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 wind episode 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.