Oakape interesting, thanks foe that. I had assumed (i know) that you just fed in the time and the machine took care of everything.
For me, having a required time at a pre-descent point & then a fixed speed schedule for descent would be the easiest way of ensuring everyone gets where they are needed at the correct time. Any corrections required due to wind changes on descent could then be dealt with by ATC speed control or vectoring.
A very similar idea has been floated. Simply move the fix much further out, i.e. prior to descent point. The descent phase then becomes mush easier for us as everyone is doing the same speed on descent (just like the old days pre FF's). The downside is that it gives you a lot less time to achieve the requires time.
Capn Bloggs and
hung start when we went to MAESTRO in PH a conscious decision was taken not to issue FF times until with the inner sectors (134.2/133.8 from the north, 135.8/118.2 from the east).
MAESTRO will recalculate the entire sequence every time a single piece of information is changed (aircraft estimates, RWY config, acceptance rate). For a RWY change this is a massive advantage over the manual system, but other time its can be a disadvantage.
MAESTRO starts sequencing (i think) at least two hours out, but it will be in a state of flux as estimates change and short leg flights depart etc. The closer you get to arriving the more stable the times become. It was decided that prior to the inner sectors, rather than issuing a time that will in all likelihood change more than once, we would give speed reduction based on the current MAESTRO time. I can't remember the exact values but it's something like up to 5 minutes - no reduction, 5-10 reduce by M0.05, grater than 10 reduce by M0.05 or greater, expect an X minutes delay.
One side advantage of this is that two aircraft in trail given a delay will both reduce by the same amount, reducing the different profile/speed issue. None of these procedures are set in stone however, and can be altered if that's what industry wants.
Its the same reason we changed to giving a hold departure time rather than 'depart the hold to make the fix time'. That way between the holding point and the fix everybody will be doing the same speed.
With regard to the speeding up to 250 after the fix, I was very surprised that jets would do less than 250 prior. I was expecting a 250 knot descent and than if that was not enough a request for vectors/holding. I didn't even know a jet could do 210 on descent, I was always taught 250 was a minimum we could assign, if we had really messed up we could ask nicely for 240 or maybe 230.