (usually those that have been built on green-field sites). Where exactly would you stand to look at Saarinen's New York building? On a concrete abutment between off-ramps and on-ramps, subject to the flaming abuse of New York cabbies?

The conditions of modern travel mean that the vast majority of an airport's users will reach the inside without ever noticing the exterior – either because they've been funnelled in through the sealed pipes of the embarkation bridges or because they've transferred seamlessly from one interior to another, whether they've arrived by car or train. And that puts a premium on getting some of the qualities you might traditionally associate with an exterior – scale, perspective, a sense of mass – inside the building.

From the outside, Terminal Five is a giant shed. A pretty stylish one, as you might expect, as it's been designed by Richard Rogers Partnership, but a shed nonetheless. It's only inside that you find more elaborate kinds of drama – in the giant steel arches that encompass the space and the huge knuckled struts to which they connect. And, to further add to the paradox, the biggest seduction the interior offers you is the implicit sensation that you're still outside, achieved partly by wrap-around glass walls on every side. "So light, modern and spacious that it's hard to believe that it's an airport terminal at all," says British Airways on its website, implicitly acknowledging another of the problems an airport architect has to contend with – which is that the first thought of most people entering one will be how quickly they can get out again.

It isn't alone in that, of course. Station termini are a means to an end, too, but the aesthetic of transition in a railway station is quite different from that of an airport. A station plugs you into the heart of a city and has to find a façade that will manage that junction between functional speed and street life. As a result, the dream of all transport architects – that their buildings might be worth visiting in their own right – has actually come true in several cases.

However often the wish is expressed for an airport, though, it's vanishingly unlikely to happen. An airport isn't a destination, it's a digestive system – dedicated to the ingestion of large numbers of people and their efficient expulsion, after screening for murderous intent and the extraction of reasonable amounts of cash.

As a result, one of the main tasks of an airport terminal – in aesthetic terms at least – is pain relief. The analgesic most favoured by BAA is shopping, because they make a huge amount of profit from it, but architects themselves favour slightly more high-minded tranquillisers – a soothing clarity of function, so that the traveller can maintain the illusion of free will, and surroundings that don't actively add to your distress when that illusion is shattered by another delay or a security queue.

I didn't get to see Terminal Five with its most critical component – passengers – in place. But it looks as if it will do a pretty good job of reducing the aggravation of air travel to a point where it doesn't actually induce dread. Bizarre, really: the architect's best result is to allow you to forget you were ever in the building at all.