Avionics: Thunderstorm Avoidance Tools

Equipment for your best defense

April 1, 2008

Flying in or near convective weather is serious business and pilots should consider their alternatives carefully before tangling with the severe turbulence, heavy rain, hail and lightning commonly associated with mature thunderstorms.

Often, delaying or even canceling the flight might be the best course of action, but once the “go” decision has been made, the pilot is obliged to plan his route with weather avoidance as the top priority. He or she also must be psychologically prepared to accept wide deviations around visible cloud buildups, and around thunderstorms embedded within stratus clouds or other instrument conditions.

To do this with greater reliability and confidence, all instrument pilots should strive to better acquaint themselves with the weather avoidance tools and equipment that might already reside on their instrument panels, or are readily available in the aftermarket. The most common of these systems are radar, datalink weather, and lightning detectors. Each has its own unique strengths and weaknesses, and are best when used together.

Radar picture sharpens

Airborne radar is likely the pilot’s best defense against inadvertent penetration of a thunderstorm cell. It is also the most expensive ($8,000 to $25,000) and maintenance-intensive, and general aviation aircraft can typically accommodate antennas no larger than 12 inches—too small for practical long-range viewing.

Modern digital systems can detect precipitation at ranges of more than 200 nautical miles, but GA radars are generally most valuable within 40 or 50 miles of a storm. Still, radar can be a challenge to interpret—even for seasoned veterans—and a misread return could be worse than none at all.

“Radar can only tell you three things about a thunderstorm—what it is, where it is, and where it’s going to be when you get there,” said Archie Trammell, a longtime radar instructor. “Red doesn’t mean anything except heavy rain; it’s what kind of red that’s important. Is it stratiform or convective? You’ve got to make judgments based on the actual synoptic situation, not just the color on the screen.”

Airborne radars are getting better, and the newest turbine aircraft systems offer a preview of what piston aircraft pilots have to look forward to. One of the first of these next-generation systems, the Rockwell Collins RTA-4100 MultiScan radar, automatically scans ahead of an aircraft out to a distance of 300 nm, while eliminating ground return.

Computer algorithms analyze the returns to provide a fairly complete weather picture, not just the precipitation rates. In addition, the radar automatically corrects for aircraft attitude and antenna tilt, while providing turbulence detection and alerting out to a distance of 40 nm.

“Our goal is to reduce the amount of fiddling a pilot has to do with the radar,” said Jeff Finley, a radar engineer at Rockwell Collins. “This will be a wonderful feature for GA pilots getting into VLJs and other high performance aircraft, who may not have spent the last 40 years learning how to set the tilt.”

Datalink—the game changer

At the moment, the biggest news in weather avoidance is datalink technology, which allows radar images and other weather information to be displayed in the cockpits of even small aircraft. One popular example is Garmin’s 496 handheld GPS receiver, which displays Nexrad radar data attained through the XM Satellite Weather subscription service. The entire package can be had for less than $3,000.

As well as datalink systems work, however, pilots need to be aware that the radar information they display is a strategic or broad-brush view, rather than the tactical picture needed to support rapid decision-making. And, remember the in-flight datalink refresh rate will be five minutes or longer—an eternity for a pilot negotiating a line of rapidly developing thunderstorm cells.

Nexrad image quality is also a function of antenna tilt at the ground-based radar site, which is not always consistent, nor is it always evident to the user. In some cases the pilot may only be seeing a one-degree tilt that shows only the situation at ground level. Trammell called this the “Weather Channel” return. A composite scan is better, he said, because it shows the echo strength within the area of highest reflectivity.

Other datalink information sources, such as satellite photography, can help datalink users build on the situational awareness gleaned from the radar imagery. METAR temperatures at airports along the route could also provide an early indication of thunderstorm activity. A wide spread between temperature and dew point is a good warning sign, as is a significant shift in the low-level winds, or a wind shear advisory. Still, we’re only just beginning to tap into datalink’s potential.

One of the newest datalink platforms is Avidyne’s MLB700, a dual-channel receiver capable of receiving data and audio via the Sirius satellite broadcast network. In addition to providing the Sirius music and entertainment service, this S-Band system offers a full range of WSI InFlight weather products. These include the NOWrad mosaic of the U.S. Doppler Weather Radar network, graphical and textual METARs, TAFs, TFRs, lightning detection, cell movement tracker, echo tops (cell height), winds and temperatures aloft, and graphical airmets and sigmets.

“We’re only five or six years old when it comes to this stuff, but we’re learning fast,” said Glen Gray, director of aviation services for WxWorx, a Florida-based provider of datalink products. “Think about how far we’ve come from those first, highly pixilated Nexrad images; now we’ve got fairly high resolution. Our goal is to listen to pilots and give them what they want. They’ve asked for a horizontal view of radar picture and that’s in the works. We’re also looking at increasing satellite bandwidth availability. But at this stage we’re still walking, not running.”

Lightning detection recharged

The Stormscope lightning detection system, in contrast, has reached a high state of evolution since the core technology debuted in the 1970s. Once the sole thunderstorm avoidance tool available to GA operators, these relatively inexpensive ($2,500 on up) devices began as stand-alone units. While these are still popular, the best-selling variation is the WX-500, a remote unit compatible with various multi-function displays (MFDs).

According to L-3 Communications, which obtained the device with its March 2003 acquisition of BF Goodrich Avionics Systems in Grand Rapids, Michigan, the system is unique in its ability to detect thunderstorms during their developing stage, when lightning might precede the emergence of the main cell. Stormscopes also detect the cloud-to-cloud lightning of the mature cell, while datalink displays only register the cloud-to-ground lightning the storms emit while dissipating.

“We look at the lightning because it’s present, but also because it provides a very accurate indication of thunderstorm-related weather phenomena,” said Kim Stephenson, director of aftermarket business development for L-3 in Grand Rapids. “By happening in real time and showing all types of lightning, the Stormscope is tactical, like a radar. Datalink, in comparison, is strategic because of its latency, but this makes it better as a planning tool. The truth is, they really work well together.”

Even radar-equipped airplanes have a use for the Stormscope, according to Stephenson, because it allows pilots to check behind radar shadows and other signs of beam attenuation. “They use it to look and see if there’s another storm waiting behind the big storm in front of them,” she said.

Avidyne recently unveiled its TWX670, a new lightning detection system that takes the Stormscope concept a step farther. Strikes are depicted in the form of an intuitive, three-color view not unlike the one associated with radars and datalinks. TWX670 also provides accurate strike information from zero to 25 miles, an area masked out on most MFD interfaces. Imagery generated by this system—which debuted this year aboard the new Piper Matrix—may be piped through an MFD or a smaller, individual display.

When using Stormscopes and other lightning detectors, it’s not uncommon to see lightning strikes in areas where no precipitation is evident. This happens because the lightning is present during the formative or cumulus stage of the thunderstorm, which is typically rain-free. As the storm dissipates so can its tendency to produce electrical discharges, so Stormscope indications could fall off during this phase, even though heavy precipitation could still be in your path. Never fly into a towering cumulus cloud, even if there’s no lightning present.

Playing it safe—and smart

Unless the thunderstorms are embedded in stratus clouds, pilots are often able to pick their way through them visually, by keeping their distance from cumulus and cumulonimbus clouds. A popular rule of thumb recommends avoiding these buildups by at least 20 nautical miles, but experience is no doubt the best teacher.

The relative danger of thunderstorms can vary by region. Late afternoon CBs over south Florida may look like a mesoscale convective complex in west Texas or Oklahoma, but the latter is far more dangerous. Be careful about maneuvering between the tops of growing cumulus clouds, as they can climb far faster than most aircraft. Below isn’t necessarily a safe path either, because of up- and downdrafts, hail, and lightning. Never hesitate to deviate or even cancel when convective weather looms—the airlines do so as a matter of course.

Sometimes the best tactic is to turn back, or to land at the nearest safe airport. If penetrating a thunderstorm cell is unavoidable, reduce speed below V A (maneuvering speed), turn up the panel lights, and ensure seat belts are firmly fastened. Do a 180-degree turn if you think it will put you back in the clear, and let the aircraft rise and fall with the turbulence. If the radar indicates a shadow or hook-shaped echo with scalloped or magenta edges, fly a heading that will keep you upwind of it if you can—this could mean the difference between a rough ride and catastrophic hail damage.

“This is serious stuff, this weather flying, and there’s a level of emotional maturity that goes into it,” said Bruce Landsberg executive director of the AOPA Air Safety Foundation. “In air carrier operations, none of the decisions regarding thunderstorms, including whether to fly at all, rest with a single person. They’ve got management, dispatch, and two-pilot crews. But in GA it’s all up to us. We all have to understand that the equipment, however sophisticated, doesn’t make the unflyable, flyable.”

E-mail the author at paul.richfield@aopa.org.