Evenin'
I believe another of the reasons we concern ourselves with ranges is so that we provide a good service to you flyboys; it helps you to know where you are and what you need to do in order to achieve a good descent profile.
Having been invloved at the ATMDC I am able to pass on a little bit more: One of the tools we are working on is SASS (Stack Advisory Support System). It uses the landing order, known speeds (corrected for TAS), wind data and "standard" tracks off the stacks to compute stack departure times. They are VERY good. Once off the stack the time changes to a range which is computed from the same factors. The ranges count down as the aircraft gets closer.
An interesting point has come up in the development process: If your track to touchdown is 24 miles long and have a strong tailwind for a significant part of that track then, because your groundspeed is high, you spend less TIME in the air than if there were no wind at all. The question is therefore, when we give you a TRACK mileage to run do you assess what the wind is when you set up your rate of descent? If not, how do you know what rate to set up? Would you prefer to be given AIR miles to run?
This piece of kit is very good, to the extent that it will "regularise" the intermediate approach and reduce much of the doglegging. It is also less likely that we will miss the correct stack departure which means there will be fewer occasions where we chase the approach, which will reduce the chance of a rushed approach. It therefore enhances safety.
Twenty point four.
[ 18 August 2001: Message edited by: 120.4 ]