Airport expansion: Which options will be cleared for take-off?
The interim report by the Airports Commission has shortlisted three options for expanding UK airport capacity. These are:
- A third runway at Heathrow
- A second runway at Gatwick
- Lengthening an existing runway at Heathrow, increasing capacity
The commission, led by businessman Sir Howard Davies, was set up by the government in September 2012 with the final report due in 2015.
So what do we know about the shortlisted proposals?
The Davies Commission has shortlisted a plan for a new 3,500m-long runway to the north-west of Heathrow's existing runways, after the airport submitted three optional locations.
Extending the existing northern runway by at least 6,000 metres to the west and allowing it to operate as two separate runways has also been shortlisted.
Proponents of Heathrow expansion have long argued that Britain needs a larger hub airport to compete with European rivals like Amsterdam Schiphol and Paris Charles de Gaulle.
However, expanding the airport has many opponents, chiefly on environmental grounds.
Mayor of London Boris Johnson has put forward various plans for a brand new four-runway hub in the Thames Estuary, but Heathrow's managers say their options are "quicker and cheaper".
Heathrow's submission said a new runway could be in place by 2029, allowing 260,000 more flights.
The airport is one of the world's busiest, handling 70 million passengers last year. More than a third transfer to other flights, making it a major hub airport.
But Heathrow operates at 98% of its capacity within a current limit of 480,000 flights per year, much closer to capacity than other major London airports and rival hubs in Europe.
Lack of spare capacity can mean delays and cancellations can be caused by small disruptions such as bad weather.
Switching Heathrow to 'mixed mode' operation has also been proposed as a medium-term option. This would see both runways used for arrivals or departures.
However, this would have major implications for noise levels around the airport.
The Davies Commission has rejected it in the short term, saying it would "make sense only as part of a longer-term scenario for adding new capacity."
Heathrow's owners, Heathrow Airport Holdings, (formerly BAA) have also said that they do not support mixed mode operation.
Pros: Seen as the best short-term option to keep Britain competitive with its European rivals, a third runway could be completed relatively quickly. The airport is a big employer and supporters cite a knock-on effect on businesses in the area.
Cons: The environmental impacts. Susan Pearson, 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 people already living under the flight path. Added to that, there would be a loss of homes.
Comment: "A third runway at Heathrow is the most disastrous option in terms of noise and air pollution, but it would deliver far more useful flights than any other solution and we need something fast," said David Learmount, of aviation experts Flightglobal.
The Davies Commission has backed Gatwick Airport's proposal for a new +3,000m runway south of the existing one.
The runway would be built sufficiently far from the existing runway to allow for fully independent operation.
Gatwick is the world's busiest single-runway airport, but relatively few passengers - about 5% - use Gatwick to transfer to other flights.
Gatwick says its case is "robust and compelling". The expansion would cost between £5bn and £9bn, and be financed privately.
Airport chiefs say that the new runway could be opened by 2025 and would have less of an environmental impact than a third runway at Heathrow.
Depending on how the second runway is used, passenger numbers would be expanded from about 34.2 million passengers a year to between 67 and 87 million.
Gatwick backs a "constellation" approach in which London would be served by three main airports, each with two runways.
Pros: If Gatwick added a second runway, fewer people would be affected by noise and pollution than the number affected by a third runway at Heathrow, which is closer to central London. Gatwick already has 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, expensive project.
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," said Laurie Price, an aviation consultant and former government adviser.
Other airport options not shortlisted
Mayor of London Boris Johnson has long advocated a new four-runway hub airport to replace Heathrow, but the Davies Commission has not shortlisted the proposal "at this stage".
Options have included an artificial island in the Thames Estuary - dubbed "Boris Island" - a major expansion of Stansted, or an airport at the Isle of Grain in north Kent.
One version of the Isle of Grain plan, designed by the company of architect Sir Norman Foster, includes new high-speed rail links and a Thames crossing and barrier. It is estimated the cost would be £50bn.
Mr Johnson has said that London needed to match foreign cities which had constructed "mega airports", and that anyone who believed that was possible at Heathrow was "quite simply crackers".
His plan also includes creating a new town housing up to 250,000 people on the Heathrow site once the airport closed.
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, and there is a 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. The cost and length of time it would take to complete are also cited as problems.
Comment: "The Thames option could provide a solution but it would also be the most costly and would take the longest. There is also a severe risk from migrating birds. Planes survive most bird strikes but the cost to the airlines is huge and occasionally, like in the Hudson River crash, they stop all the engines," said David Learmount.
Options for expansion at Stansted has also been ruled out, with the Davies Commission citing falling passenger volumes and existing spare capacity.
The current terminal opened to passengers in 1991 and is now the UK's fourth busiest airport. Boris Johnson has previously included it on his list of three possible four-runway hubs.
It is the UK base of low-budget airline Ryanair, whose chief executive Michael O'Leary has called for a second runway at Stansted to help increase capacity in the South East.
A Ryanair spokesman said the government lacked a "progressive strategy" on airport expansion and said ministers should "immediately sanction" a third runway at Heathrow and second runways at both Gatwick and Stansted.
"Anti-growth nimbys cannot be allowed to oppose national infrastructure on the basis of their narrow vested interest," the spokesman added.
The Davies Commission suggests a second runway could become a viable option in the 2040s.
Pros: Fewer people would be affected by noise than at Heathrow and there is already land to build a second runway.
Cons: Poor transport infrastructure makes it a difficult trip from London, and a huge increase in passenger numbers would make this worse. Improving transport links 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?" said Laurie Price.
Birmingham airport's chief executive, Paul Kehoe, authored a report last year outlining why the government should utilise existing airports outside the South East to take the strain off Heathrow.
He said Birmingham, which currently has 9m passengers a year, could build an extra runway and take up to 70m a year passengers in the future.
Mr Kehoe said the UK needed a "network of long-haul national airports" which would help "rebalance" the economy.
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," said 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 British European Airways - a predecessor of present-day British Airways - handling as many as 50,000 take-offs and landings in 1957.
The Davies Commission said it "recognises the value of the role Northolt currently plays in respect of civilian traffic, but does not include significantly increased use of Northolt among its short or medium term recommendations."
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's a ridiculous idea. It's surrounded by suburbia, and where's the infrastructure to support it?" said Flightglobal's David Learmount.
Transport Secretary Patrick McLoughlin has said the country needs to increase its aviation hub capacity to compete with international competitors. It is a view echoed by some other politicians, business leaders and aviation experts.
But there are others who say no credible case has been made to increase the UK's airport capacity.
Friends of the Earth's head of campaigns Andrew Pendleton said: "The region doesn't need more airport capacity; London already has more flights to the world's top business centres than any of its European competitors."
Susan Pearson, communications director of Airport Watch, a group opposed to airport expansion that might harm the environment, said more effort should be made to utilise existing capacity.
"We don't have any hard evidence yet on demand," she said.
And Cait Hewitt, deputy director of the Aviation Environment Federation, said: "The idea that the UK is facing an airport capacity crisis is a myth.
"Architects seem to be looking around for work and airports want to impress their shareholders with grandiose expansion plans, but the truth is that we don't need any new runways."