No please do, I build complex transport systems as part of my day job ! You won't lose me
With due respect, I suspect your expertise won't cover detailed understanding of Air Traffic Control procedures, Air Traffic Management, operational airport matters and air safety?
It's a complex subject and too much to fully detail here.
I'm also just about to travel to the other side of the planet and really don't have the time, but basically there has to be safe operating distances and time separations between aircraft operating on active runways and in the air.
In the case of runway operations and landings and take-offs, there are a number of set minimum criteria to ensure aircraft are not in conflict with each other.
For landing aircraft, the possibility of a missed approach, or go-around has to be at taken into account. A missed approach or aborted landing can be initiated from as late as on the actual landing roll and the aircraft will climb away to a safe height and have to be stabilised before it can be safely flown back into the traffic pattern for another attempt at landing.
That would mean an aircraft following a missed approach, initially climbing straight ahead and if that occurs from the runway itself, or on very short finals, the aircraft is very likely to overfly the proposed extended runway in question here, before it could be safely turned away.
Nothing, including another aircraft, can be on the runway in front of a landing aircraft once it has been given clearance to land.
In the hypothetical case of the proposed extra long runway as a solution at Heathrow, if aircraft were to be lined up on the extended runway, even beyond the normal safe landing distance of other aircraft behind them on the landing part of the runway, account has to be taken of a possible over run or a missed approach being carried out. The second of these would be problematic and take-off clearances would have to be timed to ensure there is no loss of the minimum required separation between a departing aircraft and one that is performing a "go around" following a missed approach or aborted landing.
It's that timing and required separation which will restrict the capacity of that particular runway arrangement.
In low visibility conditions, the situation will even more constrained.
Just for the record, IMHO HAL have shot themselves in the foot with the limited options and proposals they submitted to the enquiry.
There are possible runway layouts and modes of operation that would have resulted in a new parallel runway that would not only provide the most optimal capacity, but in some cases meant no overflying of additional areas of west London than currently is the case. Unfortunately the horse has bolted on those more pragmatic solutions.