FMS Guide,
The VS shown on the DES page of the FMS gives you the average vertical speed required to reach the next waypoint on your LEGs page with an altitude restriction, and this only at present groundspeed.
Let's assume you have a flightplan in the FMS with the only altitude restriction at your GS intercept waypoint, totally unrealistic but for the understanding required.
Further, let's assume this GS--27 (as we want to call this waypoint now) shows a restriction with 2000 ft.
You commence your flight and as soon as you climb through 2000 ft the FMS starts calculating your required vertical speed to reach this GS--27 waypoint at the 2000 ft restricted height. This is purely based on known aircraft data plus groundspeed data. There are no inputs of extra mileage requirements of a possible speed 250 at 10000ft restriction etc.
In VNAV mode the calculation basically works likewise, however, the FMS calculates a completely engine idle approach, in ideal conditions this will be a CDA approach with the thrust levers advancing just before reaching the GS--27 waypoint (assuming you followed the flap schedule as the FMS will give you).
However, the FMS needs a few more data to be more accurate. On the DES forecast page you need to complete the empty fields with winds at various altitudes, the OAT deviation and the QNH. The FMS will provide an idle power descend from your cruising level, it will automatically reduce speed at the pre-programmed altitude (250 / 10000ft) and of course for the flap schedule it calculates.
The calculation is based of the programmed routing, the ambient conditions at your present level as well as at the landing field as per forecast page.
In VNAV you will have a high vertical speed at intitial descend, expect not more than 1700 ft below 10000 ft in clean config.
This happens, of course, when following the descend profile in SPEED mode. Using PATH mode the FMS will follow the path indicated and should adjust thrust required to maintain this path. In older software versions this may happen a tad too late and a speed disconnect may occour, reverting the system into MCP mode which means idle power, high V/S rate...followed by a pilot's "What the f.
" announcement.....