Airborne Weather Radar — Page 227, Lesson 209

We're starting a brand-new topic today: the Airborne Weather Radar, or AWR. This is the radar you'll use in the cockpit to see weather ahead and to map the terrain. Let's begin with the big picture. The AWR has four main components. We have the Tx / Rx, which is the transmitter and receiver unit. Then we have the antenna, the indicator which is the display screen, and the control unit which is the panel you'll use to operate it. Its two primary functions are turbulence detection and navigation. Now, the principle of operation. The radar works by sending out a pulse of energy. The echo — the time it takes for that pulse to bounce back — gives you the range to the target. The sweep of the antenna gives you the relative bearing, meaning the direction of the target relative to your aircraft's heading. There are two different beam shapes used. For weather detection and for long-range mapping beyond 60 nautical miles, we use a pencil beam. This is a narrow, focused beam. For short-range mapping, we use a cosecant² beam, which is a fan-shaped beam that spreads out. The antenna is attitude stabilized, meaning it's gyroscopically held level with the horizon regardless of the aircraft's pitch and roll. The beam width is dependent on the antenna size. A larger antenna produces a narrower beam. And the beam width directly affects your resolution — a narrower beam gives you better resolution, meaning you can distinguish between two targets that are close together. The system operates at a frequency of 9375 MHz. This frequency is chosen because it's the best for detecting large water droplets and hail. That's the key to weather radar — it's not really seeing clouds, it's seeing precipitation. Now, let's talk about interpreting the weather. The most dangerous turbulence is found where the rainfall gradient is steepest — that's where the precipitation intensity changes most rapidly over a short distance. You'll get few returns from precipitation with a wavelength of greater than 10 cm, so those targets won't show up well. On the colour display, the returns are shown in a specific order of intensity. From least to most intense, the colours are black, green, yellow, red, and magenta. Black means no return, and magenta is the most severe. And you must beware of certain shapes on the display: U's, fingers, scallops, and hooks. These are all indicators of severe weather, often associated with thunderstorms or even tornadoes. Let's move to the control unit. On the Mono Control Unit, we have several controls. The Power/Stab On switch turns the system on and stabilises the antenna in pitch and roll. When you select Stab Off, the antenna becomes locked to the aircraft axes — it moves with the aircraft instead of staying level with the horizon. The Range control has a Standby position and selections up to about 150 NM. The Tilt control adjusts the antenna's vertical angle, from ± 15 degrees. You tilt up to see weather at a higher altitude or to increase your effective range, and tilt down to see lower. The MAP mode uses the fan-shaped beam and is used for mapping up to 60 NM. The MAN mode is manual gain with the pencil beam, used for mapping beyond 60 NM. The WEA mode is weather mode, using the pencil beam with AGC, which is automatic gain control. And the CONT mode is contour mode — in this mode, black holes on the display indicate turbulence. For mapping, you tilt down for the best target presentation, and you must beware of hill shadows — areas behind high terrain that the radar can't see. For weather operation, you adjust the tilt for the best weather picture. If your tilt is too high, you'll miss thunderstorms entirely. And again, beware of the shadow area behind a storm cell, which can hide weather behind it. Finally, the Colour AWR has additional controls: Wx for weather, Wx+T for weather plus turbulence, Wx(Var) for variable weather, WxA for weather attenuation, Hold to freeze the display, and Tgt Alert for target alert. That's the complete summary of the Airborne Weather Radar. We've covered the components, the beam shapes, the frequency, the colour coding, and every control on the unit.