After 400 feet you can switch it on! You take off in a heading, T/O mode! Then the Vnav mode can either control speed or given altitudes in the FMS! For example. when in a descend to a specific point, lets call it ARTIP (that´s a IAF into Schipol AMS, Where you have to be at FL100 or below).
You write into the FMS where there is ARTIP on the left, the NAV side, FL100B on the right of it, where the VNAV side is. There you put in the speeds or in this example an altitude constrain. Than the airplane will fly to ARTIP and will be exactly at FL100 as long as you are in the NAV mode! But when you chance into the Heading mode in the Flight Guidance Controler, because of getting radar vectors of the controler, the airplane is unable for the Vpath mode like I described it to you before! So either you change into the Vertical Speed mode VS, or into the Vertical Flight Level Change Mode VFLCH, where the airplane holds a desired speed. In a descend it would be the maximum descend speed or a speed designed by ATC. In that case you would control the descend speed with the thrust setting.
This mode you can do in 2 different ways, either its called VFLCH or FLCH, the difference is in the VFLCH mode the speed is insert in the FMS and in the FLCH mode the speed is hold wich is set on the flight guidance control panel!
I hope, to helped you a little!
yes i can handle that alone. - - -famous last words