Vertical wind
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 781 on June 11, 2002. The most important
component of this estimate is the rate of change of aircraft pressure altitude,
which is approximated as a centered difference, one second either side of the
second at which the computed value is stored.
The
most simplifying approximation in the equation of motion is substituting pitch
(which is measured on the T-28) for angle-of-attack (which is not
measured).This approximation leads to
an improvement in estimated vertical wind over that obtained using solely the
rate-of-change of aircraft pressure altitude as a proxy for vertical wind, but
is not as accurate as would be a similar calculation with a measured
angle-of-attack. The technique is valid only for straight and level,
non-accelerating flight. The negative vertical windepisode beginning around 01:14 and continuing through 01:17 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 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.