Airborne Weather Radar — Page 208, Lesson 194

I want to walk you through the Airborne Weather Radar — the AWR — because this is one of the few systems in the aircraft where the pilot is an active part of the loop. The radar doesn't just tell you the answer; it gives you raw information and you have to interpret it. That's the whole point of this chapter. Let's start with the introduction. The airborne weather radar is used to provide pilots with information regarding weather ahead, as well as navigation. So it's a dual-purpose system. But here's the key difference from most other systems: it requires interpretation by the pilot, and its use is enhanced by the skill of the user. In other words, two pilots looking at the same screen can extract different amounts of useful information. The better you are at reading it, the more it gives you. Now, where does this information get displayed? It can be shown on a dedicated unit — a standalone radar indicator — or, on modern aircraft, it can be shown in combination with the aircraft route on the EFIS navigation display, the ND. EFIS stands for Electronic Flight Instrument System, and the ND is the navigation display. So on a modern aircraft, your weather picture is overlaid right on top of your route on that display. Let's talk about what's actually on the screen. The information on cloud formations or terrain features is displayed on the indicator's screen as a range from the aircraft and a bearing relative to its heading. So think of it as a polar picture centered on your aircraft: range tells you how far away the return is, and bearing tells you the direction relative to the nose of the aircraft. That's important — it's relative to heading, not to north. The presentation can be monochrome — that's a single colour, typically green — or, on modern systems, it can be in colours: green, yellow, red, and/or magenta. Now, the meaning of those colours depends on which mode you're in. In the weather mode, the colours represent the increasing variations in rainfall rate, from light to very strong returns. So green is light rainfall, yellow is moderate, red is strong, and magenta usually indicates the presence of turbulence associated with intense rainfall. That magenta is the one you really want to avoid — it's not just rain, it's turbulence. But here's the contrast I want you to hold onto: in ground mapping mode, the same colours mean something different. Green indicates light ground returns, yellow indicates medium ground returns, and red indicates heavy ground returns. So the same colour scale, but a completely different interpretation depending on whether you're in weather mode or ground mapping mode. That's a classic trap — you have to know which mode you're in before you read the colours. Now let's look at the component parts. The airborne equipment comprises four main items. First, the transmitter/receiver — that's the unit that sends out the radar pulse and receives the returning echo. Second, the antenna, and this is a critical detail: it is stabilized in pitch and roll. That means the antenna is gyro-stabilized so that as the aircraft pitches and rolls, the antenna stays pointed where you set it, rather than wobbling with the aircraft attitude. Third, the indicator — that's the display unit you're looking at. And fourth, the control unit — that's the panel where you select the mode, the range, the tilt, and so on. Those four items together make up the AWR. So what are the main functions of the AWR? There are four, and I want you to remember them as a set. First, it detects the size of water droplets, and hence deduces where the areas of turbulence are within the cloud. That's the key physics: the radar measures the droplet size, and from that you can infer where the turbulence is. Second, it determines the height of cloud tops by tilting the radar beam up or down. So by raising or lowering the antenna beam, you can scan the vertical extent of a cloud and find its top. Third, it maps the terrain below the aircraft to provide navigational information and high ground avoidance. That's the ground mapping mode we talked about. And fourth, it provides a position fix — a range and bearing — from a prominent feature. So if you can identify a distinctive piece of terrain or coastline on the display, you can get a fix on your position from its range and bearing. Let me tie that together with the figures I have here. Figure 13.1 shows the AWR components — that's your transmitter/receiver, antenna, indicator, and control unit laid out. Figure 13.2 shows a monochrome cloud display with avoidance courses — that's the single-colour presentation and how you'd plot a course around the weather. Figure 13.3 is the colour weather display, where you'd see the green, yellow, red, and magenta. And Figure 13.4 is the terrain mapping display, showing the ground returns in that mode. So the takeaway for this introduction: the AWR is a pilot-interpreted tool, not an autopilot. It gives you range and bearing relative to your heading, in either monochrome or colour, and the colour meaning flips depending on whether you're in weather mode or ground mapping mode. The hardware is four boxes — transmitter/receiver, stabilized antenna, indicator, and control unit — and the four functions are droplet-size detection for turbulence, cloud-top height via beam tilt, terrain mapping for navigation and high-ground avoidance, and position fixing from a prominent feature. That's the foundation. Everything else in this chapter builds on those four functions.