Airborne Weather Radar — Page 220, Lesson 206

We're starting a fresh topic now: Airborne Weather Radar, and specifically the operational side of it. I want to walk you through how we actually use this equipment to avoid thunderstorms, and then we'll look at the control units. Let's begin with the weather operation itself. The first principle is to select the maximum range on the radar so you detect weather formations in good time. Then you adjust the TILT control to remove ground returns. Tilt is the vertical angle of the radar beam, and ground returns are the echoes you get back from the earth's surface, which clutter the display and hide weather. So you tilt the beam up just enough to clear the ground clutter. Now, if the storm system is extensive, you make an early track adjustment, in consultation with ATC, to avoid it. That's a key point — you don't wait until you're on top of it. You talk to air traffic control and change your planned track early. If that's not possible, then as the clouds get nearer, you select the lower ranges and the CONT function. CONT is the contour mode, which I'll explain in a moment. You use it to determine the best track to avoid potential turbulence. Here's the critical operational skill: you must ensure that short-term alterations of heading steer the aircraft away from the worst areas, and not deeper into them. That's the danger — a small heading change can put you right into the heart of the storm. To achieve this, you need constant switching between short, medium, and longer ranges. That way you maintain a complete picture of the storm system, not just the part that's immediately ahead. Now let's talk about a serious hazard: the shadow area. There is a danger of not being able to map the area behind heavy rain, because no radar waves will penetrate it. The radar beam is absorbed and reflected by that heavy rain, so everything behind it is blank. That blank region is called the shadow area, and it may contain severe weather. So when you see a shadow, you must treat it as a potential threat, not as clear air. Next, the height of a storm cloud. You can ascertain it by adjusting the tilt until the radar returns from the cloud just disappear. When the beam is tilted so high that the top of the cloud is no longer illuminated, the returns vanish. Then you calculate the height of the top of the cloud using the 1 in 60 rule. That's the rule where, at 60 units of distance, one unit of height subtends one degree of angle. So if you know the tilt angle and the range, you can work out the cloud-top height. And it's worth noting that a thunderstorm may not be detected at all if the tilt setting is set at too high an angle — you can literally miss it entirely. Now, on the older AWR systems, where the conical beam is produced by a dish antenna, there is always some vertical overspill of energy. That overspill is reflected back to the aircraft and appears on the display as a "height ring." This ring roughly indicates the aircraft's height. It also serves a useful purpose: it indicates that the equipment is serviceable when there is no weather ahead. So if you see that ring, you know the radar is working. Let me now move to the colour AWR controls. The controls for a colour unit are similar to the monochrome unit in terms of range, tilt, and gain, but they may have additional features. The first is Wx, which stands for Weather. Then Wx + T, which is Weather plus Turbulence. And Wx(var), which is Weather with variable gain control. Other sets may have a WxA control, which stands for Weather Alert, and that gives a flashing display of the areas associated with turbulence. There may also be a Contour Intensity control, which enables you to adjust the display for optimum presentation. The latest AWR controls are activated by push ON/OFF switches located around the colour screen. And one of those is Test. The Test function displays the colour pattern for a pre-flight serviceability check. So before you fly, you press Test and verify the display is working correctly. Let me tie this together. The whole art of weather radar operation is managing range and tilt. You start wide to see the big picture, you tilt to clear ground clutter, you switch ranges constantly to keep the full storm system in view, and you use the contour and turbulence modes to pick the safest path. And you always remember the shadow area — what you can't see may be the most dangerous thing out there. That figure shows a basic control unit for a monochrome AWR with range scales of 20, 50, and — the caption cuts off there, but you can see the layout of the controls we've been discussing.