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

I want to walk you through the glide slope portion of the ILS now. We've covered the localizer, which gives you left-right guidance. The glide slope is the vertical counterpart — it gives you up-down guidance, the descent path down to the runway. The glide slope transmitter is a UHF transmitter. UHF stands for Ultra High Frequency. It's located to one side of the runway, approximately 200 metres from the runway edge, and 300 metres upwind of the threshold. So picture it sitting off to the side, not on the centre line, and positioned upwind of the threshold — that's the end of the runway where you touch down. The principle is exactly the same as the localizer, but there's a key difference: the carrier wave is UHF, and the lobes are arranged in the vertical plane instead of the horizontal plane. So instead of left and right lobes, you have an upper lobe and a lower lobe. The upper lobe — the large lobe — carries a 90 Hz modulation. The bottom lobe — the small lobe — carries a 150 Hz modulation. Remember those two frequencies: 90 Hz on top, 150 Hz on the bottom. The glide path itself is usually 3 degrees. ICAO requires the glide path angle to be between 2 and 4 degrees. The glide path is defined as the path where the DDM — that's Difference in Depth of Modulation — of the overlapping lobes is zero. When the DDM is zero, the ILS indicator's glide path needle will indicate zero deviation. So when you're exactly on the glide path, the needle sits centred. The radiation pattern is shown in Figure 9.9. Now, here's an important safety concept: false glide slopes. A false glide slope is defined as a path of points, in the vertical plane containing the runway centre line, at which the DDM is zero — other than the path that forms the true ILS glide path. In other words, there are other places in the sky where the needle will also centre, and those are not the real glide path. Why do these false paths occur? The twin lobes are repeated due to two causes. First, metallic structures situated at the transmission point, and ground reflections. Second, the height and propagation characteristics of the aerial. Here's the practical number you need to remember: the first false glide slope occurs at approximately twice the glide path angle. For a standard 3-degree glide path, that means the first false glide slope sits at about 6 degrees above the ground. False glide slopes always occur above the true glide slope. They should not constitute a danger, but pilots should be aware of their presence — because if you intercept one, you'll be flying a much steeper descent than you think. That's why normal flying practice is to establish on the localizer first, and then intercept the glide slope from below. Intercepting from below means you cross the true glide path first, before you ever reach a false one. However, at some airfields — London Heathrow is the example here — a continuous descent approach is used. In that procedure, aircraft are positioned by ground radar to capture the glide slope from above. Because you're coming from above, you could potentially pass through a false glide slope. So it's advisable to always confirm the aircraft height in relation to distance to go, by reference to DME, markers, locators, and so on. That cross-check protects you from being fooled by a false glide slope. Now let me move to the cockpit indications. I want to show you a typical HSI — that's the Horizontal Situation Indicator — in Figure 9.11. Let me walk you through the key components you'll see on it. There's the compass card, the heading index, the heading selector knob, and the directional gyro failure flag. There's the command track pointer and lateral deviation bar, with the command track flag. There's the ILS failure flag, the azimuth failure flag, and the loss of power flag. And importantly for this lesson, there's the ILS glide scope deviation — that's the vertical needle showing your position relative to the glide path — and the DME range readout, plus the selected course. Now, localizer indications. For a front course approach, the indications for fly left and fly right are shown in Figure 9.12. Here's the sensitivity figure you need: full scale deflection of the needle indicates that the aircraft is 2.5 degrees or more left or right of the centre line. That means the sensitivity is 0.5 degrees per dot. So each dot on the display represents half a degree of deviation. Finally, let's talk about the back beam approach. Where a localizer is designed to radiate back course information, it can serve two purposes. First, it can give azimuth guidance on overshoot from the main precision approach runway — in that case, when you're overshooting, you should obey the CDI or HSI needle. CDI is the Course Deviation Indicator. Second, it can give a back course approach to the reciprocal of the main precision approach runway. Here's the critical difference: in this case, the CDI needle will give reverse indications — meaning it works backwards, so you have to fly opposite to what the needle shows. But an HSI will give correct indications, provided that the front course QDM has been selected. QDM is the magnetic bearing to the station. So the HSI, with the front course selected, displays correctly even on the back course, whereas the CDI reverses. Let me make sure you've got the key numbers locked in: glide slope transmitter 200 metres from the runway edge, 300 metres upwind of the threshold. Upper lobe 90 Hz, lower lobe 150 Hz. Glide path usually 3 degrees, ICAO range 2 to 4 degrees. First false glide slope at about twice the glide path angle, so 6 degrees for a standard 3-degree path. Full scale localizer deflection at 2.5 degrees, sensitivity 0.5 degrees per dot. And the back course rule: CDI reverses, HSI stays correct with front course QDM selected.