Airports are about rather more than the design of check-in desks and departure lounges. They are about the design of cities, nations. So let's get one thing straight.

Any new building at Heathrow - even one that is, in effect, a new airport in itself, the massive Terminal Five, opening in March -can only ever be a temporary fix. Everybody knows the airport is in the wrong place, that it is too small and that the noise, pollution and danger of forcing planes to fly low right over the capital will get steadily more unacceptable.

Eventually, Heathrow will have to close and we will have to build a new airport way out east, something everybody has been in denial about since plans to do just that were shelved in the early 1970s.

This will involve displacing lots of geese somewhere off the Essex coast -a price well worth paying.

In the meantime, Heathrow is to carry on expanding until 2020, thus making a bad thing worse. Terminal Four, built in the mid-1980s, was meant to be the end of it.

So they almost immediately started planning T5, with Richard Rogers as architect.

Now there are plans to build a third runway and a sixth terminal. Before that, they will knock down a lot of the original congested cluster of airport buildings to make way for Norman Foster's "Heathrow East", which will be our attempt at a suitable national gateway for the 2012 Olympics.

T5 alone is a giant project, a huge work of civil engineering, with a light dusting of architecture on top, that has cost Pounds 4.3 billion. It was first suggested back in 1982. Do I hear you mutter that we could have built a whole new airport somewhere more sensible in that time? Well, of course, if this were China. In Beijing, they have built a much larger terminal than T5, designed by Foster, with a runway to serve it, in less than five years. I gather the Chinese don't go in much for public inquiries.

Having seen the Beijing terminal, which is roughly two miles long, is shaped like two Concordes parked nose to nose and contains some awe-inspiring spaces, it was inevitable I was going to be disappointed by T5, which is a great big glass and steel shed, shoehorned into too small a site. That is a little unfair. As we're stuck with Heathrow for quite a few more years, and, as we have a tradition of adapting things (think of the Royal Opera House or the British Museum), we have to judge this place on different terms. Namely: how clever is it at overcoming the problems we insist on making for ourselves?

It's ingenious, for sure. It may be compact, but T5 finally makes Heathrow feel like a serious contemporary airport rather than a tangle of knotted tinfoil tubes. Take that small site -which is actually the size of Hyde Park. Rogers's first design assumed that there would be lots more land available, and his proposed terminal accordingly sprawled across a vast area on just one level. Now, several redesigns and nearly 20 years later we have a high-rise terminal building stacked on seven levels, plunging deep into the ground to conceal its bulk. The fact that this works is miraculous. But it is just a bit Heath Robinson.

The concourse beneath the arched roof on top of the building has a grand-terminus feel to it, a 21st-century St Pancras of the air. There are some dramatic views to be had around the edges, where you can see from top to bottom past the giant robot legs that hold everything up. Once you are through security, you descend to the departure lounges. There's a catch, though. It feels like a long way down, and it is -and, if your plane is parked at the satellite terminal across the apron, you must ride a long escalator into the bowels of the earth to catch a ram there.

The architect in charge of the project, Rogers's long-term colleague Mike Davies, was also responsible for the Millennium Dome, and the two giant buildings have plenty in common intellectually. Both respond to uncertainty in the same way, by creating a big container inside which just about anything can happen. Airports are the most fluid environments on earth.

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