Instrument Landing System (ILS) — Page 157, Lesson 146

We're starting the glide slope portion of the ILS now. Let's look at the transmitter first. The glide slope UHF transmitter sits to one side of the runway, about 200 metres from the runway edge, and 300 metres upwind of the threshold. So it's not on the centreline — it's offset, and it's positioned before the threshold, upwind. The principle is exactly the same as the localizer, but with two key differences. First, it uses a UHF carrier wave. Second, the lobes are arranged in the vertical plane, not the horizontal one. So instead of left and right, we're now talking about up and down. Here's the lobe arrangement. The upper lobe — the large lobe — carries a 90 Hz modulation. The bottom lobe — the small lobe — carries a 150 Hz modulation. The glide path itself, usually 3°, is defined where the DDM of the overlapping lobes is zero. That's the same depth of modulation concept you saw with the localizer — where the two modulations balance, the difference is zero, and the glide path needle on your ILS indicator shows zero deviation. ICAO requires the glide path angle to be between 2° and 4°, and the standard is 3°. The radiation pattern is shown in Figure 9.9. Now, here's something important — false glide slopes. These are defined as the paths of points, in the vertical plane containing the runway centreline, at which the DDM is zero — other than the path that forms the true ILS glide path. So you get extra zero-DDM paths that aren't the real glide path. Why do they happen? The twin lobes get repeated for two reasons. First, metallic structures at the transmission point, and ground reflections. Second, the height and propagation characteristics of the aerial itself. The first false glide slope occurs at approximately twice the glide path angle — so about 6° above ground for a standard 3° glide path. False glide slopes always occur above the true glide slope. They shouldn't constitute a danger, but pilots should be aware of their presence. Now, normal flying practice is to establish on the localizer and intercept the glide slope from below. But at airfields like London Heathrow, a continuous descent approach is used — the aircraft are positioned by ground radar to capture the glide slope from above. Either way, it's advisable to always confirm the aircraft's height in relation to distance to go, by reference to DME, markers, locators, and so on. Let's move on to the localizer indications. On a front course approach, full-scale deflection of the needle indicates the aircraft is 2.5° or more left or right of the centreline. That means the sensitivity is 0.5° per dot. So each dot on your display represents half a degree of deviation. Finally, the back beam approach. Where a localizer is designed to radiate back course information, it can do two things. First, it can give azimuth guidance on overshoot from the main precision approach runway — in that case, you obey the CDI or HSI needle. Second, it can give a back course approach to the reciprocal of the main precision approach runway. Here's the key difference: in that case, the CDI needle gives reverse indications — fly left when you should fly right. But an HSI gives correct indications, provided you've selected the front course QDM. So the takeaway: on a back course approach, the CDI is reversed, but the HSI is not — as long as the front course QDM is selected.