Page 15 - Volume 18 Number 2

ManagementFigure 2Bottom of Beam -4°Tilt Management10nm 20nm 30nm 40nm 50nm2. That’s the preferred “Parked” position of TILT, which is why it’s called NAP, the “Normal Antenna Position”. The display will then be as in the photo directly belowNAP is TIP Minus 4oThe instant you select it you’ll most likely holler, “But, hey, I can’t see thunderstorm echoes in all that ground clutter!”Yes, you can. Thunderstorms are those red echoes that have a “Radar Shadow” behind them, as at 1:30 and 10:30 in the photo. Radar energy is like the beam of a flashlight; it can’t penetrate solid objects, like hard thunderstorms, so it casts a shadow behind them. Cities and towns are those red echoes with no shadow behind them, as at 11:00, 11:45 and 12:30. With NAP selected, thunderstormsjump right up off the display. Any echo that’s well inside where the beam intersects terrain, as at 9:30, is a thunderstorm. That’s automatic, because of the 4o downslope on the bottom of the beam.But, you’ll next say all that clutter moving back on the display is a distraction. Not true, any more than the terrain drifting by the windshield is distracting. In truth, you’re seeing exactly the same thing on radar as through the windshield. The image from radar simply gets to your brain through one path and the image through the windshield via another path. No difference. Both give you situational awareness, especially useful in terminal areas.Think about it; as a wise pilot, do you ever rely on any single source of information? Certainly not. To be certain, you must have three sources of data. In this instance, you monitor a couple of instruments and back up those indications with what’s displayed on your radar — even through darkness and cloud. Even in VMC, the view ahead should be backed up by radar. Situational awareness. And on radar it’s raw data, not possibly contaminated bysome faulty algorithm.In addition, for those flying in the flight levels, NAP is your surest protection from a fast-growing thunderstorm. Not all do, but often a storm will grow in height at a rate of 4,000, 5,000 fpm; in rare instances, even 8,000 to 10,000 fpm.You won’t be surprised by one of those devils when NAP is selected.How come? Examine figure 2. Note the storm at 30 nm is below the bottom edge of the beam’s sweep so it’ll not be detected and displayed; the taller one at 20 nm is detected and displayed and must be avoided. Obviously, when flying at higher altitudes, an echo that intrudes inside the 30-nm range could be a threat. Which leads to a reasonable rule when flying high; “With NAP selected, never allow an echo to drift back inside the 30 nm arc.”Why? Because of the ancient rule of 60. To convert an angular dimension (a degree) into a linear dimension (nm), simply multiply the angular dimension by 100. The result will be the width of a degree per nm at that distance. (It’s to rule- of-thumb accuracy.) Therefore, with the bottom of the beam sloped down 4o at NAP an echo at 30 nm will be cleared by 12,000 feet or less — 100 X 4o X 30 = 12,000.But, 12,000 feet is a lot of clearance above the hazard, isn’t it? Not really. What your radar beam detects is the “Wet Top” of a thunderstorm, the part detectable by your little radar when water drops are above a certain threshold size. The “Turbulent Top”, and/or the “Hail Top”, can easily extend 10 to 20 percent above that. At 30 nm, the “Radar Top” may beFEBRUARY 2014TWIN & TURBINE • 13t