When ATC asks me a speed to maintain, is it IAS, GS or TAS?

[ED_4]

Why would ATC use anything other than Indicated Air Speed? And have you muck up the calculations while you're in the air? I think it only makes sense to use what is available in front of you. And that is none other than Indicated Air Speed!

[flightstrike]

In the real world flying a Kingair, I have been asked to reduce by 20 knots ground speed if possible. If they ask you to reduce speed, they are refering to IAS unless they state otherwise.

[rrr7134]

IAS. All ATC speed and flow control schedules are all based on IAS.