On Wed, 07 Jul 2004 19:29:06 -0700, AES/newspost
wrote:

Listening to audio channel 9 on United, at higher altitudes and during
cruise I hear ATC say things like "What's your mach?" and "Maintain
mach 77" and at lower altitudes and during approaches "slow to 180" or
"maintain 250 for spacing".

For the jet crew out there.... What's the transition point between
mach and airspeed callouts? FL180?