Airborne Weather Radar — Page 227, Lesson 209

Right, let's get into the Airborne Weather Radar. This is a big one for you, so we'll take it piece by piece. We'll start with the summary block, which gives us the whole skeleton of the system. First, the components. The AWR, the Airborne Weather Radar, is made up of four main boxes: the Tx/Rx, which is the transmitter and receiver combined; the antenna; the indicator, which is your display; and the control unit, which is what you'll be reaching for in the cockpit. Now, its functions. It has two primary jobs: turbulence detection and navigation. So it's not just a weather toy—it's a navigation aid too, especially for mapping the ground ahead. Let's get into the principle of operation, because this is where the physics comes in. The radar works on two basic ideas. For range, it uses echo—it sends a pulse, it bounces off a target, and the time it takes to come back tells you how far away it is. For relative bearing, it uses sweep—the antenna physically scans across the sky, and the direction the antenna is pointing when it gets an echo tells you the bearing of the target relative to your nose. Now, the beam shape is critical. There are two types you need to know. First, the pencil beam. This is a narrow, focused beam. It's used for weather detection and for long-range mapping, specifically beyond 60 NM, or nautical miles. Because it's narrow, it gives you good detail but covers a small area. Second, the cosecant² beam. That's a mouthful, I know. It's a fan-shaped beam that's wide in the vertical plane. This is used for short range work. The antenna itself is attitude stabilized, which means it's gyro-stabilized to stay level with the horizon even when the aircraft is pitching or rolling. Here's a key relationship: beam width is dependent on antenna size. The bigger the antenna, the narrower the beam. And the effect of beamwidth is on resolution—a narrower beam gives you better resolution, meaning you can distinguish between two targets that are close together. A wide beam will smear them into one blob. And the frequency. The AWR operates at 9375 MHz. This frequency is chosen because it's the best for detecting large water droplets and hail. That's the whole point—you want to see the dangerous stuff. Now, let's talk about what you actually see. Turbulence is indicated where the rainfall gradient is steepest. That's a crucial phrase. It's not the heaviest rain that's the most dangerous—it's the change in rainfall intensity. Where the rain rate changes rapidly over a short distance, that's where the turbulence is. Also, you need to know that there are few returns from wavelengths greater than 10 cm. So if the precipitation particles are smaller than about a tenth of your wavelength, you won't see them well. The radar is tuned for the big drops. On the display, the colours have a strict order. From least to most intense: black, green, yellow, red, magenta. Black is no return, green is light, yellow is moderate, red is heavy, and magenta is the most severe—that's the turbulence indication. And a word of warning for you: beware of U's, fingers, scallops, and hooks. These are specific echo shapes on the display that indicate severe weather. A hook echo, for example, is a classic sign of a tornado or a very intense storm cell. You need to recognise these shapes and avoid them. Now, let's move to the Mono Control Unit. This is the panel you'll be using. Let's go through each control. First, Power/Stab On. When you switch this on, the antenna attitude is stabilised in pitch and roll. The antenna stays level with the horizon. Then you have Stab Off. When you select this, the antenna is locked to the aircraft axes. So if the aircraft pitches up, the antenna pitches up with it. This is used in specific situations, but you need to understand the difference. Next, Range. This has a Standby position and selections up to about 150 NM. So you can select your display range out to 150 nautical miles. Then, Tilt. This is a crucial control. It's adjustable ± 15°. The rule of thumb is: tilt up for increased range or to look at a lower altitude. Wait, let me re-read that. Tilt up for increased range or lower altitude. So if you tilt the beam up, you can see further, or you can see weather that's at a lower altitude relative to you. It's about where you point the beam. Now, the mode selector. This is where it gets interesting. MAP mode uses the fan-shaped beam, the cosecant² beam. You use this up to 60 NM for ground mapping. MAN is manual gain with the pencil beam. You use this to map greater than 60 NM. So when you need to see further, you switch to manual gain and the pencil beam. WEA is the weather mode. This uses the pencil beam with AGC. AGC is Automatic Gain Control—the receiver automatically adjusts its sensitivity so that the display shows a consistent picture regardless of the strength of the returns. And finally, CONT. This is the contour mode. In this mode, black holes indicate turbulence. The radar suppresses the strongest returns and displays them as black, so you can see the intense cores of storms as black holes on the display. Now, let's talk about mapping in practice. When you're using the radar for navigation, you tilt down for the best target presentation. You want the beam to hit the ground ahead of you. And you must beware of hill shadows. If there's a high hill in front of you, it will cast a radar shadow behind it—an area of no return that looks like a lake or flat ground, but could be hiding terrain or weather. For weather operation, you adjust the tilt for the best weather picture. This is a skill you'll develop. If your tilt is too high, you will miss the thunderstorms, the TS. The beam will go over the top of the storm and you'll see nothing. And again, beware the shadow area—a big storm can hide another storm behind it. Finally, let's look at the Colour AWR—the colour weather radar. The controls here are: Wx, which is weather; Wx+T, which is weather plus turbulence; Wx(Var), which is variable weather gain; WxA, which is weather attenuation; Hold, which freezes the display; and Tgt Alert, which is target alert. These are the modes you'll select on a modern colour radar. So, to tie it all together: you have a radar that transmits at 9375 MHz, uses a pencil beam for weather and long-range mapping, and a cosecant² beam for short-range mapping. You control the tilt to aim the beam, you select the mode for the task, and you interpret the colours and shapes on the display to avoid the dangerous weather. The key is understanding what each control does and how the beam shape and tilt affect what you see.