BTIZ wrote:
IIRC.. from controlling traffic more than 25 years go..

Speed control for spacing, must use Mach numbers above FL290.. not Indicated
airspeed in knots.

Below FL290, or was it FL250, use of IAS in knots is standard..

Ref: 7110.65 (which I do not have handy right now)


At or above FL240, express speed (IAS) in Mach
Below FL 240, express speed in knots

The document I read indicates the unit should still be given, like
"Maintain two eight zero knots" or "Increase speed to Mach point 72"
http://www.faa.gov/atpubs/ATC/Chp5/atc0507.html#5-7-2