Heathrow expansion: The alternatives to a third runway
In September, the government launched a review of how the UK might expand its airport capacity in the South East.
One long-standing option has been to add a third runway at Heathrow, but this plan has many opponents. Here, we look at the main arguments for and against the Heathrow plan and some of the alternatives.
Heathrow is one of the world's busiest airports, handling more than 69 million passengers a year. More than a third transfer onto other flights, making it a valuable hub airport. Those transfer passengers using Heathrow to change flights have helped create a large route network and bring economic benefits. But Heathrow is close to full capacity and delays and cancellations can be caused by small disruptions such as bad weather.
Flights can't operate 24 hours a day and an agreement with the local authority to not use the runways in "mixed mode" - where take off and landing operate from the same runway - limits Heathrow's capacity for more flights with two runways.
However, building a third runway has proved a divisive issue for many years.
Pros: Seen as the best short-term option to keep Britain competitive with its European rivals, as a third runway could be completed relatively quickly. The airport is a big employer and supporters cite a knock-on effect to businesses in the area.
Cons: The environmental impacts. John Stewart, of the Airport Watch campaign, said Heathrow would become the biggest emitter of carbon dioxide (CO2) in the country and the noise pollution would become even worse for the 725,000 already living under the flight path. Added to that, there would be a loss of homes.
Comment: "A third runway at Heathrow is inevitable because nothing else does the trick. We have left it so long, we don't have a choice," says David Learmount, of Flight Global.
Gatwick is the world's largest single runway airport where more than half the flights are low cost. Few passengers - approximately just one in 10 - use Gatwick to transfer to other flights.
There has been talk of combining a second runway at Gatwick to create a combined hub with Heathrow, dubbed "Heathwick"
Pros: If Gatwick added a second runway, fewer people would be affected by the noise and environmental concerns than a third runway at Heathrow. Plus, there are already good transport links to London.
Cons: An existing agreement means no new runway will be built at Gatwick until 2019. The north terminal was built where the second runway would have been. A high-speed link between Gatwick and Heathrow would be a long-term project and expensive.
Comment: "It would still mean you're splitting your hub between two airports, whereas at Schiphol [airport in Amsterdam], you've got one. The market isn't there for it," says Laurie Price, an aviation consultant and former government adviser.
Stansted was opened to passengers in 1991 on the site of a former airport and is now the UK's fourth busiest.
It is the UK base of low-budget airline, Ryanair, whose chief executive Michael O'Leary recently called for a second runway at Stansted to help increase capacity in the South East.
Mr O'Leary says there is already planning permission and all that is needed for the second runway is government approval.
Pros: Fewer people would be affected by noise and there is already land to build a second runway. Easier to add a second runway than at other airports like Luton.
Cons: Poor transport infrastructure makes it a difficult trip from London, especially if there would be a huge increase in passenger numbers. Updating it would be costly.
Comment: "It's the wrong side of town. If the demand is north and west, why do you do it north and east?" says Laurie Price.
Opening up the RAF airport for short-haul civilian flights has been suggested as a short-term solution while a third runway at Heathrow is being built.
Currently the RAF's main base in London, it is already used for a small number of private business flights.
It used to be the key London base for the now defunct British European Airways, handling as many as 50,000 take-offs and landings in 1957.
Pros: Located only a few miles north of Heathrow, so in a better position to serve people transferring flights than Gatwick. There is already a runway and lots of spare capacity.
Cons: The government says it needs a military airfield inside the M25, so it would be difficult to add thousands of annual civilian flights. It would add more traffic to already busy skies in the area.
Comment: "It wouldn't give much additional capacity. The runway would have to be made longer. There would be the same environmental problems as a third runway at Heathrow," says Flightglobal's David Learmount.
Three proposals have been put forward for an airport located in the Thames Estuary that would replace Heathrow. It's an option favoured by London Mayor Boris Johnson.
The mayor has supported the idea of a new man-made four-runway hub airport, dubbed "Boris Island", located at Shivering Sands in Kent.
An even more ambitious vision came from the company of architect Sir Norman Foster. His Thames Hub proposal includes a 150-million-capacity airport on the edge of the Isle of Grain, new high-speed rail links and a Thames crossing and barrier. It's estimated the cost would be £50bn.
The latest plan, by the architects Gensler, would include four floating runways tethered to the sea bed. It would cost up to £60bn.
Pros: Likely to be privately funded and provide an airport hub large enough to compete with European rivals. Create thousands of new jobs to the east of London. Solve the noise problems affecting London residents.
Cons: Critics say it is likely to devastate the economy around Heathrow. Serious risk of bird strike to aircraft using the airport. Air traffic controllers say it is in the worst spot for traffic. Environmental campaigners say it would be a threat to rare wildlife. Others say a nearby WWII wreck loaded with unexploded bombs would be a risk. Length of time it would take to complete.
Comment: "At a time when short-term results aren't very good, then something long-term and more or less guaranteed to pull in income would seem attractive to investors," says David Learmount.
Birmingham's chief executive, Paul Kehoe, recently authored a report outlining why the government should utilise existing airports outside the South East to take the strain off Heathrow.
The report says using strategic airports across the country that already have spare capacity would be better for customers and businesses.
Pros: Central England location. Work has already started on extending the runway for longer haul flights, with spare capacity for more than 27 million passengers annually by 2014. Those in favour say it will help spread the economic benefits to regions outside of the capital.
Cons: Lack of interest from airlines in using Birmingham so far. Still more than an hour away from central London by train.
Comment: "Birmingham is great, but it's only got one runway. Locationally it's terrific. But there is nothing to stop any airline flying into Birmingham today. If the market isn't there, the airlines won't go there," says Laurie Price.
The new Transport Secretary, Patrick McLoughlin, says the country needs to increase its aviation hub capacity to compete with international competitors. It's a view echoed by other politicians, business leaders and aviation experts.
Laurie Price, a consultant for Mott MacDonald, says: "We know there's probably about two million passengers a year, whose origin is the UK but are connecting in Amsterdam." He says this highlights how Heathrow's competitors are reaping the economic benefits of the UK's lack of hub capacity.
Prof Joe Nellis, of Cranfield University, points out China is building 82 airports by 2015.
But there are others who say no credible case has been made to increase the UK's airport capacity.
"There should be a moratorium on any airport expansion at all," says Friends of the Earth campaigner Jane Thomas.
"A bigger threat to our economy and our future is climate change. We need to look at the switch from short haul flights to rail."
John Stewart, who chairs Airport Watch, a group opposed to airport expansion that might harm the environment, says more effort should be made to utilise existing capacity.
"We don't have any hard evidence yet on demand," he says.
"[Former Transport Secretary] Justine Greening had the right approach, 'Is extra capacity needed? How much? Then, where?'"