Indicated, True or GS?

[Boatguy]

When ATC instructs us to "reduce speed to XXX for separation", is XXX indicated, true or ground?

[waynemcc999]

Russ, my understanding is that any speeds requested by ATC are Indicated (they correct for altitude, winds, etc).
Wayne

P.S. In my simple head this is symmetric to when they say "fly heading 120" they mean Heading (and not Course) and they correct for winds as needed.

[tjmoody]

Yes its Indicated. See FAR 4-4-12

[Mjwatlanta]

There’s a great podcast that answers questions like this one every week. Opposing Bases.

[tjmoody]

There’s a great podcast that answers questions like this one every week. Opposing Bases.

Agreed! Opposing Bases is terrific.

[ememic99]

When ATC instructs us to "reduce speed to XXX for separation", is XXX indicated, true or ground?

Indicated, as any other speed ATC asks you to comply with. Once when landing to Vienna ATC, squeezed me between A320 and EMB190 after I confirmed I can keep 165 until outer marker and 150 until short final. Alternative was 15 min in holding pattern

[Boatguy]

IAS makes the most sense, but I wanted to be sure.

In the DA40 it was usually "best forward speed", but yesterday I was instructed to slow down.

Thank you everyone!

[Boatguy]

Yes its Indicated. See FAR 4-4-12

After searching my FAR references in vain, I switched to the AIM. There it was: AIM 4-4-12b.

Thanks!

[tjmoody]

Yes its Indicated. See FAR 4-4-12

After searching my FAR references in vain, I switched to the AIM. There it was: AIM 4-4-12b.

Thanks!

My bad! There I go confusing FAR and AIM again

[Ed McDonald]

IAS hands down. Only recently do aircraft have TAS and GS with air data computers and GPS.