IN 1943 Frederick Miles, an aircraft designer, had the intriguing idea to build an airport near Gravesend, along the Kent coast, to serve London. The plan never took off - but Miles's vision lives on. Though London now has five international airports, pitches for new ones in the Thames Estuary keep coming. In March the government will consider yet another when it consults on a "sustainable framework for UK aviation." The latest plan to build a hub in east London, dreamed up by Boris Johnson, London's mayor, was until recently pooh-poohed. Now it is being seriously considered. So are plans to expand existing airports in the south-east. Ministers certainly need to solve the problem of London's airport capacity crunch, which threatens to throttle the economy. Heathrow, the world's busiest international airport, is already full. Its two runways operate at 99% capacity. Though passengers loathe the congestion, delays and often shabby facilities there, last year 68.7m of them passed through the airport, more than London's four other big airports combined. Yet Heathrow cannot grow to meet demand. If they wish to add a flight, airlines must first cancel another. BAA, its owner, says Heathrow is losing out to four European hubs, each of which has four or five runways. Frankfurt serves 245 destinations, Heathrow only 160, according to OAG, an aviation consultancy. Demand is unlikely to ease. Business travel has grown despite the rise of e-mail and videoconferencing. Stretching existing resources would help only a little. Current limits on night flying are to be revisited. Using Heathrow's runways more flexibly would allow another 60,000 flights a year, says Colin Matthews of BAA. The schedule is currently designed to give local residents some respite. Other airports may pick up trade from Heathrow, particularly if competition between them intensifies. The Competition Commission forced BAA to sell off Gatwick in 2009; on February 1st the firm lost an appeal against having to offload Stansted too. But without new runways, the Department for Transport predicts that Gatwick, Heathrow and Stansted will all be full by 2030. The key problem is whether and how to provide the capacity for a larger hub, with connections across the world. BAA says Heathrow's status as London's prime airport relies on flights proceeding to and from the same point. British Airways" experiment in the 1990s with dividing its business between Heathrow and Gatwick - "the hub without the hubbub" - failed. Still, if more runways were available at another London airport, additional airlines might set up shop there. Gatwick is already the 11th-busiest international airport and serves almost as many destinations as Heathrow, though its flights are less frequent and typically shorter haul. Governments have done surprisingly little research into the benefits of hubs, according to Tom Worsley, former economist at the Department for Transport. BAA does well out of the present one. Airlines, too, can charge premium fares for Heathrow flights because landing slots are scarce. The host country benefits little directly from air travellers who are in transit at Heathrow, but those transfer passengers help support direct access for airlines to a larger number of cities. London is also a major destination in its own right: two-thirds of passengers end their trip at Heathrow. The last Labour government's proposal for a third Heathrow runway was fought off by west Londoners irked by noisy aeroplanes and by green campaigners who fretted about noxious emissions. Action groups near other airports are geared up to resist any fresh expansion plans with similar zeal. In 2010 the coalition ruled out building new runways at London's airports. Ministers are now thinking again. Deciding where the expansion should happen is a battle in the making.