At present, Heathrow Airport Ltd. have 2 schemes for extra rwys.
No, they don't. The only proposal that Heathrow have put their name to, to date, is the long-established northern (Sipson) scheme.
The southern (Stanwell Moor) runway proposal, complete with its pin-the-tail-on-the-donkey map, comes to us courtesy of the Evening Standard, though it's true that the ES is generally regarded as an unofficial mouthpiece for Heathrow because of its editor's links with BAA's
PR machine.
Anyway we will find out one way or another in a week's time when Heathrow make their submission to the Davies Commission
But back to the northern runway(s). Nobody, present company excepted, has ever proposed
two northern runways.
There could be a 2.0 mi. rwy about 0.5 mi. north of 09R/27L, heading west on mostly open land from just south of Sipson to a point beyond the M25. This rwy would only take out relatively few houses (compared to south of the airport) and some other buildings around Hatch Lane, and possibly the relocation of Waterside. It would also require a diversion of the A4.
Let’s call it 10R/28L to be more accurate.
Another 0.5 mi. north of this of this could be the original “third rwy” scheme that takes out Sipson.
Let’s call it 10L/28R.
How they would be designated is irrelevant. Their proximity isn't.
On westerly operations, this would allow landings on 27R and 28R and vice versa and take offs on 27L and 28L and vice versa.
On easterly operations, this would allow landings on 09L and 10L and vice versa and take offs on 09R and 28R and vice versa.
This could retain segregated mode and alternation.
Look at any 4-parallel-runway airport. You will find that, for a number of good operational reasons, almost all use the two outers for landings and the inners for takeoffs, and the same arguments would apply at LHR, such as minimising the need for aircraft to cross the landing runway. That, of course, means no alternation.
As for aircraft taxiing across rwys, this would be occur in both of Heathrow Airport Ltd’s proposed rwy schemes.
Not so. The only scheme proposed to date (see above) has been a single, new northern runway operating in mixed-mode and serving (mainly) a new T6, with a minimal requirement for traffic to cross either of the current runways.
Two northern runways would, presumably, also serve a proportion of T1/T2/T3/T5 traffic (otherwise why bother with two?) and would greatly increase the number of crossings of 09L/27R compared to the single-runway proposal.
Or are you now proposing T7 as well as T6? Where will that go?