A great topic and one that is very worthwhile discussing - lots to learn on both sides, I feel.
I'll make a few points. I've worked on both sides.
I'm generalising - but from the centre's and TCA's/Approach units I've worked in - I find the following to be true.
1. To answer the original question, even though LATCC can see both selected and indicated airspeed, I'd venture that 10 seconds is a rather spritely time to jump on a perceived inadherence to a speed instruction.
2. Speed Control, for the majority of controllers, and in particular Centre/TCA controllers is at best a rather imprecise science. When the aircraft is in level flight, a la final approach, or en-route, great - apply your 160's and Mach 0.78's and expect lateral spacing accuracy - magic. Initial descent into an airport, however - 'suck it and see'. How do we make this work as controllers? 9 times out of 10, ask the lead guy what he's doing (270 kts) - restrict the guy behind to the same or slower speed. Works, but its rough science.
Fact is that from FL300 down to FL100 the changing levels, winds, IAS/TAS relationships, Mach/IAS crossover, etc. etc. make the speed control a little loose.
If you're a controller - try this - next time you're sequencing for the initial approach, take a guess at what the mach number or IAS is before you ask ... often far different from what you think.
3. Mode Selection and time to achieve assigned speed. If I'm on final, and can see a guy 3 miles ahead, I've got my ever-ready trigger finger on the speedbrake - ready to achieve that 180 or 160 knot restriction in seconds ... however, if I'm descending through FL210 and get a speed reduction of 60 knots - lets say from 280 to 200 ... that reduction is going to be slower. I'm certainly not dirtying up the airplane to achieve the restriction ... it'll happen at a nice even rate (I believe the Doc 4444 ideal is 1 kt per second, I could be wrong).
I can achieve this reduction in a variety of modes, independent of aircraft type - V/S, LVL Change, whatever .... one is not necessarily any different from another depending on the current speed and config., in terms of achieving the speed required.
4. The Mach/IAS changeover is the biggest hurdle to accurate spacing, from what I've seen. Today a perfect example, controller asks what's your Indicated speed .. colleague replies "255 kts" ... great, ... guy behind is told "250 or less indicated ..." ... Great except that we are at FL320 and 255 is actually pretty fast ....
To make it work ... with perfect spacing ... tell both guys to fly, eg. Mach 0.76, transition to 280kts on indicated .... you'll keep the spacing all the way, as we will both have the same crossover, and the same groundspeed is guaranteed.
5. The "Lie"
Trust me, this happens far more rarely that you think. Hell of a lot less than controllers lie to pilots
. Kidding.
But they key is that there are 2 guys in the cockpit. And neither is likely to say to other, hey - I know ATC just said slow it up - but f%ck it ... lets keep doing 250 and see what happens. A very simple reason - but its very unlikely that you'll get two jokers flying together. Most - even if they want to get home early to see the football - will dial in the 220 knots immediately - and select a method of speed reduction immediately - and the other guy will say checked and we all go home without any report writing.
6. The go down and slow down
Ok so its old hat, we all know it, but it really is true for most airplanes - and most airplanes are built with pathetic excuses for speed brakes. If you're within flap limiting speed you can also chuck out a bit to slow up - but often slowing up quickly is difficult .... especially at higher alts.
Just a few random thoughts .... keep 'em coming!
