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Magenta Sky
Know Your Radar
by Stan Dunn
In addressing thunderstorm flying, the Aeronautical Information Manual (AIM) begins with this gem: “Never regard any thunderstorm lightly, even when radar echoes
are of light intensity. Avoiding thunderstorms is the best policy.” It sounds like sage advice. One of those things that are great in theory, but rapidly becomes muddled in the grey shades of the real world. I have lost track of the number of times that I have landed at an airport with the thunderstorm (TS) descriptor amended to the terminal weather.
the antenna dish; the larger the antenna, the narrower the beam will be. A narrow beam gives a radar system more fidelity over distance, as the radar pulse is much more con- centrated. Think of it like a flashlight: If you have a wide beam, the light gets dim very quickly; with a narrow beam, it has a much longer range.
An 8-degree radar beam (a standard size for civil air- craft) ends up covering 40,000 feet worth of sky at 50 nm (you can calculate this via the ancient rule of 60, where 1
I do recall the first time it hap- pened. I was a raw captain on a Beech 1900 approaching Denver interna- tional Airport. I could actually see the field, and told the controller that they should get rid of the TS code present in the weather: We were not, I explained, allowed to fly through thunderstorms. The prover- bial crickets chirped a few moments before the controller calmly coun- tered that none of the 150 aircraft that had landed in the past hour had complained.
Here is the first thing to know about thunderstorms: They can kill you. Here is the second thing: For aviation to be useful as transportation, you will eventually have to fly through an area in which they are present.”
degree equals 100 feet per nm). Al- though 40,000 feet worth of radar sounds impressive, the problem is that this can begin to stretch the limits of physics. At 40,000 feet, the radar’s initial energy pulse is covering just shy of 7 nm worth of sky. This can make it easy to miss magenta danger areas at longer ranges, as radar returns become fruitlessly diffuse.
The physical limits of the radar beam also make the use tilt ex- tremely important since onboard radar covers a limited slice of the sky (particularly at shorter rang- es). At lower altitudes tilt man-
Here is the first thing to know
about thunderstorms: They can kill
you. Here is the second thing: For
aviation to be useful as transportation, you will eventually have to fly through an area in which they are present. While there are a number of different resources available to assist in making the penetration of weather safe, few are of higher value than weather radar.
agement can be fairly basic: Below 10,000 feet, dangerous weather is largely defined by the amount of precipitation that exists above the aircraft. Start from zero tilt (with half the beam above and half below the aircraft altitude) and tilt up by half of the width of the radar beam. The result will be that the bottom of the radar beam will be at your present altitude, with the radar displaying the precipitation above you. (This assumes zero tilt is properly calibrated; you can use the rule of 60 and a little math to verify this while painting the ground).
Know Your Radar
The first bit of information that you should memorize about your radar is the angle of the beam. With X-band ra- dar, the radar beam’s width is inversely related to the size of
8 • TWIN & TURBINE / September 2020