What speed does ATC use?

[GodAtum]

In P2ATC it shows GS and TAS. In planes, the speed tape shows IAS. Often, the IAS is lower then the GS, especially in high winds. ATC gets confused because I think it's using GS, but should be using IAS?

[RobW05]

Hmm, why do you think it's getting confused, can you give an example?

Generally IAS is important for your flying maneuvers as it is showing the airflow over your wings. The higher you get the lower your IAS will be, as the air gets thinner. TAS is IAS corrected for air density, so that's telling how fast you really are moving through the air. But all that is useless for navigational purposes as it won't allow you to calculate by when you are reaching waypoints. Therefore you need GS, which is TAS corrected for wind.

Therefore the only factor on which ATC can judge by when to assign you a descend or something is GS. You also need GS to figure out which vertical speed you need to reach an assigned altitude at a certain waypoint. 

[GodAtum]

For example, ATC tells me to slow to 250, on my speed tape it is 250, but GS is 280.

[RobW05]

Haven't run into that, but the speed limit under 10000 ft should be IAS. That would be a question for Dave to answer how P2ATC handles that. 

[GodAtum]

5 hours ago, RobW05 said:

Haven't run into that, but the speed limit under 10000 ft should be IAS. That would be a question for Dave to answer how P2ATC handles that. 

In my VATSIM and IVAO experience, the controllers always use IAS.

[Dave-Pilot2ATC]

P2A uses IAS when looking at Speed limits.  It only uses GS when calculating TOD and ETE's, etc.

Dave