Vertical
wind is approximated using a simplified inversion of the aircraft equation of
motion (Kopp, 1985). An example is shown
in Figure 1 for Flight 813 on
The technique is valid only for straight and level,
non-accelerating flight. The negative vertical wind episode in the figure above, beginning around
23:13 and continuing through 23:14, is actually during a turn,
and the deduced vertical wind is not valid during this time interval. Basically,
the pilot pitches the aircraft nose up and adds power during the turn to maintain altitude as
the aircraft undergoes an acceleration to change horizontal direction, and the
Kopp inversion infers a downdraft because the nose is pitched up but the
altitude is not increasing as it should if the nose is pitched up in
straight-line flight.