Instrument Landing System (ILS) — Page 163, Lesson 152

Let’s start with the glide path indications, because that’s where the excerpt begins. On the ILS, the glide path is the vertical guidance that keeps you on the correct descent angle down to the runway. The instrument shows you a fly-up or fly-down command. If you’re below the glide path, you get a fly-up indication; if you’re above it, you get a fly-down indication. The full-scale deflection on the glide path needle tells you that the aircraft is 0.7° or more above or below the glide path. That’s the angular limit of the display. The sensitivity is 0.14° per dot. So each dot on the instrument represents 0.14 degrees of deviation from the glide path. Now, here’s a critical safety limit: the maximum safe deviation below the glideslope is half full-scale deflection, which is 2.5 dots fly-up. That’s not just a display limit—it’s a safety boundary. The note in the excerpt is emphatic: if, on approach, the left/right deflections or the fly-up indications exceed half full scale, then an immediate go-around should be initiated, because safe terrain clearance may be compromised. So the half-scale mark is your decision point. Beyond that, you’re not guaranteed terrain clearance, and you must go around. Now let’s move to the ILS categories. The excerpt splits them into two sets: the ground installation performance categories and the operational performance categories. These are different things, and you need to keep them separate. The ground installation categories are about what the ground equipment can do—how far down the guidance extends. Category I ground installation provides guidance information from the coverage limit of the ILS down to the point where the localizer course line intersects the ILS glide path at a height of 200 ft, or 60 m, or less above the horizontal plane containing the threshold. So the guidance is valid down to that decision height. Category II ground installation provides guidance from the coverage limit down to the point where the localizer course line intersects the glide path at a height of 50 ft, or 15 m, or less above the threshold plane. Category III ground installation, with the aid of ancillary equipment where necessary, provides guidance from the coverage limit of the facility to, and along, the runway surface. So Category III ground guidance goes all the way down to the runway and along it. Now the operational performance categories. These are about the aircraft and the crew—what the airborne equipment and the pilot are certified to do. The improvement in the ground installations allows guidance down to the surface of the runway, and that requires a corresponding improvement in the airborne equipment. So an aircraft may be certified to operate to one of these classifications. Category I operational: an instrument approach and landing with a DH—that’s decision height—not lower than 60 m, which is 200 ft, and a Runway Visual Range, RVR, not less than 550 m. Category II operational: a precision instrument approach and landing with a DH lower than 60 m, 200 ft, but not lower than 30 m, 100 ft, and an RVR not less than 300 m. Category IIIA: a precision approach and landing with a DH lower than 30 m, 100 ft, or no DH at all, and an RVR not less than 200 m. Category IIIB: a precision approach and landing with a DH lower than 15 m, 50 ft, or no DH, and an RVR less than 200 m but not less than 75 m. So you can see the pattern: as the category increases, the decision height comes down and the required runway visual range decreases. The ground installation categories and the operational categories are paired—the better the ground equipment, the lower you can fly, but only if the aircraft and crew are certified to match. That’s the whole logic of the ILS categories.