Not related to anything to do in an airliner, but the discussion on potential examples of what I’ll call “expanded raw data,” especially the logarithmic localizer, reminded me of a very elegant solution to a problem, that I encountered in cropdusting school. In modern times, you’re guided down the swath by a GPS lightbar, which works the same as a localizer: you’ve got the center mark, and a light (horizontal column)that moves left or right, showing which way the swath is relative to you. And, it’s logarithmic! On the swath, one mark of deviation (it’s a row of LED’s) is something like a lateral foot, while if you’re significantly off (including 90 degrees away on the turnaround prior to going down for the swath) you still get a meaningful indication. (If it was 1 light per foot all the way, it would be begged if you’re anything but near-perfect already)
So, that’s beautiful guidance when you’re on the swath and not following a row crop, but how do you provide guidance in the turn-on? With what I’ve described so far, it’s still impossible to estimate your closure rate with the centerline, and you’d have to join it long prior to entering the field. Maybe 10, 20, 30 seconds to adjust? Who knows. This would be highly uneconomical when repeated hundreds or thousands of times, and could not match the historical solution of a guy with a flag that you visually roll in on.
Enter the second row of lights, under the first one. It shows the derivative of your lateral position, or the cross-track rate. It’s also logarithmic, and it’s calibrated so that on the ideal turn-on, the top bar (localizer) and bottom bar (cross rate) shrink together! So, you watch the ends, and play the bottom vs. top. If the bottom bar lags (is longer than) the top, your cross rate is too large for where you are along the turn, so you’re gonna blow past, so you increase (or slow the decrease of) your bank a bit. In the opposite situation, if you’re set to roll out too soon, the cross rate is too small (bottom bar shorter than the top) so you unbank a bit more to get your profile closer to the centerline. Of course, while flying it, you don’t think about the if-this-then-that like I just described, you just watch the end of the bottom bar and adjust your bank so as to manipulate it in the direction you want, using the end of the top bar as the reference.
I only went to school for this for a few weeks and never got a job, but to this day I’m tickled by the elegance of this setup, and how with some practice, it delivers you perfectly centered down the swath and going straight the instant you roll out.
(Of course this can all be done with a much simpler flight director, but there’s the obvious problem of looking intently at the instrument panel as you’re making sure you don’t hit the ground or the wires as you’re pulling out of a rolling dive a few feet AGL.. While this setup is on top of the nose, far away from you, not just in your field of view but also in focus, while you’re looking at the ground and obstacles.
Having said this, they could also design the bottom bar to just show a FD roll command, the goal being to keep it centered all the time. For all I know, this might even exist. Pretty sure the the regulations in that field pose a smaller obstacle to tech progress than those in the transport sector, combined with the lifecycle times of airliner development)