Let’s pick up right where the lighting system’s purpose leaves off — Page 426, Lesson 573

Let’s pick up right where the lighting system’s purpose leaves off. I want to walk you through the design logic that governs approach lighting, because everything here is built around one idea: giving you, the pilot, visual cues that let you judge your aircraft’s attitude and height as you fly down the approach plane. First, the core principle. The arrangement of lights is meant to give you an indication of aircraft attitude — that is, whether your nose is up, down, or level relative to the approach path. The same arrangement can also give you an indication of your height above the approach plane. So the lights aren’t just decoration; they’re a visual reference that tells you where you are in space as you descend. Now, the systems themselves range in complexity. At the simple end, you have a centre line and a crossbar. At the complex end, you have the highly intricate layouts used for CAT III precision instrument approach systems. The key design driver is this: the visual criteria for landing must be met by the visual acquisition of the approach light system. In plain terms, the system has to be designed so that you can see it early enough to make a landing decision, and it must cater to the most restrictive decision heights and minimum descent heights you’ll fly. Here’s the primary unit of design: the length of the segments, set by ICAO at 300 metres. That 300-metre segment length is the building block everything else is measured against. And there’s an important safety rule: any ILS or MLS azimuth antenna that protrudes through the plane of the lights must be treated as an obstacle, and it must be marked and lit accordingly. So if an antenna pokes up through the light plane, it’s a hazard and gets obstacle marking. Let me now introduce the two main system types. First, the Calvert system, named after its inventor. It’s generally used in the UK and occasionally elsewhere. A Calvert system consists of five bars and a distance-coded centre line. The distance coding is what tells you how far along the approach you are. There’s also a NATO system, which is similar but does not have the distance coding on the centre line. So the contrast is: Calvert has the coding, NATO doesn’t. Next, let’s talk about the individual lights themselves — the barrettes. The lights that make up a system can be arranged in two ways. Either as single light units, which is the Calvert method, or as groups of three or more lights arranged as a bar, which is the ICAO method. For example, the centre line of a system might be made of single point-source lights, or it might be a bar of five lights close together. That arrangement of three or more lights close together is called a barrette — pronounced “barre-et,” meaning small bar. The whole point of the name is to avoid confusion with the bar constituent parts of an approach lighting system. So a barrette is a small cluster of lights, distinct from the larger crossbars. Now let’s look at the simple approach lighting system. This is the basic one. It consists of a row of lights on the extended centre line of the runway, extending — whenever possible — over a distance of not less than 420 metres from the threshold. Then there’s a row of lights forming a crossbar, either 18 or 30 metres in length, placed at a distance of 300 metres from the threshold. The crossbar lights must be as nearly as practicable in a horizontal straight line, at right angles to the centre line, and bisected by it. So the crossbar sits perpendicular to the centre line, and the centre line cuts it exactly in half. This simple system is used on a non-instrument runway, and it may also be used on a non-precision instrument runway. Take a look at Figure 21.1 to see the layout. Now the precision approach CAT I lighting system. This is more demanding. It consists of a row of lights on the extended centre line extending, wherever possible, over a distance of 900 metres from the runway threshold. Here’s the critical reasoning: if the length is less than 900 metres — and on a 3° glide path, 900 metres coincides with CAT I system minima of 200 feet — then it’s possible that an aircraft may not be over the approach lighting at decision height, or DH. So the 900-metre length is tied directly to your ability to see the lights at the moment you must decide to continue or go around. The five crossbars are spaced 150 metres apart, and they form three segments. The inner segment runs from 0 to 300 metres. The middle segment runs from 300 to 600 metres. The outer segment runs from 600 to 900 metres. So you have five bars dividing that 900-metre run into three equal 300-metre segments — which ties back to that ICAO primary unit of design we started with. And again, the same obstacle rule applies: any ILS or MLS azimuth antenna protruding through the plane of the lights is treated as an obstacle and marked and lit accordingly. Figure 21.2 shows the CAT I Calvert five-bar and centre line system. So to tie it all together: the 300-metre segment is the design unit, the Calvert system uses five bars with distance coding, the barrette is the ICAO cluster of three or more lights, the simple system gives you a 420-metre centre line with an 18- or 30-metre crossbar at 300 metres, and the CAT I system extends to 900 metres with five bars at 150-metre spacing to match your 200-foot decision height on a 3° glide path. That’s the full picture of how approach lighting is structured.