Woodpecker
Thanks for your interest.
I haven't done the maths myself: remember, he has also been losing time whilst he has been slowing to 145 kts. Additionally, although we "discount" spacing drift inside 4nm, as the following traffic would be maintaining 160kts until it gets to 4d the time interval between them continues to reduce after the heavy gets inside 4nm. Who knows where that becomes unsafe? We don't.
In this particular case I instructed the following traffic to go around when its spacing behind the heavy was 4nm; it should have been (and had been!) 5nm. Fortunately, although the Tower controller initially indicated that the departure runway would not be available for the following traffic, they managed to change their plan to suit me and shortly after the traffic had commenced its climb I was able to offer it a late visual switch to 27L.
Please, lets be clear here: I applied the minimum legal spacing for that aircraft pair because that is the demand of the airline community; that spacing was established some 7 or 8 miles previously and I applied common speeds to maintain it. If we don't apply minimum spacing when ever we can then then Heathrow's already oversubscribed capacity will be diminished.
Heathrow's runways are scheduled at a rediculous 98% of capacity, Frankfurt, I believe, is 90% and Paris 80%. As a point of note: Heathrow approach is deliberately flowed at 2 or 3 aircraft per hour above the anticipated landing rate so as to keep pressure on to achieve minimum spacing. Under those circumstances, traffic that is at 145kts at 6nm instead of 160kts to 4 is placing the ATCO in an illegal position, and the following traffic in a dangerous position that we are not permitted to ignor.
During the week a Heathrow colleague was suspended after traffic that had read back the correct heading took up and incorrect one and infringed separation. The ATCO was not at fault but it was a number of hours before he was re-instated and this can have significant consequences for delays.
I have absolutely no desire to, nor would I ever put aircrew in a compromising position. If our standard speeds are not working for you then we have to change them because not flying them puts us in a compromising position. This cannot be right.
Mixed-mode is being looked at now; infact I was centrally involved in the concept design. There are many issues, and politics to resolve before it can be implimented. Current rules have DfT restrictions.
Hope this helps
.4