Airborne Weather Radar — Page 220, Lesson 200

We're starting the Airborne Weather Radar chapter, and I want to walk you through the control unit first. This is the panel in the cockpit that lets you operate the radar, and Figure 13.12 shows a basic control unit for a monochrome AWR — that's an Airborne Weather Radar — with range scales of 20, 50, and 150 NM. Let's look at each function in turn. First, the Power Switch. In the ON position, the system is energized, and the aerial — the antenna — is automatically stabilized in PITCH and ROLL. That means the radar beam stays level relative to the horizon even if the aircraft nose pitches up or down, or the wings bank. Now, a lopsided or asymmetric display probably indicates that the stabilization has failed. If you see that, you can switch to the STAB OFF position, which locks the scanner to the pitch and roll axes of the aircraft — so the antenna moves with the aircraft rather than being stabilized against it. Next, the Range Switch. The STANDBY position holds the equipment in readiness during periods when the AWR is not required. Selection of a range position energizes the transmitter. Now, here's a critical safety point: whilst on the ground, the STANDBY position must be maintained until it is certain that personnel and any reflecting objects, such as hangars, are not in the radar's transmitting sector. The radiation can damage health, and the reflections from adjacent structures can damage the equipment. Selection of the MAPPING beam produces the same hazards. So in poor weather conditions, you switch from STANDBY to the 0–20 NM scale as soon as the aircraft is clear of personnel and buildings, and check the weather conditions in the take-off direction. The maximum practical range for weather and for navigation is in the region of 150 NM. Now the Tilt Control. This control enables the radar beam to be tilted from the horizontal within 15° UP (+) and 15° DOWN (−). In the horizontal plane, the antenna sweeps up to 90° either side of the nose, though a sector of 60° on each side is generally sufficient for the role of weather depiction and navigation. For ground mapping, the beam has to be tilted down. In order to observe cloud formations, it is raised to reduce ground returns. And here's an important relationship: due to the curvature of the earth, the tilt should be higher when the selected range increases or when the aircraft descends to a lower altitude. Equally, the tilt setting should be lower when the selected range decreases or when the aircraft climbs to a higher altitude. That's because at longer range or lower altitude, the beam needs to point higher to clear the earth's curvature; at shorter range or higher altitude, you can point it lower. Before we move on, I want to make sure you've got the earlier figures in context. Figures 13.8 through 13.11 show radar signatures indicating hail activity: the U-shape, the finger, the scalloped edge, and the hook. These are the classic shapes you look for on the display when assessing severe weather. And Figures 13.13 and 13.14 illustrate the tilt geometry we just discussed — the sweep sector and how tilt changes with range and altitude. So to tie it together: the control unit gives you power and stabilization, range selection with a safe STANDBY procedure, and tilt management that you adjust based on range and altitude. That's the core of operating the weather radar.