B747-400 Vnav
 

Guys,
As a Gnd Eng i dont get to see VNAV in operation all that often.
I was hoping someone might be able to give a simple explanation as to which pitch mode the Autopilot will use and maybe some examples of VNAV SPD, VNAV ALT and VNAV PTH. I would just like to be able to get my head around it a little better. Thanks.

Not an exhaustive dissertation, but a few examples:

In climbs, VNAV SPD is normally operative. The autothrottles set the climb thrust (THRUST or THRUST REF mode), and speed is controlled via pitch.

When the altitude set in the MCP restricts climb or descent so that the VNAV cruise altitude is not achievable (e.g., ATC climb restriction) or the descent path cannot be maintained (ATC descent restriction), VNAV ALT is engaged. Autothrottles control speed (SPD mode), and Altitude Hold is maintained by pitch.

In cruise when the VNAV cruise altitude has been achieved, and in descent when the optimum descent path can be maintained, VNAV PATH is engaged. Autothrottles control speed (SPD, IDLE, or HOLD mode), and pitch commands adjust altitude or descent profile.