Lucky indeed are airline captains when it comes to storm avoidance. Theyve got a multi-thousand dollar radar system with a dish the size of a manhole cover and an earnest young first officer to twiddle the tilt knob and suggest deviations.
And there will be no arguments about which is better-Stormscope or radar-because airliners dont have Stormscopes. They rely exclusively (well, almost) on radar to skirt nasty weather. So if the airlines use radar to the exclusion of sferics gear, it must be far better, right?
Well, maybe yes, if you happen to be sporting around in a Boeing or a Micky-D with a big antenna spewing out enough microwave energy to bake a potato five miles downrange. The same cant necessarily be said of the rather modest radar systems available to single-engine and light-twin GA owners.
Small radar systems have insurmountable technical limitations that produce the sensation of looking at storms through a toilet paper tube; the big picture may be elusive.
Nonetheless, little airplane radar is still beneficial, but all things considered, dollar for dollar, the best value in storm detection is a spark detector.
That said, new display technology such as Avidynes new FlightMax system, Garmins GNS 430 interfaces and Apollos MX20 display are rapidly changing the rules, since these device will be-or already are-capable of painting both radar and lightning data on the same display, using data from remote sensors such as BFGoodrichs WX-500 Stormscope.
Sparks or Water?
The survival skill in storm avoidance is steering clear of turbulence, lightning and rain, in that order. Oh, and lets add hail, too. Although by definition, a thunderstorm has lightning, all four hazards may or may not be present together in a typical thunderstorm.
And neither radar nor a sferics device is capable of detecting turbulence. Both imply the existence of turbulence by seeing rain (radar) or detecting lightning (sferics.) Some sophisticated radars can detect turbulence in clear air, but you cant afford one so forget it.
Radar sees water in various forms, but thats it. Its good at seeing rain and wet hail and marginal in detecting wet or dry snow or dry hail. It cant see ice crystals in frozen clouds or small dry hail at all. Transmitted radar energy isnt really reflected from water droplets but actually energizes the droplets in a process called dipoling.
The radar antenna detects this faint return of energy and displays it on the screen, in discrete levels of some sort. Color radars use three or four colored levels: Green, yellow, red and magenta, dependent on droplet size and thus precipitation intensity. Monochrome radars-of which there are many flying-display levels by flashing the video display. The benefit of radars discrimination is also one of its fundamental weaknesses: It requires interpretation and judgment that comes only from training and experience, something thats generally lacking in the GA world. (Even an active pilot may use radar in anger only a handful of times each year.)
Flying a color radar, for example, most pilots will wisely avoid any yellow or level 2 returns, rightly reasoning that these might contain dangerous turbulence. Then again, perhaps not. Radar displays are calibrated to stratus rainfall rates and a wet stratus cloud may harbor heavy rain but no turbulence.
To the radar, precip is precip. Stratus rain is no different from convective rain. Its up to the pilot to consider what he sees on the radar against other factors, such as the type of weather system, the likelihood of instability and geography. A yellow return in Oklahoma in July doesnt necessarily represent the same hazard level as a yellow return in Florida in October. Obviously a conservative policy to avoid all level 2 or yellow returns will work, but at times, that may result in unnecessary deviations.
Beam Limitations
By far, the critical limitation for small GA radars is antenna size. Typically, for a light twin or single, antenna diameter is 10 or 12 inches, versus up to 30 or more inches for bizjets and airliners. The smaller the antenna, the wider the radar beam and the more its energy is dispersed with distance.
A 10-inch antenna, of the sort typically found in a wing-mounted pod, has a 10-degree beam, versus 3 degrees for a 30-inch dish. Leading-edge antennas buried in the wing are even wider; 16 degrees.
Think of this as the difference between a weak, broadly focused flashlight and one with a powerful, sharply focused beam. Youll see more detail at greater distance with the focused flashlight. At 60 miles, the 10-inch antenna has a beam-really a cone-60,000 feet wide; a 30-inch dish has a cone 18,000 feet in diameter.
To measure reflectivity, radars assume that all of the energy impacts the target storm. But at great distances with a small antenna, a storm wont fill the beams entire width; some of the energy will be lost as it scoots past areas where theres simply no water to see. But the radar cant compensate for this and, assuming all the energy is returning, it will thus underestimate the precip intensity. On a small-dish radar at 60 or 100 miles, a green return could in fact be a level 6 monster.
This puts the pilot of a relatively slow twin or single at a distinct disadvantage because to assess a storm accurately, hell have to fly much closer-thus filling the radar beam-and then deviate more radically to avoid the hazard. A minor deviation at great distance is usually less time and distance consuming than a larger one closer in. Knowing this limitation, you can make sound deviation decisions, but not at the distance that an airline Captain can.
Low Altitude
Small dish radars are further hobbled when their wide beams collide with the ground during low-altitude maneuvering, say during the approach. When close in to the airport and with limited tilt control, the pilot can only elevate the radar beam to see perhaps 7000 feet or so above the aircraft, an altitude thats too low to see areas where intense rain and turbulence may originate.
For that reason, radar storm strategy on approach has to be decided well ahead of descending and reaching the outer marker. This requires understanding the current display and guessing what will happen in the next five minutes. Again, training and experience help.
Radar judgment can be further flummoxed by another radar shortcoming: Attenuation. In intense storms, water droplets can reach a size that corresponds to 3/4 of the radar wavelength, which causes all of the radiated energy to be absorbed. When all the RF power is sucked up, none is returned and the radar screen is blank. What appears to be no weather could actually be a killer level 6 cell.
Attenuation areas appear as bow-shaped echoes and there have been several celebrated accidents caused by pilots venturing into intense, radar-attenuated cells. Unfortunately, the laws of physics are such that the more hazardous the weather, the less able radar can see through it.
Spark Detectors
Sferics devices-Stormscopes and Strike Finders-have none of these limitations but they do have their own warts. Sferics devices are nothing but passive listening devices that detect and display lightning strikes as dots on a display.
These instruments have come a long way since Paul Ryan pioneered the first Stormscope in 1975. Unlike radar, sferics devices dont perform speed-of-light calculations on range and azimuth of storms based on reflected energy levels, but make educated guesses based on built-in mathematical modeling of what is, in the final analysis, a chaotic event in nature.
An early limitation of the Stormscope was something called radial spread. The receiver was incapable of discriminating between a weak strike close in or strong strike far away and thus could make only the vaguest guesses about range. This resulted in pie-shaped groups of dots, with the wide end of the slice closest to the storm. The only option was a wide-but safe-deviation. Subsequent iterations of the Stormscope and now the Strike Finder have supposedly tamed radial spread but we wonder how well. In flying with both systems and radar, we have seen occasional errors in both azimuth and range, sometimes significant. Again, avoiding the dots by a generous margin would have reliably skirted the weather, but at the cost of additional flying miles.
Still, a sferics devices major advantage-other than cheap cost-is its ability to see (really hear) storms over the horizon with a degree of accuracy suitable for decisionmaking. And sferics can do something radar cant: By merely switching on the unit on ground, you can instantly obtain a 360-degree assessment of the weather before leaving the ramp.
Sferics makers counsel against using these devices for penetrating lines of storms and we see this as sound advice. Compared to radar, a Stormscope or Strike Finder does a relatively crude job of painting cells and close in, the range is too questionable to chance it, in our view. Its one thing to navigate between well-defined dot clusters on the 50-mile range but something else to try and pick your way through the dots on the 25-mile range.
As a result, any deviations made based on sferics displays are likely to be far from the hazard area. True, that adds flying miles. But it also yields a conservative policy all but guaranteed to keep you clear of bumps, rain and hail that can be present at surprising distances from the cells themselves. In other words, with radar-even small antenna radar-theres a chance you can pick through a line; with sferics, you really shouldnt.
A persistent myth about sferics is that they somehow see turbulence in clear air, not related to lightning but to static electricity caused by swirling air. Actually, the reverse is true: Stormscopes and Strike Finders-at least late model units-hear only lightening strikes and may not necessarily display all that they hear.
The strike has to meet certain criteria in the receivers built-in model, which explains why youll occasionally see a strike visually but it wont appear on the sferics display. The thing isnt broken; its designed to work that way.
Cost, Installation
In basic performance and capability, late model sferics devices and radar are close in delivered capability, that being providing reliable deviation information. Each has strengths and weaknesses. But when cost is factored into the value decision, sferics far outdistances radar, both in initial purchase price and long-term maintenance costs.
Lets look at new radar prices first: There are two practical systems for GA singles and twins, Bendix/Kings RDR 2000 and Narcos KWX 56/58, a system which began life as a Bendix product but which was sold to Narco when Bendix acquired King Radio in the mid-1980s.
If you want a spanking new KWX 56 three-color radar, plan on spending about $21,000 for installation with a 10-inch antenna, including a wing pod, if needed. For a 12-inch antenna-twins only-add another $2000. Want the four-color KWX 58? Crank the bottom line up to $24,000, installed, plus the $2000 premium for the larger antenna.
The Bendix/King line is pricier, installing for about $28,000. Hunt around in the used market and were sure you can find a bargain RDR 160 for under $6000 or even a used RDR 2000 in the $15,000 range. Whether these are a good value depends on how well they hold up without significant repairs.
Speaking of which, radar repair is generally expensive. That antenna hanging out there on the wing has many moving parts and even the best pod will allow some moisture migration in the driving precip the pod has to resist. Moisture is the chief enemy of all radar installations.
Magnetrons eventually need replacement and these are among the most expensive parts in avionics. Were told that older radar displays are also becoming increasingly difficult to repair, meaning that the Avidyne approval for radar display is all the more attractive. (See sidebar.)
By comparison, the state-of-the-art Stormscope-the WX-950, is installing these days for about $6500 complete, while the fancier WX-1000 goes for closer to $9000. Insights Strike Finder installs for just under $5000 and real bargain hunters can still put it in a WX-900 (LCD display, 100-mile range limit) for under $4000. The WX-500 remote sensor Stormscope sells for about $5995 and has the same display capabilities as the WX-950; you provide the display.
Maintenance wise, we dont hear many howls of complaint about sferics devices in general. BFGoodrich has suffered the occasional spate of component failures on Stormscopes but weve never seen any consistent patterns.
Conclusion
The bottom line on the bottom line is this: For less than one third the money of a radar set-up-maybe a lot less-a sferics device provides adequate if conservative storm avoidance, it will be easier to install and maintain and you wont have a pod hanging off the wing.
In the ideal world, we think having both a color radar and a WX-950 would be perfect. But few of us have the checkbook to pay our way through the ideal world. Besides, unless you fly serious thunderstorm weather on a weekly basis, chances are you only need storm avoidance a handful of times each year. True, when you need it, you need it badly. But does that justify a $30,000 investment? We doubt it.
New display technology has already rendered the discrete radar/sferics approach somewhat obsolete and we think any owner considering a new installation should at least price an Avidyne or Apollo MX20 system with remote storm sensing equipment.
The day may have indeed arrived when its affordable to have a single display that projects both radar and lightning data, with datalinked doppler thrown in for good measure.
Also With This Article
Click here to view the Display Checklist.
Click here to view “Real World Deviation.”
Click here to view “Garmin, Avidyne Raise the Ante on Storm Display.”
Click here to view “Scope vs. Radar: User Report.”
Click here to view the Stormscope & Radar Addresses.
-by Paul Bertorelli
