Readers of Empire magazine last week voted Harrison Ford the greatest movie star of all time. I've a confession to make: he bores the kneecaps off me. He should retire quietly while we can all still remember his last good film (Witness, from 1985).

It's the monotony of his acting that's so bothersome. He has three expressions: I'm Sucking a Lemon But Am Still a Hard Bastard (contorted eyebrows and sourly down-turned mouth); See, Iron John Cries Real Tears (lipless grimace, streaming ducts), and Huff-Puffingly Stern So Don't Push Me, (lined forehead, flyaway hair, the look of harried stupidity on overdrive). And that's all, folks. He abandoned his fourth expression, Bitchy Sarcastic Touche (used to great effect against Carrie Fisher in The Empire Strikes Back ) some time ago, possibly because it's too camp to suit the can-do heroic persona he's carefuly constructed for himself through the last decade.

Ford's new star vehicle, Air Force One (15), doesn't expand his facial repertoire, and continues his long association with politically suspect Hollywood thrillers - Clear and Present Danger, Patriot Games, The Devil's Own. On the surface, it's another film designed to get members of gun- owning rural US households cheering in the cinema aisles: the plot concerns a gang of Kazakhstani terrorists who hijack the eponymous US presidential jet in an attempt to get their imprisoned leader, General Radek, released from jail. Only one man (as they say in these things) can stop them.

Ford plays President James Marshall, a Bud-drinking football fan determined to kick anti-democratic butt. The name is a smart choice, packing a triple- whammy of butch allusions: there's marshal (as in Western lawman, air force chief), martial (as in nuke 'em, gotcha!, over the top, boys), and Marshall (as in Plan - humanitarian aid plus economic imperialism).

Primarily, it's a sort of flag-waving techno-porn: the press release gives you all the technical specifications of the plane - "Thrust: 56,700 pounds ... Length: 231 feet, 10 inches," and tells you that the latest model "is the first Air Force One to include full twin beds and a shower". Phew! And patriotism is encrypted into the very circuitry of this airborne Oval Office. When the President is trying to foil the hijackers by sabotaging the aircraft's fuel supply, he's unsure of which wires to cross. Like a Tibetan Uncle Sam, he empties his mind of everything but the Star-Spangled Banner, and slices through the green and yellow ones, leaving the red, white and blue wires uncompromised. And God Bless America, the fuel is jettisoned. Ever felt you've been plugged into all of Ronald Reagan's favourite delusional fantasies?

But the film also exhibits some interestingly Clintonian confusions. President Ford is a Vietnam vet, but not some napalm-chucking gook-waster - instead, he earned his liberal credentials by running "more helicopter rescue missions" than any of his peers. He's not Rambo, but cuddly sitcom Dad who - should duty call - can also break necks and mow down his enemies with machine gun bullets. It's just that he has the decency to be wracked with conscience before he blows the bad guys to pieces. Another actor might have made this ambivalence the key to an interesting performance. Unfortunately, Ford treats it like a hole in the plot that can be plugged with his lemon/bastard expression.

But the film's ambiguous attitude towards its villains is director Wolfgang Petersen's biggest concession to complexity. Petersen suggests that its gang of Kazakhstani gunmen - led by barnstormingly thick-vowelled Gary Oldman - are driven by a nationalism that demands some of the same respect as that of its hero. Oldman, of course, plays hijacker Ivan Korshunov as Vlad the Impaler in a cardigan, a Grade A sociopath who, unlike his opponents, doesn't wear a tie, and whose R-rollingly Slavic battle cry "Arrolease Arradek" prompts unbidden memories of Michael Palin declaring "Weleathe Wodewick" in Monty Python's Life of Brian. But the movie, nevertheless, is prepared to flirt with his political aspirations. When he rants about capitalists being shot in the streets of Moscow, and describes how American victory in the Cold War has made his homeland a haven for gangsterism and prostitution, the rhetoric is persuasive. And, tellingly, President Ford offers no counter-argument when Oldman contends that millions of Iraqis were killed in the Gulf War for the sake of a "nickel on a barrel of oil". Sylvester Stallone would have pumped him full of lead for casting such aspersions.

There are other moral-political grey areas. A double agent infiltrates the presidential staff, though what exactly he's up to is never satisfactorily explained (he might even be working for one of those internal conspiracy theory-type organisations). Down on the ground, Vice President Kate Bennett (Glenn Close, mustering all her fierce, pale, transsexual beauty) is more hampered by domestic party politics than the terror at 30,000 feet. It's just enough to generate the suspicion that this film might really want to burn its draftcard. But the sentiment is short-lived - as Petersen cranks things up to a patently ridiculous finale, critique gets drowned out by the noise of patriotic bugles. Straining like some Easter Island statue in need of a laxative, President Ford distracts attention from his rather hollow victory over Oldman's faction with an increasingly ludicrous series of action-hero stunts.

187 (18) is an equally silly piece of Americana. It starts off very promisingly, with science teacher Samuel L Jackson trying to return to work in a rough inner-city school, after having been stabbed by one of his pupils. Social realism. Nice. We're all set for a gritty re-run of Sister Act II. But halfway through, the film suddenly blunders into Wes Craven territory, as Jackson goes ga-ga and starts firing hypodermic blowdarts into class troublemakers and meting out discipline so severe that even my old deputy head Mr Mulkerrin wouldn't have approved of it. It's a shame, because Jackson's performance is a solid piece of work, and Kevin Reynolds - the auteur behind Waterworld and Robin Hood: Prince of Thieves - directs with some interesting flourishes. In one scene, he reflects Jackson's anxiety by turning his class into a myopic blur that's actually quite painful to look at. Elsewhere, he uses flary jump-cuts that have a Nike-ad immediacy. He renders the whole film in scorched, stained hues that act as an index on the moral aridity of the education system portrayed. But the movie's second half is quite astonishingly ill-judged, and it's the plot, rather than Jackson's harassed Mr. Garfield, that goes most convincingly loopy. What's the excuse? "A teacher wrote this film," is the deadpan caption that looms before the closing credits. Screenwriter Scott Yangemann has also written quiz questions for the gameshow Jeopardy. Boasting about that in an on-screen caption would have provided no less persuasive evidence of the film's veracity.

This weekend, the Barbican began a two-week season of films celebrating 70 years of Elstree studios. Although the Hollywood of Hertfordshire has lost half of its backlot to an out-of-town Tesco, it can still attract mainlining features - when I visited on Tuesday with a charabanc of critics, the water tank was being prepared for the arrival of the cast of The Avengers. Val Guest's Hell is a City (15) opens the Barbican festivities, and stars Stanley Baker in a script with a marvellously dated sense of sleaze. Guest (who later directed The Quatermass Xperiment) makes Manchester look like Manhattan, filling his images with as much neon and highrise architecture as he can find. His dialogue, too, is self-consciously American, drunk on its freedom to slip words like "bitch" and "bastard" into its staccato rhythms. All the men are low-lifes or hard-cases, all the women are slatterns (apart from a mute girl with a predilection for nocturnal Hoovering) and Baker's detective has an arrestingly casual line in chauvinism. An irresistible period piece with a savage sense of humour and a five-second cameo from Doris Speed (Annie Walker to you).

The Watermelon Woman (nc) is a witty mixture of social comedy, pseudo- documentary and self-portrait. It's a sort of African-American lesbian version of Zelig: Cheryl Dunye directs and stars as a frustrated film- maker researching the life of Fay Richards, a forgotten (and fictional) black film actress of the 1930s - known as the Watermelon Woman. Its character sketches are keenly observed, and its cultural pseudo-investigations are lent academic verisimilitude by Camille Paglia: "If the watermelon symbolises African-American culture, rightly so. Because look at what white middle-class feminism stands for - anorexia and bulimia." Dunye has constructed a chatty, sexy, good-humoured and moving meditation on race, sexuality, history and gender. Slight, but a delight.

Cinema details: Going Out, page 14.