Radar myths and misconceptions

Flying Safety, May, 2002 by Dave Gwinn

The radar tilt control obeys that same principal used for beam diffusion: 1 degree equals 1000 feet at 10 miles or 1 degree equals 6000 feet at 60 miles. Take any mileage and "00" and you have the effect of one degree at that distance.

For example, to tilt down 1 degree at 75 miles is to lower the center of the beam 7500 feet below the aircraft. Remember, one-half the beam diffusion is extending below the beam center. It may be on the ground. That's a many decibel return. Knowing how to manipulate the beam at various altitudes to place it within significant water areas is the essence of effective radar use.

At low altitudes, however, we encounter a problem. From the outer marker to the airport is typically five miles. One degree at five miles is 500 feet. Limited to a 15-degree tilt up, the pilot can only elevate the center of the radar beam to 7500 feet above the aircraft. Horrid rainfall and windshear don't originate at 7500 or 9000 feet AGL. During one radar seminar, I commented to the audience (tongue in cheek): "If you've postponed all of your radar decisions to the LOM, just turn it off. It's extremely limited at that low altitude. All of your commitment decisions must have been made at much higher altitudes, at greater distances from the airport, and with good radar investigation that's just not possible at the out maker."

Mythical Misuses

Myths about radar are many, including the notion that you can measure the tops of storms with it. Frozen water doesn't have the free molecules to dipole. Therefore, it's not practical to accurately measure the height of a thunderstorm top composed of snow, hail and cirrus clouds with airborne radar. Top determination is imaginative and one manufacturer markets a product alleged to do it, but the concept is badly flawed.

Despite the wildest application of imagination, you can't navigate with any practicality using weather radar. From every altitude and angle, cities and structures come back with vastly different levels of reflectivity. (A lake or river, however, will totally absorb the energy and its perimeters can be determined with some ease. KAL007 might not have overflown the Soviet Union if it had ground painted and recognized that it should not be crossing the Kuril Islands.) Over land, radar is a mass of purple, red and yellow. with some desensitizing (gain control), you probably can find a city. But why? Better to ask for ATC radar vectors.

Because the beam is a crude representation, you cannot calibrate your beam width or your receiver with ground returns. Radar--at least weather radar--was not designed to determine safe overflight of terrain. These techniques are all products of people's imaginations and calculator skills.

Nor does radar preclude bird strikes. The safe permissible level for human exposure to radar is typically at 15 to 25 feet. At that short distance, tissue as sensitive as retinas is not affected (consult your manual). So you're not going to tickle and redirect a bird at three miles. In our seminars, we have a photo of two bird strikes upon a Falcon. Both birds "aimed" at the radome from which 5000 watts of power was being transmitted. It doesn't hurt to turn on the radar, but it won't be your last disappointment in life when you get that bird strike.