FMS wind
The FMS wind is computed as a vectorial difference between the airspeed aligned to the aircraft heading and the ground speed aligned to the ground track. The FMS-calculated wind vector is normally displayed on the Navigation Display (ND) or on the Control Display Unit (CDU). Some FMS installations provide a decomposition of the wind vector in cross- and tailwind components. The FMS calculates the wind for the altitude the aircraft is actually flying. Note that tailwind limits and the tailwind used for field performance refer to the wind measured at a 10-meter height. The FMS wind is therefore of little value to the pilot when he makes his decision to land, i.e. at top of descent, during descent and upon initiation of the final approach. Nevertheless many pilots tend to monitor the FMS for exceedance of the maximum tailwind.
Uncertainties exist in the determination of derived inertial quantities (like ground speed and ground track) that will influence the accurate determination of the FMS wind vector. Especially the calculation of the drift angle should be treated with suspect in a dynamic environment like an approach. Secondly, the airspeed is assumed to be aligned with the heading, sideslip is not measured nor incorporated in the calculation of the FMS wind, yielding questionable results once the aircraft has commenced decrabbing in crosswind conditions that might be present as well. Finally, FMS computations are filtered, resulting in a typical time delay of 3-5 seconds. A second relevant effect of this filtering process is that gust values will not be displayed to the flight crew. For these reasons the use of FMS wind is normally accurate only in the cruise phase of the flight. However, it can be shown that, although the crosswind component determined by the FMS can be highly inaccurate in the final phase of the flight, the tailwind component is relatively insensitive to FMS errors in the determination of the drift angle. This is a direct result of the geometry of the speed vectors involved.