Visibility — Page 282, Lesson 265

I want to walk you through how we measure visibility on an instrumented runway — specifically the systems that give pilots and air traffic control precise numbers when the weather closes in. Let's start with Instrumented Runway Visual Range, or IRVR. This is the automated system that reports how far a pilot can see along the runway in low-visibility conditions. The key instrument here is the transmissometer. A transmissometer has two main parts: a light source transmitter and a photo-electric cell receiver. These two units are placed alongside the runway, separated from each other by a known distance. The transmitter sends a beam of light across that gap to the receiver. Now, the receiver generates an electrical current, and the strength of that current depends directly on the clarity of the air between the transmitter and the receiver. If the air is clear, the receiver gets a strong signal and the current is high. If fog, rain, or haze is in the way, less light reaches the receiver, the current drops, and the system calculates a shorter visibility. IRVR is only reported under specific conditions: when the normal visibility is 1500 metres or less, or when shallow fog is reported or forecast. So it's not used on a clear sunny day — it kicks in when visibility gets poor. The readings from the transmissometers are sent directly to Air Traffic Control (ATC). On a typical runway, we have three transmissometers positioned alongside it. That gives us three separate readings: one for the touch-down zone (where the aircraft first contacts the runway), one for the mid-point of the runway, and one for the stop-end (the far end). These three values are reported together. For example, you might see something like: R28L / 600 400 550. That means for runway 28 left, the touch-down zone visibility is 600 metres, the mid-point is 400 metres, and the stop-end is 550 metres. Now, transmissometers are the older technology. They are increasingly being replaced by forward scatter visibility meters. Let me explain how those work. The principle is different. A forward scatter meter has a transmitter that projects a narrow light beam forward. Then, a narrow aperture receiver is placed at an angle to that transmitter — specifically, an angle in the range of 20° to 50°. The receiver does not measure the direct beam; instead, it measures the amount of scattered light coming from the transmitter. Why does that matter? Because the amount of scattered light depends on the number and type of particles in the atmosphere — whether they are water droplets, ice crystals, or solid particles like dust or smoke. Here's a clever part: by using three sensors set at different angles, the system can automatically determine what substance is reducing the visibility — is it fog, snow, or haze? — and from that, calculate an accurate visibility value. These forward scatter meters are sited in similar positions to the transmissometers — alongside the runway at the touch-down zone, mid-point, and stop-end. And just like transmissometers, they only determine visibility in the direction of take-off and landing, which is the direction that matters most for the pilot. Let me also give you a quick summary of visibility effects that you need to keep in mind: - By day, visibility is generally poor when you look up sun — the glare reduces contrast. - By night, visibility is usually better when looking up moon, because of light reflections from water surfaces, railway lines, and other features on the ground. - In precipitation, visibility is worst in driving snow and also very poor in drizzle — that's because drizzle has a large number of very small droplets that scatter light effectively. - Night visibility is improved if the pilot does not look at bright cockpit lights, because that preserves the eyes' dark adaptation. Now, the rest of this page moves into practice questions. These are the book's practice questions — let's try them one at a time.