Computes the Visual Descent Point (VDP) distance from the runway threshold.
The VDP is the point on a non-precision approach at which a normal descent
from the MDA to the runway threshold should begin.
Parameters
glidepathAngleDeg: number
Desired glidepath angle in degrees (typically 3.0).
thresholdCrossingHeightFt: number
Height above the threshold at the VDP start (MDA - TDZE, or TCH).
Computes the Visual Descent Point (VDP) distance from the runway threshold. The VDP is the point on a non-precision approach at which a normal descent from the MDA to the runway threshold should begin.