Summer-Weather Flying

Why and when radar can be a friend or foe

Are general aviation weather radars better than lightning detection devices? Many pilots think so. Conditioned by years of television weather-watching and manufacturer propaganda, pilots seem to favor radar's colorized visual images of storms over a Stormscope's or Strikefinder's patterns of dots or crosses. But radar imagery can be deceptive, and a radar's lies can kill an unwary pilot.

To use radar effectively and safely, it's best to take a structured course in weather radar operation and interpretation. Two of the best courses are offered by AJT Inc. (903/778-2177) and Dave Gwinn Radar Seminars (913/831-3338). If you want to get the most out of radar, take one of these courses.

Radar: Con

Some of radar's shortcomings center on the nature of the equipment — things like the unit's power, the size of the radar antenna, and the nature of the radome covering the antenna. Other problems have to do with pilots' misunderstanding of how radar works, as well as misinterpretation of radar returns and certain severe weather radar signatures. Perhaps the most serious shortcoming is that there are times when even the most powerful, sophisticated radars are simply blind to severe storms.

At the heart of radar's potentially deceptive nature is the radar antenna. It collects radar energy from the unit's transmitter and sends it out in pulses over the 1,000- to 30,000-MHz-frequency band. This energy comes out in a conically-shaped beam from the antenna; the smaller the antenna, the poorer the radar's performance. That's because smaller-diameter antennas put out big, fat radar beams that do poor jobs of defining storm features. And if you're flying anything smaller than a mid-size business jet, you're flying with a small (a 10- or 12-inch-diameter) antenna. So there's one strike against you right off the bat.

To illustrate the problems of small antennas on small airplanes, let's use AOPA's Beechcraft A36 Bonanza as an example. Pilots new to this airplane get all misty-eyed when they see the wing-mounted radar pod and the King KWX-56 display screen on the instrument panel. They think this gear can get them through or around a line of thunderstorms, no sweat.

Hah! Behind that radome is a 10-inch antenna that throws out a beam 10 degrees wide. By the time that beam reaches 80 miles ahead of the airplane, it's 80,000 feet high and 80,000 feet wide. That beam goes above the tropopause at that range, and if there are any storm cells out there, their features have been distorted and merged by a process called beam smearing. "It's like trying to paint a nail head with a mop," says Archie Trammel, president of AJT, Inc. Individual cells of storm activity are blended together by beam smearing at this range, and the storm will appear to be much wider than it really is. Needless to say, the imagery you may see on the display screen will probably be useless for locating easy passages through storm systems.

Set the display for the 20- or 10-nautical-mile range, however, and the beam is narrower — a 20,000-foot or 10,000-foot disc, respectively. More useful for seeing storm and cell shapes, but still subject to some beam smearing. As for using the tilt control to scan up and down the height of a storm, forget it. At the 80-mile range, the Bonanza's beam tops the highest of thunderstorms — even if the tilt control is set at 0. Leave vertical scanning to the airliners, whose 30-inch-diameter antennas put out 3-degree beams that really can do the kind of vertical profiling that lesser general aviation units can promise only on a good day.

Another antenna-related problem comes into play when a small-diameter antenna's beam misses precipitation echoes. For example, aim a small antenna at a storm that is 40 miles away and 4 miles high by 4 miles wide. About 75 percent of the radar energy in that 8-mile-high by 8-mile-wide beam at that range will sail over, under, or around the storm. The precipitation echoes in this situation are said to be less than "beam filling," and this is not a good thing. The radar averages the radar energy returned to the antenna and therefore shows a storm that's much weaker than it really is — all because of a non-beam-filling return. Couple beam-smearing effects with an absence of beam-filling returns and you have a deceptive picture in the cockpit. The radar display can show precipitation echoes both wider and weaker than they really are.

"The bigger the antenna, the better," says radar expert Dave Gwinn, who likens the focus of small-diameter antenna beams to that of a patio light. Large 30-inch-diameter airliner-sized antennas, on the other hand, "have the focus of a spotlight" and are much more accurate in defining storm shapes. There is less beam smearing and more beam filling.

A bad radome can affect a radar's performance, too. Small antenna or large, if it's covered by a radome lovingly swathed in a metallic paint job (plastic flakes are OK) or affected by a bad epoxy lay-up, you've got a blinded radar. You want at least 90 percent of the antenna's radar energy to pass through the radome. A plain fiberglass radome — one with no overlaps or uneven blobs in the lay-up — lets the antenna send and receive the most energy. Radomes of honeycomb construction also work very well. AOPA's A36 has a fiberglass radome with one coat of nonmetallic, polyurethane paint, so its radar transmissivity is — well, we don't know because it's never been tested. In fact, most radomes aren't tested for transmissivity, mainly because so few general aviation shops have the equipment to do the work.

Norton Performance Plastics of Akron, Ohio (330/296-9948), is world-renowned for its expertise in radome design, fabrication, analysis, and testing. When it comes to poor radomes, Ben Mackenzie, Norton's director of technology and engineering, says he's seen it all. "There's a lot of junk out there," says Mackenzie. "I've even seen commuter airliner radomes so bad they made the beam show weather not as severe as it really is, producing false rainstorms on a display that aren't really there because of radar energy being reflected off the radome's upper surface and going down to the ground." Mackenzie strongly suggests that radomes with transmissivity values of less than 85 percent are next to useless and usually cause horrendous attenuation problems — an issue we'll address shortly.

Botched, incompetent field repairs to chipped, cracked, or otherwise damaged radomes are epidemic, according to Mackenzie. "So many times, I'll see a radome that looks like it was fixed by an auto body or boat hull repair shop. Honest. I've even heard guys say they were referred to a body shop specializing in Corvettes for their radome work."

Radomes must be carefully tuned, according to Mackenzie. This means that great care must be paid to the thickness of each radome's fiberglass layers, as well as the thickness of the radome core material. "If the radome's not tuned properly, it doesn't matter how good your radar components are. You could have the best Doppler radar they make, but with a bad radome, it just wouldn't see well at all. It would be as though you put a pair of Coke-bottle glasses on Michael Jordan and then asked him to go out and play basketball."

Attenuation can affect all radars, but it's much more of a problem for the lower-power, small-antenna models. Many variables affect attenuation, variables that a good radar course will delve into in detail. To be brutally brief, attenuation is the masking, or complete loss, of radar energy reflected back to the antenna.

How does it happen? As radar pulses leave the antenna, their strength diminishes with distance. That's the first part of the problem. Now, imagine that you're on instruments, in a rainshower situation, and trying to use your radar to look for embedded thunderstorms. The rain between you and any embedded cells can prevent the radar from seeing those cells because most, if not all, of the returns depicted on the cockpit display will be of the intervening rain — not the bad stuff behind it. The radar isn't powerful enough to send beams all the way through the storm complex. All you'll see will be the green returns signifying light to moderate rainfall — until you stumble into a cell. This is one of radar's deadliest ways of lying.

In cases of severe thunderstorms containing hail and heavy downpours in lines or dense clusters, even airline radars can attenuate. A 1977 crash of a Southern Airways DC-9 illustrates this point perfectly. Cockpit voice recordings indicate that the pilot chose to fly through what he felt was a narrow "soft spot" in a line of thunderstorms depicted on the DC-9's radar. The soft spot was anything but. It was the heart of the strongest cell in the line and contained hail that blew out the airplane's windshield and ultimately caused a crash landing with fatalities. The narrow spot was narrow because the radar couldn't penetrate the heavy precipitation. Behind the narrow spot, the radar screen showed no echoes at all — but it really hid the worst of the storm. No echoes were shown because no radar could get through. What the crew saw was a radar shadow, one of the biggest danger signs a pilot can see on radar. A radar shadow looks like a "clear" area behind a cluster or line of precipitation returns.

Radar: Pro

Beam smearing, low-ball precipitation averaging, crummy radomes, low power, attenuation. It makes you wonder: Is there anything good about the radars used by smaller airplanes? The answer is yes, with a few big caveats. Here are a few general tips on how to get the most out of a small general aviation weather radar.

First of all, for tactical decision making, don't bother selecting radar ranges beyond 40 miles. The beam and attenuation problems we've already discussed are the reason. The 10-, 20-, and 40-mile ranges can be more useful in identifying storm features — if attenuation isn't at work.

Have the radar turned on before entering an area of precipitation. Use the tilt control to tilt the antenna down just enough to ground map the outer third of the display screen. Ground returns follow the arc created by the sweep of the antenna; rivers, lakes, and other bodies of water will appear black. Higher terrain will show up as yellow or red.

The tilt control can be used to help avoid flying into radar shadows. If you can't see ground returns behind any areas of precipitation returns, don't press on. The radar is attenuating, and heavy rain may be blocking the "view" immediately ahead.

Examine any storm features, using the 10- or 20-mile ranges. Look for the following dangerous radar signatures:

Steep precipitation gradients. These occur when the distances between light (green), moderate (yellow), and heavy (red) returns are very closely spaced. The latest radars use five colors, including two shades of red and magenta to indicate the heaviest returns.

Hooks or fingers. These shapes, usually appended to core portions of a storm cell, may show steep gradients and heavy returns within their narrow bands. Many times, hooks prove to be tornadoes, and fingers may represent hail shafts.

Scalloped edges. These are other signs of actively growing, aggressive storms. Scallops can turn into hooks.

Pendant shapes. Another sign of severe thunderstorms, especially if combined with the features mentioned above.

Bows with no distant returns. A bowed-out (towards the antenna) echo with no apparent radar returns behind it should be taken as evidence of the kind of attenuation caused by heavy downpours — so heavy that severe weather lurks in the blank area where the "clear" radar shadow appears.

Any radar shadows. If there's a "clear" area behind any precipitation echoes, don't go there. Or anywhere near there. The general rule is that if you can't see ground returns behind an echo, assume that the "clear" area is a radar shadow.

Radar strategy

There are only a few main rules for using radar. First off, obtain a complete preflight weather briefing; radar is certainly no substitute for common sense. One of the most important radar rules is to avoid flying into radar shadows. Another is to avoid storms with steep gradients or any of the other warning signs by at least 20 nautical miles. Also, be aware of the possibility that your radar may be attenuating. Perhaps the earliest warning of this is a display filled with a wide, even band of low-level precipitation echoes that seems to remain the same and travel with the airplane. Guess what? This means that, for all practical purposes, your radar is useless — because you can't see through whatever is attenuating your signal.

Of course, an excellent strategy is to have both lightning detection and radar, and in fact this is how we fly our A36. I use the lightning detector — a BFGoodrich WX-1000 — as an early warning and gross-avoidance vectoring aid, and the radar for close-in circumnavigation, when it's not attenuating. This combination works well, strategically speaking, although there are drawbacks. The WX-1000 yields a false range (the lightning returns are shown closer than they really are), but at least it errs on the conservative side. The radar has all the potential problems we've just covered but seems to work just fine for detouring around cells when used on the 10- and 20-mile ranges.

Of course, the whole setup works best when flying in Florida and visually avoiding the kind of air mass thunderstorms so typical there. Which proves the most important point of all about summer weather flying: stay visual and make all your weather deviations in the clear.