Quote: "I've read with interest much discussion on PPrune about boris island being a fantasy and the best way to grow is by expanding LHR. I am inclined to agree but I don't see where a 3rd runway could be accomodated given the local contraints of major roads? Could someone enlighten me as to what the proposal, if any, would be?"
The third rwy will be located on land north of Heathrow between the A4, which currently forms the northern boundary of LHR, and the M4 motorway. It will be about 1 mi. north of 10L/27R (the "northern runway"), and parallel to it. Although this area is relatively unurbanised, there are some villages and one of these, Sipson, will be demolished.
The link road between LHR and the M4 will be diverted and part of it may be tunnelised. Parts of the A4 will also need to be tunnelised to allow for taxyways to the expanded airport. There will also be another terminal and associated infrastructure including a railway station, and another fire station will be mandatory.
The new rwy is intended to be about 7,000 feet long, so a little over half the length of the 2 existing rwys, and will be used by smaller and quieter aircraft. The largest jets will remain confined to the two existing rwys.
Hope this helps.