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

Let's start with the glide path indications, because that's where the excerpt opens. The glide path is the vertical guidance element of the ILS — it tells you whether you're too high or too low relative to the correct descent path down to the runway. The instrument shows you a "fly up" or "fly down" command. Think of it this way: if the needle indicates fly up, you're below the glide path and need to climb back up to it. If it indicates fly down, you're above it and need to descend. That's the basic command logic. Now, the key numbers here. Full scale deflection — that's when the needle is at the very edge of its range — indicates that the aircraft is 0.7 degrees or more above or below the glide path. So the instrument's full range covers at least 0.7 degrees of deviation in either direction. The sensitivity is 0.14 degrees per dot. A "dot" is the standard unit on these instruments — the display is divided into dots, and each dot represents 0.14 degrees of angular deviation from the glide path. So if you're one dot off, you're 0.14 degrees off. If you're two dots off, you're 0.28 degrees off, and so on. Here's the critical safety limit: the maximum safe deviation below the glideslope is half full-scale deflection, which is 2.5 dots fly up. So if you're more than 2.5 dots below the glide path, you're in dangerous territory. And that connects directly to the note in the excerpt: if, on approach, the left/right deflections — that's the localizer, the lateral guidance — or the fly-up indications exceed half full scale, then an immediate go-around should be initiated. Why? Because safe terrain clearance may be compromised. You're getting too far off the protected path, and the terrain clearance that the approach procedure guarantees may no longer be valid. So that half-scale threshold is your trigger to abandon the approach. Now let's move to the ILS categories. These are defined by ICAO, and there are two separate classification systems you need to keep straight. The first is the ILS Facility Performance Categories — that's about the ground installation. The second is the Operational Performance Categories — that's about what the aircraft and crew are certified to do. Let's take the ground installation categories first. Category I ground installation: it provides guidance information from the coverage limit of the ILS — that's the outer edge of where the signal is usable — down to the point where the localizer course line intersects the ILS glide path at a height of 200 feet, or 60 meters, or less above the horizontal plane containing the threshold. The threshold is the beginning of the runway landing surface. So a Cat I ground facility gives you guidance down to 200 feet above the threshold. Category II ground installation: same idea, but the guidance extends down to where the localizer course line intersects the glide path at a height of 50 feet, or 15 meters, or less above the threshold plane. So Cat II gets you lower — down to 50 feet. Category III ground installation: this one, with the aid of ancillary equipment where necessary, provides guidance information from the coverage limit of the facility to, and along, the runway surface. So Cat III ground guidance takes you all the way down to the runway and along it — you can be guided right onto the pavement. Now the Operational Performance Categories. 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 the aircraft must be certified to operate to one of these classifications. Category I operational: an instrument approach and landing with a decision height — that's DH — not lower than 60 meters, which is 200 feet, and a Runway Visual Range, RVR, not less than 550 meters. RVR is the distance over which the runway is visible — it's the visibility measure used for landing. Category II operational: a precision instrument approach and landing with a DH lower than 60 meters — 200 feet — but not lower than 30 meters, which is 100 feet, and an RVR not less than 300 meters. Category IIIA: a precision instrument approach and landing with a DH lower than 30 meters — 100 feet — or no DH at all, and an RVR not less than 200 meters. Category IIIB: a precision instrument approach and landing with a DH lower than 15 meters — 50 feet — or no DH, and an RVR less than 200 meters but not less than 75 meters. So you can see the pattern: as the category goes up, the decision height comes down and the required runway visual range comes down. The ground facility gets better, the aircraft equipment gets better, and the crew is allowed to fly lower and in poorer visibility before committing to land. One thing I want to make sure you hold onto: the distinction between the two category systems. The Facility Performance Categories describe what the ground installation can do — how far down it provides guidance. The Operational Performance Categories describe what the aircraft and crew are certified to do — what decision height and RVR they're allowed to use. They're related, but they're separate certifications. Let me also make sure the decision height concept is clear, since it's central here. Decision height is the height at which, if you haven't established the required visual reference to continue the approach, you must initiate a go-around. It's your last chance to decide: land or abort. And the categories are telling you how low you're allowed to take that decision. Now, the figures I have available show the localizer and the glide path indications. The localizer figures show the course line and the QDM and QDR — those are the magnetic bearings to and from the station. But since the excerpt focuses on the glide path and the categories, let me make sure you've got the glide path picture clear in your mind: the needle, the dots, the 0.14 degrees per dot sensitivity, and the critical half-scale go-around trigger. That's the glide path indication and the full category structure