What happens when you push the ALT HOLD pushbutton while the pitch FMA is engaged in 'VV+1200'?

When the ALT HOLD pushbutton is engaged while the pitch Flight Mode Annunciator (FMA) is in a vertical speed mode of 'VV+1200', the system transitions from controlling the aircraft's pitch based on a selected vertical speed to maintaining a specific altitude. By pressing the ALT HOLD button, the aircraft recognizes that it should level off at the current altitude, effectively capturing the altitude it is at when the button is pressed. The FMA will then reflect this change by displaying 'ALT HOLD', indicating that the autothrottle system is now engaged in altitude hold mode.

This functionality is essential for pilots as it allows for a smooth transition from climbing or descending to level flight, ensuring that the aircraft can stabilize at a designated altitude without further pilot input on vertical speed. The understanding of this mode is critical in situations where altitude maintenance becomes necessary after a climb or descent.