As i stated on last page a new heathrow runway will add another roughly 800 aircraft per day into the sky of the london area which is already full.
Most london airports including Gatwick, Stansted, Luton, Northolt and London City all have departure routes that conflict with that of Heathrows.
For example. Most heathrow northbound departures go to about 6,000 ft initially. Thats the highest you can go until you enter the airways system. Luton eastbounds initially go south then east and have to stay at 4,000 (below heathrow outbounds) before climbing to 5,000 where they stay below stansted outbounds.
London city northbounds have to stay at 3,000ft until clear of Luton eastbounds then climb over luton inbounds.
luton westbounds have to stay at 5 below heathrow inbounds and outbounds.
Northolt westbounds have to go under Heathrow outbounds and inbounds and they actually STOP Luton outbounds until they are clear of west london.
Heathrow has 4 'stacks' which are holding areas for inbounds. 2 per runway, one north and one south of said runway. These holds are full, all the time.
Another runway means another 1 or 2 holds. Where do they go? What town are they going to sit over? Holds are usually about 20 miles away from said airport.
The London Terminal control area is full. The traffic levels where we are at the moment are expected to increase up to about 30% by 2020. Thats without a new runway.
Having new departure and inbound routes and holds will force other airports' routes to be moved (which they need to do already to be fair). This will mean departure routes and holding patterns over towns where there are none at the moment. This means massive public consultation covering the entire london Terminal control area which spreads from bristol to cambridge and oxford to east essex. Whos going to pay for that?
Then of course you've got gatwick and stansted planning second runways (planning regardless of heathrow decision).