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

Let’s start the Instrument Landing System. This is the heart of precision approach flying, so I want you to think of it as the system that gives you guidance in both the horizontal and the vertical plane — that’s what makes it a precision approach system, as opposed to a non-precision aid that only gives you lateral guidance. The ILS has been around for over 40 years and is still the most accurate approach and landing aid in current use. It lets you carry out an instrument approach to a runway, and it even enables automatic landings. So the whole idea is: the system gives you cockpit indications that let you fly down a predetermined glide path and an extended runway centre line, all the way to your Decision Height — your DH. Let me define that term properly, because it’s central. Decision Height is the height at which you, the pilot, make the decision: if you have the required visual references and sufficient room to manoeuvre for a safe touchdown, you land. If not, you go around — that’s the overshoot — and you carry out the published missed approach procedure. So the ILS gets you down to that decision point; the decision itself is yours. Now the components. The system needs a suitable ground installation and airborne equipment. The ground installation has three distinct components: the localizer, the glide path, and the marker beacons. In some installations you may also have a back course available. Let’s take the localizer first — abbreviated LLZ. It transmits in the VHF band, and it’s located about 300 metres from the up-wind end of the runway. That’s the lateral guidance element — it defines the extended runway centre line you fly along. We’ll go through the glide path and the marker beacons next, but for now, hold this structure: localizer for horizontal, glide path for vertical, markers for distance information along the approach.