I know that you have to maintain less than 250 knots when below 10,000MSL. Is that airspeed or ground speed?
Must be airspeed. If you didn't have GPS or DME, how would you know your ground speed?
Perhaps one of the real-life pilots will weigh in. . .
The limitation is indicated air speed. If ATC needs you to be slower for traffic conflicts or traffic flow, they will issue a lower speed.
There is often the case where ATC asks for higher than normal airspeeds on approach. C-172's normally fly approaches to final at 80 knots slowing to 70 over the fence. I am often asked by ATC to maintain 100 knots to short final to allow smoother traffic flow, especially at busy air transport airports.
Yeah. That would make sense.