How do airliners get to 250 kts when descending through 10,000 feet as required by the FAA? Do you level off at 10,500 feet until speed is less than 250 and then descend further? Must pilots ask ATC permission to temporarily level off slightly above 10,000 feet to decelerate and then get further permission to continue descent? I’ve wondered for years how this works. On a 787 into SFO, we had the screens on the headrest that showed us GPS and speed etc. I remember looking at it on descent and noticing we were leveled off traveling around 145. Also, considering 250 is the maximum, do you travel around 240/245 above 10,000 feet to be sure you do not accelerate beyond 250? Thank you for the explanations.
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Note that the speed you saw would have been ground speed not air speed. But in any case, you generally descend with the autopilot in a mode that pitches to hold an airspeed, called "Speed Mode" or something similar. The other option is to descend in a mode that pitches to a vertical speed, called "VS" mode. Speed Mode is used most of the time. You set the target speed the AP will maintain with a dial on the autopilot panel that moves a bug on the speed tape.
When you start a descent you will usually dial the speed you want to descend at, engage speed mode, then pull the thrust to idle, and over you go. Say you are descending with the bug at 290, which means the bug is set to 290 on the tape and the airplane will nose up or down as required to hold that speed, and in this case you are in some kind of dive.
As 10000 ft approaches, say passing through 12000, you just dial the bug down the speed tape to 250. The airplane will pitch up a bit and start decelerating, then start pitching down again to maintain 250. You try to time it so you are hitting 250 just as you are passing 10000.
ATC doesn't know your indicated airspeed, only your ground speed, so a controller has to estimate your airspeed based on the wind and ground speed. This means being a few knots over the limit won't get you busted.
If you screw it up and are still too fast by more than say 10-20kt passing 10000 (or maybe because of some other imperative), then you'll dump some speed brakes to slow down faster. But like I said, 10kts too fast passing 10 is not a big deal.
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$\begingroup$ Great answer, but I'd like to point out that ATC actually can see your indicated airspeed, if the aircraft and the ATC facility are equipped to work with Enhanced Mode S transponders. In fact, with this technology the ATC can even see the airspeed you have selected in your autopilot window. This is an extra layer of safety, so he can confirm you are complying with his clearance. $\endgroup$– FcoMay 19, 2019 at 19:07
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$\begingroup$ Thanks I suspected that. My last IFR flight was some years ago. $\endgroup$– John KMay 19, 2019 at 19:14