"Nathan Young" wrote in message For the jet crew out there.... What's
the transition point between
mach and airspeed callouts? FL180?

As we climb, the outside air temperature drops. As the temperature drops,
the speed of sound drops. We climb at indicated airspeed (not true airspeed)
until mach climb airspeed matches indicated airspeed, at which point we
transition over from indicated airspeed to mach airspeed. For the jet I fly,
the transition occurs around FL240 (24,000') depending on outside air
temperature.

D.