Originally Posted by
Clinton McKenzie
If the 'lead' aircraft has a groundspeed that's e.g. 5 kts slower than the 'trail' aircraft on the same track, why does ATC 'care' what their respective IAS indications happen to be if ATC knows their respective positions and groundspeeds?
Because ATC issue instructions to fly an airspeed, not a groundspeed. So if they want you 10 knots slower, they can ask the airspeed and tell you to fly 10 knots slower than what you report. The same thing may apply to headings.