When I saw a notice in the latest Aviation Information Circular about replacement of outdated localizers and glide path units with new state-of-the-art equipment, I wasn't too excited. Nav aids get shut down for maintenance all the time and you just use another approach if you happen to need to go there in bad weather. It's rare, in my experience, that the weather is in between the precision and non-precision minima, so I almost browsed right past it, but then I saw this line:
As well, new ILS systems do not generate a useable back-course signal; consequently, localizer back-course procedures will be replaced with area navigation (RNAV) approaches, where applicable.
I did not know that. ILS stands for Instrument Landing System, but that's a really useless abbreviation because there are half a dozen types of instrument landing systems, of which only one is called ILS, a.k.a. precision approach. An ILS provides both lateral and vertical guidance to a runway. It's not as simple as riding a single beam down to the runway threshold, but that's the image that works for me. The part of the transmission that the pilot follows to align with the runway is called the localizer and the part that lets the pilot know if she is too high or too low is called the glideslope. The signals are usable within ten degrees of the runway centreline, but the needle that indicates position on the localizer reaches full deflection 2.5 degrees from the centreline, so to make an acceptable approach, the airplane must remain within about one degree of the centre of the beam. Although the beam is associated with one runway (that is one end of a particular piece of pavement) the signals leak out in the other direction to the opposite runway. If there are no obstacles or terrain that interfere with a safe approach, it may be possible to design a backcourse approach using the same nav aid on the opposite runway. A pilot flying a backcourse approach follows the localizer signals in the opposite direction in every sense. If the front course approach is on a track of 090 to runway 09, then the back course approach will be on a track of 270 to runway 27, and when the instrument signals that the pilot is left of the localizer, the pilot should fly left and when the instruments show the pilot is too far to the right, the pilot should fly right! (There are ways to set some instruments up backwards so you don't always have to use reverse sensing, but that's the traditional way). The glideslope signals may be received on the back course, but they are not usable at all and the pilot should ignore fly up or fly down indications, not do the opposite. Also, you get to fly the airplane pointy end first and right side up.
So this AIC is telling me that all my long ago work learning how to fly with reverse sensing is going out the window because new technology localizers don't have a back course? Cool. I wish I could tell you more, but the link to more information in the bulletin 404s. Someday people will look at the LOC REV switch on old autopilots and wonder what the heck that was for. I wonder what else I'll see fall by the wayside during my aviation career. I hope a lot of those old autopilots get replaced, but I know some of them will never go away, even if they don't ever work again.