Originally Posted by
ATCO1962
I'd stick with Mach number at or above FL280 and IAS below that. It's easier for everyone as ATCOs know the Mach number range for most jets above that level but we certainly couldn't tell you what a good, assured IAS would be above that without a few questions first as to whether you can make the speed or not. I haven't heard IAS used at all above FL280 ; that's after 38 years of controlling, mostly in upper airspace.
Given that for any aircraft at any level, assuming they're Mode S equipped which is 99.5% of our traffic, we can see what IAS they're flying regardless of whether or not they're flying on a Mach number, then we can have a reasonable stab at what might be a suitable IAS to keep them in the air. Things have changed in 38 years