For VNAV to function correctly, what conditions must be met?

For VNAV (Vertical Navigation) to function correctly, it is essential that the aircraft is in NAV mode, the aircraft is above the Top of Descent (ToD), there is an altitude preselect set, and the VNAV button is activated. This combination ensures that the aircraft can effectively track the planned vertical profile, allowing for optimized altitude and descent management during a flight.

Being in NAV mode indicates that the aircraft's navigation system is actively following a predefined route, which is crucial for the VNAV system to operate as it relies on the lateral navigation data. Being above the ToD is necessary because VNAV primarily manages descent profiles; if the aircraft were below this point, the system would not be actively engaged in descent management. The altitude preselect feature enables the pilots to set the desired altitude for either ascent or descent, while the activation of the VNAV button allows the system to take control of vertical navigation parameters according to pre-programmed flight plans.

Therefore, the specific conditions outlined in this correct option are fundamental for the proper functionality of the VNAV system in aviation operations.