Is that after you have asked for the original estimate? I have had "system estimates" that haven't been within a bull's roar of what we see in the cockpit.
TAATS/MAESTRO estimates for the descent phase are very unreliable. They use the TAS in the flight plan, forecast winds and a defined profile based on aircraft type, none of which necessarily reflect reality. Therefore for PH we ask aircraft for their estimate and then put that in MAESTRO. I'll be diplomatic and say some aircraft/airlines are more precise than others. "Unforecast headwinds" often mean aircraft can't make the estimate they gave us at 200 miles out.
Please explain? If the first aeroplane is at 230KIAS, he's ahead of where he should be on the railway line. If the following aeroplane is at 280KIAS, he's obviously behind where he should be on the railway line. If they both go through the station at 250, they will at the very least be only at the correct spacing, and probably wider than desired. What's the problem?
You would think that, but no. Sometimes the second is ahead of where that should be, and planning on slowing down later.
Honest question – why not issue a simple speed reduction, save time on the radio and tweeking about in the cockpit, power for TAS to ground speed/ distance calculation etc. Always wanted to know as it is one of my 'pet' though unspoken, very minor dislikes; especially on short-ish sectors or at short notice, or, on descent, through the lumps. It's doable, and no sweat but – I've always wanted to ask....?
Thats what was done 20+years ago in ML when I started, there is just too much traffic now. We need you over a point at a time, the thinking is your computer has all the information to calculate the required speed and descent profile, we don't. To use a ridiculous extension, we would never give you a power setting to achieve a speed, we just give you the speed and let you work it out. Carry that forward, we give you a time and let you work everything out. Plus we were told that one airline has boasted they could make fix times the the second (they can't).
I do sometimes wonder though when we are told "Industry wants..." if our people who'd don't actually control are talking to airline people who don't actually fly.
With regard to the original question, think in terms of spacing. On a good day we will run a two minute sequence. 30 seconds early puts you only 3/4 of the required distance behind. If you are a minute early and the preceding is a minute late, you are going to be sharing a piece of sky.