Mach Number

[rudderrudderrat]

Hi Papa600,

It may help to think of it like this:

Your minimum speed is always IAS.
Your Maximum speed is either VMO (IAS) at low Alt: or MMO (Mach) at high Alt.

For economy (fuel & engineering flying costs) we fly at a Mach speed at high Alt and at an IAS at low Alt.

e.g. During the climb - say we use 300 kts IAS until we reach Mach Number 0.8 then continue climbing to cruise at 0.8 Mach. Depending on FL, this may be say 250 kts IAS.