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The Big Question: What would a third runway mean for Heathrow, and is it really necessary?

Why are we asking this now?

The battle over the expansion of Heathrow airport, which would see the number flights at the airport increase from 480,000 to 700,000, is hotting up. While the Government is in favour of expanding the airport with the addition of a third runway and possible sixth terminal, opposition to the new runway is mounting, even within its own ranks.

A debate on the new runway was called yesterday after around 50 Labour MPs backed a Parliamentary motion opposing expansion. Many have environmental concerns, while others in marginal constituencies surrounding Heathrow fear that its local unpopularity could be enough to lose them their seat at the next general election. Outside Parliament, local residents already driven mad by the constant drone of planes flying over their homes gathered to protest about the Government's apparent determination to go ahead with the third runway.

While some want to limit Heathrow to its current size, others are even suggesting Heathrow should be honourably "retired", and a new airport built in the Thames estuary, where residents will not be affected, traffic congestion will be less severe and planes will not be flying over densely populated areas.

Why does the Government say a third runway is needed?

The most commonly used argument supporting an extra runway is growing demand. Passenger numbers are set to double over the next 20 years, but Heathrow is already working at almost full capacity. Despite being the world's busiest airport, it is unusual in only having two runways (a third, smaller strip is only used in emergencies). That is the same number as it had when it opened for its first commercial flight on 31 May 1946. Most other major airports have at least three. Some have four or five.

The new runway would allow an extra 220,000 flights a year to Heathrow's capacity a year by 2030, contributing as much as £9bn to the UK's economy. The business lobby say that preventing a third runway will mean that many business travellers, who use Heathrow as a connecting hub to reach destinations outside the UK, switch to alternatives such as Amsterdam, Paris and Frankfurt.

Who supports the plan?

The biggest cheerleaders for the third runway are BAA, the company which operates Heathrow, and British Airways, which has the plum take-off slots at the airport. Writing in The Independent last month, BA's chief executive Willy Walsh said: "No new runways have been built at major south-east airports in the past 60 years. In the meantime, air travel has increased exponentially and is now as vital to the success of the UK as the shipping lanes were in the early industrial era."

Ahead of yesterday's debate, the prime minister also made the case for the new runway, citing "the fact that in Paris, Amsterdam and Frankfurt, there are either four or five runways there to deal with the traffic". His words have been backed by other ministers, who have talked about the need to take "tough decisions" to keep Britain competitive.

Which groups are opposing it?

In addition to a well-organised residents' campaign against the expansion, local authorities in the area have also formed an alliance to oppose it. On top of that, the Government has some critics closer to home. As well as the 50 rebels within its ranks, cabinet ministers also have concerns. Hilary Benn, Harriet Harman, Ed Miliband and David Miliband are all said to have expressed concerns, particularly over the new runway's environmental impact.

What are the main objections to the runway?

The Government's White Paper included the caveat that the final go-ahead would only be given if the runway met the necessary noise and environmental standards. Critics say it will not. Even the Environment Agency has said that nitrogen dioxide (NO2) emissions produced by the addition of the thirds runway would cause Heathrow to breach EU regulations. The government would almost certainly have to ask the EU to delay implementation of new NO2 limits. Questions have also been asked about the equipment used to guage noise.

The extra runway would also cause major upheaval in an already heavily built-up area of West London, with 700 homes having to be demolished to make room. The roads in the area, which already struggle to cope with the traffic heading for the airport, would face even worse gridlock.

There are also holes in the economic arguments for expansion. Figures published this week showed that more than a third of passengers using Heathrow were simply doing so as a point to transfer, and contributing nothing to the UK's economy. Reducing the number of transferring passengers could remove the need for an extra runway, opponents claim.

Any other issues?

Part of the Government's evidence that a bigger Heathrow could meet noise limits is that a model of plane, much quieter than those currently landing at Heathrow, will be used in the future. There's only one problem – it doesn't exist yet. But supporters say it is fair to predict that quieter planes will be around by the time a new runway is completed.

"Technology, such as the new A380 super jumbo, shows that planes are getting bigger but are not getting noisier," said Roger Wiltshire, secretary general of the British Air Transport Association. "It is not an unreasonable assumption to make that future technology will help Heathrow meet noise limits."

What are the alternatives to a third runway?

After a long period of silence on the issue, the Conservatives are now vehemently against the third runway. Teresa Villiers, the shadow transport secretary, has gone as far as warning companies not sign any contracts related to building the runway. She has been assured by her legal team that she can rip them up should the Conservatives win the next election.

In place of the third runway, the Conservatives want to link Heathrow up to a high-speed rail network, leading to Birmingham, Leeds and Manchester. But some claim that they have missed the point – only three per cent of Heathrow flights head to Manchester or Leeds.

Are there any other solutions?

It may seem like a distant possibility, but support for replacing Heathrow with a new airport in the Thames estuary is gaining ground. Its most powerful supporter is the mayor of London, Boris Johnson, who is said to be keen to appoint the British civil engineers who helped design Hong Kong's island airport to conduct a feasibility study into the idea.

Unlike Heathrow, its location would let it operate around the clock, while it could be linked to the high-speed rail line at St Pancras and to ferry ports. But as Boris's own party opposes the scheme and tens of billions would be needed to fund the project, it still has a long way to go before becoming a realistic option.

During the Commons debate yesterday, Geoff Hoon said that the prospect of building a new airport had been looked at, but was dismissed because of fears over transport links, bird strike, the impact on the environment and raising the money for the plan. Time will tell if Boris can prove him wrong.

Should a third runway be added to the world's busiest airport?


* Demand for air travel is set to double in a generation; Heathrow's existing runways will not be able to cope

* Heathrow – with only two runways since it began in 1946 – needs at least three to compete with other airports

* Its two runways operate at more than 98 per cent of their capacity; delays have serious knock-on effects


* Heathrow was chosen as a good spot for planes to be scrambled to protect London during the war, but it's a terrible place for a commercial airport

* The Government will be in breach of NO2 limits if the runway goes ahead

* Opposition to the new runway goes all the way to the top; Cabinet members have voiced their concern