The 'do not exceed ...knts' command

Is that ground speed or airspeed, I ask because I was just at 199knts airspeed and asked not to exceed 200. I thought you can only see the ground speed of other planes as well - or can ATC see both?

ATC can only see your ground speed. If he tells you to not exceed XXX knots, that would be airspeed.

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It is usually airspeed. ATC would somehow tell if you have slowed down. Ground speed would also be affected by various variables eg winds thus it would not be the most convenient IMO to be used.

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Just wanted to say something about this.

ATC see ground speed. I like what (I think) you said, ground speed is handy for us. Since we basically separate aircrafts compared to the ground, ground speed is actually the best. Airspeed is useless to us since we aren’t flying the plane.

Planes follow the instructions in airspeed. It’s not because atc sees ground speed that you have to follow ground speed. Our commands are airspeed. So sometimes it can happen that we give « do not exceed » even though you’re already slower. (Since we see ground speed we need to somehow estimate what your airspeed is.)

If you hear that kind of command, have a look around. You’ll see quite quickly why a controller wants you to slow down. In dense traffic we really appreciate if you try and help us by going at a speed which is similar to the others. (For example if you’re a fighter jet coming in an airport that requires an approach above 10 000ft, please don’t fly at mach2…)

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That’s interesting to know. I just assumed they’d made a mistake but as they can’t see airspeed they must be guessing. I am one who is always aware of proximity to other traffic. I always get the feeling though that most people are trying to fly as fast as they can get away with.

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We have a few ways to estimate depending on altitude, but I have to admit I myself am not that precise.

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