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During the climb phase, how is the transition from constant speed (IAS) to constant Mach number handled? Does the pilot flying have to manually "push the button" on the control panel, or is the transition automatically done by the aircraft? How does the pilot / aircraft know when the transition should happen (i.e., from flight plan?)

I guess it's the same during descent?

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Does the pilot flying have to manually "push the button" on the control panel?

It's automatic, with a manual changeover button available to the pilots as shown below (label no. 2).

enter image description here

How does the aircraft know when the transition should happen?

When the target climb indicated airspeed (IAS) equals the target climb Mach number, the changeover takes place, usually around FL260. Likewise for the descent. The target speeds are managed by the FMS and can be changed.

enter image description here
Image shows the FMS CDU.

(3) Displays computed values or manually entered values for the selected mode.

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The transition happens around FL260 at which the Mach Maximum speed of the plane, intersects with the Maximum speed IAS of the plane.

For example if the maximum IAS Speed is 300 knots, and maximum Mach is 0.76, then you will fly 300 knots until Mach is 0.76, at which you will follow the Mach Number.

Descent, same thing. Follow the mach number until it intersects with maximum IAS, at which point follow the IAS down.

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  • $\begingroup$ Did you really mean MAXIMUM IAS? This sounds like Vne. Surely one doesn't normally climb at Vne. $\endgroup$ – quiet flyer Aug 1 at 18:51

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