Airborne Weather Radar — Page 208, Lesson 194

We're starting a brand-new topic now: Airborne Weather Radar. Let me walk you through what this system is and what it does. First, the name. Airborne weather radar, abbreviated AWR. Its job is to give the pilot information about the weather ahead, and also to help with navigation. Now here's the key thing that makes it different from almost every other system on the aircraft: the radar doesn't give you a finished answer. It gives you raw information that you, the pilot, have to interpret. So the quality of the information you get out of it is directly enhanced by your skill as the user. That's a really important point to hold onto — this is a tool that rewards practice. Where does the picture appear? On older aircraft, the radar information is shown on a dedicated display unit. On modern aircraft, it's combined with the aircraft's route and shown on the EFIS navigation display — that's the Electronic Flight Instrument System's ND, the navigation display. So you get weather overlaid on your route. Now, what exactly is on that screen? The display shows cloud formations or terrain features, and it presents them in two ways: as a range from the aircraft, and as a bearing relative to the aircraft's heading. So the screen is essentially a map centred on you, with things plotted by how far away they are and in which direction relative to the nose of the aircraft. The presentation can be monochrome — that's a single colour, usually green — or, on modern systems, it can be in colour: green, yellow, red, and/or magenta. And here's where the interpretation comes in. In the weather mode, those colours represent increasing variations in rainfall rate, from light returns right up to very strong returns. So green is light rain, yellow is moderate, red is heavy. And magenta — magenta usually indicates the presence of turbulence associated with intense rainfall. So when you see magenta, you're not just looking at heavy rain; you're looking at turbulence, and that's a cue to avoid that area. Now, the same colours mean something different in ground mapping mode. When you're using the radar to map the terrain below, green indicates light ground returns, yellow indicates medium ground returns, and red indicates heavy ground returns. So the colour scale is the same, but the meaning flips depending on which mode you're in. That's a classic trap for a new pilot — you have to know which mode you're in before you read the colours. Let me now take you through the component parts of the airborne equipment. There are four main units. First, the transmitter/receiver — that's the unit that generates the radar pulse and receives the returning echo. Second, the antenna, and this is important: the antenna is stabilized in pitch and roll. That means it's gyro-stabilised so that it stays pointed where you want it even when the aircraft is manoeuvring. Third, the indicator — that's the screen you look at. And fourth, the control unit — that's the panel you use to operate the system, to select the mode, the range, the tilt, and so on. So what are the main functions of an AWR? There are four of them, and I want you to remember all four. First, it detects the size of water droplets, and from that it deduces where the areas of turbulence are within the cloud. That's the weather detection role. Second, it determines the height of cloud tops by tilting the radar beam up or down. So by moving the antenna tilt, you can scan up and find where the cloud tops are. Third, it maps the terrain below the aircraft to provide navigational information and high ground avoidance. That's the ground mapping role. And fourth, it provides a position fix — a range and bearing — from a prominent feature. So if you can see a distinctive coastline or a mountain on the display, you can take a range and bearing from it and use that to fix your position. Let me just tie that together. The radar is a tool with two faces: weather detection and navigation. In weather mode, you're reading rainfall rate and turbulence. In ground mapping mode, you're reading terrain returns. And in both cases, the display is giving you range and bearing relative to your heading, and the colours tell you about the strength of the return. The antenna is stabilised so the picture stays reliable, and the whole thing is only as good as your interpretation. One thing I want to flag before we move on: the figures in the book — Figure 13.1 shows the AWR components, Figure 13.2 shows a monochrome cloud display with avoidance courses, Figure 13.3 shows a colour weather display, and Figure 13.4 shows a terrain mapping display. Those will help you visualise what I've just described, but the core concepts are what I've walked you through.