For us it is variable. Mostly with temperature. For instance, we normally
climb at 270 knots until M 0.75, and continue referencing Mach.

Also, if you select a target speed with the FMS it is IAS until 24,350 and
then the numbers automatically turn to Mach by the computer.

I've never really though too much about it. If ATC wants your speed you give
them IAS. If they want your Mach, they ask for Mach. If a chart is
referenced IAS, we use IAS. Some charts are referenced Mach, then of course,
Mach is used.

Karl

"Nathan Young" wrote in message
...
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?