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Aus dem Manual:

On this FCU, the speed, heading and altitude buttons can be pushed orpulled. To push a button, press the left mouse button. To pull, use the right mouse button.
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When you pull a button, you use the MANAGED mode. It means the FMGC will automatically take control of the autopilot to provide it the right value of speed, heading or alititude in order to follow the flight plan programmed in the FMGC. When a speed, heading or altitude is managed, no value is displayed on the FCU panel, "----" appears instead.

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