Instrument Landing System (ILS) — Page 147, Lesson 133

I want to walk you through the Instrument Landing System — the ILS. This is the system that has been in existence for over 40 years, and it is still the most accurate approach and landing aid in current use. So when we talk about precision approaches, this is the gold standard. Let me start with what the system actually does for you as a pilot. The ILS provides you with an accurate means of carrying out an instrument approach to a runway, and it gives you guidance in both the horizontal and the vertical planes. That dual guidance is the key. Because it guides you in both planes, we call it a precision approach system. That's the definition you need to hold onto — a precision approach system gives guidance in both the horizontal and the vertical plane. Now, what does that guidance actually look like in the cockpit? The ILS provides you with visual instructions — instruments you can see — that let you fly the aircraft down a predetermined glide path and along the extended runway centre line, which we call the localizer. So you have two separate pieces of guidance: one for the vertical plane, the glide path, and one for the horizontal plane, the localizer. You fly down that glide path and localizer to your Decision Height, which we abbreviate as DH. And here's the critical decision point. At decision height, you make a decision. If you have the required visual references and sufficient room to manoeuvre the aircraft for a safe touchdown, you land. If you don't have those visual references, or you don't have the room, you go around — we call that overshooting — and you carry out the published missed approach procedure. So the whole approach is built around that decision height moment. Now let's look at the components that make this system work. The ILS requires two things: a suitable ground installation and airborne equipment on the aircraft. The ground installation has three distinct components. Let me name them clearly: the localizer, the glide path, and the marker beacons. In some installations, a back course may also be available — that's an additional option, not always present. Let me give you the specifics on the localizer, which we abbreviate as LLZ. The localizer transmits in the VHF band — that's Very High Frequency. And it's located about 300 metres from the up-wind end of the runway. So think of it sitting near the far end of the runway, the end you're approaching from, transmitting that horizontal guidance that keeps you aligned with the extended centre line. That's the foundation of the ILS — the concept of precision guidance in both planes, the decision height decision, and the three ground components. We'll build on each of these as we go deeper.