mvsb1863
9th Dec 2008, 17:00
Hello friends
First of all I would like to request you to read that few lines below in order to help you understand my question.
3.5.2 Tower Wind Reporting
On short final and just before takeoff the pilot may obtain a wind report from the tower. This
wind report is also based on the same wind sensors as for the ATIS wind. However the tower
wind report is more accurate than the ATIS report since it is based on a two-minute period
preceding the pilot's contact with the control tower. As is the case for ATIS information, tower
wind reports need to be decomposed into cross- and tailwind components by the flight crew.
3.5.3 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.
Thank you very much for your patience and the question is...
Is there any company that uses the FMS Wind information during the final approach (short final) in order to make a decision of landing or not, disreguarding the Wind reported by the Tower ???
Thanks and looking forward any answer.
First of all I would like to request you to read that few lines below in order to help you understand my question.
3.5.2 Tower Wind Reporting
On short final and just before takeoff the pilot may obtain a wind report from the tower. This
wind report is also based on the same wind sensors as for the ATIS wind. However the tower
wind report is more accurate than the ATIS report since it is based on a two-minute period
preceding the pilot's contact with the control tower. As is the case for ATIS information, tower
wind reports need to be decomposed into cross- and tailwind components by the flight crew.
3.5.3 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.
Thank you very much for your patience and the question is...
Is there any company that uses the FMS Wind information during the final approach (short final) in order to make a decision of landing or not, disreguarding the Wind reported by the Tower ???
Thanks and looking forward any answer.