Hi,
There is a need for more flights to be introduced into the London area. And if not more flights, then more passengers and cargo without increasing planes and congestion into the area. The third runway increases cargo/passengers and sorts out some of the congestion.
There are various alternatives to buiding a 3rd runway at Heathrow. These include building more runways at the other London airports (Gatwick, Stanstead and Luton (+ London City Airport)). Or, making bigger planes that can carry more passengers/cargo, effectively bringing in the same amount of passengers/cargo but with fewer planes needed.
Airbus have made a big lead with this 'bigger aeroplane' theory. The proposed A380-900 could carry up to 900 people in economy-only configuration. The are also plans for an A380-1000 which can carry (as the name suggests) 1000 people (in the economy-only configuration). The A380F has also started the market for larger and larger freight aircraft. I think that this aircraft can carry 150 tonnes, whereas the B747-400F can only carry 121.
The 3rd runway at Heathrow will expand the airports future potential aswell. Currently, its runways are operating at about 98.2% capacity, whereas Paris Charles De Gaulle is running at just over 75% capacity. Ok, CDG has 4 runways to LHR's 2, but without the expansion, LHR could easily lose its 'busiest airport in the world' claim.
The new runway would only be 2200M long, as apposed the shortest current runway being 3660M. Only smaller planes could use the 3rd runway, and as it was meant for domestic flights, this would follow what was proposed for the runway. The runway would be used at about 60% capacity. This would relieve the other runways of some smaller, domestic flights, and allow smaller airlines into the airport to do the domestic flights that the runway was intended for.
Building the 3rd runway would take quite a while, providing jobs (which, lets face it, in the current economic climate are a blessing) and when the runway has been built, carry on to provide jobs. About 60,000 jobs would be created during construction and around 8000 others when the construction is complete. There would be various taxi-ways to lay, and many differently skilled people could benifit from it. Needless to say, these jobs would be created at any airport that wants to expand. From 2020-2080, its is estimated that the runway would bring about £5.5bn.
There would be less greenhouse gases due to planes, as, although the number of flights are increased, planes would not have to spend so long stacked up waiting to land. It is estimated that clearing the carbon up, would cost 2.8bn between 2020 and 2080, which is about half of the amount that the runway would bring.
There are a couple of villages under threat, and people will have to move out of their houses, however, these people have had offers of compensation worth more than their houses, and BAA have stated that the people displaced by the 3rd runway would not have their property price blighted by the runway.
I am fully for the 3rd runway, however, I do believe that the other London airports deserve to be developed before hand. This would create more jobs at different locations around London, and mean that congestion in the air is taken away from Heathrow.