One of the really cool things about modern avionics is what they can do with basic information. A common example is pushing the “Direct” button, entering a destination and seeing the device pop up the on-course heading, the current ETA and a great-circle route overlaid on a moving map. Long gone are the days when we’d ask ATC for a vector to a distant point and “follow along on the Loran.” Another cool thing many of these boxes and related software can do is calculate a descent rate or angle while we’re en route and looking for the point at which we can begin a constant-rate descent to our destination.
A similar capability exists and is used in the terminal environment, where the magic can calculate a vertical descent angle (VDA) from the final approach fix (FAF) to the runway. It’s a virtual glideslope, where no ILS exists, and never was. But none of this is foolproof, and as a recent bulletin from the FAA describes, some of the sophisticated boxes in our panels can suffer from the age-old “garbage in, garbage out” computer problem.
