I was listening to Norcal yesterday and heard a controller request an airplane increase speed to 210 knots "if able". The plane was previously at ~180 knots. The pilot responded he had flaps 2 and politely declined.
Can TRACON demand a plane increase speed if the controller already assigned a slower speed? And if yes, what would be the phraseology? "Maintain 210 knots or greater until DUMBA"? (I don't think I've ever heard that when a plane was slower.)
Yes, we can ask pilots to speed up on approach, but it is discouraged and not commonly done. This is because, as an aircraft approaches an airport, the crew will configure it for landing. This includes lowering the flaps, which lowers a number of reference speeds - essentially, making them unable to fly very fast. In order to speed up, they would have to raise the flaps again, which would result in extra workload and bad fuel economy.
The controller you heard knows this. They used the phrase "if able" to emphasize to the pilots, that they only had to comply if it was convenient for them.
ICAO provides the following guidance (form Doc 4444):
4.6.1.3 Speed adjustments should be limited to those necessary to establish and/or maintain a desired separation minimum or spacing. Instructions involving frequent changes of speed, including alternate speed increases and decreases, should be avoided.
...
4.6.3.5 Arriving aircraft should be permitted to operate in a clean configuration for as long as possible. Below 4 550 m (FL 150), speed reductions for turbojet aircraft to not less than 410 km/h (220 kt) IAS, which will normally be very close to the minimum speed of turbojet aircraft in a clean configuration, may be used.
4.6.3.6 Only minor speed adjustments not exceeding plus/minus 40 km/h (20 kt) IAS should be used for aircraft on intermediate and final approach.
4.6.3.7 Speed control should not be applied to aircraft after passing a point 7 km (4 NM) from the threshold on final approach.
SOCAL will do this pretty routinely sequencing arrivals into LAX. The most common practice is to assign 250 knots at some point, then slow you to 210, then assign 170 or 180 "to the marker" (about 5-6 mile final), at which point you'll be slowing to your target approach speed. So mostly commonly, they are only assigning speed reductions. However, there are times when they want you to close up space on the aircraft ahead, so they might speed you back up again until the extra distance is closed up -- at which point they slow you back again.
These instructions are always given with the implicit understanding that, if you can't do it, you'll say so... "XYZ123 is unable 250, I can give you 225" or something similar. If you've already extended flaps so that the higher speed would be near or over an aircraft limitation, that could be a reasonable reply. (Of course, if you extended flaps 20 miles out from the runway, maybe retracting them again & not annoying SOCAL might be a wiser course of action.)
Depending on the aircraft type (how fast you can fly clean, and how quickly you can slow to your approach speed so that you are stable for the approach at 1000' - or whatever your OpSpecs/SOP's require), you might be given something else. Light aircraft are often told to keep a fairly high speed (for them) to the marker, since otherwise they're like bicycles on the freeway & slowing down everybody behind them. On the other hand, they can often decelerate more rapidly than some jets, so staying fast until close in may work out just fine. Again, if the controller is assigning a speed that's too fast until too close, it's the pilot's job to say, "unable."
The controllers at SOCAL are excellent at knowing what the aircraft they're working with are capable of, and they are very good at timing the speed reductions so that you end up with about 3 miles separation on short final, so that the runway usage is kept high... as one aircraft is turning off the runway, the next jet is a mile or so out. Much closer than that & you risk go-arounds if somebody gets nervous or the turning-off aircraft isn't expeditious; much farther than that & you aren't landing planes as quickly as what you could be.
Some controllers match the proficiency of the ones at SOCAL; others don't compare. Of course, you don't really expect the controller at Podunk to have the same practice in setting up a string of aircraft coming in one after another as those in SOCAL, NORCAL, ATL, ORD, and so forth. The good ones will use all the tools at their disposal, and sometimes this will include having one aircraft increase their speed for a while.
