"I've got radar" is a phrase that moves pilots to boast or envy. Why? Because there's a common assumption that by avoiding precipitation echoes on a radar screen, you'll enjoy safe, smooth rides around — and maybe even through — the nastiest thunderstorm cells. Well, maybe, maybe not. Many general aviation weather radars simply don't have the large-diameter antennas, long radar pulse widths, and higher power ratings of the more capable weather radars used in larger, turbine-powered airplanes. m Most radars designed for smaller general aviation airplanes have antennas in the 8-to-12-inch diameter ranges and shorter pulse widths. Pulse width is a measurement of the amount of time that the radar's transmitter is on during the broadcast of each pulse of radar energy. The longer the pulse width and the higher the power rating, the better the radar is able to map distant returns and see through precipitation echoes. The shorter the pulse width and the lower the power rating, the better it's able to paint image details, but at the cost of poor radar penetration. Low-power general aviation radars are usually saddled with short pulse widths.
Radar's strengths
Some pilots prefer radar over lightning detection equipment. That's because radar does have some very useful features. These include:
- The ability to depict the extent of any areas of significant precipitation. The pilot can see echoes right on the radar display screen, and avoid them by keeping them at a safe distance.
- The ability to show hazardous radar signatures. These include hooks, fingers, scalloped edges, steep rainfall gradients, and echoes that bow toward the radar antenna. These signatures are signs that a violent thunderstorm may be in progress. Hooks, fingers, and scalloping are linked to the presence of tornadoes. Steep gradients (narrow horizontal distances between precipitation contours of increasing intensity) indicate strong vertical and horizontal wind shears — in addition to heavy rain.
- The ability to see red. Depending on make and model, radar paints precipitation echoes in a maximum of three to six different colors. These correspond to the amount of radar energy (measured in decibels) being reflected back to the radar's receiver by a given cell. Green usually corresponds to light precipitation, yellow to moderate rainfall, and red is assigned to stronger rainfall rates — rates described by ground-based National Weather Service weather radars as Level 3 to Level 6. Some airborne weather radars use a magenta color to signify the presence of extremely high rainfall rates of the kind found in the most dangerous storms.
The advantage here is that the pilot can see right off the bat if a storm is dangerous, just by looking at the depicted colors. Obviously, a storm with a large core of red returns and fringed with hooks or scallops is one to avoid by a very wide berth. - The ability to scan storm cells vertically. By using the radar's tilt control, it's possible to roughly determine the tops of any storm cells, as well as peer inside their vertical structure. A new generation of vertical-profiling radars is specifically designed to make top-to-bottom examinations of storm structure. If a storm's insides are red from top to bottom, that's a sure sign of a mean, mature cell.
For these reasons and more, some pilots believe radar to be more helpful than lightning detection, which simply plots the locations and numbers of lightning strokes. However, there is no free ride. Radar lovers have to be aware of the ways that radar can mislead pilots and draw them into potentially fatal traps.
Radar's limitations
Airborne weather radar works by sending radio energy out in bursts. This energy then bounces off of raindrops and other precipitation and travels back to the radar antenna, which "listens" for the intensity of the reflected returns. Trouble is, a low-powered radar with a small antenna can experience attenuation when flying in precipitation. Attenuation is a premature scattering and dilution of reflected radar energy. What's so bad about this? The radar energy detects precipitation hitting the radome and immediately ahead of the airplane, but is so weakened by the short trip out and back that more distant, dangerous radar returns are invisible on the cockpit radar display. Simply put, the radar beam can't penetrate nearby rain well enough to see far ahead.
With attenuation, the pilot would see a constant band of light rainfall that always seems to travel just ahead of the airplane. Meanwhile, the strong returns of a violent storm just a few miles away can be masked. Obviously, this can set the trap for blundering into a storm, especially if the pilot is trying to negotiate a storm-free passage in situations where thunderstorms are embedded within areas of clouds and precipitation. The higher a radar's power rating and the larger its antenna diameter, the lower the chance of attenuation.
Beam smearing is another phenomenon that affects low-powered, small-antenna general aviation weather radars. As a radar beam leaves the antenna, it fans out in a cone-shaped propagation pattern. By the time the pulse of radar energy reaches, say, the 60-mile mark, the diameter of the radar beam can be eight or more miles wide. If you're trying to examine a four-mile-wide storm cell or line of small cells at that range, the radar beam will average the intensity of the reflected radar energy coming back from those storms — making them appear less severe than they might be and smearing the return as the beam sweeps back and forth. The result: One or more strong cells can look like a single streak of low-intensity precipitation. One radar expert, Archie Trammell, explains this beam-smearing by likening it to painting a nail head with a mop. With a larger-diameter antenna and a narrower radar beam, more of the beam will be "filled" with any precipitation echoes. This produces a more accurate rendition of any storm's shape and location, because less energy travels over, under, or around a precipitation cell.
Another limitation of radar is its inability to reliably detect non-cloud-related thunderstorm threats. These include lightning, dry hail (wet hail is highly reflective, and shows up very well on radar), turbulence, and downbursts or microbursts. Sure, some manufacturers offer lightning detection with their radars, and some of the newer Doppler units have a turbulence-detecting feature, but for the majority of the radars on the market, these luxuries simply aren't there. Some users and manufacturers even doubt the accuracy and usefulness of turbulence detection.
Operational basics
It's impossible to hold a comprehensive discussion on proper radar usage in this limited space. We can, however, review the most basic of basics. Here are some of the simpler guidelines:
- The closer range settings (e.g., 20 and 40 miles) give the most accurate weather depictions. Beyond these ranges, most general aviation radars will show only weak returns, even though storms 80 or more miles away may contain heavy precipitation. That's because by the time the radar energy makes the trip out and back its strength is considerably diminished.
- Proper use of the tilt control is critical. This is a complicated subject, and formulas abound for determining cloud top height, storm dimensions, and other vital statistics derived from tilt information. However, a good rule of thumb is to tilt the radar antenna down far enough so that ground returns (these look like bands that follow the arc of the radar's sweep) fill the outer third of the display screen. Then tilt the antenna up a few degrees. This should come close to giving you a level view of any upcoming weather at your altitude. Periodically, tilt the antenna down until you see ground returns. This assures you that the radar hasn't failed.
- Avoid weather-related red radar returns. Later-model color weather radars are designed so that a 50-dBz reflectivity (which corresponds to a rainfall rate of at least a half-inch per hour) shows up in red. Stay well away from the red — at the very least, by 20 miles.
- Learn the shapes of hazardous precipitation signatures and stay well away from them.
- Avoid any radar "shadows." Radar shadows appear as precipitation-free zones, and they are most likely to show up behind strong storms with steep reflectivity gradients and hazardous radar signatures. Everything may look clear behind a bad storm (you won't even see ground returns), but just the opposite is true. That's because all the radar energy is being reflected back to the antenna; the beam isn't strong enough to punch through the strong returns, see distant returns, and reflect back to present a truly accurate picture. The radar beam is blocked from any further progress by the first heavy rainfall it encounters. For this reason, never fly toward a radar shadow. A killer storm could be lurking in the "clear air" behind it.
- Remember that the best thunderstorm avoidance tactic is to stay clear of all cloud buildups. Visual circumnavigation is an almost bulletproof means of avoiding storm hazards. Attenuation is minimal, you can spot any additional clouds with the potential to grow to thunderstorm proportions, and there's the added relief of not having to worry about stumbling into a storm while on instruments.
- Having both lightning detection equipment and radar is a good way to fly. Lightning detection has the advantage of fairly accurate range information and a wider field of view, and it can provide early warning of any distant, growing cells. Radar augments this with the promise of better image detail of precipitation patterns. Together with information gleaned from a thorough preflight weather briefing, in-flight updates from flight watch (122.0 MHz), and — for those of you flying on instrument flight plans — the center weather units at air route traffic control centers, you'll make the best use of all available help.
Get thee to school
Anyone serious about really understanding radar should attend a course given by recognized experts. This is the only way to completely understand how to operate a weather radar. We can recommend two course offerings.
Archie Trammel, a former AOPA Air Safety Foundation executive director, aviation journalist, and avionics expert, conducts his one-day, $250 airborne weather radar seminars at locations all around the United States. If a flight department attends a seminar, the first attendee is charged $250, but any additional attendees pay $200. You can schedule a slot in one of his classes by calling AjT Inc. at 800/687-5661, fax 903/778-2857, or sending an e-mail message to [email protected].
Dave Gwinn, a retired TWA captain with extensive expertise in both radar and lightning detection equipment, also holds radar courses. Gwinn's seminars are run under contract to avionics manufacturer Honeywell, Inc., and they take place in the United States, Europe, and South America. The cost of a one-day seminar is $200; a half-day refresher course is $125. Specially scheduled courses can be held at sites of a client's choosing, but at extra cost. To register, contact Honeywell's Sharon Butash at 602/436-8966, or Fred Polak at 602/436-8972. E-mail information requests can be sent to [email protected]. To make fax requests, dial 602/436-8310.
Wx Briefs
Automated Weather Observing Systems (AWOS) manufacturer Vaisala announced that its Artais Division was recently awarded a $985,000 contract to install AWOS units at 16 airports in Texas. By the end of 1999, AWOS IIIs should be installed at Brenham Municipal Airport, Casparis Municipal Airport, Hemphill County Airport, Gillespie County Airport, Gainesville Municipal Airport, Bell Airport, Kerrville Municipal Airport, Fayette Regional Air Center, Lampasas Municipal Airport, Harrison County Airport, Palestine Municipal Airport, Hale County Airport, Winston Field, Avenger Field, Garner Field, and Wilbarger County Airport.
BFGoodrich just received certification of its RGC250 radar graphics computer. The RGC250 can depict navigation and radar data, TCAS I or Skywatch traffic advisories, and lightning data (from BFG's WX-1000E Stormscope or WX-500 lightning sensor) on a wide range of radar displays made by Collins, Honeywell, and AlliedSignal.
Aviation Week and Space Technology awarded Larry Cornman and Tenny Lindholm of the National Center for Atmospheric Research (NCAR) one of its annual Laurels in the electronics category. Cornman and Lindholm developed a system whereby turbulence data, automatically collected by airliners, is datalinked to ground sites and used to improve forecast models and warn pilots in flight. The research was sponsored by the National Science Foundation and the FAA's Aviation Weather Development Program. This was Cornman's second such award. In 1990 he was recognized for his work developing the Low Level Wind Shear Alert System, an NCAR project funded by the FAA. — TAH
E-mail the author at [email protected].
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