And, though most clich?deplete our responses (and effectively tranquillise our emotions) there’s something about this one that can often make us more responsive to the constructed anxieties of cinema. We might briefly imagine ourselves to be getting God’s view of the ground, but we never feel immune from falling. This is true of nearly all bird’s-eye tracks – not just this particular form. We are removed from a perspective in which we imaginatively share space with the actors to one in which we’re unmistakably above and separated from them (as in the overhead tracking shot which Martin Scorsese uses when viewing the aftermath of a shooting in Taxi Driver).Perhaps most importantly, though – and certainly at the heart of its eager adoption by an art-form devoted to sensation – is the sense of vertigo that the shot delivers – its sense of precarious poise above a gulf.

It can be a very pleasurable shot this, a dream of flying – but it’s almost never without a nightmarish element.The rigid geometry of the shot does something else too, though – it makes this camera-movement more self-consciously scientific and detached than other kinds of tracking (which are always prone to become anthropomorphised – as something eager or obedient or excited).The city glides beneath us like a specimen slide being mechanically scanned beneath a microscope lens – evidence to be inspected. Looking perpendicularly down we feel something different – the bombardier’s terror of the bombing run, when evasion is disallowed and the attacker is at his most vulnerable. Since it more closely approximates a natural outlook – ahead, at the oncoming landscape – it’s more easily absorbed. A forward-looking helicopter shot (another addictive clich?or the jobbing director) delivers a quite different sense of superiority to the audience – because we can see what’s coming.

We can see the city beneath us with a predator’s eye (one reason that the shot is used frequently in thrillers) but we simultaneously understand that we are too far above the streets to be able to grasp the essential details.There’s something unnerving about the locked-off camera, too. But it did get me wondering about the potency of that particular clich?Why is it that it works on us so effectively – and what has made it so successful in the Darwinian ecology of film language? One answer would obviously have to do with its hint at omniscience – at a perspective from which the floor plan of human life is laid out like a map – and yet it is a shot which, curiously, mixes potency and helplessness. As always when it is employed we feel a sense of the ineluctable and fated, which is hardly inappropriate to the subject matter of the film. As a result, most scenes in United 93 are carefully uninflected.

When the first plane hits the World Trade Center you see the resulting fire from the windows of Newark Control Tower, as something distant and difficult to grasp, glimpsed over the shoulders of people who don’t yet know what it means.
There is, as far as I can remember, just one exception to the film’s extended repudiation of visual panache – and it occurs at the very beginning, when Greengrass employs a much-loved clich?f modern cinema: a tracking shot of Manhattan from directly above, so that the city blocks roll past beneath.This doesn’t really damage his attempt to purge United 93 of calculated or artful spectacle because we haven’t even begun the task of suspending our disbelief – and the shot does what it is intended to do, which is to offer a kind of key-signature for what follows. But it is also a determination that the events of that day shouldn’t be coloured by any kind of hindsight – artistic hindsight being a particularly distorting version of that disability. There are things here that relatives don’t want to see and which the rest of us should not – even if an ignoble part of us hankers after the sight. This isn’t just a matter of his well-publicised avoidance of recognisable Hollywood stars or the studious low-key naturalism of the dialogue.